BACTERIAL FERMENTATION OF SUGARS 75 



If we consider sugars generally from the point of view of 

 the capacity of yeast to originate alcoholic fermentation in them, 

 we may say that the simpler the constitution of the sugar the 

 more easily is it fermented. Thus the monosaccharides are 

 more easily acted on by yeast than the di- or polysaccharides. 

 Usually an independent process resulting in the splitting of the 

 higher into the lower is preliminary to the alcoholic fermentation. 

 Thus yeast first inverts cane sugar into glucose and fructose and 

 then acts on these products. From what is known it is probable 

 that similar facts hold with regard to the action of bacteria. 



Besides sugars other alcohols with large molecules may be 

 broken down by bacterial action, and these bodies have been 

 used for differentiating the properties of allied bacteria. Among 

 these substances may be mentioned the trihydric alcohol glycerol 

 (glycerin), the tetrahydric erythritol and the hexahydric dulcitol 

 (dulcite), mannitol (mannite), and sorbitol (sorbite). 



Similarly certain glucosides, such as salicin, coniferin, etc., 

 have been used for testing the fermentative properties of 

 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 sub- 

 stances 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 (b) 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 g'as-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 (v. p. 38) 

 or a dextrose-free bouillon (v. 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. 



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 



