FOOD OF MICROORGANISMS 157 



oxygen concentration is raised to thirty times the atmospheric pressure. 

 This would mean a concentration of 0.027 per cent. It shows that 

 oxygen is about as poisonous as formaldehyde or bichloride of mercury. 



Some bacteria are extremely sensitive to oxygen, and wiU die if 

 exposed to ordinary atmospheric oxygen. They grow only if oxygen 

 is almost completely removed. These organisms are called the 

 strictly anaerobic or obligate anaerobic bacteria. They are contrasted 

 with the facultative anaerobic bacteria which thrive with oxygen as well 

 as without, and the strictly aerobic bacteria which have to have oxygen 

 for their normal life processes. 



No strict limits can be drawn between aerobic and anaerobic 

 bacteria. Even the most sensitive of organisms will be able to tolerate 

 traces of oxygen, while the strictly aerobic bacteria can multiply also 

 if the oxygen concentration is below that of a saturated solution. The 

 limits of growth for the anaerobic bacteria are the limit 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: 



Fig. 100. — 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 les's 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 Beggiatoacece, belong to type 7. Type 

 9 is said to be representative of B. abortus. 



