Effects of Temperature on Soil Conidia of Blastomyces Dermatitidis.

A strain of Blastomyces dermatitidis, originally isolated from soil (Denton et al., Science 133: 1126, 1961), was grown in sterile, rich, garden soil in a laboratory-model terrarium developed for ecological studies of pathogenic fungi. An 8-g sample of the infected soil was suspended in 50 ml of physiological saline in a test tube (38 X 200 mm), agitated vigorously, and allowed to settle for 1 hr. A 10-ml sample of the supernatant was removed at the interface with the sediment. Microscopy of the sample revealed that it contained numerous conidia free from mycelial elements. Portions (0.5 ml) of the spore suspension were exposed for 1 hr in test tubes (16 X 150 mm) to temperatures varying, at intervals of 10 C, from 40 to 100 C. Two portions of 0.1 ml from each spore suspension were then plated, each on a separate Sabouraud agar plate. Another 0.5-ml portion of the suspension was held at 37 C for 1 hr, diluted 1:10, and 0.1-ml samples planted on duplicate Sabouraud plates. An unheated control was treated in a like manner. The plates were incubated at room temperature until all viable spores germinated. The effect of the various temperatures on the conidia is shown in Table 1. The unheated control plates showed that the original suspension contained approximately 16,550 viable soil conidia per ml. In the tubes which were subjected to 37 and 40 C, there was 100% survival of the spores. At 50 C, there was a dramatic decrease in the number of spores surviving, to only 10% of the original number. None of the spores survived exposure to -temperatures ranging from 60 to 100 C. The remainder of the undiluted spore suspension was frozen and stored at 0 C. During the next 3 months, the spore suspension was thawed, plated, and refrozen several times. This resulted in a gradual increase in the number of conidia germinating until the viable spore count stabilized at approximately 24,500 per ml. This suspension was then used to determine more TABLE 1. Survival of soil conidia of Blastomyces dermatitidis exposed to various temperatures for I hr