USE OF PHAGES IN EPIDEMIOLOGICAL STUDIES 419 



to be characteristic of particular types established on a biological 

 basis (that is, the miiis, intermedius, and gravis varieties of C. 

 diphtheriae) . Thibaut and Fridericq (1956) independently 

 produced a typing scheme for this organism in which 8 temperate 

 phages define 9 types. These workers claim that 5 of the 

 typing phages in their scheme are different adaptations of a 

 single phage. 



10. Bacterial Typing by Identification of Carried Phages 



The typing methods described above depend on the different 

 sensitivities of bacterial strains to a battery of selected phages. 

 It has been shown that these sensitivities are frequently an 

 expression of resistance patterns produced in the bacteria by the 

 presence of prophages, the identity of which varies from type to 

 type. It is thus evident that the identification of these carried 

 phages presents another avenue of approach for bacterial typing. 

 Such an approach has been used by Boyd (1950, 1952), Boyd, 

 Parker, and Mair (1951) and Boyd and Bidwell (1957) for typing 

 S. lyphimiirium, and by Scholtens (1950, 1955, 1956) for S. para- 

 typhi B, and there is good evidence to show that the method is 

 reliable. Its value as a routine procedure depends on the ease 

 with which phages can be identified. If this is a simple matter 

 entailing the determination of plaque morphology and host 

 range of the phages concerned, or of the specific changes in 

 phage resistance brought about in selected indicator strains by 

 lysogenization (Anderson, 1956; Anderson and Williams, 1956; 

 Boyd and Bidwell, 1957), the method presents no great technical 

 difficulties, although it is much slower than the more con- 

 ventional operation of applying a battery of typing phages to 

 the strains to be investigated. If, however, serological identi- 

 fication of the phages and determination of their physical proper- 

 ties are required, the method is too slow for routine use, because 

 the epidemiological value of phage typing, especially in Entero- 

 bacteriaceae, is often determined by the speed with which results 

 can be obtained. 



One drawback of this method is that the phage resistance 



