USE OF PHAGES IN EPIDEMIOLOGICAL STUDIES 397 



1. Historical 



The practicability of phage typing depends on one of the most 

 important properties of phages, their host specificity. This 

 property was recognized at an early date in phage history. 

 Sonnenschein (1925, 1928) isolated specific phages for Salmonella 

 paratyphi B and S. typhi, and suggested their use for the rapid 

 identification of these organisms. This method was employed 

 by, among others, Schmidt (1931a, b), who found that Sonnen- 

 schein's original paratyphoid B phage could be "adapted" to 

 S. typhimurium, S. cholerae suis, and S. enteritidis. The resulting 

 phages were on the whole specific for the serotypes on which thev 

 had been grown, but the phage adapted to S. enteritidis was able 

 to lyse S. cholerae suis. Marcuse (1931) used specific phages to 

 identify strains oi Shigella jlexneri, and the same author (1934a, b) 

 used Sonnenschein's phages for the identification of S. paratyphi 

 B and .5". typhi. Marcuse confirmed the specificity of these 

 phages. He found that 30 per cent of 469 typhoid strains ex- 

 amined were resistant to the original typhoid phage, but by adap- 

 tation of the phage to resistant strains he was able to obtain 

 preparations lysing the remainder. Successive phage adapta- 

 tion to resistant cultures resulted in the production of five phages, 

 dividing the strains of S. typhi into as many groups. This, then, 

 was an early phage typing scheme for the typhoid bacillus. 



It will be noted that the word "adaptation" is used above in 

 relation to the propagation of phages on resistant strains. It 

 should be remembered, however, that many if not all salmonellas 

 are lysogenic, and that, in attempts at phage adaptation, there is 

 always the risk of contamination of the original phage with phages 

 carried by strains on which propagation is being carried out. 

 Such contaminating temperate phages may be able to lyse 

 strains resistant to the original phage, and high-titer prepara- 

 tions having new host ranges may then result which would er- 

 roneously be regarded as adaptations of the starting phage. 

 This phenomenon certainly proved to be a source of confusion in 

 later phage-typing schemes, and there are strong reasons for 

 suspecting that it played a part in the apparent phage adapta- 



