98 S. E. LURIA 



ship, common structural pattern in X-ray diffraction studies, and similarity of 

 gross chemical composition, but showed differences in finer chemical composi- 

 tion. 



The low rate of adsorption of the mutant bacterial viruses by their new hosts 

 may be attributed to a less satisfactory fitting of the surface structures of virus 

 and bacterium, lowering their affinity. 



A possible alternative explanation must be mentioned — namely, that the 

 acquired ability of a mutant virus to be adsorbed by a new host may be due to 

 a change in only one out of a number of surface structures of the virus particle. 

 This would explain the smaller adsorption rate of the mutant viruses by the 

 mutant bacteria, while the rate of adsorption by the normal bacteria remains 

 the same as for normal viruses. 



The evolutionary implications that the results discussed above present for 

 the system bacterium-virus are of some interest. Because of the lytic activity of 

 most bacterial viruses, a bacterial strain is doomed to destruction once it has 

 come in contact with a virus active upon it. The only chance of survival of the 

 bacterial strain is the occurrence of mutations to virus-resistance. As we have 

 seen, however, this occurrence does not necessarily protect the strain, because 

 its mutation to virus-resistance may be compensated for by an independent 

 complementary mutation of the virus. In. certain cases (Sertic 1929) para- 

 sitism may be maintained by two parallel series of complementary mutations 

 in the host and the parasite. 



Mutations of bacterial viruses enlarging their host range need not always be 

 limited to activity upon closely related bacterial strains, but may conceivably 

 render a virus active on strains belonging to different species. It is interesting, 

 from the standpoint of bacterial taxonomy, that while bacterial viruses may be 

 active on species belonging to different genera, chiefly within the family 

 Enterobacteriaceae, no virus has ever been found to be active on members of 

 different families. 



From two bacterial viruses, a and 7, two new viruses a' and 7' were isolated, 

 differing from a and 7 by their ability to attack bacterial strains which by 

 mutation had become resistant to virus a or to virus 7. 



An analysis of the distribution of the particles of a new virus in a series of 

 similar cultures of normal virus proved that the new virus arises by mutation 

 from the particles of normal virus in the course of their growth on sensitive 

 bacteria. 



Each mutant is indistinguishable from its parent virus in serological proper- 

 ties and in its activity on the common bacterial host. The latter property was 

 utilized to study the interference between similar virus particles. The results 

 confirmed the conclusion that only one of the infecting particles succeeds in 

 growing in each bacterial cell. 



The mutant viruses are poorly adsorbed by their new hosts. Their growth on 

 these was investigated. 



236 



