122 BACTERIOPHAGES 



2. Host Ranges of Bacteriophages 



D'Herelle (1926) noted that some phage strains were highly 

 specific attacking only certain strains in a single species of bac- 

 terium, while other phage strains possessed "multiple virulences" 

 enabling them to attack bacteria in different genera. He noted 

 one pure phage strain which attacked several species of shigella, 

 several strains of E. coli, and some strains of paratyphoid B. 

 Phage strains capable of attacking only a single bacterial species 

 were termed "monovalent," while phages capable of attacking 

 two or more bacterial species were called "polyvalent" (Kalman- 

 son and Bronfenbrenner, 1 942) . The difficulty with this termi- 

 nology is that the term polyvalent phage was also applied to 

 mixtures of phages prepared for therapeutic use, and it is often 

 difl[icult to tell in the early literature whether a "polyvalent 

 phage" was a "pure line phage" or a mixture of phages. 



It is evident that the host ranges of some phages cut across the 

 lines of bacterial classification. A Pasteurella peslis phage, for 

 instance, also lyses some salmonella and some shigella species 

 (Lazarus and Gunnison, 1947). Such extensive host ranges are 

 also observed with certain strains of plant and animal viruses. 



The host range is a useful characteristic for the identification 

 of phage strains. For instance the closely related coliphage 

 strains T2, T4, and T6 are similar in most respects but may be 

 readily distinguished by their host ranges on appropriate 

 mutants of their common host, strain B of E. coli (Demerec and 

 Fano, 1945). Mutant B/6 is resistant to T6 but susceptible to 

 T2 and T4, whereas mutant B/3,4 is resistant to T4 but suscepti- 

 ble to T2 and T6. Similarly B '2 is resistant only to T2. 



The host range is not an invariant characteristic of a phage 

 strain, however, because the host range can alter as a result of 

 phage mutation, or as a result of phenotypic modification. Also 

 the phage susceptibility of the indicator bacterial strains may be 

 changed by several methods. These modifications of host range 

 will be discussed later. 



