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Opportunistic fungi fail to induce disease in most immunocompetent persons but can do so in those with impaired host defenses. There are five genera of medically important fungi: Candida, Cryptococcus, Aspergillus, Mucor, and Rhizopus. Important features of the opportunistic fungal diseases are described in Table 50–1.

Table 50–1Important Features of Opportunistic Fungal Diseases

Additional information regarding the clinical aspects of infections caused by the fungi in this chapter is provided in Part IX entitled Infectious Diseases beginning on Chapter 70.



Candida albicans, the most important species of Candida, causes thrush, vaginitis, esophagitis, diaper rash, and chronic mucocutaneous candidiasis (CMC). It also causes disseminated infections such as right-sided endocarditis (especially in intravenous drug users), bloodstream infections (candidemia), and endophthalmitis. Infections related to indwelling intravenous and urinary catheters are also important.

Candida glabrata is the second most common cause of disseminated candidal infections and is more drug resistant than C. albicans. Candida auris causes serious bloodstream infections and is highly antibiotic resistant.


C. albicans is an oval yeast with a single bud (Figures 50–1 and 50–2). It is part of the normal flora of mucous membranes of the upper respiratory, gastrointestinal, and female genital tracts. In tissues, it appears most often as yeasts or as pseudohyphae (Figures 50–1 and 50–3). Pseudohyphae are elongated yeasts that visually resemble hyphae but are not true hyphae. True hyphae are also formed when C. albicans invades tissues. C. albicans forms germ tubes whereas most other candidal species do not.

Figure 50–1

Candida albicans. A: Budding yeasts and pseudohyphae in tissues or exudate. B: Pseudohyphae and chlamydospores in culture at 20°C. C: Germ tubes at 37°C. (Reproduced with permission from Brooks GF, Butel JS, Ornston LN. Jawetz, Melnick & Adelberg’s Medical Microbiology, 20th ...

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