CHAPTER V. 

 THE CHEMISTRY OF BACTERIA. 



All living cells take in food, build up protoplasm and 

 throw off waste — or metabolic products. In a multi-cellular 

 organism, such as the higher plant or animal, it is not pos- 

 sible to trace each of the various chemical compounds which 

 are given ofE back to the cell which made them. In the case 

 of the uni-cellular organisms this can, as a rule, be easily 

 done. Although a number of different species may be grow- 

 ing together in a liquid it is possible to separate the species 

 from one another and to obtain thus pure cultures. Th e chem- 

 ical products of each species can then be studied, unmixed 

 with the products of other organisms. 



A large number of bacterial species may grow upon one 

 and the same nutrient medium. The chemical products, 

 however, will vary with each species. Thus, one organism 

 growing upon a given medium may produce an acid whereas 

 another species growing upon the same medium may give 

 rise to an alkaline reaction. In some instances differences 

 of this kind are very marked and are utilized for the pur- 

 pose of distinguishing one species from another. Thus, the 

 colon bacillus produces acid and gaseous products and in- 

 dol, whereas the typhoid bacillus, which otherwise resem- 

 bles the former very much, does not elaborate either one of 

 these products. 



A given micro-organism always produces a considerable 

 number of different chemical compounds. ThUs, the yeast 

 plant is said to produce alcohol, but it must not be inferred 

 that this is the only substance which it elaborates. On the 

 contrary it produces gases, several kinds of alcohol, a num- 



