HOST SPECIFICITY 129 



carried phage, but also to a number of unrelated virulent phages. 

 Several other examples of this phenomenon are known and will 

 be discussed later on. Infection with a latent phage would ap- 

 pear to be a valuable protective mechanism to the bacterium. 

 The intimate details of this acquired resistance to virulent 

 phages are unknown, but must be rather specific since the bac- 

 terium remains susceptible to lysis by certain other phages. 



Lysogenization may produce phage-resistance in still another 

 way. Salmonella strains of group E2 (somatic antigens 3,15) 

 are lysogenic for a phage which can be demonstrated by means 

 of indicator strains from group Ei (somatic antigens 3,10). 

 Strains of group Ej, which become lysogenic for such a phage, 

 lose at the same time the ability to adsorb the phage (Uetake, 

 Nakagawa, and Akiba, 1955). In this case lysogenization is 

 accompanied by a change in the surface of the host cell, such 

 that adsorption is blocked. The change can be demonstrated 

 serologically, since the lysogenized strains have also lost antigen 

 10 and gained antigen 15. If such lysogenic strains are exposed 

 to antiserum specific for antigen 15, antigenic variants can be 

 obtained which have lost antigen 15, and show now antigen 10. 

 These variants are not lysogenic any more, and are susceptible 

 to the phage, thus confirming the close association of the lysogenic 

 condition with the change in the cell surface. 



7. Phage Mutations Affecting Host Range 



D'Herelle (1926) considered the extension of host range of a 

 phage preparation to be an adaptive process which he called 

 "acquisition of virulence." His procedure of adaptation was to 

 grow two bacterial strains in mixed culture, add a phage active 

 for one strain and watch for lysis. If lysis was not evident the 

 culture was filtered and the filtrate used to inoculate a fresh 

 mixed culture, this process being repeated until lysis occurred. 

 Then the "adapted" phage was isolated by repeated subculture 

 on its new bacterial host. A better method is to spread the same 

 mixture of bacterial strains and phage on the surface of agar 



