Ultraviolet Irradiation of Bacteriophage During Intracellular Growth.

In the course of attempts to produce mutations in bacteriophage by ultraviolet irradiation of bacteria infected with the phage, we found that suppression of the ability of an infected bacterium to liberate phage required higher doses than either sterilization of noninfected bacteria or inactivation of free phage. . The doses required to suppress phage liberation varied in the course of the interval between infection and liberation, during which intracellular phage growth takes place. These observations were in agreement with the fact that phage multiplication can take place in bacteria recently sterilised by irradiation (Anderson, 1944; Rouyer and Latarjet, 1946). They suggested that the observed effect of radiation on the infected bacteria might depend on ~activa~on of intracellular phage. Variations of this ef3tect might then reflect the changes in number and properties of phage particles during ~t~ce~~ growth-that is, in the course of processes that lead to the produ~~on of over 100 phage particles from each infected barters (Delbtiek, 1946). An analysis of the changes in ~traviolet sensitivity during the period of intracellular growth could then be expected to supply ~fo~ation on the rne~~srn of growth. The rate of ultra~olet ~activation of free phage is a simple exponential function of the dose of radiation (one-hit ~activation, see Latarjet and Wahl, 1945). If inactivation of the gradual ~tra~ell~ar phage particles followed the same function, and if the ability of a bacterium to liberate phage depended on the survival in it of at least one active particle, then the number of intracellular particles per bacterium at the time of ovation should influence the rate of suppression of phage liberation. Instead of an exponential one-hit inactivation curve, as for free phage, we should find for the infected bacteria a multiple-hit curve, the number of hits refleeting the number of active particles present at the time of irradiation. The feasibility of this analysis was suggested by some preliminary observations of this type by Anderson (1944). His data, whmh he kindly discussed with us, seemed to indicate a shift from one-hit to multiple-hit type of curve in the inactivation curves for phage-infected bacteria during intracellular phage growth. The process of intrace~ular phage growth-in particular, of the kinetics of p~eproduction-h~so farescaped every attempt at clarification made eitherby breaking down infected bacteria or by electron microscopy. This problem is of