INDOL-FORMATION BY BACTERIA 81 



collects in the upper part of the closed limb, the medium being 

 displaced into the bulb. If the limb be graduated the amount 

 of gas evolved can be measured, and rough chemical tests can be 

 applied, e.g., the presence of carbonic acid gas can be tested for 

 by absorbing it with a solution of caustic soda and that of 

 hydrogen by ignition (see under b. coli). 



The development of an acid reaction is demonstrated by the 

 addition of an indicator to the medium, litmus or neutral-red 

 being generally used. The details of the composition of such 

 media have already been given. In Hiss's serum water media 

 the production of acid also leads to coagulation of the medium. 

 Sometimes acid is formed very slowly from sugars, so that it is 

 well to keep the cultures under observation for several days. 



Acid and gas-formation may be simultaneously tested for, by 

 placing the fluid medium containing the indicator in Durham's 

 tubes. 



In all tests in which sugars are used, a control uninoculated 

 tube ought to be incubated along with the bacterial cultures, as 

 changes in reaction sometimes spontaneously occur in media 

 containing unstable sugars. Tests in which sugars are used are 

 best carried out in Jena glass tubes. 



The capacity of an organism to produce acid may be measured 

 by taking a standard amount of a fluid medium and allowing 

 growth to take place for a standard time, and then adding an 

 amount of, say, decinormal soda solution sufficient to bring the 

 litmus back to the tint of the original medium. 



The Observation of Indol - formation by Bacteria. — The 

 formation of indol from protein by a bacterium sometimes 

 constitutes an important specific characteristic. To observe 

 indol production the bacterium is grown, preferably at incubation 

 temperature, in a fluid medium containing peptone. The latter 

 may either be sugar-free bouillon or preferably peptone solution 

 (see p. 39). Any medium containing sugars must be avoided, 

 as the presence of these substances may inhibit the production 

 of indol. Two methods are in use for the detection of this 

 body. 



(1) The Nitroso-indol Method. — Indol is here recognised by 

 the fact that when it is acted on by nitric acid in the presence of 

 nitrites, a nitroso-indol compound is produced, which has a rosy 

 red colour. Some bacteria (e.g., the cholera vibrio) produce 

 nitrites as well as indol, but usually in making the test (e.g., in 

 the case of b. coli) the nitrites must be added. This is effected 

 by adding to an ordinary tube of medium 1 c.c. of a -02 per 

 cent, solution of potassium nitrite, and testing with pure nitric 



