320 F. JACOB AND E. L. WOLLMAN 



is brought about by bacteriophages that rapidly and completely lyse cultures 

 of susceptible bacteria: infection with such phages leads irrevocably to death 

 and dissolution of the host. 



It was soon recognized, however, that another type of relationship could 

 exist between bacteria and bacteriophages: this type is represented by 

 lysogenic bacteria, which apparently can be grown indefinitely while releasing 

 phage into their culture medium. 



These two situations therefore appear to be completely opposed: in the 

 first case, bacteriophage is the agent of an acute and always fatal disease of 

 the host, while, in the second case, it behaves as a perfectly weU tolerated 

 parasite. For many years, the nature and mode of action of bacteriophage 

 were subjects of controversy; conflicting theories were proposed, depending 

 on which facts, whether belonging to one or the other situation, had to be 

 accomited for. It is only in the last decade, after weU-defined cases have 

 been thoroughly analyzed, that a complete and harmonious picture has been 

 reached. 



Those bacteriophages which always kUl the infected bacteria are now called 

 virulent phages, and their properties and reproduction have been the subject 

 of previous chapters in this volume. As to the phages which are produced by 

 lysogenic bacteria, infection of sensitive indicator bacteria with such phages 

 results either in multiplication of the phage and lysis of the infected cell, or 

 it may lead to the formation of a stable association between the host and the 

 virus. The infected bacterium survives and becomes hereditarily endowed 

 with the property of producing new phage particles identical with those that 

 have been used for the initial infection. A new lysogenic system has thus 

 been estabHshed. Phages which are able to perform this process of lysogeniza- 

 tion are called temperate phages. 



Lysogeny may thus be defined as the hereditary property of certain bac- 

 teria which enables them to produce bacteriophage in the absence of external 

 phage particles. A lysogenic bacterium possesses and transmits to its progeny 

 the capacity to produce phage. Lysogeny is a stable character and, in a 

 lysogenic culture, each bacterium will give rise to a lysogenic clone (Fig. 1), 



"Artificial" lysogenic strains, that is, the lysogenic derivatives obtained 

 by lysogenization of sensitive bacteria by temperate phages, are, in aU their 

 properties, similar to the lysogenic strains isolated from natural sources. A 

 lysogenic strain is thus represented by the symbol of the bacterial strain, 

 followed by the symbol of the phage in parenthesis. For example, a strain of 

 Escherichia coli, called K12, which is lysogenic for a temperate phage called 

 A, is represented as K12(A). The fact that infection of a sensitive bacterium 

 with a temperate phage may lead to the formation of a stable lysogenic 

 system shows that the genetic information introduced by the infecting 

 particle is thereafter indefinitely perpetuated in every descendant of the 



