Effect of low temperature on the growth and fine structure of Bacillus subtilis

Logarithmically growing cultures of Bacillus subtilis transferred from 37 to 15 C present atypical growth curves, and ultrathin sections of such cells reveal structural modifications involving mesosome deterioration and double cell wall formation. After a time, optical density and viable count increase, and cells regain the appearance typical of control cells, indicating a recovery from thermal stress. Subcultures of such recovered cells continue to grow well at 15 C. Cultures transferred from 37 to 12 C show atypical growth and fine structure, although no recovery from this stress is seen. Cultures previously grown at 15 C continue to grow at 12 C, and, furthermore, do not show the ultrastructural alterations seen in similar cells with a 37 C thermal history. The results of these studies suggest that low temperatures induce structural modifications in B. subtilis, that the response of a population to thermal stress may change during the period of the stress, and that thermal history may influence the response of a population to thermal stress.