BIOLOGY OF BACTERIA. 
11 
From what has been said of the properties of 
bacteria it is possible to make a number of classifica¬ 
tions; for example, there are the spore-forming and 
non-spore-forming bacteria, the motile and non-motile, 
fermenting and non-fermenting, acid forming and 
alkali forming, etc. By observing these properties of 
bacteria it is possible to identify them. 
Like all plants bacteria require food, which must 
be in very simple form to enable them to assimilate it. 
Oxygen, carbon, nitrogen, hydrogen, and chemical 
salts form their chief food. They derive the oxygen 
from the air, although some varieties of bacteria take 
it.from substances in which the oxygen is combined 
with other chemical elements. The bacteria that take 
their oxygen from the air are called aerobic bacteria, 
while those taking it from substances containing it in 
combined form are called anaerobic bacteria. The line 
of demarcation between the aerobic and the anaerobic 
bacteria is not fixed, as sometimes bacteria thriving 
best under aerobic conditions will, nevertheless, grow 
in the absence of free oxygen and vice versa. These 
are spoken of as facultative anaerobes or aerobes, as 
the case may be. The carbon is obtained from pro- 
teids, carbohydrates (starchy substances), or fats. 
The hydrogen is derived for the most part from water. 
The nitrogen is obtained from proteids such as albumin. 
The salts required for nutrition are sodium, potas¬ 
sium, and magnesium. 
Certain conditions of environment exert a great 
deal of influence upon the life and growth of bacteria. 
