The “opportunistic fungi” are usually found as members of the resident human microbiota or as saprophytes 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 46–1Agents of Opportunistic Mycoses ||Download (.pdf) TABLE 46–1 Agents of Opportunistic Mycoses
|ORGANISM ||TISSUE ||CULTURE AT 25°C ||CULTURE AT 37°C ||SOURCE ||INFECTION |
|Candida ||Yeast (hyphae)a ||Yeast (hyphae)a ||Yeast ||Endogenous ||Skin, mucous membranes, urinary, disseminated |
|Aspergillus ||Hyphae (septate) ||Mold ||Mold ||Environment ||Lung, disseminated |
|Zygomycetesb ||Hyphae (nonseptate) ||Mold ||Mold ||Environment ||Rhinocerebral, lung, disseminated |
|Pneumocystis ||Elliptical spores ||Nonec ||Nonec ||Unknown ||Pneumonia |
CANDIDA: GENERAL CHARACTERISTICS
Candida species grow as 4 to 6 μm, budding, round or oval yeast-like cells (Figure 46–1) under most conditions and at most temperatures. Under certain growth conditions, including those encountered during infection, certain pathogenic Candida species can also form hyphae. Of the over 150 Candida species, fewer than 10 cause human infections. Particular attention is given to the differentiation of C albicans from other species, because it is by far the most common cause of disease. For serious infections, identifying Candida isolates to the species level is important for prognostic and treatment decisions (Figure 46–1).
Candida albicans. This scanning electron micrograph demonstrates dimorphism with both yeast-like blastoconidia and hyphae. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th edition. McGraw-Hill, 2008.)
Formation of hyphae and chlamydoconidia are distinguishing features
Carbohydrate assimilation and fermentation help clinical labs to distinguish between Candida species
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. Germ tube–negative strains may be further identified biochemically or reported as “yeast not C albicans,” ...