BACTERIAL FERMENTATIONS IN SUGARS. 



79 



f- 



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

 appear round the colonies. These frequently coalesce to form 

 bubbles of a larger size, 

 and those which are super- 

 ficial in process of time 

 diffuse out of the medium. 

 This method is very fre- 

 quently used for studying 

 gas formation by B. coli. 



(2) Durham's Tubes 

 (Fig. 44, b). The plug of 

 a tube which contains about 

 one-third more than usual 

 of a liquid medium is re- 

 moved, and a small test- 

 tube is slipped into the 

 latter mouth downwards. 6 

 The plug is replaced and FIG. 44. 

 the tube sterilised thrice 



at 100 C. The air re- 

 maining in the smaller tube 

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

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

 part of the inner tube. 



(3) The Fermentation Tube (Fig. 44, 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 

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

 displaced into the bulb. 



H. W. Hill has advantageously modified the ordinary fer- 

 mentation tube of Smith (Fig. 45) by having the bulb made larger 

 so as to accommodate twice the quantity of fluid contained in the 

 branch, thus avoiding wetting of the plugs during sterilisation ; 

 also, in having replaced the sealed end of the branch by a snugly 

 fitting, hollow, ground-glass thimble, or stopper, permitting one 

 to examine the contents of the branch, either chemically or bac- 

 teriologically, without contamination by fluid in the bulb, such as 

 happens when using the ordinary form of the tube. In carrying 

 out this examination it is obviously necessary to first replace 



Tubes for demonstrating gas-formation 

 by bacteria. 



a, tube with " shake " culture. 



b, Durham's fermentation tube. 



c, ordinary form of fermentation tube (Smith's). 



