DETECTION OF PATHOGENIC BACTERIA IN WATER 273 



drops of the water under investigation are added, and 

 after being shaken and the contents thoroughly mixed 

 the tubes are again placed in the incubator. If after 

 twenty-four hours' incubation any of the tubes appear 

 turbid, they should be submitted to ordinary plate- 

 cultivation, and the resulting colonies carefully examined 

 for the characters which they exhibit with a view to 

 their identification. In using this method for the de- 

 tection of typhoid bacilli in water, it has, however, 

 been found by one of us that the above incubation must 

 be prolonged for forty-eight or even seventy-two hours 

 when only few typhoid bacilli are present. 



A further distinction which serves for the differentia- 

 tion between the coli and typhoid bacilli is the so-called 

 indolrreaction. This is best applied in the following 

 manner, as recommended by Kitasato : ^ — 



To 10 c.c. of the culture in ordinary alkaline peptone- 

 broth (see p. 25) of the organism under examination, 

 and which has been growing for twenty-four hours in 

 the incubator, add 1 c.c. of a solution of potassium or 

 sodium nitrite (containing '02 gram in 100 c.c), and 

 then a few drops of concentrated sulphuric acid. If 

 indol is present^ a rose to deep-red coloration is produced^ 

 depending on the interaction of nitrous acid with indol 

 to form nitrosoindol nitrate, which is of red colour. On 

 applying this test to the B. coli communis an indol- 

 reaction is obtained, whilst the typhoid bacillus oives a 

 negative result. 



Vincent's Method!^ — In this method, which is very 

 similar to Parietti's above, the attempt is made to 



^ Zeitsch. f. Hygiene, vol. vii., 1889," p. 518. Through a very serious 

 printer's error in this paper, potassium nitrate is repeatedly described 

 as being used instead of nitrite. Kitasato corrects this mistake in ibid, 

 vol, viii., 1890, p, 61. 



^ Comptes rendiis hebdomadai/res des Seances de la Societe de 

 Biologie, 1890, No. 5. 



T 



