228 NUTRITION AND METABOLISM 



absence of oxygen is very high, the organisms have to decompose 

 much more food. This accounts, to a great extent, for the enormous 

 destructive power of bacteria, when comparisons of the great quantity 

 of food decomposed are made with the very insignificant weights of 

 cells. It has been estimated that the lactic bacteria decompose their 

 own weight of sugar in one hour. 



Summing up the relation of oxygen to microorganisms, some 

 bacteria, and especially the molds, are found depending upon oxygen as 

 an indispensable part of their food. Three groups are recognized: 

 Those, a large number, organisms in the presence of oxygen producing 

 oxidations; those able to sustain life without oxygen; and those de- 

 pending entirely upon decompositions which require no oxygen. 

 The lactic bacteria and the butyric bacteria belong in the last group. 



In considering the oxygen requirements, it is customary to in- 

 clude another influence of oxygen upon bacteria. This has really 

 nothing to do with its food value, but deals with the poisonous qualities 

 of oxygen. Oxygen in this light may well be called a poison as it will 

 kill bacteria in very low concentrations. Ordinarily it is regarded as 

 constituting over 20 per cent of our atmosphere. But if a study is 

 made of its effect upon bacteria, it is necessary to measure it in the 

 same way food is measured, and consider the concentration in which 

 it is offered to the cell. Microorganisms obtain their oxygen not as 

 gas, but as dissolved oxygen. The solubility of oxygen is very small, 

 about 0.0009 P er cent at 20. Practically all bacteria die readily if the 

 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 will 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 



