Ultravioletinduced abnormal growth in Escherichia coli; the influence of yeast extract and of photoreactivation.

Growth of the ultraviolet-irradiated cell is of special interest. Growthmay offer a simpler measure of radiation effect than survival. Growth after irradiation is a reflection of events in the latent or middle period between the irradiation and the end biological result (Kelner, 1954; Latarjet and Gray, 1954; Kelner et al., 1955; Barner and Cohen, 1956). During this period the ultraviolet damage is partly photoreversible and reactions leading to delayed mutation are preventible (Kelner, 1949; Newcombe, 1956). There may be a relation between abnormal growth processes in the irradiated cell and radiation-induced effects which are characteristically delayed. Since ultraviolet light (2537 A) probably affects the cell nucleus specifically, study of the growth of the irradiated cell may help in solving the problem of the immediate and delayed control of cell growth by the nucleus. The literature on the effects on microbial growth of ultraviolet light (Kelner, 1953; Errera, 1954; Kanazir and Errera, 1956; Barner and Cohen, 1956; Iverson and Giese, 1957; Deering and Setlow, 1957), and of ionizing radiations (Billen et al., 1953a, b; Latarjet, 1954; Powers and Pomeroy, 1958) may be summarized as follows: cells continue to grow after irradiation at a rate dependent upon dose, but with a dose killing as much as 90 per cent of the cells, the rate is little less than the control. Oxygen consumption is little affected for about the first 90 min, and the synthesis of deoxyribonucleic acid is inhibited more than that of ribonucleic acid,