MUTATIONS OF BACTERIAL VIRUSES 85 



generally found to be complete: the virus is not adsorbed by the bacterial cells 

 and does not grow in their presence. 



We have adopted in this paper the convention of naming the mutant bac- 

 terial strains by the Roman letter corresponding to the original strain, fol- 

 lowed by the Greek letter corresponding to the virus in the presence of which 

 they have developed as secondary growth after lysis. For example, strain 

 Ba indicates a mutant from B isolated as secondary growth after B was lysed 

 by virus a. This mutant will generally be resistant to virus a (unless otherwise 

 specified). Strains isolated in the presence of the same virus, but different in 

 some properties (colonial morphology, range of sensitivity, etc.), are distin- 

 guished by sub-indexes: Bai, Ba 2 , ■ ■ ■ Mutants obtained from other mutants 

 are named by adding the Greek letter corresponding to the virus in the presence 

 of which the new mutant has been isolated: Bay, for example, will be a mutant 

 from Ba isolated after lysis of Ba by virus 7, and will generally be resistant to 

 both viruses a and 7. This notation offers the advantage that the name of each 

 mutant strain mirrors its previous history, indicating the strain of origin and 

 the virus, or viruses, whose selective action has brought about its isolation. 



From strain B, one can easily isolate two mutants, Bai and Ba 2 , differing in 

 some growth characteristics but both resistant to virus a and sensitive to virus 

 7. Very exceptionally, one obtains from B a mutant B7, resistant to virus 7 

 and sensitive to virus a. 2 



Plating very large amounts of virus 7 with a suspension of cells B7 often pro- 

 duces some small, clear plaques, similar to the regular 7-plaques produced on 

 B. The ratio of the number of plaques thus produced to the number of plaques 

 produced by the same virus suspension when plated with strain B is generally 

 very small (of the order of io~ 7 , occasionally up to io -5 ). For a given suspen- 

 sion of virus, this ratio remains constant, at least for several months. The 

 plaques produced on B7 being clear, they cannot be attributed to non-homo- 

 geneity of the bacterial suspension, with presence of some 7-sensitive cells: the 

 action of a virus on a bacterial mixture partially sensitive results in the forma- 

 tion of plaques of turbid appearance, due to the growth of the resistant cells. 



If some of the contents of one of the few plaques produced by a suspension 

 of virus 7 on B7 are picked up and immediately plated again with a suspension 

 of B7, a large number of plaques are obtained. This confirms the expectation 

 that the plaque contained a large number of virus particles attacking B7. By 

 means of repeated one-plaque isolations and platings with B7, a pure virus 

 strain is obtained which can be grown in liquid cultures of B7. This pure virus 

 strain we called 7'; its properties will be described in the following section. 3 



It may be stated here that the presence of virus y' has been found in concen- 



2 The strains B7 and Bai have been called A and C in previous papers (Dele-ruck and Luria 

 1942), in which the interest was focused on their use as indicator strains for plating mixed virus 

 suspensions rather than on their mutational origin. 



3 Several of the experiments with virus 7' were done using a strain B7« 2 instead of strain B7, 

 because our stock of strain B7 (obtained from strain B in 1941) had recently, in the course of 

 subculturing, become partially sensitive to virus 7. Strain B7a 2 , derived from B7 before this oc- 

 curred, behaved toward viruses 7 and 7' exactly like the original strain B7. 



223 



