FOOD OF MICROORGANISMS 



229 



limits of growth for the anaerobic bacteria are the limits of tolerance of 

 the poisoning oxygen; the lower limit of growth for the aerobic bacteria 

 is a question of too scanty food supply. The relation between bacteria 

 and oxygen is graphically represented in the following diagram, after 

 Kruse: 



Oxygen Pressure* 

 o.i OH o.k oS 10 



2.0 



3.0 



i 



i 



3 



f 

 S 



FIG. 112. Influence of oxygen upon microorganisms. 



The lines indicate the oxygen concentrations where growth is possible. Line 

 i is a strict anaerobe; 2 is not quite so strict; 3 is still less sensitive though it 

 cannot grow if exposed to direct influence of the atmosphere; 4 is a facultative 

 bacterium such as B. coli; 5 is another one which can tolerate still more oxygen; 

 6 can grow only with oxygen but can get along with very little : it might be one 

 of the urea bacteria; 8 is more dependent upon oxygen and the line would corre- 

 spond to average molds; 7 is a peculiar type needing oxygen and yet being very 

 sensitive to it. The sulphur bacteria, e.g., the Beggiatoacea, belong to type 7. Type 

 9 is said to be representative of B. abortus. 



i.o indicates the normal atmospheric oxygen content (about 21 per cent by volume). 



