84 METHODS OF CULTIVATION OF BACTERIA 



used is a gas-forming one, then, as growth occurs, little bubbles 

 appear round the colonies. 



In this method the gas-formation results from fermentation 

 of the glucose naturally present in the medium from transforma- 

 tion of the carbo-hydrates of muscle. The amount of glucose 

 naturally present, however, varies much, and therefore glucose 

 should be added to the medium if the effects on this sugar are 

 to be observed with certainty. The shake culture method may 

 be utilised for observing fermentation in other sugars by adding 

 to peptone solution containing the sugar 10 to 15 per cent, of 

 gelatin. 



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 albumin 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 



