Rapid Quantitative Method for Measuring the Sensitivity of Blastomyces Dermatitidis to Fungistatic Agents.

Guidry, D. J. (Louisiana State University School of Medicine, New Orleans) and Findlay Maier. Rapid quantitative method for measuring the sensitivity of Blastomyces dermatitidis to fungistatic agents. J. Bacteriol. 85:504-508. 1963.-Isolates of Blastomyces dermatitidis (yeast phase) were grown in continuous shake liquid culture. When sufficient cells were added to impart an initial optical density of 0.100 to the inoculated medium, growth was rapid, and the time required to assay fungistatic agents was reduced to 48 hr. Standard curves on light transmittance and direct cell count facilitated preparation of inoculum and measurement of growth. Generation time was determined for various growth phases of B. dermatitidis, and the data were applied to selection of the optimal period of growth for assaying fungistatic agents. Four isolates of B. dermatitidis were tested for their sensitivity to amphotericin and 2-hydroxystilbamidine. An amphotericin B concentration of 1.4 mug/ml was sufficient for maximal inhibition of all isolates tested; there were differences among isolates in sensitivity to lower concentrations of the drug. A 2-hydroxystilbamidine concentration of 62.5 mug/ml was sufficient for maximal inhibition of two isolates; the two remaining isolates required a concentration of 125 mug/ml. A latent period in inhibition of growth occurred when amphotericin B and 2-hydroxystilbamidine were added to cultures of B. dermatitidis. This latent period, together with generation time of the yeast cells, determined the maximal degree of inhibition which could be attained.