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Metabolic flux in cellulose batch and cellulose- fed continuous

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					Microbiology (2001), 147, 1461–1471                                                                                                                                                       Printed in Great Britain




                                                                                Metabolic flux in cellulose batch and cellulose-
                                                                                fed continuous cultures of Clostridium
                                                                                cellulolyticum in response to acidic
                                                                                environment
                                                                                     $
                                                                                Mickael Desvaux, Emmanuel Guedon and Henri Petitdemange

                                                                                Author for correspondence : Henri Petitdemange. Tel : j33 3 83 91 20 53. Fax : j33 3 83 91 25 50.
                                                                                e-mail : hpetitde!lcb.uhp-nancy.fr


   Laboratoire de Biochimie                                                     Clostridium cellulolyticum, a nonruminal cellulolytic mesophilic bacterium, was
            !
   des Bacteries Gram j,                                                        grown in batch and continuous cultures on cellulose using a chemically defined
   Domaine Scientifique
   Victor Grignard, Universite!                                                 medium. In batch culture with unregulated pH, less cellulose degradation and
                   !      !
   Henri Poincare, Faculte des                                                  higher accumulation of soluble glucides were obtained compared to a culture
   Sciences, BP 239, 54506                                                      with the pH controlled at 72. The gain in cellulose degradation achieved with
                 '
   Vandœuvre-les-Nancy
                                                                                pH control was offset by catabolite production rather than soluble sugar
     !
   Cedex, France
                                                                                accumulation. The pH-controlled condition improved biomass, ethanol and
                                                                                acetate production, whereas maximum lactate and extracellular pyruvate
                                                                                concentrations were lower than in the non-pH-controlled condition. In a
                                                                                cellulose-fed chemostat at constant dilution rate and pH values ranging from
                                                                                74 to 62, maximum cell density was obtained at pH 70. Environmental
                                                                                acidification chiefly influenced biomass formation, since at pH 64 the dry
                                                                                weight of cells was more than fourfold lower compared to that at pH 70,
                                                                                whereas the specific rate of cellulose assimilation decreased only from 1174 to
                                                                                1013 milliequivalents of carbon (g cells)N1 hN1. The molar growth yield and the
                                                                                energetic growth yield did not decline as pH was lowered, and an abrupt
                                                                                transition to washout was observed. Decreasing the pH induced a shift from an
                                                                                acetate-ethanol fermentation to a lactate-ethanol fermentation. The
                                                                                acetate/ethanol ratio decreased as the pH declined, reaching close to 1 at pH
                                                                                64. Whatever the pH conditions, lactate dehydrogenase was always greatly in
                                                                                excess. As pH decreased, both the biosynthesis and the catabolic efficiency of
                                                                                the pyruvate-ferredoxin oxidoreductase declined, as indicated by the ratio of
                                                                                the specific enzyme activity to the specific metabolic rate, which fell from 98
                                                                                to 18. Thus a change of only 1 pH unit induced considerable metabolic change
                                                                                and ended by washout at around pH 62. C. cellulolyticum appeared to be
                                                                                similar to rumen cellulolytic bacteria in its sensitivity to acidic conditions.
                                                                                Apparently, the cellulolytic anaerobes studied thus far do not thrive when the
                                                                                pH drops below 60, suggesting that they evolved in environments where acid
                                                                                tolerance was not required for successful competition with other microbes.


                                                                                       Keywords : cellulolytic bacteria, flux analysis, environmental pH, cellulose degradation,
                                                                                                  chemostat


INTRODUCTION                                                                                                                                        cant in swamps, marshes, sediments, composts and
                                                                                                                                                    anaerobic waste treatment, and in the intestinal tracts of
Biological degradation of cellulosic materials is signifi-                                                                                           herbivorous animals and insects (Ljungdahl & Eriksson,
.................................................................................................................................................   1985). Within the biosphere, cellulolytic clostridia par-
Abbreviations : AADH, acetaldehyde dehydrogenase ; AK, acetate kinase ;                                                                             ticipate significantly in this process (Bayer & Lamed,
ADH, alcohol dehydrogenase ; ATP-Eff, efficiency of ATP generation ; Fd,                                                                             1992 ; Leschine, 1995 ; Tomme et al., 1995), which is
ferredoxin ; G1P, glucose 1-phosphate ; G6P, glucose 6-phosphate ; LDH,                                                                             strongly linked to the global carbon cycle (Wolin &
lactate dehydrogenase ; meq C, milliequivalent of carbon ; PFO, pyruvate–
ferredoxin oxidoreductase ; PTA, phosphotransacetylase ; R, ratio of
                                                                                                                                                    Miller, 1987).
specific enzyme activity to metabolic flux.                                                                                                           High concentrations of fermentation acids and, as a

0002-4608 # 2001 SGM                                                                                                                                                                                       1461
M. D E S V A UX, E. G U E D O N a n d H. P E T I T D E M A N GE


