366 BACTERIOPHAGES 



On the contrary, in lysogenic bacteria, each cell can potentially 

 produce phage. Even after many successive reisolations (Bail, 

 1925; Bordet, 1925) or after prolonged growth in antiphage 

 serum (MacKinley, 1 925) every bacterium gives rise to cultures 

 containing bacteriophage. A strain can therefore be considered 

 lysogenic if, on repeated isolations, its cultures or their filtrates 

 regularly form plaques when plated on indicator bacteria. 

 Obviously, the lysogenic character of a strain can be ascertained 

 only if suitable indicator bacteria are available. 



Lysogeny appears to be widely spread in nature. When sys- 

 tematic investigations have been carried out, it has been de- 

 tected in many strains of various species and genera {Staphylococcus, 

 Vibrio, Pseudomonas, Salmonella, Bacillus, Corynebacterium, etc.). 

 In certain species of Salmonella (Burnet, 1 932) and Staphylococcus 

 (Williams Smith, 1948), almost every strain is lysogenic. A 

 single bacterial strain may release up to five diff'erent types of 

 phage (Williams Smith, 1948; Rountree, 1949a). 



2. Prophage 



Phages released by lysogenic bacteria are able to establish 

 new lysogenic systems when they infect sensitive cells of the in- 

 dicator bacterial strain. Such phages are called temperate and 

 the process by which they re-establish lysogeny is called ly- 

 sogenization. Lysogenic clones produced in this way in the lab- 

 oratory differ in no significant respect from the ones isolated 

 from nature. The phage released by lysogenized bacteria 

 remains identical to the phage used for the initial infection. 

 Lysogeny is therefore specific and, after lysogenization, every 

 cell of the progeny carries in some form the genetic information 

 necessary for the biosynthesis of a given type of phage particle. 



Phage is not maintained as such within lysogenic bacteria, 

 however. No infective particles can be detected after disrup- 

 tion of the cells (Burnet and McKie, 1929; Wollman and 

 Wollman, 1936b; Gratia, 1936a). One is led therefore to con- 

 clude that lysogenic bacteria do not contain phage particles, 

 but carry the information necessary for the production of phage 



