Emerging and Emerged Pathogenic Candida Species: Beyond the Candida albicans Paradigm

Abstract

Many ascomycete yeast species from the Candida genus are widely distributed in nature and act as common saprophytic constituents of the normal human microflora. However, some of these fungal species can also become opportunistic pathogens following a transition from a commensal to a pathogenic phase, induced by alterations in the host environment. Candida species thereby rarely trigger infection in healthy people, but take advantage of a locally or systematically impaired immune system to proliferate in the host and cause diseases termed ‘‘candidiasis.’’ Such fungal infections can be subdivided into three major groups: cutaneous (skin and its appendages), mucosal (oropharyngeal, esophageal, and vulvovaginal) and systemic (bloodstream infections, i.e., candidemia and other forms of invasive candidiasis [IC]). While superficial candidiasis (cutaneous and mucosal) is often observed in AIDS patients, oropharyngeal thrush and vaginitis are more frequently seen in immunocompetent infants and adult women, respectively. Candidemia and IC are common in cancer patients or in transplant individuals following immunosuppression. Candidiasis currently represents the fourth leading cause of nosocomial infections, at 8% to 10%, and mortality due to systemic candidiasis remains high, ranging from 15% to 35% depending on the infecting Candida species [1]. Although Candida albicans remains the most frequently isolated agent of candidiasis, non-albicans Candida (NAC) species now account for a substantial part of clinical isolates collected worldwide in hospitals. NAC species of particular clinical importance include Candida glabrata, Candida tropicalis, Candida parapsilosis, and Candida krusei (synonym: Issatchenkia orientalis), as well as the less-prominent species Candida guilliermondii, Candida lusitaniae, Candida kefyr, Candida famata (synonym: Debaryomyces hansenii), Candida inconspicua, Candida rugosa, Candida dubliniensis, and Candida norvegensis (Table 1). A complementary set of about 20 opportunistic NAC species is also known, but exhibits lower isolation rates [2].