Heat Activation Inducing Germination in the Spores of Thermotolerant and Thermophilic Aerobic Bacteria

Abstract

The spores of many thermotolerant and thermophilic bacteria were found to be susceptible to heat activation. In the absence of pregermination heating (usually 95[degree] C for 10 min.) a large proportion of the potentially viable spores failed to germinate. Of 11 cultures (5 of which were Bacillus subtilis, 2 B. calidolactis, and 1 B. coagulans), isolated from 10 outbreaks of spoilage of commercially processed evaporated milk, nearly all were heat-activatable. Activation was measured by pour plates made before and after heating. The proportion of spores which respond to preheating depends on the amt. of heat, the nature of the heating medium, and the temp. at which the spores are formed and subcultivated. At 95 [degree]C in skim milk, activation was nearly complete in 15 min. The heating mediums, arranged in the order of their effectiveness upon heat activation, were: glucose or lactose (0.5%) > peptone (0.5%) > skim milk > glucose nutrient agar > beef extract (0.3%) > dist. water > glucose nutrient broth > NaCl (0.5%). The conc. of glucose most favorable to heat activation was ap-prox. 0.1%. The depressing action of salt upon heat activation increased with conc. in the 0.05-1% range. The response to preheating was lower at the higher temps. of subcultiva-tion. Preheating of susceptible spores serves to lower their minimum temp. of germination. This applies where non-lethal heating is concerned and usually to spores which survive drastic heat treatment. When thermotolerant spp. are involved, heat activation operates to bring the temp. of germination of the majority of spores within the usual storage and distribution temp. range of canned foods. Heat activation in spores has an important bearing on certain laboratory and commercial practices. Some understanding of heat activation is essential to the accurate enumeration of viable spores in many thermotolerant and thermophilic spp.