UNIT 6_Introduction to Mycology

							                                   UNIT 6: Mycology
Mycology- the study of fungi

Structural-functional relationships

    1. The fungi are more evolutionarily advanced forms of microorganisms, as
       compared to the prokaryotes (prions, viruses, bacteria).
    2. They are classified as eukaryotes,
          a. They have a diploid number of chromosomes and
          b. A nuclear membrane and
          c. Have sterols in their plasma membrane.
    3. Have complex structural features that are used in speciation.
    4. Divided into two basic morphological forms:
          a. Yeasts and
          b. Hyphae.

 Yeasts are unicellular fungi which reproduce asexually by blastoconidia formation
(budding) or fission.
Hyphae are multi-cellular fungi which reproduce asexually and/or sexually.

 Dimorphism is the condition where by a fungus can exhibit either the yeast form or
the hyphal form, depending on growth conditions.
Most fungi occur in the hyphae form as branching, threadlike tubular filaments.
These filamentous structures either lack cross walls (coenocytic) or have cross walls
(septate) depending on the species.
In some cases septate hyphae develop clamp connections at the septa which connect
the hyphal elements.
Conidia: Aerial hyphae often produce asexual reproduction propagules1
termed conidia (synonymous with spores).
Macroconidia - Relatively large and complex conidia
Micro conidia -smaller and simpler conidia.
Endospores - When the conidia are enclosed in a sac (the sporangium), they are
called endospores.
 The presence/absence of conidia and their size, shape and location are major features
used in the laboratory to identify the species of fungus in clinical specimens.
Mycelium- A mass of hyphal elements is termed the mycelium (synonymous
with mold).




1
 a part of a plant or fungus, e.g. a bud or a spore, that becomes detached from the rest and forms a new
organism
                                         Fungi




                     Yeast form                        Hyphal form




                                                 Coenocytic           Septate




                                                                Clamp            No
                                                      Clamp


Asexual reproduction- via conidia formation, does not involve genetic recombination
between two sexual types
Sexual reproduction - involves genetic recombination between two sexual types.

Metabolism
   All fungi are free living, i.e., they are not obligate intracellular parasites.
   Do not contain chlorophyll and cannot synthesize macromolecules from
     carbon dioxide and energy derived from light rays.
   Therefore all fungi are heterotrophs, living on preformed organic matter.
A. Yeast cells reproducing by blastoconidia formation; B. Yeast dividing by
fission; C. Pseudohyphal development; D. Coenocytic hyphae; E. Septate
hyphae; F. Septate hyphae with clamp connections
Source: Medical Microbiology, 1990, Murray, et al., p. 299, Fig. 28-1.
   Aspergillus; B. Penicillium; C. Geotrichum; D. Trichophyton; E.
   Microsporum; F. Epidermophyton and G. Rhizopus. From Medical
   Source: Microbiology, 1990, Murray, et al., p. 300, Fig. 28-2.




Classification:
The fungi pathogenic for animals and man are generally grouped according to one of
two methods:
   1. based upon the area of body they infect
   2. based upon the morphology of the infecting fungus

1. Based on the area of body they infect
      a. Superficial/Cutaneous mycoses: Dermatophytes are the only obligatory
          parasites that live in keratin layers of the skin and cause ringworm. Such
          infections are called superficial mycoses. Important dermatophyes which
          cause ringworm in man and animals are the Genus Microsporum and
          genus Trichophyton.

       b. Subcutaneous mycoses: infection with these occurs following their entry
          into the skin through wounds where they grow in mycelial form, producing
          localized lesions which usually do not spread.

       c. Deep/systemic mycoses: some fungi establish in normal individuals. When
          the individual is debilitated and immunocompromised, they become highly
          susceptible to the diseases. E.g., Histoplasmosis, aspergillosis
2. Based on the morphology of the infecting fungus
      a. Candidiasis
      b. Histoplasmosis
      c. Coccidioidomycosis

Generally, fungi are classified into following classes:
 Myxomycetes
 Phycomycetes
 Ascomycetes
 Basidiomycetes
 Deuteromycetes

Myxomycetes- commonly known as slime molds.
Not known to produce animal disease.

Phycomycetes- Absidia, Mucor and Rhizopus cause disease in animals which are not
communicable but are contracted from the environment. Cuase mycotic abortion in
animals
Grow rapidly on Sabouraud’s medium at 25ºC their growth is inhibited by
Cyclohexamide

Ascomycetes- Claviceps purpurea causes Ergotism

Basidiomycetes- not of much importance in disease causation in animals

Deuteromycetes- major group which cause disease in man and animals. e.g., Candida
sp., Aspergillus sp.


Common fungal infections in animals
     Disease Name             Causative fungus                 Animals affected
Ringworm                  Microsporum spp.                All domestic animals and
                          Trichophyton spp.               man
Aspergillosis             Aspergillus spp.                Cattle, horse, poultry
Thrush                    Candida albicans                Avian spp.
                                                          Also in dogs, cats, man,
                                                          pigs.
Histoplasmosis                Histoplasma                 Man and domestic animals
Aflatoxicosis                 Aspergillusflavus           All animals and man
                              ASpergillus parasiticus