result, low pH conditions are often found in anaerobic            systems were used to study the effects of pH on growth
habitats (Ljungdahl & Eriksson, 1985 ; Goodwin &                  and metabolism of C. cellulolyticum.
Zeikus, 1987). Yet due to their particular pattern of
intracellular pH regulation (Huang et al., 1985) com-             METHODS
pared with other low-GjC Gram-positive anaerobes,
                                                                  Organism and medium. Clostridium cellulolyticum ATCC
mainly the so-called lactic acid bacteria, the clostridial-
                                                                  35319 was isolated from decayed grass by Petitdemange et al.
type bacteria are generally considered as restricted to           (1984). Germination of stocks of spores and anaerobic cell
less acidic ecological niches (Russell et al., 1996).             culture were performed as described by Desvaux et al. (2000).
Cellulolytic clostridia digest cellulose through extra-           The defined medium used in all experiments was a modified
cellular multienzyme complexes (Be! guin & Lemaire,               CM3 medium as described by Guedon et al. (1999a) containing
1996 ; Bayer et al., 1998). These cellulosomes are found          various amounts of cellulose MN301 (Macherey-Nagel) as
at the surface of the bacteria and allow both cell                specified in Results.
adhesion to cellulose fibres (Bayer et al., 1996) and very         Growth conditions. Clostridium cellulolyticum was grown
efficient degradative activity against crystalline cellulose        either in batch or in continuous culture with cellulose as sole
due to a high synergism of the different cellulase                 carbon and energy source. All experiments were performed in
                                                                  a 1n5 l working volume fermenter (LSL Biolafitte) as previously
components (Boisset et al., 1999).                                described (Guedon et al., 1999a, b) ; the temperature was
Clostridium cellulolyticum is a low-GjC Gram-posi-                maintained at 34 mC and the pH was controlled by automatic
tive nonruminal cellulolytic mesophilic bacterium be-             addition of 3 M NaOH or 1 M HCl as specified in Results.
longing to the clostridial group III, and also classified in       The inoculum was 10 % (v\v) from an exponentially growing
family 4, genus 2, of a new proposed hierarchical                 culture.
structure for clostridia (Collins et al., 1994). Using            Batch cultures were prepared as previously described
cellobiose, a soluble substrate, several advances in              (Desvaux et al., 2000). The chemostat system used was a
understanding of the metabolism of this bacterium have            segmented gas–liquid continuous culture device as described
been made, such as (i) a better control of catabolism in          by Weimer et al. (1991b) with some modifications (Desvaux et
                                                                  al., 2001). The cultures were maintained for a period of eight
a mineral salt-based medium (Payot et al., 1998 ; Guedon          to nine residence times (Desvaux et al., 2001) ; for each
et al., 1999b), (ii) recognition of major differences              condition the data were the mean of at least three samples.
in regulatory responses in cellobiose-limited and
                                                                  Analytical procedures. Biomass, cellulose concentration,
cellobiose-saturated chemostat cultures (Guedon et al.,           gases, extracellular proteins, amino acids, glucose, soluble
2000b), and (iii) the importance of glucose 6-phosphate           cellodextrins, glycogen, acetate, ethanol and lactate extra-
(G6P) and glucose 1-phosphate (G1P) nodes in regu-                cellular pyruvate were assayed as described by Desvaux et al.
lation of metabolic fluxes (Guedon et al., 2000a). Earlier         (2001). Pyridine nucleotides, coenzyme A (CoA), acetyl-CoA,
investigations using cellulose have been mainly devoted           G1P and G6P were extracted and fluorimetric determination
to cellulolytic performance and bacterial behaviour               performed as previously described (Desvaux et al., 2001). ATP
towards insoluble substrates (Giallo et al., 1985 ;               and ADP were measured using the luciferin–luciferase
Gelhaye et al., 1993a, b). A few studies, however, have           luminescence system (Microbial Biomass Test Kit, Celsis
focused on the metabolism of this bacterium on cellu-             Lumac) (Guedon et al., 2000a).
lose : recent investigation of cellulose fermentation             Enzyme assays. Cell extracts were prepared and enzyme
performed in batch culture (Desvaux et al., 2000)                 assays performed as previously described (Guedon et al.,
indicated (i) variation of metabolite yields as a function        2000a). Pyruvate-ferredoxin (Fd) oxidoreductase (PFO) (EC
of initial cellulose concentration, and (ii) an early growth      1;2;7;1), lactate dehydrogenase (LDH) (EC 1;1;1;27),
                                                                  phosphotransacetylase (PTA) (EC 2;3;1;8), acetate kinase
inhibition related to pyruvate overflow as with cello-             (AK) (EC 2;7;2;1), acetaldehyde dehydrogenase (AADH)
biose (Guedon et al., 1999a). A study in a cellulose-             (EC 1;2;1;10) and alcohol dehydrogenase (ADH) (EC
limited chemostat indicated that bacterial metabolism             1;1;1;1) were assayed as described by Desvaux et al. (2001).
was not as distorted as with cellobiose and C. cellulo-           Calculations and carbon flux analysis. The metabolic path-
lyticum appeared well adapted to a cellulolytic lifestyle         ways and equations of cellulose fermentation by C. cellulo-
(Desvaux et al., 2001).                                           lyticum, expressed as n hexose equivalents (hexose eq)
Whereas the effects of acidic conditions on the growth of          corresponding to n glucose residues of the cellulose chain,
cellulolytic rumen bacteria have been the subject of              were previously reported (Desvaux et al., 2000).
considerable research (Russell & Dombrowski, 1980 ;               The qcellulose is the specific rate of hexose residue fermented in
Kalachniuk et al., 1994 ; Russell & Wilson, 1996 ; Russell        mmol (g cells)−" h−". qacetate, qethanol and qlactate are the specific
& Diez-Gonzalez, 1998), little is known of how these              rates of product formation in mmol (g cells)−" h−".
conditions affect the metabolism of cellulolytic clostridia        qextracellular pyruvate is the specific rate of extracellular pyruvate
                                                                  formation in µmol (g cells)−" h−". qNADH produced and
(Duong et al., 1983 ; Mitchell, 1998). The aim of the             qNADHconsumed are the specific rates of NADH production and
present work was to investigate the cellulose degra-              NADH consumption respectively in mmol (g cells)−" h−"
dation and metabolic changes of C. cellulolyticum on              and were calculated as follows : qNADH produced l qpyruvate and
insoluble cellulose caused by environmental pH con-               qNADH consumed l 2 qethanoljqlactate. The specific rate of acid
ditions. Since conditions in natural environments most            production (O’Sullivan & Condon, 1999), was calculated as
likely resemble those somewhere between a closed batch            follows : qH+ l qacetatejqlactatejqextracellular pyruvate.
culture and an open continuous culture system                     The molar growth yield (Yx/s) is expressed in g cells (mol
(Kova! rova! -Kovar & Egli, 1998), both types of culture          hexose eq fermented)−". The energetic yield of biomass (YATP)

1462
                                                                                                                                                                                   C. cellulolyticum carbon flux in acid environment




.................................................................................................................................................................................................................................................................................................................
              Fig. 1. Carbon flux distribution within the central metabolic pathways of C. cellulolyticum when grown on cellulosic
              substrate (n is the number of hexose residues inside the biopolymer). Carbon flow corresponds to the equations given in
              Methods.


