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 ||Download (.pdf) TABLE 50–1 Important Features of Opportunistic Fungal Diseases
|Genus ||Form in Tissue Seen by Microscopy ||Geographic Location ||Important Clinical Findings ||Laboratory Diagnosis |
|Candida ||Yeast forms pseudohyphae (also hyphae) ||Worldwide ||Thrush in mouth and vagina; endocarditis in intravenous drug users ||Gram-positive; culture grows yeast colonies; Candida albicans forms germ tubes; polymerase chain reaction (PCR) assay; MALDI-TOF assay |
|Cryptococcus ||Yeast with large capsule ||Worldwide ||Meningitis ||India ink stain shows yeast with large capsule; culture grows very mucoid colonies; PCR assay |
|Aspergillus ||Mold with septate hyphae ||Worldwide ||Fungus ball in lung; wound and burn infections; indwelling catheter infections; sinusitis ||Culture grows mold with green spores; conidia in radiating chains |
|Mucor and Rhizopus ||Mold with nonseptate hyphae ||Worldwide ||Necrotic lesion formed when mold invades blood vessels; predisposing factors are diabetic ketoacidosis, renal acidosis, and cancer ||Culture grows mold with black spores; conidia enclosed in a sac called a sporangium |
Additional information regarding the clinical aspects of infections caused by the fungi in this chapter is provided in Part IX, entitled Infectious Diseases.
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, central lines, 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.
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 ed. New York, ...