Fungi or the Eumycota 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 90 000 known species, fewer than 200 have been reported to produce disease in humans. These diseases have unique clinical and microbiologic features and are increasing in immunocompromised patients.
Fungi are eukaryotes with a higher level of biologic complexity than bacteria. They are spore bearing; reproducing both sexually and asexually. Fungi may be unicellular or may differentiate and become multicellular by the development of long-branching filaments. They acquire nutrients by absorption but lack the chlorophyll of plants. The diseases caused by fungi are called mycoses. They vary greatly in their manifestations but tend to be subacute to chronic with indolent, relapsing features. Acute disease, such as that produced by many viruses and bacteria, is uncommon with fungal infections.
Cell organization is eukaryotic
The fungal cell has typical eukaryotic features, including a nucleus with a nucleolus, nuclear membrane, and linear chromosomes (Figure 42–1). The cytoplasm contains a cytoskeleton with actin microfilaments and tubulin-containing microtubules. Ribosomes and organelles, such as mitochondria, endoplasmic reticulum, and the Golgi apparatus, are also present. Fungal cells have a rigid cell wall external to the cytoplasmic membrane, which differs in its chemical composition from that of bacteria and plants. An important difference from mammalian cells is the sterol makeup of the cytoplasmic membrane. In fungi, the dominant sterol is ergosterol; in mammalian cells, it is cholesterol. Fungi are usually in the haploid state, although diploid nuclei are formed through nuclear fusion in the process of sexual reproduction.
A yeast cell showing the cell wall and internal structures of the fungal eukaryotic cell plan. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
Presence of a nucleus, mitochondria, and endoplasmic reticulum
Ergosterol, not cholesterol, makes up cell membrane
The chemical structure of the cell wall in fungi is markedly different from that of bacterial cells in that it does not contain peptidoglycan, glycerol, teichoic acids, or lipopolysaccharide. In their place are the polysaccharides mannan, glucan, and chitin in close association with each other and with structural proteins (Figure 42–2). 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. They are major determinants of serologic specificity because of variations in the composition and linkages of the polymer side chains. Glucans are glucosyl polymers, some of which form fibrils that increase the strength of the fungal cell wall, found to be often in close association with chitin. Chitin is composed of long, unbranched chains ...