226 COMPOSITION OF THE DIGESTIVE JUICES. [CH. VIII. 



Now peptone does not contain free tryptophane, and some 

 specimens of peptone only contain small quantities of 

 combined tryptophane, as can be determined by the 

 failure to get a vivid glyoxylic reaction (Ex. 23). Before 

 the bacteria can make indol it is probable that they have to 

 multiply considerably so as to produce the various enzymes 

 concerned. If the nutrient medium contains free trypto- 

 phane and is otherwise suitable, the formation of indol 

 starts almost immediately. Cole and Onslow* have de- 

 scribed a " tryptic broth, "prepared by digesting a solution 

 of commercial casein with trypsin, as being eminently 

 suitable for making indol tests. It contains an abundance 

 of tryptophane and other amino-acids in the free state. 

 The growth of the organisms is usually very luxuriant, 

 and the element of uncertainty about the appearance of 

 indol is abolished. With this medium the production of 

 indol by a typical indol former can be detected in four or 

 five hours. 



One of the best known indol formers is B. coli, an almost 

 universal inhabitant of the intestine. Organisms closely 

 allied to it, such as B. typhosus and B. paratyphosus are 

 unable to transform tryptophane into indol or scatol. 



It is interesting to note that the simultaneous fermenta- 

 tion of glucose inhibits indol production. This is not due 

 to the fact that the medium becomes acid owing to the 

 carbohydrate fermentation. It indicates that the meta- 

 bolism and enzyme production of the organisms are 

 profoundly influenced by the available sources of energy. 

 Since a large number of diseases are caused by the products 

 of bacterial action, a complete study of the chemistry of 

 bacterial growth is of great importance. 



For the following experiments the appliances and technique of a bacterio- 

 logical laboratory are required. 



Preparation of " tryptic broti." A 10 per cent, solution of commercial 

 casein is prepared as described in Ex. 87, A (i.) to (vi.). Toluol is added as an 

 antiseptic, but the sodium fluoride is omitted. Trypsin is then added and the 



* The Lancet, July I, 1916, p. 9. 



