Beta-lactam antibiotics and other
cell wall synthesis inhibitors
Last update: 11.06.2008 1/94
• In 1928 Alexander Fleming observed that a culture
plate on which Staphylococci were being grown had
become contaminated with a mold of the genus
Penicillium, and that bacterial growth in the vicinity
of the mold had been inhibited.
• He isolated the mold in pure culture and
demonstrated that it produced an antibacterial
substance, which he called penicillin.
Extraction and synthesis
• This substance was extracted by Florey and Chain in
1940 from Penicillium notatum and they showed that
it had powerful chemotherapeutic properties in
infected mice and that it was non-toxic.
• Its remarkable antibacterial effects in man were
clearly demonstrated in 1941 in a policeman who had
Staphylococcal and Streptococcal septicemia with
• Ten years later virtually unlimited quantities of
penicillin G were available for clinical use.
• After the isolation of the nucleus, 6-aminopenicillanic
acid, numerous semisynthetic penicillins were
developed that are stable at acid pH, resistant to β-
lactamase, and active against both Gram-positive
and Gram-negative bacteria.
• The penicillins are classified as β-lactam drugs
because of their unique four-member β-lactam ring.
• Structural integrity of the 6-aminopenicillanic acid
nucleus is essential for the antibacterial activity.
Site of substitution
Clinical limitations of penicillin G
• It is unstable at acidic pH
• It is susceptible to destruction by β-lactamase
• It is relatively inactive against Gram-negative bacilli.
Mechanism of action
• Penicillins, like all β-lactam antibiotics inhibit bacterial
growth by interfering with a specific step in bacterial
cell wall synthesis.
• Cell wall is composed of a complex cross-linked
• The polysaccharide contains alternating amino
sugars N-acetylglucosamine and N-acetylmuramic
acid. A five amino-acid peptide is linked to the N-
acetylmuramic acid sugar. This peptide terminates in
• Penicillin-binding proteins catalyze the
transpeptidase reaction that removes the terminal
alanine to form a cross link with a nearby peptide,
which gives cell wall its structural rigidity.
• β-lactam antibiotics are structural analogs of the
natural D-Ala-D-Ala substrate and they are covalently
bound to penicillin binding proteins.
Cell envelope of Gram-negative bacterium
Cell wall synthesis inhibitors
Comparison of the structure of cell walls
Bectericidal and bacteriostatic effects
• With penicillins, transpeptidation reaction is inhibited,
peptidoglycan synthesis is blocked and the cell dies.
• Furthermore, autolysins, bacterial enzymes that
remodel and break cell wall are activated.
• Penicillins and cephalosporins are classically
bactericidal. However, the cell should be actively
growing and synthesizing cell wall.
• But some organisms have defective autolytic
enzymes and are inhibited but not lysed, thus
penicillins are bacteriostatic in these organisms.
Resistance to penicillins
The production of ß-lactamases
• There are more than 100 different types of this
enzyme. The process is genetically controlled
commonly with plasmids.
• β-lactamase production is particularly important in
Staphylococci but other organisms such as Neisseria
gonorrhoeae and Hemophilus species also produce
these enzymes where as β-hemolytic Streptococci do
• In developed countries at least 80% of Staphylococci
now produce β-lactamase.
Other resistance mechanisms
• A reduction in the permeability of the outer
Thus there is a decreased ability of the drug to
penetrate to the target site.
• The occurrence of modified penicillin binding sites.
This mechanism is responsible in methicillin
resistance in Pneumococci.
• When given orally different penicillins are absorbed to
differing degrees depending on their stability in acid
and their absorption on to food.
• Parenteral administration of penicillin G can be
intramuscular or intravenous.
• Intrathecal administration is inadvisable particularly
with benzylpenicillin as it can cause convulsions.
• The drugs are widely distributed in the body fluids,
passing into joints, into pleural and pericardial
cavities, into the bile, the saliva, the milk and across
• However they are lipid insoluble and they do not
enter mammalian cells and do not cross intact blood-
• If the meninges are inflamed, they may reach
therapeutically effective concentrations in the
• Elimination of most penicillins is mainly renal and
occurs rapidly, 90% being by tubular secretion.
• Excretion of penicillin by tubular secretion can be
partially blocked by probenecid, which raises the
plasma concentration and prolongs the action.
• Penicillin G has a relatively short half life so it must
be frequently dosed, intravenously infused or slow
release preparations such as procaine penicillin or
benzathine penicillin should be given.
