STAGES IN PHAGE MULTIPLICATION 171 



Step growth experiment. The average burst size depends on the 

 phage strain, the host cell, and the environmental conditions. 

 J3ifTerent phages growing in the same host strain may have quite 

 different burst sizes (Delbriick, 1 946b) . The same phage growing 

 in different host strains may also have different burst sizes 

 (Barry and Goebel, 1951). An effect of the physiological state 

 of the host cells was demonstrated by Delbriick (1940b) who ob- 

 tained an average burst size of 20 with resting bacteria and 170 

 with growing bacteria. Heden (1951) studied the burst size 

 during the period of transition from the resting phase to the phase 

 of logarithmic growth of the host cell. He found the maximum 

 yield per cell at about the time of onset of bacterial division. 

 This was also the time when the bacterial cell size and the con- 

 tent of ribonucleic acid per cell were maximal. This applied to 

 bacteria grown in broth or in synthetic iriedium. The burst size 

 is generally much larger in media such as nutrient broth than it is 

 in synthetic media. 



Under conditions of lysis inhibition, where the time of lysis is 

 delayed, the burst size may be more than doubled (Doermann, 

 1 948a) . Since delaying lysis of the cell permits inore phage par- 

 ticles to be produced, it would appear that lysis rather than ex- 

 haustion of materials interrupts phage growth. 



The phage yield from individual bacteria can be determined 

 by the single burst technique. A suspension of infected bac- 

 teria is diluted sufficiently so that, when samples are distributed 

 into separate tubes, only a small proportion of the tubes will con- 

 tain infected cells, mostly only one. The samples are incubated 

 until the bacteria have lysed, and then each is plated to deter- 

 mine its phage content. In experiments of this type with phage 

 Tl, Delbriick (1945a) found that burst sizes ranged from below 

 20 to over 1,000. Since the distribution of burst sizes was much 

 broader than either the distribution of host cell dimensions or the 

 distribution of latent periods, the enormous range in burst sizes 

 was probably not due to either of these factors. Similar observa- 

 tions have been reported by Delbriick (1945c) and by Hershey 

 and Rotman (1949). There is no adequate explanation at 

 present for the wide range of burst sizes in a culture. 



