66 THE CHEMISTRY OF BACTERIA 



of sodium, potassium, sulphur, calcium, and magnesium. Foods to 

 be fully suitable for bacterial needs, therefore, should contain these 

 elements. It should be stated, however, that the food requirements 

 of bacteria vary within wide limits, but the above statements are 

 generally applicable. 



2. Sources of Food. (a) Nitrogen. The nature of the compounds 

 in which nitrogen must be presented to bacteria as food varies greatly 

 among the different groups. The nodule bacteria found in the nodules 

 on the roots of many leguminous plants actually utilize atmospheric 

 nitrogen: nitrifying bacteria found chiefly in the soil derive their 

 nitrogen requirement chiefly from mineral salts which are oxidized 

 through their activities to nitrites and eventually to nitrates. From 

 this very simple source of nitrogen these bacteria are able to synthesize 

 the complex nitrogen-containing proteins of their bodies. 



The majority of bacteria, including not only the saprophytic organ- 

 isms but most of those pathogenic for man, animals, and plants as 

 well, thrive in media in which nitrogen is presented to them as peptones, 

 albumoses, or even certain amino-acids; in other words, upon the pro- 

 ducts of protein digestion. The more strictly pathogenic organisms, 

 as the gonococcus, may require nitrogen in the form of highly specific 

 tissue proteins. Generally speaking, animal protein or its derivatives 

 is more easily utilized by bacteria than protein of vegetable origin. 



(b) Carbon. The simplest carbon compound which occurs naturally, 

 CO 2 , cannot be used by bacteria, except certain nitrifying bacteria, 

 as a source of energy, for it is already fully oxidized. The carbon 

 of proteins and their derivatives, of carbohydrates, and of fats, on the 

 contrary, is readily utilizable by most bacteria. As a rule, hydro- 

 carbons of the aliphatic series are not attacked by the microorganisms, 

 but compounds containing oxygen as well as carbon and hydrogen are 

 better adapted for microbial food. Organic acids, as acetic acid, 

 aspartic, tartaric, and many oxyacids are utilizable by some bacteria. 

 The simpler alcohols can be used, but by very few bacteria. The 

 complex alcohols, like glycerin and mannite, on the other hand, are 

 available food materials for many. 



The best nitrogen-free food compounds for microorganisms are the 

 carbohydrates, particularly those containing six and twelve carbon 

 atoms, the hexoses and bioses respectively. Carbohydrates containing 

 four, five, or any number of carbon atoms not a multiple of three are 

 usually not readily attacked by bacteria. Starches and cellulose are 

 not generally utilizable, although certain types of organisms, notably 



