INDOLE 25 



then leucin, tyrosin, and glycocol, and basic compounds 

 to which the name of ptomine has been given ; next 

 indole, skatole, and phenol, and volatile fatty acids ; and 

 lastly, mercaptans, sulphuretted hydrogen, marsh gas, 

 ammonia, carbonic acid, and hydrogen. 



In view of its practical importance in bacteriological 

 analysis and the identification of species, indole may here 

 be referred to at some length. 



Indole. Indole (C 8 H 7 N) is a product of the putrefactive 

 decomposition of proteins containing a tryptophane 

 nucleus and is formed during the growth of many organisms, 

 and, since one species may produce it and another allied 

 one may not, the determination of its presence or absence 

 in the culture may be of value in the identification of 

 organisms. The detection of indole is based on the 

 reaction with nitrous acid, with which it gives a fine 

 purplish-red coloration. In order to test for it, the 

 organism is grown in a fluid medium for twenty-four to 

 forty-eight hours or longer, 1 c.c. of a 0-1 per cent, solution 

 of sodium nitrite is added to every 10 c.c. of the culture, 

 and a few drops of pure concentrated sulphuric acid or 

 of hydrochloric acid are allowed to trickle slowly down the 

 side of the test-tube, which is inclined with its mouth 

 away from the operator. As the acid runs down, it is 

 mixed with the fluid ; a colour varying from pale pink to 

 pale purple indicates the presence of indole. A control 

 tube, uninoculated, should also be similarly tested to make 

 sure that the reaction is due to the products of the growth 

 of the organism. The culture fluid usually employed is 

 peptone water, preferably 2 per cent., but some samples 

 of " peptone " occasionally fail to give the indole reaction 

 when organisms are grown in media prepared from them ; 

 the right kind of peptone must, therefore, be used. As 

 the dilute solution of sodium nitrite is unstable, a stock 

 5 per cent, solution may be kept ; 2 c.c. of this solution 