was (Desvaux et al., 2000) : YATP l concnbiomass\(1n94                                                                                                          qpyruvate l
concnacetatej0n94 concnethanolj0n94 concnlactatej 0n94                                                                                                          qacetatejqethanoljqlactatejqextracellular pyruvatejqcarbon dioxide
concnextracellular pyruvate). YATP is expressed in g cells (mole ATP
                                                                                                                                                                The turnover of a pool (h−") was calculated from specific rate
produced)−". qATP is the specific rate of ATP generation in
                                                                                                                                                                and pool size expressed in moles or in carbon equivalents
mmol (g cells)−" h−", calculated by the following equation
                                                                                                                                                                (Holms, 1996). The ratio R corresponded to the ratio of
(Desvaux et al., 2000) : qATP l 1n94 qacetatej0n94 qethanolj
                                                                                                                                                                specific enzyme activity to metabolic flux (Holms, 1996 ;
0n94 qlactatej0n94 qextracellular pyruvate. The energetic efficiency
                                                                                                                                                                Desvaux et al., 2001).
(ATP-Eff) corresponding to the ATP generation in cellulose
catabolism is given by the ratio of qATP to qcellulose (Miyagi
et al., 1994).                                                                                                                                                  RESULTS
Distribution of the carbon flow by stoichiometric flux analysis                                                                                                   Kinetic profile in batch cellulose fermentation
(Papoutsakis, 1984 ; Desai et al., 1999a, b) was determined by
adapting the model developed by Holms (1996) to C.                                                                                                              C. cellulolyticum was grown in a bioreactor on defined
cellulolyticum metabolism (Desvaux et al., 2001).                                                                                                               medium with 6n7 g cellulose l−" under non-pH-con-
                                                                                                                                                                trolled and pH-controlled conditions (Fig. 2). In the
At steady state, the carbon flux through each enzyme of                                                                                                          non-pH-controlled run, the pH dropped to 5n3 after 6 d
the known metabolic pathway, as indicated in Fig. 1,                                                                                                            culture (Fig. 2Ia). Compared to the batch culture
was calculated in milliequivalents of carbon (meq C)                                                                                                            controlled at pH 7n2, the final biomass attained in the
(g cells)−" h−", as follows :                                                                                                                                   non-pH-controlled condition was lower, as was the
qG P l 0n63 qcellulose                                                                                                                                          percentage of cellulose degradation, which reached
   "                                                                                                                                                            67 %, as against 86 % in the pH-controlled fermentation
qglucose l 0n37 qcellulose
                                                                                                                                                                (Figs 2Ib and 2IIb). The maximum specific growth rate,
qbiosynthesis l qbiomasskqglycogenjqamino acidjqextracellular protein                                                                                           however, was similar in non-pH-controlled and pH-
qG P l qbiosynthesisjqpyruvate                                                                                                                                  controlled batch culture, i.e. 0n053 and 0n056 h−" re-
  '
qphosphoglucomutase l qG Pkqglucose                                                                                                                             spectively.
                         '
qexopolysaccharide l qG Pkqphosphoglucomutasekqglycogen                                                                                                         The metabolite production profiles were clearly different
                       "                                                                                                                                        in the two fermentation modes (Fig. 2Ic and 2IIc). With
qacetyl-CoA l qacetatejqethanol                                                                                                                                 pH regulation, higher final levels of acetate and ethanol
qcarbon dioxide l (qacetatejqethanol)\2                                                                                                                         were reached, whereas the lactate and extracellular

                                                                                                                                                                                                                                                                                                     1463
M. D E S V A UX, E. G U E D O N a n d H. P E T I T D E M A N GE


                                                                             No pH control                                                                                                                                With pH control
                                                             8                                                                                                                                         8
                                                             7                                                                                                                                         7



                                     pH




                                                                                                                                                                               pH
                                                             6                                                                                                                                         6
                                                                      (Ia)                                                                                                                                      (IIa)
                                                             5                                                                                                                                         5
                                                                      (Ib)                                                                                                                                      (IIb)




                                                                                                                                           Residual cellulose (g I–1)




                                                                                                                                                                                                                                                                        Residual cellulose (g I–1)
                                                            600                                                                  6                                                                    600                                                     6
                                   Biomass (mg I–1)




                                                                                                                                                                             Biomass (mg I–1)
                                                            500                                                                  5                                                                    500                                                     5
                                                            400                                                                  4                                                                    400                                                     4
                                                            300                                                                  3                                                                    300                                                     3
                                                            200                                                                  2                                                                    200                                                     2
                                                            100                                                                  1                                                                    100                                                     1
                                                             0                                                                   0                                                                     0                                                      0




                                                                                                                                               Extracellular pyruvate (µM)




                                                                                                                                                                                                                                                                           Extracellular pyruvate (µM)
                                                                      (Ic)                                                                                                                                      (IIc)
                                                                                                                                 600
                                                                                                                                                                                                                                                              600
                                     Product (mM)




                                                                                                                                                                               Product (mM)
                                                            20                                                                                                                                        20
                                                                                                                                 400
                                                                                                                                                                                                                                                              400

                                                            10                                                                                                                                        10
                                                                                                                                 200
                                                                                                                                                                                                                                                              200


                                                             0                                                                   0                                                                     0                                                      0
                                                                      (Id)                                                                                                                                        (IId)
                                     Soluble glucide (mM)




                                                                                                                                                                               Soluble glucide (mM)


                                                            1·5                                                                                                                                       1·5


                                                            1·0                                                                                                                                       1·0


                                                            0·5                                                                                                                                       0·5


                                                              0                                                                                                                                         0
                                                                  0          50          100                            150                                                                                 0             50         100             150
                                                                                   Time (h)                                                                                                                                    Time (h)
.................................................................................................................................................................................................................................................................................................................
              Fig. 2. pH variation (a), growth and residual cellulose concentration (b), metabolite concentrations (c), and soluble
              glucide accumulation (d) during cellulose batch fermentation without (I) and with (II) pH control. X, pH ; #, cellulose ; $,
              biomass ; , acetate ; >, ethanol ; =, lactate ; 
, extracellular pyruvate ; W, glucose ; 4, cellobiose ; 5, cellotriose.



pyruvate concentrations attained in the course of fer-                                                                                                                       longer cellodextrins could not be detected either by
mentation were higher in a non-pH-controlled run.                                                                                                                            HPLC or by TLC techniques. Inasmuch as cellulose was
Moreover, with the pH controlled at 7.2, pyruvate was                                                                                                                        degraded to a greater extent in the pH-controlled
reconsumed in the course of the fermentation more                                                                                                                            culture, the difference in accumulation of sugars may
rapidly than without pH control. Thus once growth                                                                                                                            reflect a difference in the cellulose substrate, e.g. its
stopped, biocatabolic activity of cells ceased in the non-                                                                                                                   surface area (Weimer et al., 1990, 1991a ; Fields &
pH-controlled culture whereas in the pH-controlled                                                                                                                           Russell, 2000), at the time cultures entered stationary
condition the degraded cellulose was further metab-                                                                                                                          phase, and\or differences in metabolism of stationary-
olized by resting cells.                                                                                                                                                     phase cells.
The kinetic profile for individual soluble glucide ac-                                                                                                                        Effect of pH on the growth and metabolite
cumulation indicated that it was a non-growth-                                                                                                                               production of C. cellulolyticum in cellulose
associated event (Figs 2Id and 2IId). A higher level of                                                                                                                      continuous culture
soluble cello-oligosaccharides was achieved in fermen-
tation without pH control : up to 1n56 mM cellobiose                                                                                                                         Growth parameters, notably end products measured at
was detected as well as 0n36 mM cellotriose. With pH                                                                                                                         each steady state, as a function of the pH value are
control, the sugar level in the broth medium was very                                                                                                                        compiled in Table 1. C. cellulolyticum was grown in
limited since only 0n16 mM cellobiose was found and no                                                                                                                       continuous culture under cellulose-limited conditions
cellotriose could be detected. In both culture conditions,                                                                                                                   and at a constant dilution rate of 0n053 h−" with pH

1464
                                                                                                                                                                                                                                    C. cellulolyticum carbon flux in acid environment

Table 1. Fermentation parameters from continuous steady-state cultures of
C. cellulolyticum as a function of environmental pH at D l 0n053 h−1
.....................................................................................................................................................................................................................................

