BACTERIAL FERMENTATION OF SUGARS 83 



downwards. The plug is replaced and the tube sterilised thrice 

 for ten minutes at 100 C. The air remaining in the smaller 

 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) The Fermentation Tube (Fig. 36, c.). This consists of a 

 tube of the form shown, and the figure alst) indicates the extent 



c- 





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



a, tube with "shake" culture. 



b, Durham's fermentation tube. 



c, ordinary form of fermentation tube. 



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. 36, 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 



