30 A MANUAL OF BACTERIOLOGY 



organisms are concerned in this process, particularly a 

 group to which Hauser gave the name of Proteus. The 

 first changes which occur are the formation of proteoses 

 and peptone, 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 hydro- 

 gen, 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 putrefac- 

 tive decomposition of proteins containing a tryptophane 

 group and is formed during the growth of many organ- 

 isms, 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. One method for the detection of indole is 

 based on the reaction with nitrous acid with which it gives 

 a purplish -red coloration. In order to perform the test 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 " may fail to yield indole when organisms 

 are grown in media prepared from them ; the right kind of 



