Biochemical serogrouping of clinical isolates of Cryptococcus neoformans

Three hundred twenty-three clinical isolates of Cryptococcus neoformans of diverse geographic origins were biochemically serogrouped using glycine-cycloheximide-phenol red agar (GCP), the same medium less cycloheximide (GOP), and glycine-L-canavanine bromothymol blue agar (CGB). Twenty isolates gave positive reactions on all three media typical of the B and C serotypes. Three were from the Peoples' Republic of China; three each were from Michigan (two patients) and Louisiana; two each were from California, Georgia, and Virginia; and one each was from Alabama, Florida, North Carolina, Oklahoma, and Tennessee. Two hundred seventy-six isolates were identified as belonging to the A/D serogroup; 272 were of American origin and four were from China. Twenty-seven isolates were biochemically ungroupable. Evaluations of the reactions on all three media were open to subjective interpretations. Utilization of glycine was the most frequent atypical variable; 36 of 276 (13%) A/D isolates utilized glycine while being inhibited by either GCP or CGB or both. Significant differences between A/D and B/C serogroups in terms of susceptibility to 5-fluorocytosine but not to amphotericin B were observed; B/C serogroup isolates appeared to be less susceptible to 5-fluorocytosine in vitro than were the A/D serogroup isolates. These results provided new evidence on the distribution of B/C serogroup isolates of C. neoformans in America and demonstrate the difficulties of using biochemical tests for serotyping purposes. They also offer a possible explanation for the apparent more refractory therapeutic responses of infections caused by B and C serotypes to conventional antifungal chemotherapy.