LYSOGENY 323 



prophage may be visualized as a specific structure whose replication is co- 

 ordinated with, the division of the host bacterium, wliich it provides with 

 the information necessary for the synthesis of phage particles. 



As already stated, the lysogenic character of a strain is generally as stable 

 as any other genetic character. Spontaneous loss of this character has, how- 

 ever, been observed in exceptional cases, such as the classic strain of B. 

 megatherium (den Dooren de Jong, 1931). Accumulation of nonlysogenic 

 variants can be favored by the prolonged growth of this strain under condi- 

 tions which prevent the readsorption of free phages (Clarke, 1952). Isolation 

 of nonlysogenic derivatives has also been reported in other cases, as, for 

 instance, in that of several strains of Salmonella typhi (Anderson, 1951). The 

 frequency of such occurrence may be, to a certain degree, increased by 

 exposure of a lysogenic population to heavy doses of ultraviolet hght 

 (Lederberg, 1951). 



As it will be seen later (Section VII, B), the prophage appears to be the 

 genetic material of the phage located at a specific site of the bacterial chromo- 

 some. Loss of lysogeny would therefore correspond to the loss or inactivation 

 of this specific structure. Lysogenization, on the contrary, corresponds to the 

 acquisition of this new hereditary determinant. In a lysogenic bacterium, 

 host and virus genomes are integrated and rephcated as a single unit during 

 the course of bacterial multiphcation. The presence of the prophage ap- 

 parently does not affect growth or multiphcation of the host bacterium: the 

 prophage behaves as a normal cell constituent. Only when the prophage- 

 bacterium relationship is disturbed do lysogenic bacteria produce phage (see 

 reviews by Lwoff, 1953; Jacob, 1954). 



III. Detection and Occurrence of Lysogeny 



The lysogenic character of a bacterial strain can be recognized only when 

 there is available a sensitive indicator strain on wliich the produced phage 

 can multiply and form plaques. When investigating whether a given bacterial 

 strain is or is not lysogenic, cultures of this strain must therefore be assayed 

 by usual phage techniques, on a variety of other strains of the same or of 

 closely related species. When plaques are observed on one or several of 

 these strains, and are constantly formed by cultures grown from serial 

 isolations of single colonies, the strain under test may be considered as being 

 lysogenic. 



AVhen systematic investigations of this kind have been carried out, a large 

 proportion of the bacterial strains examined, whether recently isolated from 

 natural sources or taken from laboratory collection, were found to be lyso- 

 genic. In a careful study of 34 strains of Salmonella enteritidis, Burnet (1932, 

 1934) was able to recognize lysogeny in 27 strains. From these 27 lysogenic 



