32 A MANUAL OF BACTERIOLOGY 



Peptone water is not always a good culture medium, 

 and broth may therefore be employed, but it should be 

 free from dextrose. Peptone water with the addition of 

 a little rabbit's serum is perhaps the best culture medium 

 for the production of indole. 



The presence of dextrose, saccharose, glycerin, or 

 lactose in quantity exceeding about 0-25 per cent, prevents 

 the formation of indole in broth by bacteria. Broth 

 prepared 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 nutrient beef-broth prepared 

 from it. The dextrose is consumed, no indole is formed, 

 and the colon bacilli are eliminated by the subsequent 

 sterilisation (T. Smith). 



Homer * suggests that in a medium containing glucose 

 there is lessened indole production because of the forma- 

 tion of a glucose-tryptophane complex which is not 

 attacked so readily as tryptophane. 



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 



1 Journ. of Hygiene, vol. xv. 1916, p. 401. 