The cellulose input was 0n37 % (w\v). Values are the means of samples at steady state and are
shownpstandard deviation where appropriate. Where no standard deviation is given, individual
values did not vary from the mean by more than 10 %.

    Parameter                                                                                                                       Results obtained at pH value of

                                                                                    7n4                                              7n2                                                                  7n0                      6n8                                                         6n4

    Biomass (g l−")                   0n129p0n011 0n207p0n018 0n234p0n021 0n164p0n014 0n056p0n005
    Cellulose consumed                 4n75p0n22 7n46p0n38 8n14p0n41 5n31p0n25 1n79p0n09
     (mM hexose eq)
    qcellulose [mmol (g cells)−" h−"]      1n95        1n91        1n84        1n72        1n69
    qpyruvate [mmol (g cells)−" h−"]       2n76        2n63        2n46        2n31        2n18
    Product yield ( %)*
      Acetate                                                                    70n3                                               68n11                                                                67n2                    62n3                                                         47n7
      Ethanol                                                                    29n3                                               30n4                                                                 30n8                    33n4                                                         44n8
      Lactate                                                                     0n5                                                1n5                                                                  2n0                     4n4                                                          7n5
    qextracellular pyruvate             3n91                                                                                         9n47                                                                 7n29                   13n90                                                        32n42
     [µmol (g cells)−" h−"]
    Glycogen [mg (g cells)−"]       91n5p2n6                                                                         96n8p2n9                                                  107n3p4n0                                   78n8p2n3                                                       62n7p1n7
    Extracellular proteins (mg l−") 11n3p0n7                                                                         18n5p0n9                                                   18n2p0n8                                   15n4p0n6                                                        4n0p0n3
    Free amino acids (mg l−")       33n4p1n6                                                                         63n3p3n6                                                   62n9p4n0                                   47n1p2n1                                                       13n9p0n8
    Carbon recovery (%)                94n7                                                                             94n2                                                       92n3                                       95n3                                                           91n4

* The product yields were expressed as percentages of qpyruvate.




                                                                                                40                                                                       100
                                                                                                                                                                                                                                                        qproduct [mmol (g cells)–1 h–1]




                                                                                                                                                                                                                                                                                               (a)
                                                                                                                                     YX/S [g cells (mol hexose eq.)–1]




                                                                                                                                                                                                                                                                                          2
                                                                                                       YATP [g cells (mol ATP)–1]




                                                                                                                                                                                Degraded cellulose (%)




                    200                                                                                                                                                  80
                                                                                                30
 Biomass (mg I–1)




                                                                                                                                                                         60
                                                                                                20
                                                                                                                                                                                                                                                                                          1
                    100                                                                                                                                                  40

                                                                                                10
                                                                                                                                                                         20
                                                                                                                                                                                                                                                                                          0
                     0                                                                       0                                                                            0                                                                                                                    (b)
                           7·5                  7·0                   6·5                  6·0
                                                                                                                                                                                                                                        Ethanol/acetate ratio




                                                  pH                                                                                                                                                                                                                         1·5
                                                                                                                                                                                                                                            H2/CO2 ratio




.................................................................................................................................................
Fig. 3. Dry weight of cells, percentage cellulose degradation,
and molar and energetic growth yields during growth of C.                                                                                                                                                                                                                    1·0
cellulolyticum in continuous culture (D l 0n053 h−1) under
various pH conditions and with cellulose as sole carbon
and energy source. $, Biomass ; #, percentage cellulose                                                                                                                                                                                                                      0·5
degradation ; , YX/S ; 
, YATP.

                                                                                                                                                                                                                                                                                          0
                                                                                                                                                                                                                                                                                                7·5   7·0     6·5                  6·0
values ranging from 7n4 to 6n4. The primary metabolic                                                                                                                                                                                                                                                    pH
end products of the cellulose fermentation were acetate,
                                                                                                                                                                                                                 .................................................................................................................................................
ethanol, lactate, H and CO . In addition to carbon
                    #          #
conversion into biomass, amino acids and extracellular                                                                                                                                                           Fig. 4. Specific production rate (a) and product ratio (b) during
                                                                                                                                                                                                                 growth of C. cellulolyticum in cellulose-fed continuous culture
proteins were also detected in the supernatant (Table 1).                                                                                                                                                        (D l 0n053 h−1) under various pH conditions.              , qacetate ; >,
Exopolysaccharides were readily observable by micro-                                                                                                                                                             qethanol ; =, qlactate ; $, qH+ ; #, H2/CO2 ; 
, ethanol/acetate ratio.

                                                                                                                                                                                                                                                                                                                                                      1465
M. D E S V A UX, E. G U E D O N a n d H. P E T I T D E M A N GE


Table 2. Redox and energetic balance of C. cellulolyticum cells at steady state

   Parameter                                                                                                                                                          Results obtained at pH value of

                                                                                                                    7n4                                       7n2                                      7n0                                      6n8                                      6n4

   NADH [µmol (g cells)−"]                                                                                4n75p0n85                                4n68p0n90                                4n90p0n97                                 5n26p1n02                                5n93p1n14
   NAD+ [µmol (g cells)−"]                                                                               11n31p2n22                               12n01p2n36                               11n40p2.25                                11n22p2n17                               11n69p2n33
   NADH\NAD+ ratio                                                                                            0n42                                     0n39                                     0n43                                      0n47                                     0n51
   qNADH produced [mmol (g cells)−" h−"]                                                                      2n76                                     2n63                                     2n46                                      2n31                                     2n18
   qNADH used [mmol (g cells)−" h−"]                                                                          1n63                                     1n64                                     1n56                                      1n64                                     2n11
   qNADH produced\qNADH used ratio                                                                            1n69                                     1n61                                     1n57                                      1n41                                     1n03
   Pool ATPjADP [µmol (g cells)−"]                                                                            5n13                                     4n79                                     5n39                                      5n92                                     6n11
   ATP\ADP ratio                                                                                              0n50                                     0n46                                     0n43                                      0n39                                     0n34
   qATP [mmol (g cells)−" h−"]                                                                                4n53                                     4n26                                     3n97                                      3n61                                     3n08
   ATP-Eff*                                                                                                    2n32                                     2n23                                     2n15                                      2n10                                     1n82

* ATP-Eff is the ATP generation efficiency.


Table 3. Estimation of carbon flow based on the steady-state values from a cellulose chemostat of C. cellulolyticum at
D l 0n053 h−1 as a function of environmental pH
.................................................................................................................................................................................................................................................................................................................

Carbon flow was calculated as specified in Methods. See Fig. 1 for the metabolic pathways. The values are expressed both as meq C
(g cells)−" h−" and, in parentheses, as a percentage of the specific rate of cellulose consumed (qcellulose).

