132 BACTERIOPHAGES 



that host range mutations of phage T2 occurred at two different 

 gene loci, and that three different mutant alleles might occur at 

 one of these loci. 



The host range mutants of phages Tl and T2 just mentioned 

 have gained the ability to adsorb to indicator strains of bacteria. 

 Phage mutants are also known that are able to overcome the 

 immunity produced by the lysogenic condition, which has a 

 different basis. The temperate phage P2, mentioned in the 

 preceding section, does not lyse lysogenic cells Sh(P2), although 

 it does absorb onto them. Rare mutants of phage P2 are found, 

 which are able to lyse Sh(P2) cells. This mutation is commonly 

 associated with the loss of the ability to establish lysogeny in 

 susceptible cells (Bertani, 1953a). In a similar case involving 

 the temperate coliphage lambda, it is possible to demonstrate 

 genetic recombination between the factor imparting virulence 

 and other mutations affecting the plaque type (Jacob and Woll- 

 man, 1954). 



These studies have demonstrated conclusively the mutational 

 nature of some examples of host range variation in bacteri- 

 ophages. It has become evident in recent years that host range 

 modifications may occur by mechanisms other than mutation 

 of the virus. These phenotypic modifications of host range will 

 be discussed next. 



8. Phenotypic Modification of Host Range 



A phenotypic modification of the properties of an organism 

 implies a nonhereditary change, usually attributable to some 

 environmental influence. A phenotypic modification is re- 

 versible in the immediate descendants of the organism in the 

 absence of the causal environmental factor. Numerous examples 

 of phenotypic alterations in phages have been discovered in 

 recent years. Some examples involving host range are described 

 below. 



a. Phenotypic Mixing 



The first clear example of a phenotypic modification in host 

 range was brought to light in consequence of an observation re- 



