178 ELEMENTS OF WATER BACTERIOLOGY 



B. coli, but it has never gained wide acceptance in Eng- 

 land or Germany. The usual practice has been to 

 incubate for 4 days at 37° and to test for nitrites by 

 adding a drop of each of the following solutions in suc- 

 cession : 



A. Sulphanilic acid .5 gram 



Acetic acid (25% sol.) 150.0 c.c. 



B. Naphthylamine chloride o. i gram 



Distilled water 20 . o c.c. 



Acetic acid (25% sol.) 150.0 c.c. 



A red or violet coloration indicates the presence of nitrites. 



In making the nitrite test it is important to remember 

 the possibility that appreciable amounts of nitrite may 

 be present in the media — either derived from the air or 

 from the use of impure peptone (Wherry, 1905), In the 

 case of the nitrite reaction control tubes should always 

 be tested from the same batch of media and only a 

 distinct red color should be considered positive. The 

 nitrite test is particularly subject to variations of un- 

 explained origin. Of two duplicate tubes inoculated in 

 the same way, one may show a strong reaction and the 

 other none. 



The reduction of neutral red has been extensively used 

 in England and less in this country. It has been re- 

 ferred to in Chapter VI. as one of the tests suggested for 

 use as a presumptive indicator of the colon group as a 

 whole. MacConkey (1909) concludes that both the ni- 

 trate test and the neutral red test should be dropped 

 from the procedure used in identifying colon bacilli, since 

 so many organisms give these reactions that they have 

 little significance. Houston, however, in the important 

 investigations which will shortly be discussed, used the 



