94 NUTRITION AND METABOLISM. 



hydrogen sulphide. Organisms which depend upon oxidizing processes 

 for their supply of energy will not be able to grow without oxygen. This 

 is the case with the acetic bacteria, with the nitrifying bacteria, and with 

 many molds which are generally noted for their strong oxidizing properties. 



The question arises whether the organisms which provide for their 

 energy without oxidation will need oxygen for life processes other than 

 energy supply. The urea bacteria require no free oxygen for their 

 fermentation, but they require it for other life functions. Certain chem- 

 ical changes can take place within the cell of these bacteria only if oxygen 

 is present. They require very little oxygen, but they cease to grow if it 

 be removed completely. There are, however, certain other micro- 

 organisms which can live in a complete absence of oxygen and there are 

 some which die in the presence of oxygen. 



The organisms which require free oxygen are called aerobes; those 

 which can live without free oxygen are called anaerobes. Among the 

 latter are the obligate anaerobes, which develop only in the absence of 

 free oxygen, and the facultative anaerobes which can grow either with or 

 without free oxygen. Even the aerobic microorganisms can tolerate the 

 absence of oxygen for a considerable length of time, perhaps for years. 

 They will not multiply but remain dormant until they come in contact 

 with free oxygen again. 



The influence of free oxygen upon the obligate anaerobic bacteria is 

 remarkable; it does not only prevent their growth, but it kills them. 

 Some butyric bacilli die if exposed to air for fifteen hours, while the spores 

 of these bacilli are quite resistant. The study of these organisms is quite 

 difficult, since it takes considerable effort to remove the last traces of 

 oxygen from the culture media. Usually they are cultivated in an 

 atmosphere of hydrogen; carbon dioxide and coal gas do not give as good 

 results because these gases affect the growth of the bacteria. The best 

 gas for the cultivation of anaerobes is nitrogen, because it is absolutely 

 neutral, but it is difficult to obtain it free from oxygen. Anaerobic organ- 

 isms may also be grown in a vacuum. 



Even the obligate anaerobic organisms can tolerate a certain amount 

 of oxygen. We can speak of a maximum concentration of oxygen for 

 the various organisms, and also of a minimum concentration. The 

 minimum of oxygen for the anaerobic bacteria is nil. This is the definition 

 of all anaerobic bacteria. The maximum concentration of oxygen varies 

 with the species. Certain bacteria are known to be killed by very small 



