The fungal kingdom encompasses a diverse and rich group of organisms ranging from microscopic yeasts to mushrooms. Most fungi are free-living in nature where they function as decomposers in the energy cycle. Of the more than 90 000 known fungal species, fewer than 200 have been reported to produce disease in humans. Once considered clinical rarities, human fungal infections are becoming increasingly common, especially among immunocompromised patients. Therefore, it is important to understand the unique clinical and microbiological features of these diseases.
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 lack the chlorophyll of plants, therefore needing to acquire nutrients from the external environment. The diseases caused by fungi are called mycoses. These infections vary greatly in their manifestations but tend to present with subacute or chronic features, often relapsing over time. Acute disease, such as that produced by many viruses and bacteria, is uncommon with fungal infections.
✺ Cell organization is eukaryotic
The fungal cell has many 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 the cell walls of bacteria and plants. In addition to the cell wall, another important difference from mammalian cells is the sterol makeup of the cytoplasmic membrane. In mammalian cells, the dominant membrane sterol is cholesterol; in fungi, it is ergosterol. Fungi are usually haploid in their DNA content, although diploid nuclei are formed through nuclear fusion in the process of sexual reproduction. Interestingly, the generation of polyploid/aneuploid nuclei is a strategy used by some fungi to generate genetic diversity as a response to cell stress, such as antifungal therapy.
Presence of a nucleus, mitochondria, and endoplasmic reticulum
✺ Ergosterol, not cholesterol, makes up cell membrane
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.)
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 complex polysaccharides such as mannans, glucans, and chitins in close association with each other and with structural proteins (Figure ...