MUTATIONS OF BACTERIAL VIRUSES AFFECTING 

 THEIR HOST RANGE 1 



S. E. LURIA 



Indiana University, Bloomington, Indiana 



Received July 8, 1944 

 INTRODUCTION 



WHEN susceptible bacteria are spread on a solid culture medium with a 

 large amount of a bacterial virus, complete lysis occurs after incubation, 

 except for the appearance in some cases of colonies consisting of virus-resistant 

 cells. These are the descendants of bacteria that had undergone a mutation 

 from virus-sensitivity to virus-resistance prior to the action of the virus (Luria 

 and Delbruck 1943). The virus-resistant bacteria do not adsorb the virus. 

 Their resistance is generally specific, not extending to unrelated viruses. Con- 

 versely, when a virus i<= plated with a suspension of bacteria resistant to its 

 action, it generally doo not affect the bacteria; a uniform layer of bacterial 

 growth results. We observed, however, that plating very large amounts of a 

 virus with a resistant bacterial mutant strain occasionally results in the forma- 

 tion of a few clear "plaques" — that is, of a few virus colonies. From these 

 plaques a new virus strain may be isolated that is active on the bacterial mu- 

 tant resistant to the normal virus. A study of the origin of the new virus proved 

 that it arises by mutation from the normal virus. A mutation of the virus can 

 thus compensate for a mutation of the bacterial host. The present paper is con- 

 cerned with the study of these virus mutations and of their relation to bacterial 

 mutations. 



Mutations affecting characters of bacterial viruses have been described be- 

 fore (Gratia 1936a; Burnet and Lush 1936). In 1929, Sertic clearly recog- 

 nized the occurrence of true breeding variants of bacterial viruses capable of 

 attacking bacterial strains resistant to the original virus. He appears to have 

 considered such variants as the result of an adaptation of the virus when in the 

 presence of resistant bacteria. 



EXPERIMENTAL 



Material and basic findings 



The material for the experiments described in this paper consisted originally 

 of a strain of Escherichia coli B and of two viruses, a and 7, active on strain B 

 (Delbruck and Luria 1942). Virus a gives large plaques, virus 7 small 

 plaques when plated with B on solid media. From strain B, a series of mutant 

 strains can be isolated, some sensitive to virus a and resistant to virus 7, others 

 sensitive to virus 7 and resistant to virus a. These strains are obtained as sec- 

 ondary growths after lysis of B by virus 7 or virus a. They are easily purified 

 by repeated streak platings, and their resistance to one of the two viruses is 



1 Aided by a grant from the Dazian Foundation ior Medical Research. 



[Reprinted by permission from Genetics 30:84-99, January, 1945] 



