420 BACTERIOPHAGES 



pattern of a lysogenic bacterial strain depends on two factors 

 mentioned above: the character of the nonlysogenic ancestor, 

 and that of the carried phage. If the nonlysogenic precursor 

 of all lysogenic cultures of a given bacterial species always showed 

 the same pattern of phage resistance, identification of the tem- 

 perate phages would give as much information as would the 

 examination of the sensitivity of the strains to typing phages. 

 Work on the typhoid Vi-types has shown, however, that 

 tc|i cwV different nonlysogenic precursors may carry the same determi- 

 ning phage, and the resulting types react quite diflferently with 

 the Vi-typing phages. Identification of the temperate phage 

 only would erroneously indicate that these types were identical 

 with each other. Moreover, it is not always possible to isolate 

 temperate phages from cultures (this is commonly found in 

 S. typhi) and in such instances the method of typing by charac- 

 terization of temperate phages fails altogether, whereas the 

 examination of sensitivity patterns of a strain to a range of typing 

 phages is still effective. It is evident that a combination of the 

 two methods offers the most precise means of strain characteriza- 

 tion, but for routine use the determination of the spectrum of 

 phage sensitivity of bacterial strains is the more practical and 

 reliable approach. 



