INDOLE REACTION 27 



in the ordinary way usually contains a little dextrose 

 derived from the glycogen in the meat, and this probably 

 explains why the indole reaction is generally much more 

 marked in a peptone water than in a broth culture, although 

 the latter is a better nutrient soil. In order to prepare a 

 soil free from dextrose, the acid beef broth used in the 

 preparation of nutrient broth should be inoculated with 

 the colon bacillus and incubated for twenty-four hours, 

 and the peptone beef broth prepared from it. The 

 dextrose is consumed and no indole is formed. 1 



Some bacteria not only form indole but also produce 

 nitrites in the culture medium by the reduction of the 

 nitrates present in the peptone, etc., used in making the 

 nutrient medium, in which case the addition of pure 

 sulphuric or hydrochloric acid alone suffices to bring 

 out the pink indole reaction. This forms, therefore, an 

 additional means of distinguishing organisms, and is 

 employed especially for the recognition of the cholera 

 spirillum, which, if grown in peptone water, gives the 

 indole reaction (or, as it has been termed, " the cholera 

 red reaction ") on the addition of acid alone. The reaction 

 can be obtained as early as twelve hours after inoculation, 

 and becomes very marked in twenty-four to forty-eight 

 hours. 



If indole is formed only in small quantities, 100 c.c. 

 of the culture may be distilled ; the first 20 c.c. of the 

 distillate will contain the bulk of the indole. 



This " indole -reaction " is not necessarily always due 

 to indole ; the writer has shown 2 that the indole-like 

 reaction obtained with cultures of the diphtheria and 

 pseudo-diphtheria bacilli is owing to the presence of 

 skatole-carboxylic acid. This substance is distinguished 

 from indole by being non- volatile. To make sure of the 



1 T. Smith, Journ. of Exper. Med., vol. ii, 1897, p. 543. 



2 Trans. Path. Soc. Lond., vol. lii, pt. ii, 1901, p. 113. 



