In vitro susceptibility testing and DNA typing of Saccharomyces cerevisiae clinical isolates.

Saccharomyces spp. are widely distributed in nature and may colonize the normal human gastrointestinal tract. Although Saccharomyces cerevisiae isolates have been previously considered nonpathogenic, they appear to be increasingly associated with infections in immunocompromised or otherwise debilitated patients. The antifungal susceptibility and epidemiology of S. cerevisiae are poorly defined at present. A series of 76 isolates (mostly stool surveillance and throat swab isolates) from 70 bone marrow transplant patients hospitalized at two different medical centers were characterized by antifungal susceptibility testing and restriction endonuclease analysis of chromosomal DNA. For DNA typing, digestion with NotI followed by pulsed-field gel electrophoresis was applied. Typing results revealed 62 distinct DNA types among the 76 clinical isolates. Despite this genomic diversity, clusters of identical isolates were identified among different patients hospitalized concurrently in the same unit, indicating possible nosocomial transmission. The MICs of amphotericin B, 5-fluorocytosine, fluconazole, and itraconazole were determined by a broth microdilution method, as recommended by the National Committee for Clinical Laboratory Standards. The MICs at which 90% of the strains were inhibited were as follows: amphotericin B, 1.0 micrograms/ml; 5-fluorocytosine, 0.25 micrograms/ml; fluconazole, 8.0 micrograms/ml; and itraconazole, 1.0 micrograms/ml. The relative resistance of S. cerevisiae to fluconazole and itraconazole may promote the emergence of this species as a pathogen among immunosuppressed patients.