SYLLABUS 131 



seen (20) that T5 is adsorbed in the absence of calcium but progress is 

 arrested at an early stage. Yet, in bacteria to which T5 has been ad- 

 sorbed in the absence of calcium, the multiplication of unrelated viruses 

 is excluded (Adams, personal communication). Similarly, in bacteria 

 simultaneously infected with Ti and T2, Ti is always excluded. Thus, 

 T5 and T2 establish exclusion rapidly. On the other hand, Ti 

 establishes exclusion more slowly. If T2 is added four minutes later 

 than Ti to a bacterial suspension, there is still a considerable proportion 

 of bacteria in which T2 multiplies to the exclusion of Ti. 



The exclusion does not occur at the stage of adsorption. On the 

 contrary, as long as the two viruses are not in very highly multiple 

 excess of the bacteria (100 fold or more), they are each adsorbed as if 

 the other were not there. That they are actually adsorbed to the same 

 bacteria can be shown by a quantitative analysis of the adsorption rates 

 and by the depressor effect described below. 



When T2 is irradiated with X-rays, its ability to exclude Ti is 

 destroyed at the same rate as its ability to kill bacteria. This suggests 

 that a phage particle must invade the bacteriimi in order to exclude 

 (Watson, 1950). 



The same conclusion is reached by studying mutual exclusion in 

 buffer; phage adsorbed in buffer does not exclude others added after 

 considerable time intervals (Delbriick, unpublished). The experiments 

 discussed in 35 indicate that phage adsorbed in buffer does not progress 

 beyond adsorption. 



In mixed infections with Ti and T2 exclusion is one-sided. T2 

 wins out in almost every bacterium, and lysis occurs after 21 minutes, 

 the latent period characteristic for T2. In the few bacteria in which Ti 

 wins out lysis occurs after 13 minutes, the latent period characteristic 

 for Ti . If the two phages are not given simultaneously, the time is to be 

 reckoned from the moment at which the phage was added which is 

 liberated by the particular bacterium. 



The excluded virus, though adsorbed, is not recovered upon lysis. 

 That its attack has actually gone beyond the stage of adsorption is 

 indicated by its influence on the ^^eld of the successful virus. In the 

 case of mixed infection wdth Ti and T7, this effect of the excluded virus 

 on the yield of the successful virus consists in a very considerable re- 

 duction of the yield. This has been called the depressor effect. 



In the study of mixed infections with Ti and T7 it was found that 

 the depressor effect itself can be mitigated by the addition of specific 

 antiserum active against the excluded virus, added after adsorption of 



