76 METHODS OF CULTIVATION OF BACTERIA 



dextrose, and allow the latter to act for forty-eight hours. The bouillon 

 is then filtered and re-sterilised. A sample is tested for another period of 

 forty-eight hours with b. coli, to make certain that all the dextrose has 

 been removed. If no fresh gas - formation is observed, then to the 

 remainder of the bouillon the sugar to be investigated may be added. 

 It is preferable that the addition should be made in the form of a sterile 

 solution. If the sugar in solid form be placed in the bouillon and this 

 then sterilised, there is danger that chemical changes may take place 

 in the sugar, in consequence of its being heated in the presence of 

 substances (such as the alkali) which may act deleteriously upon it ; in 

 any case sterilisation should not be at a temperature above 100 C. 



For the observation of gas-formation either of the following 

 methods may be employed : 



(1) Durham's Tubes (Fig. 38, 6). The plug of a tube which 

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



It a c 



FIG. 38. Tubes for demonstrating gas-formation by bacteria. 



a, tube with "shake" culture, 

 fe, Durham's fermentation tube, 

 c, ordinary form of fermentation tube. 



removed, and a small test-tube is slippsd into the latter mouth 

 downwards. The plug is replaced and the tube sterilised thrice 

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

 tubs 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. 



(2) The Fermentation Tube (Fig. 38, c). This consists of a 

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



