LYSOGENY 375 



block in the multiplication of homologous phages as a conse- 

 quence of the presence of the prophage. Its mechanism is yet 

 unknown. 



The genetic material of a temperate phage which has pene- 

 trated inside an immune bacterium has a low probability of 

 becoming a prophage (Bertani, 1953a). Whereas lysogenic 

 bacteria carrying several prophages may easily be produced by 

 successive infections with unrelated temperate phages, there 

 exists an incompatibility between related prophages. As a 

 rule, bacteria successively infected with two temperate mutants 

 of the same phage carry only one type of prophage. Excep- 

 tionally, however, the infecting phage may substitute for, or be 

 added to, the original prophage. In the latter case doubly 

 lysogenic bacteria carrying two related prophages are formed, 

 and genetic recombination can be observed between the pro- 

 phages (Bertani, 1953b; Appleyard, 1954b). 



Incompatibility between mutant prophages suggests that, in 

 a lysogenic bacterium, the number of receptor sites specific for 

 a given type of prophage is limited. 



7. Defective Lysogenic Bacteria 



Phage production and immunity constitute the two criteria 

 by which the presence of a prophage may be detected. In 

 some lysogenic strains, a mutation of the prophage can prevent 

 the formation of infectious particles. Strains carrying such mu- 

 tant prophages are called defective lysogenic strains. 



Defective lysogenic bacteria have been isolated either after 

 infection of sensitive bacteria with temperate phages (Jacob, 

 1950; Lwoff and Siminovitch, 1951) or among the survivors of 

 normal lysogenic bacteria exposed to ultraviolet light (Leder- 

 bergand Lederberg, 1953; Appleyard, 1954b; Jacob and Woll- 

 man, 1956). In the cultures of these defective bacteria no, 

 or very few, phage particles can be detected. If inducible 

 defective bacteria are exposed to inducing agents, mass lysis 

 of the culture occurs, but phage particles are produced by only 

 a small fraction of the bacteria. The presence of a prophage in 



