Physiological properties of Bacillus thuringiensis endospores

Through use of an ±n^ vitro model, survival of vegetative cells and spores of Bacillus thuringiensis within insect midgut conditions was tested. Vegetative cells would not grow in 0.1 M carbonate-buffered media above pH 9 and growth was severely impeded by incorporation of thioglycollate. Outgrowth of spores was inhibited by long-term exposure to pH 8.5 or above, but short-term exposure to carbonate buffer enhanced viable counts. A number of basic chemicals and buffers improved outgrowth with best results obtained with 0.1 M potassium carbonate at a pH of 10. The combination of trypsin, carbonate, and thioglycollate yielded the highest outgrowth rate observed. Germination rates of spores were optimized by heat shock or incubation with carbonate buffer, though most spores did not require activation for germination. To further investigate spore properties, spores from four modifications of tryptic soy agar, two low in glucose and two rich in glucose, were tested for viability and resistance to heat, UV light, and octanol. In addition, spore sizes and germination rates were determined. Spores from low-glucose media (type A spores) were smaller and less resistant to heat and UV light than were spores from high-glucose media (type B spores). Type A spores also were faster germinating and had a greater percentage viable spores than their counterparts from high-glucose media. The contributions of these traits toward adaptation to the ecological niche for spores produced within insects or within the soil is proposed.