74 METHODS OF CULTIVATION OF BACTERIA 



thoroughly closed, and if these vessels be kept in a cool place in 

 the dark. A layer of sterile toluol about half an inch thick 

 ought to be run on to the top of the filtered fluid to protect the 

 latter from the atmospheric oxygen. 



Instead of being filtered off, the bacteria may be killed by 

 various antiseptics, chiefly volatile oils, such as oil of mustard 

 (Roux). These oils are stated to have no injurious effect on the 

 chemical substances in the fluid, and they may be subsequently 

 removed by evaporation. It is not practicable to kill the bacteria 

 by heat when their soluble products are to be studied, as many 

 of the latter are destroyed by a lower temperature than is required 

 to kill the bacteria themselves. 



Bacteria can be almost entirely removed from fluid cultures 

 by spinning the latter in a centrifuge of very high speed (e.g. 

 C. J. Martin's turbine centrifuge), and this method is sometimes 

 adopted in practice. 



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. These bodies are now known to be (to 

 use the definition of Holleman) aldehyde or ketone alcohols 

 containing one or more hydroxyl groups, one of which is 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 monosaccharides or monoses, disaccharides (dioses), 

 and polysaccharides (polyoses). The members of the last two 

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

 or more molecules of a rnonosaccharide with the elimination of 

 water (e.g. 2C 6 H 12 O 6 = C 12 H 22 O U + H 2 O). 



Monosacc/iarides. These are classified according to the 

 number of C atoms they contain. The pentoses ordinarily used 

 are arabinose (obtained from gum arabic), rhamnose and xylose 

 (from wood). Among the hexoses are glucose (dextrose) with 

 dextro-rotatory properties. Glucose is an aldehyde alcohol 

 (aldose). In fruit there is also a ketone alcohol (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^H^On). The ordinary members of this 

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

 (sucrose, saccharose). 



Polysaccharides. Examples are starch, raffmose, inulin (from 

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



