A Life Cycle of a Thermophilic Organism.

Abstract

Numerous investigators have described organisms capable of growing at 62°C. This is the accepted pasteurization temperature for milk. In all of the descriptions of such thermophilic phenomena, especially where milk is concerned, one characteristic has been quite apparent—that is, sharp rather than gradual increases in bacterial numbers. Our observations have been in accord with those described in the literature. Calculation of the generation time for these increases has disclosed values of 5 to 8 minutes. It was thought that such short generation times were in little accord with accepted views. The only alternative was to consider the phenomena as evidence of something other than growth. Spores are usually present at the same time vegetative cells appear in smears of milk. It has been assumed that these spores are germinating rather than that the vegetative cells are sporulating. If spores exist in milk which shows thermophilic growth they must be present in large numbers.

In our experiments, milk diluted to 1 part to 100,000,000 disclosed micro-organisms in the very highest dilutions. The protocol of such an experiment is given in Table I. It will be noted that there is a change of flora between the fourth dilution and the seventh, leaving a gap of 2 dilutions. This gap is rather characteristic of our experiments.

Continuous observations of this character have led us to the belief that there is a definite life cycle in the organism which we have been studying. It would be interesting if similar cycles were shown to exist for other organisms. Briefly, the cycle is as follows:

1. An invisible spore; present in large numbers, heat resistant, incapable of growth on ordinary laboratory media either at 37°C. or at 60°C.

2. A vegetative cell resulting from germination of (1); easily destroyed by heat, capable of growth upon ordinary laboratory media.