134 BACTERIOPHAGES 



sively diluted, this effect is lost without ever passing through a 

 level of dilution at which discrete plaques are formed. The 

 phage particles adsorb normally to the mutant bacteria, and the 

 infected bacteria lyse after the usual latent period. They also 

 liberate phage particles which, however, cannot multiply fur- 

 ther. After considerable search it was found that strain Sh of 

 Sh. dysenteriae was a suitable indicator for the phage particles 

 liberated on lysis of B/3,4,7(2,6). After passage through strain 

 Sh the phage progeny had the properties of normal T2 or T6 

 phage. 



In summary, phage T2 infects bacterial strain B '3,4,7(2,6) to 

 produce a phenotypically modified phage progeny symbolized 

 as T2*. Phage T2* does not produce plaques on B/3,4,7(2,6), 

 or on B, but does produce plaques on strain Sh. Phage T2* 

 after passage through strain Sh reverts to phage T2. Phage T2* 

 adsorbs to strains B, B/4, and B/6 and kills them, but the infec- 

 tion is abortive. 



These experiments indicate that the phenotypic change in 

 T2* is different from that found in the phage liberated from the 

 bacteria mixedly infected with T2 and T4. In the latter case 

 the modification is in the specificity of adsorption. In the case 

 of T2* the adsorption specificity is the same as that of T2, so 

 the phenotypic modification must involve some stage in multi- 

 plication subsequent to adsorption. It may be noted that the 

 changes in this instance are not adaptive. 



An adaptive type of host-induced modification was reported 

 by E. S. Anderson and Felix (1952), involving the Vi typing 

 phages of S. typhi. The original Vi II phage strain, phage A, 

 when tested at the limiting test dilution gives confluent lysis with 

 Type A typhoid strains only. When tested at higher concentra- 

 tions with a bacterial lawn of a Type C strain, a few plaques may 

 be found. When these plaques are picked and subcultured on 

 Type C bacteria, an "adapted" typing strain is obtained, which 

 at the limiting test dilution will give confluent lysis with both 

 Type C and Type A typhoid strains but only partial or no lysis 

 with all other strains of typhoid bacteria. So far, the behavior is 



