HOST SPECIFICITY 135 



typical of that of host range mutants and these adapted typing 

 phage strains were referred to as mutants by Craigie (1946). 



The significant observation of Anderson and Felix is that when 

 the "adapted" phage C is plated on a lawn of Type A typhoid, it 

 reverts completely to the characteristics of phage A. For this and 

 other reasons, Anderson and Felix designated this adaptive re- 

 sponse "phenotypic modification of host range." The general 

 phenomenon is very common and exhibits other remarkable 

 features that are discussed in Chapters XVI and XXI. 



9. Summary 



Most taxonomic groups of bacteria include strains which are 

 susceptible to some bacteriophage. The host range of a given 

 phage is often restricted to a single bacterial genus but exceptions 

 to this rule are common. Changes in the surface of a bacterium 

 may block the adsorption of a phage otherwise able to multiply 

 in such a host. These changes may occur by mutation, and are 

 often associated with antigenic changes. Multiplication of a 

 phage in a given host may be blocked at a stage past adsorption. 

 This is often a consequence of the lysogenic condition of the host. 

 Both types of blocks may be overcome by mutation of the 

 phage. The host range of the phage may be affected by the 

 conditions of its growth in a way not involving mutation and 

 selection. One class of such effects is called phenotypic mixing. 

 It is observed when two closely related phages differing in adsorp- 

 tion-specificity multiply in the same bacterium, and affects pri- 

 marily adsorption-specificity. The effects are promptly reversed 

 when the phage multiplies in pure culture. A second class of 

 such effects is called host-induced modification, and affects host 

 range independently of adsorption-specificity. It is conditioned 

 by the genotype (including carried prophages) of the bacterial 

 host. This class of effects is likewise reversed by transfer of the 

 phage to another host. 



