ISOLATION OF SPECIFIC PATHOGEN ES. 51 



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 Wathe- 

 let (Wathelet, 1895) found that of 600 colonies isolated 

 from typhoid stools and having the appearance charac- 

 teristic of B. coli and B. typhi only 10 belonged to the 

 latter species. 



At the other end of the process the identification of the 

 pure cultures isolated is again subject to considerable 

 uncertainty. The typhoid bacillus belongs to a large 

 group which contains numerous varieties differing from 

 each other by minute degrees. The inability to repro- 

 duce the disease by inoculation in available test animals 

 owing to their natural immunity is a serious drawback; 

 and the specific biochemical characters of the organism 

 are, as it happens, mostly negative ones, as shown by com- 

 parison with B. coli, to which it is supposed to be allied. 



COMPARISON OF THE CHARACTERS OF B. COLI AND B. TYPHI. 



(Horrocks, 1901.) 



C. coli. B. typhi, 



(1) Surface Colonies, Gelatin (1) Much thinner than those 

 Plates. — Thicker, and grow more of B. coli, and grow more slowly, 

 rapidly than those of B. typhi. After forty-eight hours' incuba- 

 After forty-eight hours' incubation tion at 22° C. they are hardly 

 at 22 C. they are usually large ^visible to the naked eye. 

 and characteristic. 