• Both slow-release preparations are applied
• By one intramuscular injection of procaine penicillin
clinically useful concentrations are maintained for 12-
• After one intramuscular injection of benzathine
penicillin, the plasma concentration of penicillin G is
sufficient to treat Streptococcal infection for 10 days,
and is sufficient to protect against β-hemolytic
Streptococci for 3 weeks.
• One of the remarkable features of the penicillins is
their relative freedom from direct toxic effects.
• Carbenicillin can produce hypokalemia, and because
it contains 4.7 mEq Na+ per gram, it should perhaps
be used with circumspection in cardiac disease.
• Hemostatic defects have been reported with this drug
and with penicillin G.
• The main unwanted effects of the penicillins are
hypersensitivity reactions, the basis of which is the
fact that degradation products of penicillin combine
with host protein and become antigenic.
• These are cross-reactions between various types of
• Very high doses of penicillin G can cause seizures in
Allergic reaction types
• Skin rashes of various sorts, fever, delayed types of
serum sickness with fever, urticarial skin eruptions, in
severe cases generalized edema, multiple joint
effusions, enlargement of spleen and lymph glands
• Much more serious is acute anaphylactic shock
which may in some cases be fatal, but is fortunately
very rare (probably 3-4/10000).
• Others include, vasculitis, interstitial nephritis, and
various hematologic disturbances.
• The occurrence of these allergic reactions is
unpredictable. Fortunately it is very rare in small
• Another side effect of the penicillins, particularly the
broad spectrum type given orally, is alternation of the
bacterial flora in the gut.
• This can be associated with gastrointestinal
disturbances and, in some cases, with suprainfection
by microorganisms insensitive to penicillin.
• Cephalosporins are similar to penicillins chemically,
in mechanism of action, and toxicity.
• Cephalosporins are more stable to many β-
lactamases and therefore have a broader spectrum
• Large-scale production of the common nucleus
7-aminocephalosporanic acid has made possible of a
vast array of cephalosporins.
7-Aminocephalosporanic acid nucleus
Sites of substitution 43/94
First generation cephalosporins
Second generation cephalosporins
Third generation cephalosporins
• Cephamycins (fermentation products of
Streptomyces) and some totally synthetic drugs such
as moxalactam resemble cephalosporins.
• Latamoxef is a synthetic cephomycin compound.
Cephalosporins and cephamycins
Mechanism of action for cephalosporins
• The same as that of penicillins.
• Resistance to this group of drugs has increased due
to plasmid-encoded or chromosomal β-lactamase.
There are certain kinds of β-lactamases which are
more active in hydrolyzing cephalosporins than
• Now forth generation of cephalosporins (e.g.
cefepime) are available, these are comparable to
third-generation but more resistant to some β-
• Resistance also occurs if there is decreased
penetration of the drug due to alternations to outer
membrane proteins or mutations of the binding-site
• Some cephalosporins may be given orally but most
are given parenterally (IM or IV).
• They are widely distributed in the body like penicillins.
• Some such as cefoperazone, cefotaxime,
cefuroxime, ceftriaxone, and ceftazidime also cross
the blood-brain barrier and are drugs of choice for
meningitis due to Gram-negative intestinal bacteria.
• Excretion is mostly via kidney, largely by tubular
secretion but 40% of ceftriaxone and 75% of
cefaperazone is eliminated in the bile.
• Hypersensitivity reactions very similar to those that
occur with penicillins may be seen.
• Nephrotoxicity and intolerance to alcohol (disulfiram
like reaction) has been reported.
• Diarrhea may occur with oral forms. Many second
and particularly third generation cephalosporins are
ineffective against Gram-positive organisms,
especially methicillin resistant Staphylococci and
• During treatment with such drugs, these resistant
organisms as well as fungi, often proliferate and may
Carbapenems and monobactams
• These drugs were developed to deal with β-
lactamase producing Gram-negative organisms,
which were resistant to broad spectrum and extended
• Carbapenems are derived from Streptomyces
species and one example is the semisynthetic
imipenem which acts in the same way as the other β-
• It has a very broad spectrum of antimicrobial activity
being active against many aerobic and anaerobic
Gram-positive and Gram-negative organisms
including Listeria, Pseudomonas, and most
• Meticillin resistant Staphylococci are less susceptible.
• Imipenem is partly broken in the kidney by a
dehydropeptidase in the proximal tubule, and is
therefore given in combination with cilastatin, a
specific inhibitor of this enzyme.
• Pamipenem is under investigation.
• It is similar to imipenem.
• It is not degraded by dehydropeptidase, thus no
cilastatin is needed.
• Excessive levels in kidney failure can cause seizures
with imipenem but not with meropenem.
