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 présence 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, ete. It has also been 
found that some, at least, of the aérobic bacteria can 
be accustomed to grow without oxygen, and that some 
of the anaérobics 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 
