324 F. JACOB AND E. L. WOLLMAN 



strains, three antigenically different types of phage, A, B, and D, were iso- 

 lated. Phage A was also frequently isolated from lysogenic strains of S. 'para- 

 typhi A, S. paratyphi B and S. paratyphimurium, whereas phages B and D, 

 as well as another, phage N, were found to be more rarely produced by 

 strains belonging to these species. 



Such findings have been made in a large number of bacteria species 

 (sporulated soil bacteria, Enterobacteriaceal, Pseudomonas, Vibrio, Coryne- 

 bacterium, Staphylococcus, etc.). Certain strains of lysogenic bacteria are able 

 to release two or more different types of phages. Among the strains of S. 

 enteritidis studied by Burnet, two yielded both phages A and B, and one 

 phage A and D. Some lysogenic strains of staphylococci may produce as 

 many as five different types of phages (WiUiams-Smith, 1948; Rountree, 

 1949). Lysogeny, therefore, appears to be widely distributed among bacterial 

 species. Moreover, it must be pointed out that the non lysogenic character 

 of any given strain can never be asserted. It is impossible to know whether 

 such a strain, which did not give plaques on the indicator strains employed, 

 would not have proved to be lysogenic if a suitable indicator strain had been 

 available. 



IV. Phage Production by Lysogenic Bacteria 



A. Spontaneous Production 



Cultures of lysogenic bacteria in liquid medium generally contain particles 

 of free phage. In a population of lysogenic cells, the ratio "free phage : 

 bacteria" remains approximately constant durmg growth of the culture 

 (Fig. 2). As shown by Lwoff and Gutmann, with single cells, phages are 

 produced by the lysis of a small fraction of the population. The rarity and 

 the discontinuity of phage production in a lysogenic population may also be 

 demonstrated by a modified single-burst experiment, which allows an analysis 

 of the production of phage by a large number of smaU populations during 

 a short time of growth (Bertani, 1951; Jacob, 1952c). The bacteria which 

 produce phage during the period of observation appear to be randomly 

 distributed among the populations sampled, which indicates that the factors 

 which determine phage production appear randomly in rare individuals. 



The constant ratio "free phage : bacteria" in a growing population ex- 

 presses the fact that, per generation time, a constant fraction of the popula- 

 tion lyses and releases a burst of phage. The ratio is a function of the burst 

 size and of the frequency of lysis, which can be expressed as the probability 

 per bacterium, per generation time, to produce phage. This rate of spon- 

 taneous production can be accurately measured by single-burst experiments. 

 For a given lysogenic strain grown in given conditions, the rate of spontaneous 