• The main monobactam is aztreonam, which is
resistant to most β-lactamases. It has an unusual
spectrum being active only against Gram-negative
aerobic rods including Pseudomonas, N.
menengitidis and H. influenza.
• It has no action against Gram-positive organisms or
• New injectable monobactams under investigation are
carumonam and tigemonam.
β-lactamase inhibitors (clavulanic acid,
sulbactam, and tazobactam)
• These agents resemble β-lactam molecules but they
have very weak antibacterial action.
• They are potent inhibitors of many bacterial β-
lactamases and can protect hydrolyzable penicillins
from inactivation by these enzymes.
Fixed combinations with β-lactamase
• They are available only in fixed combinations with
• Ampicillin + sulbactam
• Amoxicillin + clavulanic acid
• Ticarcillin + clavulanate potassium
• Piperacillin + tazobactam sodium
• It is an antibiotic
• It is only active against
Mechanism of action
• Vancomycin inhibits cell
wall synthesis by binding
firmly to the D-Ala-D-Ala
terminus of nascent
peptide. This inhibits the
Administration and pharmacokinetics
• It is administered orally only for antibiotic associated
enterocolitis, caused by Clostridium difficile.
Otherwise administered IV.
• The drug is widely distributed in the body and in
cerebrospinal fluid when meninges are inflammated.
• 90% is excreted by glomerular filtration.
• It is very similar to vancomycin, but it can be
administered IM as well as IV.
• It inhibits a very early stage of cell wall synthesis.
• As an analog of phosphoenolpyruvate, it is struc-
turally unrelated to any other antimicrobial agent.
• It inhibits the cytoplasmic enzyme enolpyruvate
• It is active against both Gram-positive and Gram-
negative organisms and is approved for use in
uncomplicated urinary tract infections.
• In vitro synergism occurs when phosphonomycin is
combined with β-lactam antibiotics, aminoglycosides,
• It is available in both oral and parenteral formulations.
• It inhibits cell wall formation by interfering with
dephosphorylation in cycling of the lipid carrier that
transfers peptidoglycan subunits to the growing cell
• It is markedly nephrotoxic if administered systemically.
• It is poorly absorbed in topical use. Thus topical
application results in local antibacterial activity without
significant systemic toxicity.
• It is a structural analogue of D-alanine.
• It inhibits the incorporation of D-alanine, into
peptidoglycan pentapeptide inhibiting alanine
racemase, which converts L-alanine to D-alanine.
• Cycloserine inhibits many Gram-positive and Gram-
negative organisms, but it is used almost exclusively
to treat tuberculosis resistant to first line agents.
Similarity of cycloserine and alanine
Cell wall synthesis inhibitors
• The fluoroquinolones are synthetic antibiotics
recently introduced into clinical practice.
• They include the broad spectrum agents
ciprofloxacin, ofloxacin, norfloxacin, enoxacin,
lomefloxacin and narrower-spectrum drugs used in
urinary tract infections cinoxacin and nalidixic acid.
• The last one is the first introduced quinolone and is
Mechanism of action
• These agents inhibit
topoisomerase II (a DNA
gyrase), the enzyme that
produces a negative
supercoil in DNA,
• Ciprofloxacin is the most commonly used
fluoroquinolone and is described as the type agent.
• It is a broad spectrum antibiotic effective against
many organisms resistant to penicillins,
cephalosporins, and aminoglycosides, including;
Enterobacter, H. influenzae, penicillinase producing
N. gonorrhea, Campylobacter, and Pseudomonas.
• Of the Gram-positive organisms Streptococci and
Pneumococci are only weakly inhibited.
Antibacteriel spectrum (continued)
• It should be avoided in methicilin-resistant
Staphylococci. Intracellular organisms such as M.
tuberculosis, Mycoplasma, Chlamydia, Legionella,
and Brucella species are inhibited to a variable extent
and there is only low activity against anaerobic
• To prevent emergence of resistance, ciprofloxacin
should be reserved for organisms resistant to other
Pharmacokinetics of flourokinolones
• They are well absorbed when given orally.
• Ciprofloxacin and norfloxacin » 3.3 h
• Ofloxacin. » 5 h.
• Lomefloxacin. » 8 h
• The drugs concentrate in many tissues particularly in
the kidney, prostate and lung also in phagocytes.
• Most do not cross the blood-brain barrier except for
ofloxacin, which reaches in the cerebrospinal fluid
90% of its serum concentration.
• Infrequent, usually mild and reversible.
• Such as gastrointestinal disorders, skin rashes,
artropathy, CNS symptoms (headache, dizziness),
photosensitivity, hypersensitivity reactions.
• They are contraindicated in patients under 18
because of the risk of chondrotoxicity and