1952] ULTRAVIOLET LIGHT INDEX IN BACTERIOPHAGE 65 



such a low multiplicity of infection and still have a measurable concentration 

 of survivors after growth, dilution, and irradiation, it is necessary to start with 

 a high concentration of bacteria (1 X 10^ per ml). Concentrations either half 

 or twice this value were found to give the same results. 



PRELIMINARY CONSIDERATIONS 



1. Ability of irradiated bacteria to support phage growth. It is necessary to know 

 the validity, under the conditions used, of the basic assumption that the irradia- 

 tion of the bacteria does not affect their ability to support phage growth. For 

 this reason Anderson's experiments were repeated, using phage T2r. Starved 

 bacteria, prepared as previously described, were irradiated in buffer before addi- 

 tion of phage. The number of infective centers produced per adsorbed phage was 

 determined by plating and compared to the result with unirradiated bacteria. 

 At the intensity of ultraviolet here used, 4,000 seconds of irradiation are required 

 to reduce to one-half the number of bacteria capable of liberating phage after 

 infection with T2r. Since the largest doses used in irradiating infected cells were 

 1 ,000 seconds, inactivation of the bacterial part of the complex should have had 

 a negligible effect upon the survival curve. It is conceivable, however, that the 

 sensitivity of the bacterial part of the complex does not remain unchanged 

 throughout the latent period. Indeed, it becomes difficult to separate the complex 

 into phage and bacterial components once growth has started. 



For T7, it is likewise found that the highest dose used (350 seconds) has 

 negligible effect upon the ability of the bacteria to yield phage after infection. 



2. Effect of irradiation of complexes upon the latent period and burst size of the 

 survivors. It is found that a progressive lengthening of the latent period and 

 decrease in burst size are produced by increasing doses of ultraviolet. For T2 

 monocomplexes irradiated at mid-latent period with a dose such that 5 per cent 

 survive to form visible plaques, the latent period of the survivors is doubled 

 and the burst size is reduced to 10 per cent of normal. This effect results in a 

 smaller and more variable size of the plaques formed by surviving monocom- 

 plexes. The degree of the effect appears to be determined primarily by the dose 

 of ultraviolet rather than the percentage inactivation of the complexes. There- 

 fore, it is most apparent at times during the latent period when the largest doses 

 are required for obtaining survival curves. In plotting survival curves and at- 

 tempting to analyze them by target theory, we are assuming that inactivation 

 of a complex is an all-or-none phenomenon. It must be realized that the delay 

 in lysis and reduction in burst size of the surviving infective centers could also 

 lead to failure of some of them to produce visible plaques. However, for the 

 doses used in this paper, the plaques observed with T2r and T7 do not taper 

 gradually down to zero in diameter; probably few infective centers fail to be 

 counted for this reason. 



RESULTS 



Experiments with T2r. T2 (used by Luria and Latarjet) and its mutant T2r, 

 which has the same sensitivity to ultraviolet as T2, gave similar results. It is 



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