Structural-functional relationships
The fungi are more
evolutionarily advanced forms of microorganisms, as compared to the
prokaryotes (prions, viruses, bacteria). They are classified as eukaryotes,
i.e., they have a diploid number of chromosomes and a nuclear membrane and
have sterols in their plasma membrane. Genetic complexity allows
morphologic complexity and thus these organisms have complex structural
features that are used in speciation.
Fungi can be divided into
two basic morphological forms, yeasts and hyphae. Yeastsare
unicellular fungi which reproduce asexually by blastoconidia formation
(budding) or fission. Hyphaeare 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. Very few fungi exhibit
dimorphism. 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.
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
From Medical Microbiology, 1990, Murray, et al., p. 299, Fig. 28-1.
Reproduced with permission.
A mass of hyphal elements
is termed the mycelium (synonymous with mold).
Aerial hyphae often produce asexual reproduction propagules termed conidia(synonymous
with spores). Relatively large and complex conidia are
termed macroconidia while the smaller and more simple
conidia are termed microconidia. 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.
A. Aspergillus; B. Penicillium; C. Geotrichum; D. Trichophyton; E. Microsporum; F. Epidermophyton and G. Rhizopus. From Medical Microbiology, 1990, Murray, et al., p. 300, Fig. 28-2. Reproduced with permission.
Asexual reproduction, via conidia formation, does not involve genetic recombination between two sexual types whereas sexual reproduction does involve genetic recombination between two sexual types.
Metabolism
All fungi are free living, i.e., they are not obligate intracellular parasites. They 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. For medical purposes the important aspects of fungal metabolism are:1. The synthesis of chitin, a polymer of N-acetyl glucosamine, and other compounds, for use in forming the cell wall. These induce immune hypersensitivity.
2. The synthesis of ergosterol for incorporation into the plasma membrane. This makes the plasma membrane sensitive to those antimicrobial agents which either block the synthesis of ergosterol or prevent its incorporation into the membrane or bind to it, e.g. amphotericin B.
3. The synthesis of toxins such as
a. Ergot alkaloids- these are produced by Claviceps purpurea and cause an alpha adrenergic blockade
b. Psychotropic agents - these include psilocybin, psilocin and lysergic acid diethylamide (LSD)
c. Aflatoxins - these are carcinogens produced by Aspergillus flavus when growing on grain. When these grains are eaten by humans or when they are fed to dairy cattle and they get into the milk supply, they affect humans.
4. The synthesis of proteins on ribosomes that are different from those found in bacteria. This makes the fungi immune to those antimicrobial agents that are directed against the bacterial ribosome, e.g., chloramphenicol.
5. The ability of certain metabolites to alter morphology of yeast and/or be assimilated by yeast with concomitant clinical identification affects.
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