86 S. E. LURIA 



trated suspensions of virus 7 prepared directly from one 7-particle (one 7- 

 plaque transferred to a liquid culture of B, incubated until lysis, lysate fil- 

 tered). This proves that virus 7' does not represent an initial non-homogeneity 

 of the stock of virus 7, but arises from normal particles of virus 7. 



We have said that at least two different mutants, B«i and Ba 2 , can be iso- 

 lated after lysis of B by virus a. Plating any amount of virus a with bacteria 

 Bai does not result in any effect on the bacterial growth. We found, however, 

 that plating very concentrated suspensions of virus a with cells of strain Ba 2 , 

 characterized by slow and limited growth on nutrient agar, often results in the 

 formation of a few clear plaques. These plaques contain a true breeding virus 

 a', active on strains B and Ba 2 , but not on Ban. The properties of this virus a' 

 will be discussed in a later section. 



The culture media used in this study were: nutrient broth +0.5 per cent 

 NaCl for liquid cultures; the same plus 1.1 per cent powdered agar for platings. 

 All experiments were performed in water-baths or incubators at 37°C. The 

 technique for growth curves of bacteria, growth experiments of the viruses, and 

 experiments on interference between viruses have been described in detail in a 

 previous paper (Delbruck and Luria 1942). The meaning of certain terms 

 used hereafter, however, may be recalled: constant period = minimum time be- 

 tween infection of a bacterium by virus and its lysis with virus liberation; 

 burst size = average yield of virus particles per lysed bacterium; infective cen- 

 ter = anything that produces one plaque when plated with sensitive bacteria. 

 An infective center, therefore, can be either a free particle of virus or a bac- 

 terium infected by the virus; both of them will give just one plaque when 

 plated with sensitive bacteria; efficiency of plating = the ratio between the 

 number of plaques produced by a virus suspension in a given plating and the 

 maximum number of plaques which that suspension can give when plated un- 

 der optimum conditions. The latter has been shown to correspond very closely 

 to the actual number of active virus particles present (Delbruck and Luria 

 1942). 



Properties of virus y' 



Each one of the plaques obtained by plating a suspension of virus 7 with 

 B7 is capable of yielding a strain of virus 7', which can be purified by repeated 

 platings with B7 and one-plaque isolations, since normal virus 7 does not grow 

 on bacteria B7. After isolation and purification, virus 7' can be grown in 

 liquid cultures of either bacteria B or B7; filtrates of such cultures yield stable 

 stocks. Whether grown on strain B or on strain B7, the particles of virus y' 

 show exactly the same properties, as hereafter described. 



The plaques produced by virus y' on B are small, sharp edged, and indistin- 

 guishable from those produced by virus 7. Those produced on B7 can be dis- 

 tinguished with some experience mainly because of their slightly larger size. 



An interesting feature of virus y' is that of giving a smaller number of 

 plaques when plated with strain B7 than with B. The efficiency of plating on 

 B7 varies from 0.2 to 0.6 of that on B. It seemed important to investigate 

 whether or not this difference in the efficiency of plating was due to non- 



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