Heat Activation and Heat-induced Dormancy of Bacillus stearothermophilus Spores1

Abstract

Heat-induced dormancy was observed when spores of two strains of Bacillus stearothermophilus were heated in distilled water at 80, 90, and 100 C. At temperatures above 100 C, true activation occurred; however, maximal activation was not achieved until temperatures of 110 to 115 C were employed. A heat treatment of 115 C for 3 min was required to induce maximal activation in one suspension of strain 1518 spores, whereas a heat treatment of 110 C for 7 to 10 min was adequate for the other suspension of strain 1518 spores. Spores from both strain M suspensions required heat treatments of 110 C for 9 to 15 min for maximal activation. The degree to which the spores could be activated was strain dependent and variable among spore suspensions of the same strain. The germination and outgrowth of all spores, regardless of strain and suspensions source, were significantly reduced when the spores were heated in M/120 phosphate buffer at maximal or near maximal activating temperatures. It was suspected that phosphate lowered the heat resistance of the spores to the extent that the heat treatments were lethal to a portion of the populations. The association of heat with spore germination has a unique place in the biology of the bacterial endospore. Curran and Evans (1944, 194.5) were the first workers to demonstrate systematically that sublethal heat (62 to 95 C) could induce dormant spores to germinate. For most of their work, thermotolerant, mesophilic strains were used; however, selected thermophilic strains were also investigated, including strain 1518 of Bacilius stearothermophilus. This thermophile is of particular interest to the food processor and food microbiologist because the spore form is extremely heat resistant and the vegetative form, which is capable of growth at temperatures of 70 C or slightly higher, is responsible for flat sour spoilage of low acid, canned foods. The first reference to activation of endospores at temperatures greater than 100 C was made by Brachfeld 1 Contribution from the Missouri Agricultural Experiment Station. Journal series no. 2383. Approved by the Director.

The Effect of Incubation Temperature on the Recovery of Spores of Bacillus subtilis 8057

“…The lysozynie method of Finley & Fields (1962) was used to purify the spore suspension. The spores were suspended in 50 ml of distilled water containing 0.5 mg/rnl of lysozynie and incubated a t 52" for 2 h with stirring.…”

ULTRA‐HIGH TEMPERATURE EFFECT ON B. STEAROTHERMOPHILUS DURING PUFFING OF RICE1

“…Activation has also been shown to cause apparent increases in populations of some spore suspensions (Gurney and Quesnel 1980). Finley and Fields (1962) used a temperature of 1 15°C for 2 to 4 min to obtain the maximum spore counts of B . stearothermophilus.…”

“…Titus (1957) also observed endospore thermal activation at 1 10°C for 6 to 10 min. Finley and Fields (1962) used a temperature of 115°C for 2-4 min to obtain the maximum colony count of B . stearothermophilus for one spore suspension; otherwise, they used a temperature of 110°C.…”

A Model System for Testing the Microbiological Stability of Foods Processed in Laminated Flexible Pouches

“…thermosaccharolyticum. Supplemented Nutrient Agar (Finley & Fields 1962) was used for spores of B. stearothermophilus.…”