APPENDIX 491 



Presumably in this instance resistance is the result of failure to 

 adsorb the phage particles. However, if this variant is used to 

 assay the other phages of the T group and the results are com- 

 pared with parallel assays on strain B of E. coli, the efficiency of 

 plating of the other phages is as high on the variant as on B. 

 This variant is then resistant to T6 but susceptible to all the other 

 phages of the group. This property is transmitted unchanged 

 to the descendants of the cell ev^en though the bacteria are culti- 

 vated in the absence of virus T6. The variant is a mutant of 

 strain B and is called B/6. Had the mutant been found to be 

 simultaneously resistant to T6 and Tl it would have been called 

 B/6,1. Had the latter mutant been isolated by use of Tl instead 

 of T6 it would have been called B/1,6. 



If we have a mixture of equal numbers of phage T2 and phage 

 T6 and plate an appropriate dilution of the mixture on strain B, 

 the plaque count will be the sum of the T2 and T6 plaques; 

 moreover the T2 plaques cannot be distinguished morphologi- 

 cally from the T6 plaques. However, if the mixture is plated on 

 B/6, only the T2 phage will form plaques and be counted. 

 Similarly, plating of the mixture on B/2 will enable one to assay 

 the T6 phages in the mixture. Such phage-resistant mutants are 

 called indicator strains since they enable one to assay any phage to 

 which the strain is susceptible in the presence of any phage to 

 which the strain is resistant. Some of the uses to which indi- 

 cator strains may be put are discussed later. 



In the example cited, all 12 colonies on test are found to be B/6 

 and all identical. In this particular culture of B, the frequency 

 of B/6 mutants in the population was found to be 12/10" cells. 

 A series of samples taken from the same adsorption tube and 

 plated in the same way would have differed from this only by 

 the expected sampling error. 



If a sample of less than 1 0*^ bacteria had been taken from this 

 culture, probably no mutants would have been found, so it is ob- 

 vious that the sample size is important. If a mutant occurs in- 

 frequently, a much larger sample must be taken. The bacterial 

 culture can be concentrated in the centrifuge and the sample can 



