130 BACTERIOPHAGES 



plates, because the presence of a host range mutant is promptly 

 made evident as a plaque. 



Unfortunately the earlier experimenters did not examine 

 critically the properties of the "adapted" phage strains and con- 

 sequently certain of their claims should probably be discounted. 

 For instance d'Herelle claimed to have adapted a staphylococcus 

 phage to lyse a strain of Shiga's bacillus. It is probable that this 

 "adaptation" was the result of phage contamination, especially 

 as the fully adapted phage had lost its lytic power for the staphy- 

 lococcus strain. No such radical alteration of host range has 

 been reported in recent years. 



What appears to be the first clear description of the isolation 

 of host range variants of a phage was given by Sertic (1929b). 

 His experiments will be briefly summarized using his nomencla- 

 ture for the phage and host cell variants. Cells of strain Fb of 

 E. coli were lysed by strain beta of phage Fez except for a few 

 cells of a phage resistant bacterial mutant called Fbrl. If a 

 lawn of variant Fbrl was inoculated with phage strain beta, iso- 

 lated plaques were obtained with a frequency about 1/1,000 that 

 of the plaques obtained on the parent host strain Fb. By suc- 

 cessive single plaque isolations on strain Fbrl, a pure culture of 

 host range variant beta/rl was obtained, which was able to lyse 

 both bacterial strains, Fb and Fbrl. By inoculating host strain 

 Fbrl with phage variant beta/rl a second phage resistant mutant 

 Fbr2 was obtained. Mutant bacterial strain Fbr2 was com- 

 pletely resistant to phage strain beta, but when inoculated with 

 phage beta/rl yielded a few plaques of a new variant, beta/r2. 

 Sertic pointed out that by this two stage process he was able to 

 obtain a variant phage virulent for a bacterial strain which was 

 completely resistant to the original phage. 



Many reports of host range mutants of bacteriophages have 

 since appeared, many of them dealing with adaptation of phages 

 to attack new host strains in the development of phage 

 typing methods. In most cases the authors have presented no 

 evidence to prove that the "adapted" phage was actually derived 

 from the supposed parent. 



