Response of yeast and mycelial phases of Histoplasma capsulatum to amphotericin B and actinomycin D.

The dimorphic fungus Histoplasma capsulatum exists in two phases: a unicellular yeast form at 37 C and a mycelium at 25 C. We have found that these two phases have selective drug susceptibilities. The mycelial form of H. capsulatum was much more susceptible to the polyene antibiotic amphotericin B than the yeast form; in contrast, the yeast form was more susceptible to the antibiotic actinomycin D. The changes in susceptibility occurred early in the transition between the two phases and permitted the transitions to be blocked by sublethal concentrations of the appropriate drugs.