GROWTH AND PRODUCTS OF BACTERIA 21 



Food. Because of the absence of chlorophyll the 

 majority of forms of bacteria are unable to obtain their 

 food-supply of carbon and nitrogen from the air as do 

 higher plants, but must obtain it from the organic 

 matter on which they grow, as the tissues of animals or 

 the more highly organized plants. So it is that when 

 bacteria are grown artificially, it must be on substances 

 containing these necessary food elements, as gelatin, 

 blood-serum, animal matter, or agar-agar and potatoes, 

 highly organized plant matter. 



Oxygen. Different bacteria differ in their behavior 

 toward oxygen. 



Aerobic bacteria are those which cannot exist without 

 the presence of oxygen. 



Anaerobic bacteria are those to whom oxygen is fatal. 



Facultative bacteria are those which can exist either 

 with or without oxygen. Those thriving best with 

 oxygen, but which can exist without, are called facul- 

 tative aerobes, while those thriving best without oxygen, 

 though existing with it, are termed facultative anaerobes. 



Light. Direct sunlight and strong electric light are 

 both fatal to most bacteria, hence the importance of an 

 abundance of sunlight in buildings designed for human 

 habitation. 



Products of Bacteria. The products of bacterial 

 growth are varied and complex. Some produce acids, 

 as the butyric and lactic acid produced respectively by 

 the Bacillus butyricus and Bacillus acidi lactici, and 



