BACTERIAL FERMENTATION OF SUGARS 77 



to which it ought to be filled. It is inoculated in the bend with 

 the gas-forming organism, and when growth occurs the gas 

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

 displaced into the bulb. 



For the observation of the effect of an organism on glucose 

 the following method may be employed : 



Gelatin Shake Cultures (Fig. 38, a). The gelatin in the tube 

 is melted as for making plates ; while liquid it is inoculated 

 with the growth to be observed, and shaken to distribute the 

 organisms throughout the jelly. It is then allowed to solidify, 

 and is set aside at a suitable temperature. If the bacterium used 

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

 round the colonies. 



In this method the gas -formation results from fermenta- 

 tion of the glucose naturally present in the medium from 

 transformation of the glycogen 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 

 10-15 per cent of gelatin. 



The development of an acid reaction is demonstrated by the 

 addition of an indicator to the medium, litmus being generally 

 used. The details of 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 with the bacterial cultures, as changes 

 in reaction sometimes spontaneously occur in media containing 

 unstable sugars. 



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 con- 

 stitutes an important specific characteristic. To observe indol 



