On the Difference between the Lethal Effects of H 3 and P 32 in Bacteria

Growing cultures of bacteria have a division cycle markedly different from that of cells of higher animals and plants. In growing coliform bacteria (1), the cytologically observable division of nuclear material takes place approximately halfway through the cell division cycle. Thus, in such cultures a considerable proportion of the cells are multitarget. This would appear to preclude a pure type of killing by agents that inactivate genetic material. Yet it has been found in a number of laboratories that H3-containing cells of exponentially growing cultures are inactivated with single-hit kinetics during low-temperature storage (2-5). Suicide due to p32 gives killing curves (6, 7) with extrapolation numbers in the range of 1.5 to 3.5 and usually much nearer the higher value. This had been interpreted by the Stent group to indicate that this number corresponded to the number of nuclear bodies per cell. They based their argument on previous cytological observations obtained under different experimental conditions by another author. Seemingly, support for this conclusion was added by the observation that the apparent hit number was reduced on phosphorus starvation (8), since starvation generally decreases the number of nuclear bodies per cell. However, the cell doubling time in the enriched but phosphorus-deficient medium H employed for P32 labeling was reported to be about 50 minutes, which is nearly the same as the doubling time in the minimal salts medium used for H3-thymine labeling. There is reason to believe (9) that the average number of nuclear bodies per cell depends only on the growth rate and not on the qualitative composition of the growth medium. Consequently, similar numbers of nuclear bodies per cell would be expected. This is indeed so (Table I) for the Escherichia coli strains 15 T-A-Uand B, the strains employed in the studies quoted.