78 Elements of Water Bacteriology. 



time the use of caffein has apparently been followed by 

 the best results, and it seems likely that of the precipita- 

 tion methods that employing the oxychloride of iron is the 

 best. Lubenau (Lubenau, 1907) has made some inter- 

 esting comparisons, using media containing malachite 

 green and caffein and caffein alone, in which the advan- 

 tage is decidedly in favor of the latter. 



After this part of the process is completed the identifi- 

 cation of the pure cultures isolated is subject to consider- 

 able 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 suppose'd to be aUied. 



COMPARISON OF THE CHARACTERS OF B. COLI AND 

 B. TYPHI. 



(HORROCKS, 1901.) 

 B. coli. B. typhi. 



(i) Surface Colonies, Gelatin (i) 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. 



(2) Gelatin-stab. — Quick growth (2) Slow growth on the surface 

 on the surface and along the line like the colonies; along the line of 

 of inoculation. inoculation the growth is much 



thinner, and often ends below in a 

 few white points consisting of dis- 

 crete colonies. 



