The fungi considered in this chapter are usually found as members of the resident human microbiota or as saprophytes in the environment. With the 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 common inhabitant of the gastrointestinal and genital floras, and the mold Aspergillus, widespread 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 46–1.
TABLE 46–1Agents of Opportunistic Mycoses ||Download (.pdf) TABLE 46–1 Agents of Opportunistic Mycoses
| || ||GROWTH || || |
|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 typical 4 to 6 μm, budding, round, or oval yeast cells (Figure 46–1) under most conditions and at most temperatures. Under certain conditions, including those found in infection, they can form hyphae. The Candida cell wall contains the same chitin and carbohydrate elements found in other fungi. 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.
Candida albicans. This scanning electron micrograph demonstrates dimorphism with both 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 determine 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,” depending on their apparent clinical significance.