124 BACTERIOPHAGES 



Mutation with loss of Vi antigen makes such strains susceptible 

 to phage. 



Further examples of a relationship between host antigenic 

 structure and phage susceptibility are found in the dysentery 

 bacteria (Burnet and McKie, 1930), the typhoid bacterium 

 (Craigie and Yen, 1937; Sertic and Boulgakov, 1936a), Vibrio 

 cholerae (Asheshov, Asheshov, Khan, and Lahiri, 1933) and E. 

 coli (Sertic and Boulgakov, 1937). A good review and discussion 

 of the role of bacterial antigens in protecting bacteria against the 

 attack of certain phages is given by Nicolle, Jude, and Diverneau 

 (1953). In most cases the protective antigens seem to act by 

 preventing the phage from adsorbing to the receptor sites by 

 mechanical interference. This may be, however, an over- 

 simplified concept. The evidence for a higher order of chemical 

 specificity in the adsorption of phage to host cell will be pre- 

 sented in Chapter X. 



Bacterial cultures are not necessarily homogeneous in their 

 ability to adsorb a given phage. Wahl (1953) has called "semi- 

 resistant" those bacterial strains which appear to be heterogene- 

 ous in receptivity. Such strains contain bacteria that are able 

 to adsorb the phage and bacteria that are not. The cell types 

 do not breed true and must therefore be considered either as 

 phenotypic modifications or as genetic changes recurring at high 

 rates, similar perhaps to the diphasic variation of salmonella 

 strains. 



4. Host Specificity in Infection 



Some bacteria are able to adsorb phages very well but fail to 

 serve as host cells for phage multiplication. Lysogenic bacteria, 

 for example, usually adsorb the phage they carry in the latent 

 (prophage) state, but are not affected by it. Lysogenic bacteria 

 are therefore said to be immune to superinfection. Lysogenic 

 bacteria are susceptible to lysis by other phages however. 



Some phage-resistant bacterial mutants retain the ability to 

 adsorb the phage to which they are resistant. For instance, a 

 mutant of £". coli called B/1 adsorbs coliphage Tl in a reversible 



