MUTATIONS OF BACTERIAL VIRUSES 89 



(multiple infection), the result is the same as with single infection, as if only 

 one particle could grow in a cell. Direct proof of this "self-interference" was 

 difficult to obtain, since the offspring of a virus particle is indistinguishable 

 from that of another particle of the same virus. Interference with the growth 

 of virus 7 by an excess of ultraviolet inactivated virus 7 (Luria and Delbruck 

 1942) was an indirect confirmation of the occurrence of self-interference. 



The availability of virus 7', identical with virus 7 in its behavior toward 

 cells of strain B, but traceable by its activity on strain B7, offered an oppor- 

 tunity for further study of self-interference. The experiments were done by 

 adding both viruses 7 and 7', the former in excess, to cells B, then using B7 as a 

 plating indicator for virus 7'. If virus 7 interferes with the growth of virus 7', a 

 bacterium infected first with virus 7, then with virus 7', will not liberate any 

 virus y' and will not give a plaque when plated with B7. A loss of infective cen- 

 ters will result. 



We give here the data from such an experiment, in which the results were 

 made more clear by the use of anti-virus serum to eliminate the free virus. 

 Delbruck (1944) has found that anti-virus serum inactivates free virus but 

 generally does not affect the growth of virus particles already adsorbed by bac- 

 teria. 



A culture of B was divided into two portions. At time zero one portion re- 

 ceived an excess of virus 7, and one minute later a smaller amount of virus y' . 

 The other portion received only virus y' . After five minutes, the cultures were 

 diluted into tubes containing a dilution of anti-7 serum sufficient to reduce the 

 amount of free virus (both 7 and 7') to about 1 per cent in three minutes (see 

 a later section). At the eighth minute the mixtures were highly diluted into 

 broth to stop the action of serum, and the infective centers of virus y' were 

 measured by plating with B7 at 1 2 and 1 7 minutes — that is, before liberation of 

 new virus had taken place. Other platings were done to determine the amounts 

 of free viruses. The results may be summarized as follows: 



I portion II portion 

 Bacteria/cc 8 X io 7 8 X io 7 

 Absorbed virus 7/cc 80X10 7 



Adsorbed virus 7'/cc 2X10 7 2X10 7 



Infective centers y'/cc o . 2 X io 7 2 X io 7 



It is seen that in this experiment about 90 per cent of the bacteria infected 

 with both viruses 7 and y' failed to liberate any virus y' . Other experiments, in 

 which anti-virus serum was not used, gave results of the same type, although 

 the presence of free virus y' made the loss of infective centers less conspicuous. 

 If we are justified in considering virus 7' as identical to virus 7 in its action on 

 strain B, we can view these experiments as a direct proof of the occurrence of 

 interference between similar virus particles adsorbed by the same cell. 



Properties of virus a' 



Virus a , as isolated from the few plaques obtained by plating virus a with 

 Ba 2 , is indistinguishable from virus a in its activity on bacteria of strain B. The 



227 



