CONDITIONS OF GROWTH. 55 



for the full development 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. 



Sulphur and phosphorus are two important food-stuffs 

 required by bacteria. Either calcium or magnesium 

 and sodium or potassium are also usually required for 

 bacterial growth. Iron is demanded by but few varie- 

 ties. 



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 food- 

 stuffs, that the relative proportion of each form and 

 the total concentration are of great importance. It is, 

 nevertheless, true that very wide differences can exist 

 with but slight effect upon the development of bacteria, 

 the development of the bacteria usually ceasing through 

 the accumulation of deleterious substances in the cul- 

 ture media rather than through food exhaustion. 



The reaction of the nutritive media is of very great 

 importance. Most bacteria grow best in those that are 

 slightly alkaline or neutral. Only a few varieties require 

 an acid medium, and none of these belong to the parasitic 



