STUDIES UPON BACTERIAL SPORES I. THERMAL RESISTANCE AS AFFECTED BY AGE AND ENVIRONMENT

Abstract

This investigation was undertaken to throw light on the changes in resistance to heat that have been observed to occur in spores of individual species of bacteria, as having a bearing on problems in food preservation. The organism selected for the work was Bacillus mycoides. The experiments were confined primarily to a determination of the influence of age, temperature, and humidity upon variations in thermal resistance of spores, and data were obtained at 30-day intervals upon spores stored under 9 different sets of environmental conditions. A study of the spore cycle of the organism under test; the development of special cultural methods in order to provide for maximum spore formation and the preparation of satisfactory spore suspensions; and the development of a more satisfactory technique for making the resistance tests, were necessary parts of the investigation. The following conclusions are drawn from the experimental data: The bacterial spore is not dormant under ordinary conditions, as has commonly been supposed, but is instead sluggishly active. The resistance of spores to heat is not a fixed property but a variable one, the degree of resistance being influenced by age, the temperature and humidity of the environment, and possibly other factors. The highest resistance to heat develops Under conditions of moderate temperature and humidity, and is probably reached by the time the spores are 60 days old. Spores of different species may be expected to vary somewhat in this respect. Change in resistance takes place most slowly when spores are dry and cold, but low temperature accompanied by high humidity results in the development of a high degree of resistance. In determining the thermal death points of spores that are to serve as the basis of processing schedules for canned foods the bacteriologist must take into account the change in resistance of spores under various conditions, and be as certain as possible that the resistance shown by the test spores represents the highest degree attainable by them. An annotated bibliography is appended.