LYSOGENY 335 



be rather frequent. They can be recognized only by comparison of the pro- 

 perties of lysogenic and nonlysogenic derivatives of the same strain. More- 

 over the presence of a prophage is likely to be overlooked in cases such as the 

 defective lysogenic strains in which a mutation of the prophage prevents the 

 formation of infectious particles (see Section VI, D). The genetic determinant 

 of any physiological character controlled by a defective prophage would 

 undoubtedly be thought to be, not a prophage, but a bacterial gene. 



VI. Characters of Temperate Phages 



A. General Properties 



The general properties of temperate phages may be studied, using standard 

 phage techniques, by infection of a suitable sensitive indicator. Although, 

 upon infection, a certain fraction of the infected bacteria becomes lysogenic, 

 this proportion is generally too low to alter significantly the quantitative 

 analysis of the results. When plated on a sensitive indicator, temperate 

 phages produce, in general, turbid plaques, as a result of the growth of lyso- 

 genic clones during the formation of the plaque. The degree of turbidity is, 

 to a certain extent, a measure of the lysogenizing power of the phage imder 

 study. For any temperate phage, such properties as adsorption, stability, 

 latent period, and burst size, may thus be determined. They are found to be 

 as characteristic of any given phage, and as variable from phage to phage as 

 among virulent phages. In general, however, temperate phages are found to 

 be more difficult to work with than virulent phages like those of the T series: 

 lysis in liquid medium is rarely complete, high titer stocks more difficult to 

 obtain, the latent period is longer. 



Temperate phages are frequently excluded by virulent phages. For example, 

 when lysogenic E. coli K12(A) in which the development of the temperate 

 phage A has been induced by UV light, are infected with T5 particles, A is 

 excluded by T5. This is true even if infection with T5 occurs only a few 

 miautes before intracellular appearance of A mature particles (Weigle and 

 Delbriick, 1951). 



Like other phages, temperate phages are composed of DNA and protein. 

 As they are of a smaller size than the T-even phages, they contain less DNA 

 (Smith and Siminovitch, in Lwoff, 1953) and are therefore more resistant to 

 radiations. One important property of temperate phages is their inability, 

 when inactivated by UV hght, to kiU sensitive bacteria on which they 

 adsorb. Their protein coat appears to be devoid of lethal properties, a condi- 

 tion which is indeed necessary for a phage to be temperate: that is, to be able 

 to establish, upon infection, a lysogenic system. 



