Because fungi (yeasts and molds) are eukaryotic organisms, whereas bacteria are prokaryotic, they differ in several fundamental respects (Table 47–1). Two fungal cell structures are important medically:
The fungal cell wall consists primarily of chitin (not peptidoglycan as in bacteria); thus, fungi are insensitive to certain antibiotics, such as penicillins and cephalosporins, that inhibit peptidoglycan synthesis. Chitin is a polysaccharide composed of long chains of N-acetylglucosamine. The fungal cell wall contains other polysaccharides as well, the most important of which is β-glucan, a long polymer of D-glucose. The medical importance of β-glucan is that it is the site of action of the antifungal drug caspofungin.
The fungal cell membrane contains ergosterol, in contrast to the human cell membrane, which contains cholesterol. The selective action of amphotericin B and azole drugs, such as fluconazole and ketoconazole, on fungi is based on this difference in membrane sterols.
TABLE 47–1Comparison of Fungi and Bacteria ||Download (.pdf) TABLE 47–1 Comparison of Fungi and Bacteria
|Feature ||Fungi ||Bacteria |
|Diameter ||Approximately 4 μm (Candida) ||Approximately 1 μm (Staphylococcus) |
|Nucleus ||Eukaryotic ||Prokaryotic |
|Cytoplasm ||Mitochondria and endoplasmic reticulum present ||Mitochondria and endoplasmic reticulum is absent |
|Cell membrane ||Sterols present ||Sterols absent (except Mycoplasma) |
|Cell wall content ||Chitin ||Peptidoglycan |
|Spores ||Sexual and asexual spores for reproduction ||Endospores for survival, not for reproduction |
|Thermal dimorphism ||Yes (some) ||No |
|Metabolism ||Require organic carbon; no obligate anaerobes ||Many do not require organic carbon; many obligate anaerobes |
There are two types of fungi: yeasts and molds. Yeasts grow as single cells that reproduce by asexual budding. Molds grow as long filaments (hyphae) and form a mat (mycelium). Some hyphae form transverse walls (septate hyphae), whereas others do not (nonseptate hyphae). Nonseptate hyphae are multinucleated (coenocytic). The growth of hyphae occurs by extension of the tip of the hypha, not by cell division all along the filament.
Several medically important fungi are thermally dimorphic (i.e., they form different structures at different temperatures). They exist as molds in the environment at ambient temperature and as yeasts (or other structures such as the spherules of Coccidioides) in human tissues at body temperature.
Most fungi are obligate aerobes; some are facultative anaerobes; but none are obligate anaerobes. All fungi require a preformed organic source of carbon—hence their frequent association with decaying matter. The natural habitat of most fungi is, therefore, the environment. An important exception is Candida albicans, which is part of the normal human flora.
Some fungi reproduce sexually by mating and forming sexual spores (e.g., zygospores, ascospores, and basidiospores). Zygospores are single large spores with thick walls; ascospores are formed in a sac called ascus; and basidiospores are formed externally on the tip of a pedestal called a ...