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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.

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Table 45–1 Agents of Opportunistic Mycoses 
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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.

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Formation of hyphae and chlamydoconidia are distinguishing features
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Carbohydrate assimilation and fermentation determine species
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Figure 45-1.
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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.)

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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 ...

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