An Outline of the Pattern of Bacterial Generation Times

SUMMARY: The generation times of four species of organisms have been measured, each under several sets of conditions: Aerobacter cloacae, Serratia marcescens, Streptococcus faecalis and Pseudomonas aeruginosa. Minor variations in the experimental conditions appear to affect the mean generation time less in large samples than in small. This can be explained as a result of association between the generation times of closely related organisms. Positive correlation between the generation times of sisters, cousins and perhaps second cousins shows that the influence of an ancestor is felt through two or three generations. The observed correlation between mothers and daughters is usually small, probably because of bias due to the interval between fission of cytoplasm and fission of cell wall. The coefficient of variation of generation time is not a constant for the species but it is stable under given circumstances. It is possibly related systematically to the chemical complexity of the growth medium. In unhampered growth, less than 1 % of the organisms produced are non-viable. There is positive association between the viabilities of sisters, and between the viability of an organism and the generation time of its mother. The distribution of generation times can be represented by a Pearson Type III or else a Pearson Type V distribution; both are convenient in applications. The generation time of an individual is considered to be determined partly by molecular accidents, partly by heredity.