Medium for susceptibility testing and yeast phase conversion of Blastomyces dermatitidis.

Cottonseed protein agar and a modified Tween-albumin casein hydrolysate (TAC) medium were compared for the yeast phase conversion of Blastomyces dermatitidis strains including fresh isolates as well as strains maintained in long-term storage. It was found that both media converted all the B. dermatitidis (mycelial phase) strains studied to yeast phase in three days. The TAC medium has the added advantage that it is clear and the growth can be recognized earlier than in the opaque cottonseed agar medium. The conversion in most cases was more than 95% and the morphology of the yeast cells was uniformly typical with broad base budding. There was a striking difference between the sensitivity of the yeast and mycelial phases of B. dermatitidis strains. The yeast phase was usually more sensitive to Amphotericin B than the mycelial phase of B. dermatitidis. Similarly, the yeast phases of four out of six strains were more sensitive to ketoconazole than their respective mycelial phases, while two strains showed identical sensitivity in cottonseed agar. The yeast phase organism was more susceptible to Amphotericin B when cottonseed medium was used whereas the yeast phase showed more susceptibility to ketoconazole in TAC medium. Since the sensitivity among the various strains differed, it is necessary to determine the antifungal susceptibility of the pathogenic phase of the organism for initiating proper therapy and monitoring effectiveness.