Isolation of Specific Pathogenes. ' 69 



t 

 to 30 on Fuller's scale. In all cases, however, tke chance 

 of success is small, as is well shown by the experiments 

 of Laws and Andrewes (Laws and.Andrewes, 1894), who 

 entirely failed to isolate the typhoid bacillus from the 

 sewage of London and found only two colonies of the 

 organism on a long series of plates made from the sewage 

 of a hospital containing forty typhoid patients. So 

 Wathelet (Wathelet, 1895) found that of 600 colonies 

 isolated from typhoid stools and having the appearance 

 characteristic of B. coH and B. typhi, only 10 belonged 

 to the latter species. 



In the last five years considerable progress lias been 

 made in the development of new methods for isolating 

 the typhoid bacillus. These fall in four distinct groups: 

 first, the direct isolation by differential, frequently colored, 

 media; second, enrichment methods; third, methods 

 based on concentration of the organisms by agglutination 

 with typhoid serum; and fourth, methods based on concen- 

 tration by chemical precipitation. 



In all excepting the first of these groups differential 

 media are usually employed as a second step in the iso- 

 lation. Combinations of methods have been employed 

 in many instances, and have often been successful in the 

 isolation of the typhoid bacillus from artificially infected 

 emulsions of feces and waters. 



Direct Isolation. — Drigalski and Conradi (Drigalski 

 and Conradi, 1902) prepared a medium primarily for the 

 isolation of typhoid bacilli from excreta, which may also 



