Download CHAPTER 41 The Nature of Fungi

CHAPTER 41
The Nature of Fungi
Fungi are a distinct class of microorganisms, most of which are free-living in nature
where they function as decomposers in the energy cycle. Of the more than 200,000
known species, fewer than 200 have been reported to produce disease in humans. These
diseases, the mycoses, have unique clinical and microbiologic features and are increasing
in immunocompromised patients.
I. Mycology
A. STRUCTURE
1. Typical eukaryotic features including nucleus, mitochondria, and endoplasmic
reticulum
2. Ergosterol, not cholesterol, makes up cell membrane
3. The chemical structure of the cell wall is of polysaccharides mannan, glucan,
and chitin
4. Mannoproteins are mannose-based polymers (mannan) found on the surface
and in the structural matrix of the cell wall, where they are linked to protein
5. Glucans are glucosyl polymers, some of which form fibrils that increase the
strength of the fungal cell wall
6. Chitin is composed of long chains of poly-N-acetylglucosamine and is inert,
insoluble, and rigid
B. METABOLISM
1. Heterotrophic metabolism uses available organic matter
2. Photosynthetic mechanisms are lacking
3. Most fungi are aerobes
C. REPRODUCTION
1. Anamorphic asexual reproduction forms conidia by mitosis
2. Teleomorphic meiosis forms sexual spores in specialized structures
D. FUNGAL MORPHOLOGY AND GROWTH
1. Vary from bacterial size to multicellular mushrooms
a. YEASTS AND MOLDS
1. Yeasts produce blastoconidia by budding
2. Molds produce septate or nonseptate hyphae which are tube-like
extensions with thick, parallel walls
3. Vegetative mycelium acts as a root
4. Aerial mycelium bears reproductive conidia or spores
5. Pseudohyphae are less rigid
6. Morphology of reproductive conidia and spores used for identification
7. Conidia and conidiophore arrangements determine names
8. Ascospores are borne in ascus sac
E. DIMORPHISM
1. Fungi able to grow in either yeast or mold form
2. Temperature usually triggers shift between phases
3. Shift from mold to yeast begins with heat shock response
4. Metabolic shift is toward sulfhydryl compounds in yeast form
5. Global regulator controls process
6. Dimorphism is reversible and linked to virulence
II. CLASSIFICATION
1. Taxonomy is based on sexual spores and septation of hyphae
2. Telomorph form is unknown for most pathogens
3. rRNA genes are used for classification
4. Four phyla the Chlytridiomycota, Zygomycota, Ascomycota, and
Basidiomycota
5. Medically important genera fall mostly into the Ascomycota, with a few in the
Basidiomycota, and Zygomycota
6. Medical grouping organized by biological behavior in humans
7. Systemic fungi infect previously healthy persons
III. LABORATORY DIAGNOSIS
A. Direct Examinations
1. KOH digests tissue but not fungal wall
2. Some yeasts are Gram-positive
3. Calcifluor white enhances detection
4. Often visible in histologic H&E preparations
5. Silver stains enhance detection
B. Culture
1. Growth in culture is simple but slow
2. Selective media allow isolation in the presence of bacteria
3. Sabouraud's agar optimal for fungi but poor for bacteria
4. Selective media make use of antimicrobics
5. Cultures incubated at 30°C for primary isolation
6. Yeast identified biochemically
7. Molds identified by morphology and culture features
8. Lactophenol cotton blue stains mycelia, conidia, and spores
9. Temperature variation demonstrates dimorphism
10. DNA probes are more rapid
C. Antigen and Antibody Detection
1. Serologic tests are useful for systemic fungi
2. Antigen detection shows promise