HOST SPECIFICITY 127 



phage T3 is designated B/3. If this mutant is found to be resist- 

 ant also to phages T4 and T7 it is called B/3, 4, 7. If a subsequent 

 mutation confers resistance also to T6, it is named B/3, 4,7/6, 

 each mutational step being distingished by a bar. 



Mutations to phage resistance have been extremely important 

 in the development of bacterial genetics, because of the ease 

 with which they can be selected, and because of their great 

 variety. 



Certain mutations to phage resistance are accompanied by the 

 loss of the ability to synthesize growth factors such as tryptophan 

 (E. H. Anderson, 1946) or proline (E.-L. Wollman, 1947). 

 Selecting for phage resistance in these cases is thus an easy way 

 to obtain biochemical requirements in bacterial strains. Since 

 rates of mutation to phage resistance are easily measured, this 

 type of mutation has been extensively used to test for the ability 

 of various physical and chemical agents to induce mutations (see, 

 for instance, Novick and Szilard, 1951b). Because of the variety 

 of properties which may be associated with resistance to a given 

 phage (resistance to certain other phages, biochemical require- 

 ments, colony type, etc.), often several mutant types can be 

 easily distinguished among all the resistant mutants that are 

 selected with a given phage. One obtains thus a "mutation 

 pattern" which can be used as a test of the specificity of various 

 agents capable of inducing mutations (Bryson and Davidson, 

 1951). More information on this general subject will be found 

 in the book by Braun (1953). 



Bacterial mutations involving a change from resistance to 

 sensitivity are more difficult to study because of lack of selective 

 agents for isolation of the mutant strains. An exception is found 

 in the phages specific for rough and smooth strains of salmonella 

 studied by Burnet (1929b), which can be used for selection in 

 both directions. In other cases phage-sensitive variants can be 

 selected because of morphological differences from the phage- 

 resistant parent stocks; for example, in E. coli (Nelson, 1927), 

 Sh. dysenteriae (Arkwright, 1924), and Sh. sonnei (Miller and 

 Goebel, 1949). 



