777 /; CLASSIFICATION OF BACTERIA 41 



compounds. Their carbon they derive from albumin, peptone, sugar, 

 and other allied carbohydrates; glycerin, fats, and other organic sub- 

 stances. It is an interesting fact that even compounds which in con- 

 siderable concentration are extremely poisonous, can, when in sufficient 

 dilution, provide the necessary carbon; thus some derive it from car- 

 bolic acid in very dilute solutions. 



The value of substances as a source of nutrition is often influenced 

 by the presence of other materials, as, for instance, the value of asparagin 

 is increased by the presence of sugars. Further, materials from which 

 nitrogen and carbon cannot be directly obtained still become assimilable 

 after being subjected to the influence of bacterial ferments. The pro- 

 found and diverse changes produced by the different ferments make it 

 almost impossible to establish, except in the most general way, the nutri- 

 tive value of any mixture for a large number of bacteria through a simple 

 knowledge of its chemical composition. The special culture media, 

 such as bouillon, blood serum, etc., for the development of bacteria, 

 will be dealt with in a later chapter. 



Relation of Bacteria to Oxygen. The majority absolutely require 

 oxygen for their growth, but a considerable minority fail to grow 

 unless it is excluded. A knowledge of this latter fact we owe to Pas- 

 teur, who divided bacteria into aerobic and anaerobic. Between these 

 two groups we have those that can grow either with or without the access 

 of oxygen. 



Some at least of the strict anaerobic bacteria require for the full devel- 

 opment of their life functions the presence of fermentable substances, 

 such as sugars, from which they obtain oxygen. Among bacteria can 

 be found all gradations between those bacteria which develop only in 

 the presence of oxygen to those which develop only in its absence. In 

 so far as for any variety the amount of oxygen present is unfavorable 

 there will be more or less restriction in some of the life processes of these 

 bacteria, such as pigment and toxin production, spore formation, etc. 

 It has also been found that some, at least, of the aerobic bacteria can 

 be accustomed to grow without oxygen and that some of the anaerobics 

 can be accustomed to grow with it Free oxygen kills the vegetable forms 

 of the anaerobic bacteria in a few hours, but hardly injures the spores. 



Sulphur and phosphorus are two important foodstuffs required by 

 bacteria. Either calcium or magnesium and sodium or potassium are 

 also usually required for bacterial growth. Iron is demanded by but 

 few varieties, among which is the influenza bacillus. 



When we consider the more complex culture media, either those 

 naturally existing, such as blood serum, or those created by us for the 

 cultivation of bacteria, we find, beyond the necessary amount of soluble 

 foodstuffs, that the relative proportion of each form and the total con- 

 centration are of great importance. It is, nevertheless, true that very 

 wide differences can exist with but slight effect upon the development 

 of many bacteria, the development of the bacteria usually ceasing 

 through the accumulation of deleterious substances in the culture media 

 rather than through food exhaustion. 



