DETECTION OF PATHOGENIC BACTERIA IN WATER 269 



B. coli communis alone master of the field. It is true 

 that, growing in artificial cultures side by side, there 

 are certain difierences which individualise these or- 

 j^anisms, for the B. coli communis f^rows more luxuri- 

 antly in the various culture media employed than does 

 the typhoid bacillus, but yet on gelatine-plates there 

 are always colonies to be found which in every respect 

 resemble the latter, whilst even on potatoes, which used 

 to be considered an important test, the B. coli communis 

 may and does exhibit under certain conditions growths 

 which are undistinguishable from those produced by the 

 typhoid bacillus. In two media however, according to 

 Dunbar,^ a marked difference is found in the behaviour 

 of these two organisms. Whilst in sterile milk the 

 typhoid bacillus renders the liquid slightly acid and 

 never brings about its coagulation, the B. coli communis 

 at the temperature of the body coagulates the milk in 

 from twenty-four to forty-eight hours and at the same 

 time renders it strongly acid. Again, when grown in 

 sterile fluid meat-extract the B. coli communis at the 

 above temperature produces in the course of from three 

 to twelve hours a quantity of gas (consisting of hydrogen 

 and carbonic anhydride), whilst no formation of gas 

 has ever been observed in the case of the typhoid 

 bacillus. 



This distinction has been shown by one of us to be 

 available in an extremely simple form for the differen- 

 tiation of the two organisms ; thus, on inoculating them 

 respectively into test-tubes of melted gelatine-peptone 

 (the ordinary mixture, see p. 9), then allowing the 

 latter to solidify, and maintaining them at the ordinary 



^ ' Ueber den Typhusbacillus und den Bacillus Coli communis,' 

 Zeitschrift filr Hygiene, 1892, p. 491. See also State Board of Health 

 Massachusetts Report, 1891, p. 637, ' The Differentiation of the BaciUus 

 of Typhoid Fever j' by George N. Fuller. 



