80 METHODS OF CULTIVATION OF BACTERIA 



It is to be noted, however, that a bouillon rendered " dextrose-free " by 

 b. coli may still contain carbohydrate fermentable, for example, by a 

 streptococcus. 



For the observation of gas-formation either of the following 

 methods may be employed : — 



(1) Durham's Tubes (Fig. 32, b). — The plug of a tube which 

 contains about one-third more than usual of a liquid medium 

 is removed, and a small test-tube is inverted and slipped 

 down into the medium. The plug is replaced and the tube 

 sterilised thrice for ten minutes at 100° C. The air remaining 





■ ■ ■■■. 



6 a c 



Fig. 32. — Tubes for demonstrating gas- formation by bacteria. 



a, tube with "shake" culture. 



b, Durham's fermentation tube. 



c, ordinary form of fermentation tube. 



in the smaller tube is thereby expelled. The tube is then in- 

 oculated 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 satisfactory result can be obtained with only 1 c.c. 

 of medium. 



(2) The Fermentation Tube (Fig. 32, 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 



