82 METHODS OF CULTIVATION OF BACTERIA 



bacteria. Other substances allied to sugars (e.g., inosite) have 

 also been used. 



The end products of bacterial fermentations may be various. 

 They differ according to the sugar employed and according to 

 the species of bacterium under observation, and frequently a 

 species which will ferment one sugar has no effect on another. 

 The substances finally produced, speaking roughly, may be 

 alcohols, acids, or gaseous bodies (chiefly carbon dioxide, 

 hydrogen, and methane). For the estimation of the first groups 

 complicated chemical procedure may be necessary. The tests 

 usually employed for the detection of ordinary fermentative 

 processes depend on two kinds of changes, namely, (a) the evolution 

 of gases and (6) the formation of acids. Generally speaking, we 

 may say that such tests are reliable, and the methods to be 

 pursued are simple. Besides such gases as those named, some 

 organisms give rise to sulphuretted hydrogen by breaking up the 

 proteid. The formation of this gas can be detected by the 

 blackening of lead acetate when it is added to the gas-containing 

 medium. 



In testing the effect of a bacterium on a given sugar it is 

 essential that this sugar alone be present ; the basis of the 

 medium ought therefore to be either peptone solution (vide p. 39) 

 or a dextrose-free bouillon (vide infra). The sugar or other 

 substance is added in the proportion of from a half to one 

 per cent., and care is taken not to overheat during sterilisation. 



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 alkalies) which may act deleteriously upon it ; in any 

 case sterilisation should not be at a temperature above 100 0. 



To obtain a "dextrose-free" bouillon it is usual to inoculate ordinary 

 bouillon with some organism, such as b. coli, which is known to ferment 

 dextrose, and allow it 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 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. 36, 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 slipped into the latter, mouth 



