5 o8 JOSHUA LEDERBERG 



been reported by Anderson (1946) for the corresponding mutants of E. coli B. 

 In this paper, the designation V x r will be used for the more frequent TVresist- 

 ant mutant, which is also resistant to T5. The symbol \\ a r is reserved for the 

 r5-sensitive, Tz-resistant mutant, but the evidence that distinct loci are in- 

 volved will be presented in extenso in another place. 



In addition to Vf and V u r , just mentioned, a third type of "secondary 

 colony" has been found among populations treated with the virus Ti. This 

 type, Vib r is characterized by an exceedingly slimy or mucoid colony confor- 



Figuee i. — The phenotypes of the four combinations of Lac and V are illustrated. In order 

 the}' are: Lac + Vi r ; Lac + Vi s ; Lac'Vf; Lac~V\ s . An EMB-lactose agar plate was first streaked 

 vertically with the virus Ti. Subsequently, each of the bacterial types was streaked, from left to 

 right, perpendicularly across the virus streak. After 16 hours incubation, both the Lac and Vj 

 phenotypes are well developed. Developing in the zone where Lac'V V has been lysed can be seen 

 two colonies of resistant mutants: Lac~V{. 



mation. Recombination studies on this mutant are complicated by its genetic 

 instability; V\h T rapidly reverts to the wild type, and in addition may also be 

 strongly selected against in competition with V^*. However, the locus of 

 Fib can be distinguished from the locus of the other V\ mutants by the demon- 

 stration of a different recombination frequency with Lac. These data are sum- 

 marized in order to emphasize the importance of genetic tests to insure the 

 allelic identity of phenotypically similar mutants. 



It is particularly fortunate that resistance tests can be conducted on EMB 

 agar, since this allows the characterization of a strain with respect to virus- 

 resistance and to lactose fermentation with a single streaking (see fig. 1). 



Mutants resistant to sodium chloroacetate (Cla r ) were obtained by streaking 

 a large number (about io 7 ) of bacteria on nutrient agar to which filter-sterilized 

 chloroacetate has been added to make a final concentration of 2 mg/ml At 

 this concentration, the wild type is substantially inhibited, while resistant 

 mutants grow luxuriously. This mutation is accompanied by deficiencies in 



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