132 GENERAL PHYSIOLOGY 



here. One class, known as the strict or obligate aerobes, can 

 grow only under strict aerobic conditions; the obligate 

 anaerobes can grow only under strict anaerobic conditions ; the 

 facultative anaerobes grow best under aerobic conditions, but 

 have the faculty of growing under anaerobic conditions ; and 

 the facultative aerobes grow best under anaerobic conditions, but 

 have the faculty of growing under aerobic conditions. Anae- 

 robes are widely distributed in nature. They are found, for 

 example, in the soil constantly and in great numbers. Further- 

 more, they occur in the upper layers of the soil under condi- 

 tions where oxygen is constantly present ; and it is a matter 

 of speculation as to how the conditions favorable for their 

 development are secured. Upon our artificial culture media 

 they are obligate anaerobes. It is thought by some that 

 perhaps in their natural habitat they are not so particular in 

 their oxygen requirements or, what is quite as likely, they 

 may enter into partnership with certain other forms of life 

 which furnish them anaerobic conditions. But whatever the 

 explanation, it is a well-known fact that these forms are widely 

 distributed and among them some of the disease-producing 

 bacteria, as, for example, B. tetani (lockjaw) and Bact. welchii 

 (gaseous emphysema). 



Anaerobic Culture Methods. In order to cultivate these 

 germs on artificial media it is necessary to employ special 

 methods. A great many different methods have been described 

 but they can all be grouped under the following heads : 

 i. Replacement of air. In this case air is replaced by some 



