INACTIVATION OF BACTERIA BY DECAY OF INCORPORATED RADIOACTIVE PHOSPHORUS

Cultures of Escherichia coli will not grow in media containing very high specific activities of radiophosphorus P(32), the inhibition of growth being due to the decay of assimilated P(32) atoms. Experiments with a differentially labeled thymineless strain of E. coli show that the P(32) disintegrations which occur in the bacterial deoxyribonucleic acid, i.e. in the nucleus, are mainly responsible for the inactivation of the cell. The kinetics with which radioactive bacterial populations are inactivated indicate that the function of several nuclei per bacterial cell must be eliminated by P(32) decay before the ability to generate a colony is lost. The efficiency with which each P(32) disintegration inactivates the nucleus in which it has occurred is calculated to be 0.02 (at –196°), i.e., similar in magnitude to the killing efficiency of P(32) decay in bacteriophages. P(32) decay and thymine starvation cooperate in bringing about the death of individuals of the thymineless strain, from which observation it is inferred that "thymineless death" is likewise a nuclear inactivation. The descendants of a non-radioactive bacterial culture grown for several generations in the presence of P(32) and the descendants of a radioactive culture grown in the absence of P(32) are inactivated by P(32) decay in a manner which indicates that the phosphorus atoms of bacterial nuclei are dispersed among the progeny nuclei in their line of descendance.