   Carbon flow                                                                                                                                          Results obtained at pH value of

                                                                                         7n4                                            7n2                                             7n0                                            6n8                                            6n4

   qcellulose                                                                 11n74 (100n0)                                  11n46 (100n0)                                   11n04 (100n0)                                  10n56 (100n0)                                  10n13 (100n0)
   qG P                                                                        7n39 (63n0)                                    7n22 (63n0)                                     6n95 (63n0)                                    6n65 (63n0)                                    6n38 (63n0)
      "
   qglycogen                                                                   0n18 (1n5)                                     0n19 (1n7)                                      0n21 (1n9)                                     0n16 (1n5)                                     0n12 (1n2)
   qexopolysaccharide                                                          0n41 (3n5)                                     0n42 (3n7)                                      0n60 (5n5)                                     0n51 (4n8)                                     0n63 (6n2)
   qglucose                                                                    4n35 (37n0)                                    4n24 (37n0)                                     4n09 (37n0)                                    3n91 (37n0)                                    3n75 (37n0)
   qphosphoglucomutase                                                         6n80 (57n9)                                    6n60 (57.6)                                     6n14 (55n6)                                    5n99 (56n7)                                    5n63 (55n5)
   qG P                                                                       11n15 (94n9)                                   10n85 (94n7)                                    10n23 (92n6)                                    9n90 (93n7)                                    9n38 (92n6)
      '
   qextracellular pyruvate                                                     0n01 (0n1)                                     0n03 (0n2)                                      0n02 (0n2)                                     0n04 (0n4)                                     0n10 (1n0)
   qbiosynthesis                                                               2n84 (24n2)                                    2n94 (25n7)                                     2n84 (25n7)                                    2n92 (27n6)                                    2n78 (27n4)
   qpyruvate                                                                   8n31 (70n7)                                    7n90 (69n0)                                     7n39 (66n9)                                    6n98 (66n1)                                    6n60 (65n2)
   qacetyl-CoA                                                                 5n50 (46n9)                                    5n17 (45n1)                                     4n81 (43n6)                                    4n43 (41n9)                                    4n01 (39n6)
   qlactate                                                                    0n04 (0n3)                                     0n12 (1n0)                                      0n15 (1n4)                                     0n30 (2n9)                                     0n49 (4n8)
   qCO                                                                         2n75 (23n4)                                    2n59 (22n6)                                     2n41 (21n8)                                    2n21 (21n0)                                    2n01 (19n8)
        #
   qethanol                                                                    1n62 (13n8)                                    1n60 (13n9)                                     1n51 (13n7)                                    1n54 (14n6)                                    1n94 (19n2)
   qacetate                                                                    3n88 (33n1)                                    3n58 (31n2)                                     3n30 (29n9)                                    2n88 (27n3)                                    2n07 (20n4)



scopic examination but could not be measured as                                                                                                                 (Fig. 3). When the pH value was decreased from 7n4 to
previously described (Payot et al., 1998) due to the                                                                                                            7n0, the percentage of cellulose degradation increased
significant interference with cellulose fibres leading to                                                                                                         from 20n6 to 34n2 % and then dropped to reach 7n3 % at
erroneous estimation of their concentration. Taking                                                                                                             pH 6n4. Whereas C. cellulolyticum showed depressed
into account amino acids, proteins, fermentative end                                                                                                            dry weight of cells at pH values higher than 7n0, the
products and biomass concentration, the carbon                                                                                                                  observed cell yields (YX/S) did not decline, even at a pH
balance ranged between 91n4 and 95n3 % (Table 1).                                                                                                               value close to washout (Fig. 3) ; YX/S increased from 27n1
As the pH was lowered from 7n4 to 7n0, the dry weight of                                                                                                        to 31n3 g cells (mol ATP)−" between pH 7n4 and 6n4. The
cells increased (Fig. 3) ; with a further pH decline,                                                                                                           energetic yield of biomass (YATP) changed in the same
however, the cell density decreased and a steady state of                                                                                                       way as YX/S ; such a result means that approximately the
the culture could not be established at pH 6n2 since                                                                                                            same amount of ATP was used for the process implicated
washout occurred. Whatever the pH conditions, C.                                                                                                                in cell growth regardless of the pH value.
cellulolyticum always left some cellulose undigested                                                                                                            In all runs, acetate was the main fermentative end

1466
                                                                                                                                                           C. cellulolyticum carbon flux in acid environment


                                                                  30                                                                                grown at the different pH values, continuously decreased
                                                                       (a)                                                                          from 1n95 to 1n23 mmol (g cells)−" h−" (Fig. 4a).


                                  G1P or G6P [µmol (g cells)–1]
                                                                                                                                                    Redox and energetic balance in a cellulose-fed
                                                                  20
                                                                                                                                                    chemostat at various pH values
                                                                                                                                                    The NADH balance calculated from the known meta-
                                                                  10
                                                                                                                                                    bolic pathways producing and consuming reducing
                                                                                                                                                    equivalents,i.e.qNADH produced\qNADH used,continuously
                                                                                                                                                    decreased with pH from its value of 1n69 at pH 7n4 and
                                                                                                                                                    was approximately 1 at pH 6n4 (Table 2). Whereas
                                                                   0                                                                                NAD+ molecules are reduced by glyceraldehyde 3-
                                                                       (b)                                                                          phosphate dehydrogenase during ethanol, lactate or
                                      G6P/G1P ratio




                                                                                                                                                    acetate formation, only the ethanol and lactate pro-
                                                                  1
                                                                                                                                                    duction pathways allow the regeneration of the NAD+
                                                                                                                                                    pool via dehydrogenase activities. Despite the apparent
                                                                                                                                                    imbalance at pH values higher than 6n4, the intracellular
                                                                                                                                                    NADH\NAD+ ratio was always lower than 1 (Table 2).
                                                                  0                                                                                 At pH values higher than 6n4, qlactate and qethanol were
                                                                       (c)                                                                          then not sufficient to regenerate NADH to NAD+. This
                                                                  8                                                                                 result correlated with the H \CO ratios, which were
                                                                                                                                                    always higher than 1 (Fig. #4b), #
                         Acetyl-CoA or CoA
                          [µmol (g cells)–1]




                                                                                                                                                                                       suggesting that low
                                                                                                                                                    lactate and ethanol production was balanced by H gas
                                                                  6
                                                                                                                                                    formation via NADH-Fd reductase and hydrogenase     #
                                                                  4                                                                                 activities.
                                                                                                                                                    The stoichiometry of ATP generated over hexose eq
                                                                  2                                                                                 fermented (ATP-Eff) decreased from 2n32 to 1n82 as the
                                                                                                                                                    pH was lowered from 7n4 to 6n4 ; qATP also declined,
                                                                  0                                                                                 from 4n53 to 3n08 mmol (g cells)−" h−" (Table 2). The
                                      Acetyl-CoA/CoA ratio




