120 BACTERIOLOGY. 



Indol Production. — The production of products 

 other than those which give rise to alterations in the 

 reaction of the media, and whose presence may be de- 

 tected by simple chemical reactions, is now a recognized 

 step in the identification of diiferent species of bacteria. 

 Among these chemical products there is one which is 

 produced by a number of organisms, and whose presence 

 may easily be detected by its ciiaracteristic behavior 

 when treated with certain substances. 



Indol, when acted upon by reducing agents, is seen 

 to become of a more or less conspicuous rose color. This 

 body was recognized some time ago as one of the pro- 

 ducts of growth of the comma bacillus of Asiatic cholera, 

 and for a time was thought to characterize the changes 

 produced in the media through the growth of this organ- 

 ism. It has since been found that there exist other 

 bacteria which also possess the property of producing 

 this body in the course of their development. 



The method employed for its detection is as follows : 

 Cultivate the organism for twenty-four to forty-eight 

 hours at a temperature of 37° C, in the simple peptone 

 solution known as "Dunham's solution" (see formula 

 for this medium). This solution is preferred because 

 its pale color does not mask the rose color of the reac- 

 tion when the amouut of indol present is very small. 



Four tubes should always be inoculated and kept 

 under exactly the same conditions for the same length of 

 time. 



At the end of twenty-four or forty-eight hours the 

 test may be made. Proceed as follows : To a tube con- 

 taining 7 c.c. of the peptone solution, but which has not 

 been inoculated, add 10 drops of concentrated sulphuric 

 acid. To another similar tube add 1 c.c. of a 0.01 per 



