Antisepsis, Disinfection, and Sterilization. Gerald E. McDonnell

FIGURE 1.2 Typical fungal structures. (A) Filamentous fungus (mold). Hyphae are shown as long lines of unseparated cells, with the development of a fruiting body with attached spores. (B) Typical unicellular fungal (yeast) cells. The cells are generally polymorphic. In one case, a budding cell is shown.

Unicellular fungi (yeasts) do not generally form hyphae and produce growth that appears similar to bacteria (see section 1.3.4.1). Asexual reproduction of yeasts can occur by binary fission (e.g., in Schizosaccharomyces), similar to bacterial fission, or by budding directly from the parent cell (e.g., in Saccharomyces [Fig. 1.2]). In addition, some fungi are dimorphic, growing as either unicellular or hyphal (or pseudohyphal) forms. Common fungi are listed in Table 1.5.

The fungal protoplasm is surrounded by a rigid cell envelope consisting of the plasma membrane, periplasmic space, and outer cell wall (Fig. 1.3).

Many studies have investigated the structure and function of the yeast cell envelopes of Saccharomyces cerevisiae and Candida albicans, but much less is known about the range of various fungal structures. The plasma membrane is a lipid bilayer, similar to bacterial membranes (see section 1.3.4.1), but also includes some unique sterols, such as ergosterol and zymosterol. The membrane contains many integral proteins that are involved in various processes, such as cell wall synthesis and solute and/or molecule transport. Examples include various chitin and glucan synthases. Between the membrane and the outer cell wall is a narrow periplasm that can contain various mannoproteins, including enzymes such as invertase and acid phosphatase, which play a role in substrate uptake by the cell. The cell wall is a cross-linked, modular structure that varies between different molds and yeasts. It is a major component of the cell, typically comprising 15 to 25% of the cell and consisting of ~80 to 90% polysaccharide. The basic structure consists of chitin (~5% of the cell wall) or, in some cases, cellulose fibrils within an amorphous matrix of various polysaccharide glucans with associated proteins and lipids. Chitin is a polysaccharide of acetylglucosamine and gives the cell wall rigidity. In yeasts, the chitin fibrils are normally located toward the inner surface of the cell membrane, associated with various mannans and the cell membrane; however, only some species, such as C. albicans, have chitin, while others do not. The outer layers of the cell wall are primarily composed of β-1,3- and β-1,6-glucan fibrils, with various associated proteins, mannoproteins, and lipids. In some cases, like that of the ascomycetes, a defined protein layer has been described between the outer glucans and the inner chitin fibrils. Overall, fungal cell walls are predominantly (80 to 90%) composed of polysaccharides. The various mold cell walls have a similar, but overall more rigid, structure than those of yeasts.

TABLE 1.5 Examples of common fungi