USE OF PHAGES IN EPIDEMIOLOGICAL STUDIES 403 



absolute and, in the absence of other confirmatory tests, lysis of 

 a strain by a single phage is not accepted as offering final proof 

 of species or group identification. 



Virulent phages are the most convenient for phage typing, be- 

 cause they produce clear lysis which makes the results easy to 

 read. Naturally, typing phages which are isolated from lyso- 

 genic cultures are temperate and, if the rate at which they lyso- 

 genize sensitive strains is high, it may be difficult to detect the lysis 

 they produce because of heavy secondary over-growth by lyso- 

 genized cells. It is thus advisable to isolate virulent variants of 

 such phages when possible. 



b. The Routine Test Dilution 



An important principle now observed in phage typing is the 

 use of the "routine test dilution" of the typing phages. In the 

 typing scheme for Salmonella typhi, modification of the host range 

 of Vi-phage II does not affect its capacity for adsorption to the 

 host cell, and all the adapted typing preparations are adsorbed 

 by all the Vi-types of the typhoid bacillus irrespective of their 

 specificity (Craigie, 1940). Phage multiplication occurs only 

 when the type of organism and the specificity of the phage coin- 

 cide. However, adsorption of a Vi-phage II preparation to a 

 heterologous type of the typhoid bacillus is lethal to the cells 

 (Anderson and Fraser, 1956). Thus, when the phage is applied 

 in sufficiently high concentration to a lawn of cells, mass destruc- 

 tion of the bacteria results, and after incubation the culture con- 

 cerned appears to have undergone phage lysis. It can easily be 

 demonstrated that this is not so by titrating the phage in decimal 

 dilutions on the particular strain concerned. The apparent 

 lysis will then be found to disappear abruptly between two suc- 

 cessive dilutions, and no individual plaques can be found. In 

 addition to this direct lethal effect of phage, concentrated lysates 

 may contain bacteriocins which may produce a similar effect 

 to that described above ; this also disappears on dilution without 

 yielding discrete plaques. Finally, typing phages often contain 

 particles that can undergo host-induced modification by heterol- 



