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The “opportunistic fungi” are usually found as members of the resident human microbiota or on decaying matter in the environment. With the breakdown of host defenses, they can cause infections ranging from skin/mucous membrane involvement to life-threatening, systemic disease. The most common opportunistic infections are caused by two species: the yeast Candida albicans, a common inhabitant of the gastrointestinal and genital microbiota; and the mold Aspergillus fumigatus which is widespread in the environment. Pneumocystis, a frequent cause of pneumonia in AIDS patients, is an unusual fungus that used to be considered a parasite on morphologic grounds. However, it too is a frequent colonizer of the human respiratory tract. The diseases caused by these opportunistic fungi are summarized in Table 46–1.

TABLE 46–1


Candida species grow as 4 to 6 μm, budding, round or oval yeast-like cells (Figure 46–1). Under certain growth conditions, including those encountered during infection, certain pathogenic Candida species can also form hyphal-like structures. Of the over 150 Candida species, fewer than ten cause human infections. For serious infections, identifying Candida isolates to the species level is important for prognostic and treatment decisions. Particular attention is given to the differentiation of C albicans from other species, because it is by far the most common cause of disease.

FIGURE 46–1.

Candida albicans. This scanning electron micrograph demonstrates dimorphism with both yeast-like cells and hyphae. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th ed. New York, NY: McGraw Hill; 2008.)


Most Candida species grow rapidly on Sabouraud’s agar and on enriched bacteriologic media such as blood agar. These fungi also grow readily in standard blood culture systems. Smooth, white, 2 to 4 mm colonies are produced on blood agar or other solid media after overnight incubation.

Candida species grow in multiple morphologic forms, most often as a budding yeast. C albicans is also able to form hypha-like structures triggered by changes in conditions such as temperature, pH, and available nutrients. When observed in their initial stages of germination from the yeast cell, these nascent hyphae resemble sprouts and are called “germ tubes” (Figure 46–2A). Most C albicans strains produce germ tubes when incubated in the presence of serum, allowing a rapid ...

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