Isolation of Cryptococcus-Like Yeast From Natural Environments
Background: Cryptococcus (C. gatti and C. neoformans) causes meningitis and pneumonia in immunocompromised and immunocompetent hosts, and several cases are diagnosed at Aurora Health Care each year. Cryptococcus is infrequently isolated from the environment, despite being the presumed infection source, with no known isolations in Wisconsin since 1964. C. gatti-endemic areas are expanding worldwide, and it is commonly isolated from tree hollows.
Purpose: To isolate pathogenic Cryptococcus from Wisconsin natural sites.
Methods: Samples were obtained from tree structures and natural and built surfaces in northern and southern Wisconsin (103 samples) and northeast Ohio (8) from April 2017 to December 2017. Cotton (38 samples) and liquid Amies elution (73) swabs were used to collect material for incubation at 35°C (after first 18 samples were at 20°C) on Staib (birdseed) agar. Suspicious colonies were further incubated on Sabouraud dextrose and brain-heart infusion agar at 20°C and 37°C, respectively, and on urea agar. Colonies were further examined microscopically with India ink.
Results: Use of liquid Amies elution swabs and isolation at 35°C reduced background mold growth. Of 111 samples, 2 isolates of Cryptococcus-like yeast were identified from the same weeping willow tree in Greendale, Wisconsin. These tan isolates on Staib agar were very similar in appearance, grew at 37°C, and were urease-positive, but had thin, rather than broad, capsules. One isolate tested at ACL Laboratories (Milwaukee, WI) using matrix-assisted laser desorption/ionization (MALDI) technology did not match with any database organism, which, combined with phenotypic findings, suggests that these isolates likely represent nonpathogenic environmental Cryptococcus species. No putative pathogenic Cryptococcus was isolated from these samples, consistent with the 0–10% isolation success reported in the literature.
Conclusion: Isolation of these Cryptococcus-like yeasts suggests that further isolation attempts with this technique may result in isolation of pathogenic Cryptococcus strains from the environment in Wisconsin.