56 BACTERIOLOGY. 



able to live only in the presence of free oxygen. 

 Between these extreme types lie the larger 

 number of species, which can live as well with 

 as without free oxygen according to the nature 

 of food-material present ; these are designated 

 as facultative anaerobes or facultative aerobes. 

 Aerobiosis and anaerobiosis must not be 

 considered ontologically. They are results 

 simply of adaptation to the energetical pro- 

 cesses of nutrition. Substances poor in nutri- 

 tional value may, in the presence of air, yield 

 more energy through oxidation than the rich- 

 est substance through simple splitting. On 

 the other hand, a material capable of being 

 split up easily may at first, in spite of the 

 access of oxygen, remain exempt from oxida- 

 tion and only later and secondarily come to be 

 oxidized, for the reason that the quantity of 

 energy yielded by simple splitting is sufficient 

 for the need of the microbe. As a matter 

 of fact, it is possible to accustom obligatory 

 anaerobes to life in the air and air-living 

 bacteria to anaerobiosis. A strict anaerobe, 

 Spirillum rubrum, which came into my 

 possession and which produced its red pig- 

 ment only in the absence of oxygen, could in 

 a short time be cultivated as an aerobe ; the 

 same measures have succeeded with the Acti- 

 nomyces fungus, and Kitt has cultivated 



