The genus Candida encompasses >150 species, only a few of which cause disease in humans. With rare exceptions (although the exceptions are increasing in number), the human pathogens are C. albicans, C. guilliermondii, C. krusei, C. parapsilosis, C. tropicalis, C. kefyr, C. lusitaniae, C. dubliniensis, C. glabrata, and C. auris. Ubiquitous in nature, they inhabit the gastrointestinal tract (including the mouth and oropharynx), the female genital tract, and the skin. Although cases of candidiasis have been described since antiquity in debilitated patients, the advent of Candida species as common human pathogens dates to the introduction of modern therapeutic approaches that suppress normal host-defense mechanisms. Of these relatively recent advances, the most important is the use of antibacterial agents that alter the normal human microbiota and allow nonbacterial species to become more prevalent in the commensal flora. With the introduction of antifungal agents, the causes of Candida infections shifted from an almost complete dominance of C. albicans to the common involvement of C. glabrata and the other species listed above. The non-albicans species now account for approximately half of all cases of candidemia and hematogenously disseminated candidiasis. Recognition of this change is clinically important, since the various species differ in susceptibility to the newer antifungal agents.
Candida is a small, thin-walled, ovoid yeast that measures 4–6 μm in diameter and reproduces by budding. Organisms of this genus occur in three forms in tissue: blastospores, pseudohyphae, and hyphae. Candida grows readily on simple medium; lysis centrifugation enhances its recovery from blood. Species are identified by biochemical testing (currently with automated devices) or on special agar (e.g., CHROMagar).
Candida are present in humans as commensals, in animals, in foods, and on inanimate objects. In developed countries, where contemporary medical therapeutics are commonly used, Candida species are now among the most common nosocomial pathogens. In the United States, these species are the fourth most common isolates from the blood of hospitalized patients. In a recent point-prevalence study, Candida species were the most common organisms infecting the bloodstream of hospitalized patients. In regions where advanced medical care is rarely available, mucocutaneous Candida infections, such as thrush, are more common than deep-organ infections, which rarely occur. However, the incidence of deep-organ candidiasis increases steadily as advances in health care—such as therapy with broad-spectrum antibiotics, more aggressive treatment of cancer, and the use of immunosuppression for sustaining organ transplants—are implemented. In aggregate, the global incidence of infections due to Candida species has risen steadily over the past few decades. Of great recent concern has been the global emergence of C. auris; certain strains of this organism are resistant to all classes of antifungal agents, and mortality rates from infection have been very high.
In the most serious form of Candida infection, the organisms disseminate hematogenously and form microabscesses and ...