78 METHODS OF CULTIVATION OF BACTERIA 



The Observation of Bacterial Fermentation of Sugars, etc. 

 — The capacity of certain species of bacteria to originate fermenta- 

 tions in sugars constitutes an important biological factor. It 

 is well to consider, this factor in relation to the chemical con- 

 stitution of the sugars. The true sugars are aldehydes or ketones, 

 one or more of the carbon atoms of which is united to a 

 hydroxyl group, one being directly linked to a carbon atom in 

 union with carbonyl. The group characteristic of a sugar is 

 thus - CHOH - CO - . The sugars are divided into mono- 

 saccharides, disaccharides, and polysaccharides. The members 

 of the last two groups may be looked on as derived from the 

 combination of two or more molecules of a monosaccharide with 

 the elimination of water {e.g., 2C 6 H 12 6 = C 12 H 22 O n + H 2 0), 

 but their chemical constitution may be more complex than 

 that of the first group. 



Monosaccharides. — These are classified according to the 

 number of C atoms they contain. The pentoses ordinarily used 

 are arabinose (obtained from gum arabic), xylose (from wood), 

 and rhamnose (which is really a methylpentose). Among the 

 hexoses are glucose (dextrose) with dextro-rotatory properties. 

 Glucose is ah aldehyde (aldose), but in fruit there is also 

 a ketone (ketose) called fructose, which from its laevo- 

 rotatory properties is also known as laevulose. Other hexoses 

 are mannose (from the vegetable ivory nut) and galactose (a 

 hydrolytic derivative of lactose). 



Disaccharides (C 12 H 22 O n ). — The ordinary members of this 

 group are maltose (derived from starch), lactose, and cane sugar 

 (sucrose, saccharose). 



Polysaccharides. — Examples are starch, raffinose, inulin (from 

 dahlia roots), dextrin, arabin, glycogen, cellulose. 



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 poly-saccharides. 

 Usually an independent process resulting in the splitting of the 

 higher into the lower is preliminary to the alcoholic fermenta- 

 tion. 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, closely allied bodies which are alcohols with 

 large molecules may be broken down by bacterial action, and 

 these have been used for differentiating the properties of allied 



