Germination of blastospores ofHistoplasma capsulatum

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Abstract
Factors affecting blastospore germination by the dimorphic fungus Histoplasma capsulatum were studied. Those incubated at 23°C in distilled water, physiological saline, phosphate buffered saline, or in distilled water with KCl, CaCl2 and MgCl2 did not germinate. None of the tested carbohydrates, or keto- and fatty acids stimulated germination. Less than 10% of the blastospores germinated in media with glutamic acid, isoleucine, serine, or valine, and no germination occurred on media with any of the other 13 amino acids tested. The best germination percentages were obtained on commercial casein hydrolysate (acid hydrolysed) where 75% of the yeasts germinated by 72 hr. of incubation at 23°C. The highest percentage of germination was observed on solid subtrates of casein hydrolysate at pH 7.0, which were inoculated with from 5 × 106 to 5 × 107 yeasts/ml and incubated at 23°C. Under optimal conditions, however, only 25% of the yeasts germinated on a synthetic medium composed of the 17 amino acids reported to be in commercial casein hydrolysate. The relative concentration of each amino acid in this synthetic medium corresponded to the manufacturer's analysis. Addition of vitamins or minerals to the synthetic medium did not improve germination percentages. A new 17 amino acid medium was prepared in which the concentration of each amino acid was different from that of commercial casein hydrolysate. This medium supported germination of 50% of the blastospores in 72 hr which suggests that the relative concentration of amino acids in a medium is critical to germination.