SEGREGATIONS IN ESCHERICHIA COLI 507 



Mutations for resistance to specific bacteriophages or bacterial viruses 

 have proven to be exceedingly useful. They are readily obtained as spontaneous 

 mutants by plating a large number of sensitive bacteria with the particular 

 virus in question; only resistant mutants escape lysis and may be recovered as 

 "secondary" colonies (fig. 1). Resistant mutants are readily freed from residual 

 virus by serial single colony isolation. Resistance to a given virus may be 



Table 2 

 A summary of the mutants used. 



* Lac 2 in mutant Y-87 differs from Lac in mutant Y-53 and its derivatives in the greater 

 reverse-mutability of the latter. Lac" and Lac 2 ~ are otherwise similar, and allelic. 



scored by streaking a loopful of bacteria on an EMB or nutrient agar plate at 

 right angles to a previous streak of the virus suspension (Demerec and Fano, 

 1945, see fig. 1 of the present report.) 



It was found, however, that mutations for resistance to a given virus are 

 not entirely specific, but that resistant mutants display "cross-resistance," 

 i.e., are also resistant to other viruses. For example, most Tz-resistant types 

 are also resistant to T5. (For the nomenclature of the bacterial viruses used in 

 this investigation, and a detailed account of the cross-resistance patterns of 

 another strain, E. coli B, see Demerec and Fano, 1945). The cross-resistance 

 patterns of K-12 are similar to those of E. coli B with the exception that Ti- 

 resistant mutants which are sensitive to T5 are not tryptophaneless, as has 



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