                                                                       (d)                                                                          ATPjADP pool fluctuated between 4n79 and 6n11 µmol
                                                                  5
                                                                  4
                                                                                                                                                    (g cells)−", while the ATP\ADP ratio decreased from
                                                                                                                                                    0n50 to 0n34 with environmental acidification (Table 2).
                                                                  3
                                                                                                                                                    This variation was probably correlated with the change
                                                                  2                                                                                 of the end products, namely the decrease of acetate
                                                                  1                                                                                 biosynthesis.
                                                                  0
                                                                       7·5   7·0             6·5                 6·0
                                                                                                                                                    Metabolic flux analysis of cells grown on cellulose in
                                                                                pH
                                                                                                                                                    an acid environment
.................................................................................................................................................
Fig. 5. Effect of pH on the pools (a, c) and ratio (b, d) of G1P
                                                                                                                                                    The metabolism of C. cellulolyticum when grown on
and G6P (a, b), and of acetyl-CoA and CoA (c, d), when C.                                                                                           cellulose is depicted in Fig. 1. With acidification of the
cellulolyticum is grown on cellulose in continuous culture at                                                                                       growth medium from pH 7n4 to 6n4, the rate of cellulose
D l 0n053 h−1. #, G6P ; $, G1P ; =, G6P/G1P ; 
, acetyl-CoA ;                                                                                       consumption declined from 11n74 to 10n13 meq C (g
  , CoA ; >, acetyl-CoA/CoA.                                                                                                                        cells)−" h−" (Table 3). The proportion of the cellulose
                                                                                                                                                    converted into biomass, free amino acids and extra-
                                                                                                                                                    cellular proteins, i.e. qbiosynthesis, varied from 24n2 to
product : it represented between 70n3 and 47n7 % of the                                                                                             27n4 % with this pH decline (Table 3) ; considering each,
carbon directed towards catabolites (Table 1). With                                                                                                 the biomasss formation increased from 17n9 to 20n7 % of
lowering of pH, the specific production rate of ethanol                                                                                              the original carbon while cellulose conversion into
increased while that of acetate declined, the latter always                                                                                         amino acid and extracellular protein accounted for
remaining the higher (Fig. 4a). The ethanol-to-acetate                                                                                              around 5n3 and 1n6 % of the carbon uptake respectively.
ratio increased with decreasing pH and reached 0n94 at                                                                                              Another part of the carbon flow was directed towards
pH 6n4 (Fig. 4b). In contrast, the H \CO ratio followed                                                                                             metabolite fermentation, i.e. acetate, ethanol, CO ,
                                           #
a downward trend with decreasing#pH (Fig. 4b). Lactate                                                                                              extracellular pyruvate and lactate, which as a whole     #
was also produced at all pH values, but whereas the                                                                                                 decreased from 70n7 to 65n2 % of the cellulose consumed
specific production rate (qlactate) was low at pH 7n4, i.e.                                                                                          (Table 3). Metabolism distributed carbon differently
0n01 mmol (g cells)−" h−", it increased as the pH                                                                                                   over known catabolic routes as the pH declined. Carbon
decreased, reaching 0n16 mmol (g cells)−" h−" at pH 6n4                                                                                             flow through the CO and acetate formation pathways
(Fig. 4a). As the pH was lowered, the specific rate of acid                                                                                                                 #
                                                                                                                                                    declined from 23n4 to 19n8 % and from 33n1 to 20n4 %
production (qH+), calculated from the spectrum of acid                                                                                              respectively. However, ethanol formation increased
end products formed in cellulose chemostat culture                                                                                                  from 13n8 to 19n2 % of the carbon consumed ; lactate

                                                                                                                                                                                                         1467
M. D E S V A UX, E. G U E D O N a n d H. P E T I T D E M A N GE


Table 4. Specific enzymic activity and flux relative to available enzyme activity in C. cellulolyticum cell extract at steady
state

 Enzyme*                                                          Results obtained at pH value of

                            7n4                        7n2                       7n0                      6n8                  6n4

                     SEA†           R‡           SEA              R        SEA           R          SEA          R       SEA          R

 PTA              0n83p0n08        19n1      0n97p0n07        24n0      0n71p0n08       19n2   0n61p0n05        18n8   0n46p0n06     19n5
 AK               0n88p0n09        20n6      0n83p0n10        20n1      0n64p0n06       17n2   0n49p0n05        15n2   0n30p0n03     12n9
 AADH             0n12p0n01         6n3      0n18p0n02        10n1      0n21p0n01       12n4   0n19p0n01        11n3   0n22p0n03      9n8
 ADH              0n29p0n02        15n7      0n28p0n03        15n6      0n25p0n02       14n9   0n23p0n01        13n3   0n27p0n03     12n3
 LDH              0n15p0n01       529n5      0n19p0n03       215n3      0n18p0n01      155n4   0n14p0n01        62n9   0n17p0n02     46n1
 PFO              0n61p0n05         9n8      0n66p0n03        11n2      0n34p0n04        6n3   0n13p0n02         2n5   0n08p0n01      1n8

* PTA, phosphotransacetylase ; AK, acetate kinase ; AADH, acetaldehyde dehydrogenase ; ADH, alcohol dehydrogenase ; LDH, -lactate
dehydrogenase ; PFO, pyruvate-Fd oxidoreductase.
† SEA is the specific activity of enzyme expressed in µmol min−" (mg protein)−".
‡ R is the ratio of specific enzymic activity to metabolic flux through the considered metabolic pathway. Flux was previously expressed
as µmol (mg protein)−" min−".


