Germination of the Macroconidium of Histoplasma capsulatum

Abstract

Optimal conditions for germination of the macroconidia of H. capsulatum were found to be a temperature of near 30[degree]C, a substrate pH in the range of 6.5-8.0, and a relative humidity near 100%. Conidia rarely germinated in less than 24 hours. On agar substrates, germination was markedly inhibited by acidities lower than 6.0, and failed to occur within 48 hours at pB?s below 5.0. Washed conidia germinated on aqueous agar, but increased germination percentages were obtained on enriched media. Highest germination percentages were obtained on cysteine-dextrose blood agar. Exposure to heat was ineffective in stimulating germination. Conidia in aqueous suspension and colonies on agar slants survived exposures to 55[degree]C for 7 minutes, but longer exposures at that temperature and all exposures at higher temperatures were lethal. Budding was observed from conidia on several media when incubated at 37 [degree]C, indicating that increased temperature alone can induce development of the yeast form.