122 ELEMENTS OF WATER BACTERIOLOGY 



of which those containing " neutral red," one of the 

 safranine dyes, have been somewhat fully studied. 

 Rothberger (Rothberger, 1898) first pointed out that B. 

 coli reduces solutions of this substance, the color chang- 

 ing to canary-yellow accompanied by green fluorescence. 

 Makgill (Makgill, 1901), Savage (Savage, 1901), and 

 other English observers, as well as Braun (1906), in 

 France, report favorable results from the use of this 

 test; but according to American standards, Irons 

 (Irons, 1902) and Gage and Phelps (Gage and Phelps, 

 1903) conclude that the group of organisms giving a 

 positive neutral red reaction is too large a one to give 

 very valuable sanitary information. 



Stokes (1904) urged the use of lactose broth with 

 the addition of neutral red, and believed that the pro- 

 duction in this medium of 30-50 per cent of gas with a 



2 



— gas formula and the change of neutral red to canary 



yellow in the closed arm of the fermentation tube was 

 characteristic for B. coli. 



The Lactose Bile Test for the Colon Group. On the 

 whole, by far the most satisfactory results in making a 

 rapid test for the colon group have been obtained by 

 the use of media containing bile salts, a procedure 

 the development of which in this country we owe 

 principally to Jackson (1906). 



MacConkey (1900) long ago suggested the use of 

 media containing bile salts (sodium taurocholate) 

 for the differentiation of B. coli and B. typhi, and bile- 

 salts media have been used by various English observers 

 (MacConkey, 1901; MacConkey and Hill, 1901) for 