formation also increased, from only 0n3 % at pH 7n4 to                       expense of qpyruvate, which decreased from 70n7 to
4n8 % at pH 6n4. At the same time, the proportion of                         65n2 meq C (g cells)−" h−" (Table 3) and (ii) the decrease
carbon flowing towards extracellular pyruvate rose from                       of carbon flowing through acetate production from 33n1
0n1 to 1 % with lower environmental pH.                                      to 20n4 % whereas it increased only from 13n8 to 19n2 %
As the pH decreased, the G6P pool slightly increased                         through the ethanol pathway as pH declined (Table 3).
(Fig. 5a) ; in terms of turn-over, this pool varied from
116n4 to 86n5 h−" from pH 7n4 to 6n4. This result was                        Enzymic activities as a function of environmental pH
correlated with the concomitant decrease of the carbon                       The influence of environmental pH on the specific
flow through glycolysis on the one hand and the increase                      activities of the enzymes studied is compiled in Table 4.
of the carbon flow through the biosynthesis pathway on                        In vitro, PFO, PTA, AK and AADH activities were
the other, as the environmental pH fell. The qG P
decreased from 11n15 to 9n38 meq C (g cells)−" h−" with   '                  higher under conditions giving higher in vivo specific
                                                                             production rates (Table 4). When the carbon flow was
acidification, which represented 94n9–92n6 % of the                           expressed as µmol (mg protein)−" h−" from previously
original carbon metabolized (Table 3). As for the G1P                        calculated values (Table 3), R (the ratio of the specific
pool, it increased with decreasing pH (Fig. 5a) which                        enzyme activity to metabolic rate) could be calculated
was expressed as a decrease of the turn-over of this pool                    (Holms, 1996). In the metabolic branch leading to
from 91n3 to 42n7 h−" from pH 7n4 to 6n4. As a result the                    acetate production through PTA and AK, R fluctuated
G6P\G1P ratio ranged from 1n19 to 0n73 with decreasing                       between 18n8 and 24n0, and between 20n6 and 12n9,
environmental pH (Fig. 5b). G1P is an important node in                      respectively, as pH decreased (Table 4). R for the
cellulose metabolism since carbon could either flow                           enzymes of the ethanol pathway varied between 6n3 and
down glycolysis via phophoglucomutase or stored as                           12n4, and between 15n7 and 12n3 for AADH and ADH,
glycogen, or be converted into exopolysaccharide.                            respectively (Table 4). At each step in the central
qphosphoglucomutase decreased from 6n80 to 5n63 meq C (g                     metabolic pathways, the intracellular concentration of
cells)−" h−" with lowering of pH ; when expressed as a                       substrates, products, cofactors or effector molecules as
percentage of qcellulose, this flux ranged from 57n9 to                       well as intracellular ionic strength, redox potential or
55n5 % (Table 3). The proportion of carbon through                           pH can influence the partition and regulation of the
qglycogen varied from 1n2 to 1n9 %, while qexopolysaccharide                 carbon flux (Holms, 1986). Nevertheless, the fact that
represented 3n5 % of the carbon uptake at pH 7n4 to                          fluxes were much less than the available enzyme activity
reach 6n2 % at pH 6n4 (Table 3).                                             indicated that the carbon flows were determined by the
The CoA pool was fuelled by phosphotransacetylase                            concentration of substrate available rather than the
and acetaldehyde dehydrogenase, whereas acetyl-CoA                           enzyme activity (Holms, 1996). Despite the variation of
was formed via pyruvate-Fd oxidoreductase activity                           enzyme biosynthesis, the amount of these enzymes was
(Fig. 1). With lowering of the pH, both pools increased                      always sufficient to catabolize the flowing carbon since
(Fig. 5c), however the acetyl-CoA\CoA ratio decreased                        R was much higher than 1. At pH 7n4, for the lactate
from 4n46 to 3n06 (Fig. 5d). Such results were correlated                    formation pathway, R was very high, i.e. 529n5. This
with (i) the rerouteing of carbon flow towards bio-                           indicated that although LDH was readily available, little
synthesis, which increased from 24n2 to 27n4 % at the                        carbon was catabolized by this metabolic pathway. As

1468
                                                                    C. cellulolyticum carbon flux in acid environment


the pH decreased, however, R declined to 46n1, indi-         conditions of uncoupling between catabolism and anab-
cating that the LDH was more and more implicated in          olism encountered during ammonium-limited chemostat
carbon conversion (Table 4). As for the metabolic route      performed with cellobiose (Guedon et al., 2000a), the
through PFO, R was in the same range as PTA, AK,             excess of carbon at the G1P–G6P branch point was here
AADH or ADH for environmental pH values between              limited ; in fact, exopolysaccharides and glycogen could
7n4 and 7n0, i.e. R between 6n3 and 11n2 (Table 4),          represent up to 16n0 and 21n4 % respectively of the
indicating that the fluxes were much less than the            cellobiose consumed and cellotriose was detected extra-
available enzyme activity. Yet for a pH lower than 7n0,      cellularly (Guedon et al., 2000a).
R markedly decreased and reached 1n8 at pH 6.4 (Table
                                                             The increase of the acetyl-CoA pool was corroborated
4). Then both biosynthesis and catalytic efficiency of
                                                             by the analysis of carbon flux ; the proportion of
PFO declined with pH since the specific enzyme activity
                                                             cellulose consumed flowing through PFO diminished as
and qacetyl-CoA decreased (Tables 3 and 4).
                                                             the pH declined, as did the ratio of specific enzymic
                                                             activity to metabolic flux (R), and the flux was rerouted
DISCUSSION                                                   away from acetate production. The acetyl-CoA\CoA
                                                             ratio decrease was paralleled by decreases in H \CO
Contrary to what was first observed by Giallo et al.                                                             #
                                                             and qNADH produced\qNADH used. Despite the variation     #
(1983), pH control during batch culture fermentation of
                                                             of the NADH balance, calculated from catabolic path-
C. cellulolyticum greatly influenced cell growth and
                                                             ways producing and consuming reducing equivalents,
metabolism. In fact, maintaining the pH at 7n2, increased
                                                             the intracellular NADH\NAD+ ratio was well regu-
cell density, enhanced ethanol and acetate production
                                                             lated. Such a result is in good agreement with the model
and raised the extent of cellulose hydrolysis, but did not
                                                             of Decker et al. (1976), where the NADH-Fd reductase is
increase the amount of soluble glucides formed, contrary
                                                             activated by the acetyl-CoA and inhibited by CoA and
to what was observed in non-pH-controlled fermen-
                                                             which underlines that the fates of NADH and acetyl-
tation. As previously observed with increasing the
                                                             CoA regulation are interwined. From acetyl-CoA, acet-
concentration of cellulose (Desvaux et al., 2000), the
                                                             ate was mainly formed but the flux split differently as the
maximum rate of cellulose degradation observed in
                                                             environment was acidified, favouring ethanol pro-
pH-controlled cultivation reflects the higher cell mass
                                                             duction. In addition, as pH declined, the level of lactate
compared to a non-pH-controlled culture. Cellulolysis
                                                             production rose and coincided with the pyruvate leak,
continued after the cessation of growth and a high level
                                                             indicating that PFO could no longer support carbon
of soluble glucide accumulation was only observed in a
                                                             flowing from glycolysis, R decreasing to 1n8 at pH 6n4.
non-pH-controlled culture. The gain in cellulose degra-
                                                             Whatever the pH, LDH was always biosynthesized.
dation achieved under pH control was offset by catabo-
                                                             This enzyme operated mainly as the pH declined but
lite production rather than soluble sugar accumulation.
                                                             always remained in excess since even at pH 6n4, R was
In continuous culture, maximum cell density was              46n1. In these experimental conditions, LDH allowed
obtained at pH 7n0, but as the pH declined from 7n0 to       draining off part of the pyruvate surplus. At high pH
6n4 at constant D, biomass was lowered more than             values, H \CO ratios higher than 1 suggested that H
fourfold. At the same time, the specific rate of cellulose               #     #
                                                             was produced via NADH-Fd reductase and hydrogenase       #
consumption, however, decreased only from 1n84 to            activites in addition to pyruvate-Fd oxidoreductase and
1n69 meq C (g cells)−" h−". Thus environmental acidi-        hydrogenase activities. With lower pH values, this ratio
fication influenced chiefly the biomass formation rather        decreased and was compensated by the increase of
than cellulose degradation and assimilation. C. cellulo-     ethanol production until washout occurred.
lyticum did not show depressed yields and the transition
                                                             Reinvestigation of cellulose degradation by C. cellulo-
to wash-out appeared abrupt. This result would be more
                                                             lyticum (Desvaux et al., 2000) showed marked
consistent with a direct effect on a cellular constituent,
                                                             differences in the catabolism of this bacterium as
such as the negative effect of acid on an enzyme or
                                                             compared with the first investigations carried out (Giallo
transport protein (Russell & Dombrowski, 1980 ;
                                                             et al., 1985). The present paper demonstrates that the
Russell & Diez-Gonzalez, 1998).
                                                             inhibition of growth first observed with batch culture
During cellulose catabolism by C. cellulolyticum, sol-       performed in penicillin flasks sealed with butyl rubber
uble β-glucans are first converted into G1P and G6P           stoppers and without shaking of the medium (Giallo et
(Desvaux et al., 2000). With environmental acidification,     al., 1983, 1985) is mainly the result of low pH due to acid
the G1P pool increased, since the proportion of carbon       production in the course of fermentation. The range of
flowing via phosphoglucomutase varied between 57n9            pH allowing maximum cell density is restricted ; strict
and 55n5 %. The remaining G1P was directed towards           control of pH is therefore necessary to obtain the
exopolysaccharides (up to 9n9 % of the G1P) rather than      optimum cellulolytic performance in biotechnological
glycogen synthesis (3n0 % maximum of the G1P), both          processes using C. cellulolyticum. Cellulolytic bacteria
allowing dissipation of carbon surplus (Guedon et al.,       so far investigated cannot grow at pH values sig-
2000b). As the culture pH was lowered, the flow through       nificantly less than 6n0 (Stewart, 1977 ; Russell &
glycolysis decreased while carbon directed to bio-           Dombrowski, 1980 ; Russell & Diez-Gonzalez, 1998).
synthesis increased ; as a result, the G6P pool was          However, it is well established that in anaerobic
between 15n9 and 18n1 µmol (g cells)−". Compared with        habitats, particularly in the natural environment, high

