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Fungi are eukaryotic cells, many of which can reproduce asexually (by cell division) or sexually (by the production of spores). A limited number of fungi have been implicated in plant and animal diseases (mycoses), but fungi are also widely used for bioremediation and biodegradation, product fermentation (e.g., beer, wine, and bread), and the production of biochemical products (e.g., antibiotics, enzymes, and vitamins). Due to their ubiquitous nature, they are often implicated in spoilage and as general contaminants. They are chemoheterotrophs (requiring organic nutrition), and many are saprophytes (living off dead organic matter), acquiring their food by absorption. They are generally classified as filamentous (molds) or unicellular (Fig. 1.2).

Filamentous fungi multiply by cell division, but the cells do not separate and form long tubular structures known as hyphae (singular, hypha). The further development and branching of hyphae leads to the development of a mass of fungal growth on a surface known as a mycelium (plural, mycelia). Mycelia can often grow to such an extent that they are clearly visible to the naked eye on a surface (e.g., mold growth on bread). Fragments of hyphae can break off and allow the development of further mycelia. As the mycelia develop, a variety of fruiting bodies or other structures, which contain spores, are formed. Fungal spores can be present in a variety of shapes and sizes and can be asexual and/or sexual. The various molecular structures of fungal spores have not been studied in detail, but most are surrounded by a rigid wall distinguished by its low water content and low metabolic activity and which can contain lipids and pigments, as well as nutrient reserves; these are discussed further in section 8.10.

TABLE 1.3 Comparison of general prokaryotic and eukaryotic structures^a

Structure	Prokaryotes	Eukaryotes
Basic structure
Cytoplasmic membrane	+	+
Organelles, e.g., chloroplasts, mitochondria	−	+
Nucleus defined by a membrane	−	+
Ribosomes	70S	80S
Cell wall	+/−	+/−

^aProkaryotic cells are simple, smaller structures, while eukaryotic cells are larger and more organized. Structural differences will vary, depending on the microorganism. For example, some prokaryotes (e.g., mycoplasmas) and eukaryotes (animal cells) do not have a cell wall.

TABLE 1.4 Helminths associated with disease

Species	Disease	Comments
Nematodes (roundworms)
Wuchereria bancrofti	Elephantiasis (blood or lymphatic system blockage)	Transferred via mosquitoes; can grow up to 10 cm long
Onchocerca volvulus	River blindness	Transferred via blackflies
Ascaris lumbricoides	Generally asymptomatic, but can develop into ascariasis (pneumonitis and intestinal obstruction)	From contaminated water, food, or direct surface contact; worms can grow up to 30 cm long
Enterobius vermicularis	“Pinworms”; dysentery, intestinal blockage	From contaminated water, food, or direct surface contact; worms ~1 cm long
Cestodes (tapeworms)
Taenia saginata	Generally asymptomatic, but can cause mild intestinal complications (including abdominal pain and diarrhea)	Contaminated meat; worms can be very long (> 100 cm)
Trematodes (flukes)
Fasciola hepatica	Can be asymptomatic, with complications including liver abscesses	Contaminated grasses; snails are intermediate hosts
Schistosoma spp.	Schistomiasis; can cause many complications due to growth in the bloodstream and body tissues	Water contamination; snails are intermediate hosts

FIGURE 1.1 A typical helminth life cycle (example: Enterobius vermicularis).

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

Type	Example	Comments
Filamentous	Trichophyton (e.g., T. mentagrophytes)	Dermatophytes causing superficial infections on the outer layers of skin, hair, and nails, e.g., ringworm (tinea) or athlete’s foot
Aspergillus (e.g., A. niger, A. fumigatus)	Ubiquitous in nature and often isolated as microbial contaminants; rare cause of ear infections (otitis) and pulmonary disease (aspergillosis) in immunocompromised individuals; also used in the bioremediation of tannins and for the bioproduction of citric acid
Phytophthora (e.g., P. infestans)	Causes potato blight, a plant disease
Penicillium (e.g., P. chrysogenum, P. roqueforti)	Ubiquitous in nature and often isolated as microbial contaminants (e.g., as a bread mold); rarely identified as pathogenic; some strains used for the production of penicillin and cheese
Unicellular	Cryptococcus (e.g., C. neoformans)	Ubiquitous, but can cause meningitis or pulmonary infections (cryptococcosis; valley fever)
Saccharomyces (e.g., S. cerevisiae)	Used for wine and beer production
Dimorphic	Candida (e.g., C. albicans)	Widely found as a commensal, including as part of normal human flora, but can cause candidiasis in immunocompromised patients (e.g., thrush and, in some cases, septicemia)
Histoplasma (e.g., H. capsulatum)	Causes histoplasmosis, a pulmonary disease similar to tuberculosis

The cell wall acts as a barrier for the action of biocides, and for this reason, fungi are considered relatively resistant to many antimicrobial processes in comparison to most bacteria. Some fungi (e.g., Cryptococcus) also produce a capsule structure external to the cell wall, which can act as an additional barrier. Further, fungal spores are generally more resistant than vegetative cells to biocides and heat, but not to the same extent as bacterial spores; fungal spores can be more resistant to radiation methods, as is observed when they are exposed to UV light.

FIGURE 1.3 Simplified fungal cell envelope. The cross-linked cell wall is linked to the cell membrane. The cell wall usually consists of innermost fibrils of chitin or cellulose, with outer layers of amorphous, cross-linked glucans.