DETECTION OF PATHOGENIC BACTERIA IN WATER 281 



It is necessary, therefore, that we should enter a 

 little more into detail regarding these two tests, which 

 acquire such a high importance in this connection. 



The indol-reaction, as already described (see p. 273) 

 in connection with the diagnosis of the typhoid bacilli 

 and the B. coli communis, depends upon the interaction 

 of nitrous acid with indol, resulting in the production 

 of a red colour. The formation of indol in a bacterial 

 culture can thus be readily ascertained by generating 

 nitrous acid in the latter by the addition of a small 

 quantity of a nitrite and some sulphuric acid. In the 

 case of the ordinary cultures of the cholera bacilli, 

 however, the nitrite is already present, having been 

 formed by the reduction of nitrate which is almost 

 invariably contained in the peptone used, so that the 

 addition of sulphuric acid is alone necessary for the 

 exhibition of the indol-reaction. The production of 

 this red colour on the simple addition of sulphuric acid 

 to broth cultures of cholera bacilli was originally dis- 

 covered by Bujwid 1 and Dunham - before its cause was 

 understood, and was named by them the cholera-red- 

 reaction. In order that this reaction may be obtained 

 with certainty, Koch points out that several precautions 

 must be strictly observed. It is of the greatest import- 

 ance that suitable peptone should be employed ; this 

 probably depends upon the fact that some samples of this 

 material contain an insufficient or an excessive amount 

 of nitrate respectively. 3 Secondly, the sulphuric acid 

 employed must be absolutely free from nitrous acid! 

 Thirdly, the test must only be applied to pure cultures 

 of the spirilla, as otherwise the production of the 



1 ' Eine cliemische Reaction fiir die Cholerabacterien,' Bujwid, 

 Zeitscli. f. Hygiene, vol-. ii., 1887, p. 52. 



2 ' Zur chemisclien Reaction der Cholerabacterien,' Dunham, ibid., 

 p. 337. 



3 Bleisch, Zeitscli. f. Hygiene, vol. xiv. 