                                                                                                                   1469
M. D E S V A UX, E. G U E D O N a n d H. P E T I T D E M A N GE


fermentation acid concentrations and, as a result, low                  kluyveri. In Microbial Production and Utilization of Gases, pp.
pH values are often encountered (Ljungdahl & Eriksson,                  75–83. Edited by H. G. Schlegel, G. Gottschalk & N. Pfennig.
1985 ; Goodwin & Zeikus, 1987). Since these bacteria                      $                                           $
                                                                        Gottingen : Akademie der Wissenshaften zu Gottingen.
have not developed resistance to low pH environments,                   Desai, R. P., Harris, L. M., Welker, N. E. & Papoutsakis, E. T.
this implies that they have evolved in an ecological niche              (1999a). Metabolic flux analysis elucidates the importance of
where competition for efficient metabolism in acidic                      acid-formation pathways in regulating solvent production by
conditions is not crucially important. In the same way                  Clostridium acetobutylicum. Metab Eng 1, 206–213.
that growth of C. cellulolyticum under an excess of                     Desai, R. P., Nielsen, L. K. & Papoutsakis, E. T. (1999b). Metabolic
nutrients (Guedon et al., 1999b) or with an easily                      flux analysis elucidates the importance of acid-formation path-
available carbon source, such as soluble glucides,                      ways in regulating solvent production by Clostridium aceto-
                                                                        butylicum fermentations with non-linear constraints. J Bio-
appeared as aberrations considering the natural bac-
                                                                        technol 71, 191–205.
terial ecosystem (Desvaux et al., 2000, 2001), cultures
without pH control have been shown to be detrimental                    Desvaux, M., Guedon, E. & Petitdemange, H. (2000). Cellulose
for optimum growth of this bacterium. These data from                   catabolism by Clostridium cellulolyticum growing in batch
                                                                        culture on defined medium. Appl Environ Microbiol 66,
monospecies laboratory culture must be extrapolated to                  2461–2470.
microbial ecosystems to explain the maintenance of C.
                                                                        Desvaux, M., Guedon, E. & Petitdemange, H. (2001). Carbon flux
cellulolyticum in natural environments. Clearly much
                                                                        distribution and kinetics of cellulose fermentation in steady-state
remains to be learned about the complex interactions                    continuous cultures of Clostridium cellulolyticum on a chemically
in which this bacterium takes part in microbiota                        defined medium. J Bacteriol 183, 119–130.
(Kuznetsov et al., 1979 ; Ljungdahl & Eriksson, 1985 ;
                                                                        Duong, T. V. C., Johnson, E. A. & Demain, A. L. (1983). Thermo-
Leschine, 1995 ; Costerton et al., 1995).                               philic, anaerobic and cellulolytic bacteria. In Topics in Enzyme
                                                                        and Fermentation Biotechnology, pp. 156–195. Edited by A.
ACKNOWLEDGEMENTS                                                        Weisman. New York : Wiley.
                                                                        Fields, M. W. & Russell, J. B. (2000). Fibrobacter succinogenes S85
This work was supported by the Commission of European                   ferments ball-milled cellulose as fast as cellobiose until cellulose
Communities FAIR programme (contract no. CT95-0191 [DG                  surface area is limiting. Appl Microbiol Biotechnol 54, 570–574.
12 SSMA]) and by the programme Agrice (Contract no.
                                                                        Gelhaye, E., Gehin, A. & Petitdemange, H. (1993a). Colonization
9701041).
                                                                        of crystalline cellulose by Clostridium cellulolyticum ATCC
The authors thank G. Raval for technical assistance and E.              35319. Appl Environ Microbiol 59, 3154–3156.
McRae for correcting the English and for critical reading of            Gelhaye, E., Petitdemange, H. & Gay, R. (1993b). Adhesion and
the manuscript.                                                         growth rate of Clostridium cellulolyticum ATCC 35319 on
                                                                        crystalline cellulose. J Bacteriol 175, 3452–3458.
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Payot, S., Guedon, E., Cailliez, C., Gelhaye, E. & Petitdemange, H.   Received 6 November 2000 ; revised 29 January 2001 ; accepted
(1998). Metabolism of cellobiose by Clostridium cellulolyticum        12 February 2001.




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