88 S. E. LURIA 



Table 2 

 Growth of virus y' on bacterial strains B and B-y in broth at 37°C. 



is almost as high as the number of infective ceuters on B. The differences are 

 partly accountable for by the free virus, partly by sampling errors. These ex- 

 periments prove that all particles in a suspension of virus 7' are capable of at- 

 tacking cells B7 — that is, they are all 7'-particles. 



The low efficiency of plating of virus 7' with B7 must, therefore, be due to 

 some reason other than those considered above. The most likely explanation is 

 that it is a result of the very low adsorption rate of the virus by these cells, as 

 shown below. Many particles of virus may either remain unadsorbed on the 

 agar or be adsorbed by bacteria too late to produce a fully developed, visible 

 plaque. Any such explanation is bound to be tentative, in view of our incom- 

 plete knowledge of the process of nlaque formation. 



The interaction of virus 7' with bacleria B and B7 was studied by means of 

 adsorption experiments and "one-step growth" experiments (Delbruck 1942). 

 The results are given in table 2. The interaction of virus 7' with bacteria of 

 strain B is in every respect similar to that of virus 7 (Delbruck and Luria 

 1942). Strain B7 adsorbs virus 7' much more slowly than strain B. In order to 

 obtain measurable adsorption, the experiments with B7 had to be done with 

 older cultures, containing more cells than those used for B. The cells in such 

 cultures are likely to be of smaller size, and this makes the values for the ad- 

 sorption rates and also for the burst size not strictly comparable. An experi- 

 ment with cells B grown to reach the same concentration as used for B7, how- 

 ever, showed a burst size of 95 — that is, not so low as for B7. 



It is interesting to notice that, whereas the bacterial strain B7 has a longer 

 generation time than B, virus 7' grows on B7 with the same constant period as 

 on B. The burst size, however, is smaller. The mutation B— >B7 involves, besides 

 the change in virus sensitivity, changes in the rate of bacterial division and in 

 the yield of virus 7' per cell. 



Interference of virus 7 with the growth of virus 7' 



Interference between different bacterial viruses growing on the same host 

 has been described and found to conform to the general rule that one cell liber- 

 ates only virus of one type (Delbruck and Luria 1942; Delbruck 1944). It 

 was concluded previously that interference also occurs between particles of the 

 same virus strain: when a cell is attacked by several particles of the same strain 



226 



