176 GENERAL PHYSIOLOGY 



microorganisms. When indol in the presence of a nitrite 

 is treated with sulphuric acid there is developed a pink color. 

 In order to determine then whether or not a particular cul- 

 ture has indol present or not it is only necessary to add a solu- 

 tion of nitrite and some sulphuric acid. It was originally 

 supposed that indol was produced by putrefactive bacteria 

 alone, but now it is known that it is produced by many other 

 bacteria. Sometimes it is produced in very small amounts, 

 and at other times more abundantly. Sometimes also cer- 

 tain bacteria produce in addition to the indol a nitrite. In 

 this case it is only necessary to add the sulphuric acid in order 

 to get the characteristic reaction. This reaction is known as 

 the cholera red reaction, and was at first supposed to be char- 

 acteristic of the Msp. comma of Asiatic cholera, but it is now 

 known that a considerable number of bacteria give this 

 cholera red reaction and, in making a determination of the 

 presence of indol, sulphuric acid should be added first and the 

 nitrite afterwards in order to differentiate between the simple 

 production of indol and the production of the nitroso-indol. 

 Other nitrogen by-products of great importance produced 

 by bacteria are the enzymes. These are substances that 

 stand midway between living and nonliving matter. 



Bacterial Enzymes and their Mechanism of Action. 

 Up to the present time no enzyme has been isolated in a pure 

 state. Consequently, nothing definite is known in regard to 

 the chemistry of these compounds. Enzymes are recognized 

 by the various chemical changes which they produce. They 



