396 BACTERIOPHAGES 



cient typing scheme, the organisms isolated from patients, from 

 vehicles of infection such as food or water and from the carrier 

 responsible for the outbreak of the disease under study, all react 

 identically. If an organism is widely disseminated, and es- 

 pecially if it is commonly carried by otherwise healthy persons, 

 epidemiological study is impossible without such a method. 



In order to be practically useful a typing method must satisfy 

 the following requirements: the types and the typing reagents 

 used for their recognition should be stable ; the organism should 

 be subdivisible into an adequate number of types ; the technique 

 should be simple and the results reproducible and easy to read. 

 The method should be capable of standardization in order that, 

 wherever it is employed, the results are comparable; the results 

 of the test should be available quickly; and, before being ac- 

 cepted for general use, the reliability of the method should be 

 established by exhaustive epidemiological trials. 



An early attempt to distinguish between different epidemic 

 strains of an organism was that of Kristensen and Henriksen 

 (1926) in relation to the typhoid bacillus. This method was 

 enlarged upon by Kristensen (1938). It depends on the dif- 

 ferential fermentation of L-arabinose and xylose and divides 

 Salmonella typhi into three biochemical types. Type I ferments 

 xylose but not arabinose; type II ferments neither xylose nor 

 arabinose; and type III ferments both sugars. As this scheme 

 distinguishes only three types, and as most strains belong to 

 type I, the method is not of great epidemiological value. 



Many attempts have been made to classify single bacterial 

 species by serological methods and have met with some success, 

 particularly in the group A haemolytic streptococci. In other 

 instances the number of types distinguishable is too small for 

 epidemiological purposes, or the distinction of types by antisera 

 too difficult. 



In 1938 Craigie and Yen introduced their Vi-phage typing 

 scheme for Salmonella typhi. This rapidly established itself as the 

 method of choice for the epidemiological recognition of typhoid 

 strains and served as a model for the development of all later 

 schemes for the typing of bacteria by phage. 



