LYSOGENY 



325 



production is a constant of the strain. It varies widely from strain to strain 

 (10~2 to 10~5), depending upon the type of prophage propagated. Thus, in 

 polylysogenic strains, that is, strains in which every bacterium is able to 

 produce two or more ty}3es of phages, each type of phage is generally produced 

 at its characteristic rate, irrespective of the presence of the other prophages. 

 When the spontaneous production of such strains is analyzed as described 



3 4 5 6 7 



Time in hours 



Fig. 2. Production of bacteriophages during the growth of lysogenic Pseudomonas 

 aeruginosa 13 (8). The number of bacteria and of free phages/ml. are plotted on a logar- 

 ithmic scale versus the time in hours. (From Jacob, 1954.) 



previously, it is observed that, when the phages produced are unrelated, the 

 spontaneous production of one type appears to be independent of the pro- 

 duction of the other type (Bertani, 1951; Jacob, 1952c). For strains doubly 

 lysogenic for certain types of related phages (Jacob, 1952c), however, both 

 types are often released by the same lysing bacterium. 



Thus, in a culture of lysogenic bacteria, the development of the prophage 

 into phage occurs "spontaneously" in a small but constant fraction of the 

 growing cells. The mechanism of this spontaneous production is still obscure. 



