BACTERIAL FERMENTATION OF SUGARS 81 



tube is thereby expelled. The tube is then inoculated with the 

 bacterium to be tested. Any gas developed collects in the upper 

 part of the inner tube. As some of the sugars now used for 

 fermentation tests are rather expensive, it is well to arrange the 

 Durham apparatus with very small tubes ; with these a satis- 

 factory result can be obtained with only 1 c.c. of medium. 



(2) T/te Fermentation Tube (Fig. 36, c). This consists of a 

 tube of the form shown, and the figure also indicates the extent 

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

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



FIG. 36. Tubes for demonstrating gas-fonaatiou by bacteria. 



", tube with "shake" culture. 



b, Durham's fermentation tube. 



c, ordinary form of fermentation tube. 



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. 36, a). The gelatin in the tube 

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

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

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

 ;tn 1 i- set a>ide at a suitable temperature. If the bacterium used 

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

 appear round the colonies. 

 6 



