PRODUCTS OF METABOLISM 175 



production serves as a means of differentiating closely allied 

 bacteria. The method of detecting acid production usually 

 used is to grow the germs in milk containing litmus, or in 

 sugar media to which litmus is added. In this the formation 

 of acid is indicated by the change of the blue litmus to a red 

 color. Many bacteria produce an alkali rather than an acid. 

 This is detected in the same way as the acid. Some bacteria 

 are able to produce either an acid or an alkali, depending 

 upon the nature of their food material. When the Bact. 

 diphtheriae, for example, is grown in a culture medium con- 

 taining sugar (and all media made from meat contain a small 

 amount unless special means for removing it have been em- 

 ployed) , it produces first an acid reaction, and then when the 

 sugar is used up it produces an alkaline reaction in the culture 

 medium. It has been found that when this germ is pro- 

 ducing acid it does not form its toxin readily, but when the 

 sugar is removed, then the toxin production goes on much 

 more readily, so that it becomes a matter of a great deal of 

 practical importance in the production of the toxin, to be 

 used for the manufacture of antitoxin, to have the culture 

 medium free from sugar to start with so that the germ may 

 begin its toxin production immediately. A number of by- 

 products of a good deal of importance contain nitrogen. 

 Among the simpler of the nitrogenous substances are indol 

 and skatol. Indol is very commonly produced by bacteria, 

 and, since it can be "detected by a color reaction, it is a prod- 

 uct of a good deal of importance in the differentiation of 



