The fungi considered in this chapter are usually found as members of the normal flora or as saprophytes in the environment. With breakdown of host defenses, they can produce disease ranging from superficial skin or mucous membrane infections to systemic involvement of multiple organs. The most common opportunistic infections are caused by the yeast Candida albicans, a normal inhabitant of the gastrointestinal and genital floras, and the mold Aspergillus, commonly found in the environment. Pneumocystis, a prominent cause of pneumonia in AIDS patients, used to be considered a parasite on morphologic grounds. The diseases caused by these opportunistic fungi are summarized in Table 45–1.
Table 45–1 Agents of Opportunistic Mycoses
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Table 45–1 Agents of Opportunistic Mycoses
CULTURE AT 25°C
CULTURE AT 37°C
Skin, mucous membranes, urinary, disseminated
Rhinocerebral, lung, disseminated
Candida species grow as typical 4- to 6-μm, budding, round, or oval yeast cells (Figure 45–1) under most conditions and at most temperatures. Under certain conditions, including those found in infection, they can form hyphae. Some species form chlamydoconidia in culture. Candida species identification is based on a combination of biochemical, enzymatic, and morphologic characteristics, such as carbohydrate assimilation; fermentation; and the ability to produce hyphae, germ tubes, and chlamydoconidia. Of the over 150 Candida species, fewer than 10 appear in human disease. Particular attention is given to the differentiation of C albicans from other species, because it is by far the most common cause of disease.
|Formation of hyphae and chlamydoconidia are distinguishing features|
|Carbohydrate assimilation and fermentation determine species|
Candida albicans. This scanning electron micrograph demonstrates dimorphism with both blastoconidia and hyphae. (Reproduced with permission from Willey J, Sherwood L, Woolverton C (eds). Prescott's Principles of Microbiology. New York: McGraw-Hill; 2008.)
Most Candida species grow rapidly on Sabouraud's agar and on enriched bacteriologic media such as blood agar. Smooth, white, 2- to 4-mm colonies resembling those of staphylococci are produced on blood agar after overnight incubation. Aeration of cultures favors their isolation. The primary identification procedure involves presumptive differentiation of C albicans from the other Candida species with the germ tube test (see text that follows). Germ tube–negative strains may be further identified biochemically or reported as “yeast not C albicans,” depending on their apparent clinical significance. ::: Yeasts and molds, p. 695
C albicans grows in multiple morphologic forms, most often as a yeast with budding by formation of blastoconidia. C albicans is also able to form hyphae triggered by changes in conditions such as temperature, pH, and available nutrients. When observed in their initial stages when still attached to the yeast cell, these hyphae look like sprouts and are called germ tubes (Figure 45–2A). Other elongated forms with restrictions at intervals are called pseudohyphae because they lack the parallel walls and septation of the true hyphae. There is evidence that these three forms have distinct stimuli and genetic regulation, making C albicans a polymorphic fungus. Unless otherwise specified, the term hyphae is used here to encompass both the true and pseudohyphal forms. The hyphal form also develops characteristic terminal thick-walled chlamydoconidia under certain cultural conditions (Figure 45–2B).
Candida albicans.A. When incubated at 37°C, C albicans rapidly forms elongated hyphae called germ tubes. B. On specialized media, C albicans forms thick-walled chlamydoconidia, which differentiate it from other Candida species. (Reprinted with permission from Dr. E. S. Beneke and the Upjohn Company: Scope Publications, Human Mycoses.)
The C albicans cell wall is made up of a mixture of the polysaccharides mannan, glucan, and chitin alone or in complexes with protein. A fibrillar outer layer extending to the surface contains a number of glycoproteins and complexes of mannan with protein called mannoproteins. The exact composition of the cell wall and surface components varies under different growth conditions. ::: Fungal cell wall
Clinical Capsule: Candidiasis
Candidiasis occurs in localized and disseminated forms. Localized disease is seen as erythema and white plaques in moist skinfolds (diaper rash) or on mucosal surfaces (oral thrush). It may also cause the itching and thick white discharge of vulvovaginitis. Deep tissue and disseminated disease are limited almost exclusively to the immunocompromised. Diffuse pneumonia and urinary tract involvement are especially common.
C albicans is a common member of the oropharyngeal, gastrointestinal, and female genital flora. Infections are endogenous except in cases of direct mucosal contact with lesions in others (eg, through sexual intercourse). Although C albicans is ...