Division patterns from single Escherichia coli cells.

A method has been worked out for studying the division of single bacterial cells (Escherichia coli B/r) in a uniform environment. Under optimal conditions the daughters of one single cell are found to divide at different times, a fact which indicates that they are not identical. The spread in generation times can be estimated quantitatively. When cells are irradiated with gamma rays in nutrient broth there is an increase in the spread in generation times, and the number of three-cell progeny (which require considerable difference between the daughters) rises. The results are consistent with the idea that damage to a segment of DNA has taken place and that there are three growing points on the DNA at any one time. In nutrient broth there is some evidence for repair of damage. For cells irradiated in minimal medium the pattern is different. The increase in generation times is not so marked, and only slight increase in three-cell progeny is seen. The results suggest that there is the same class of damage to the DNA, with only one growing point present and less repair. Using the criterion that no increase in generation time at all is permitted, we can estimate the total escape probability after radiation. The resulting calculation of critical target size is much closer to the whole bacterial chromosome than is found from normal plating experiments. The behavior of cells that have been exposed to irradiated medium is quite different, involving very long lags and cell death.