68 BACTERIOLOGY. 



Oxygen Requirements. 



It was at one time generally accepted that all living- 

 being's required free oxygen. In 1861, Pasteur described as 

 the cause of butyric acid fermentation a bacillus which 

 could be cultivated only in the absence of oxygen. He 

 designated those organisms which required oxygen for their 

 development as aerobic; whereas those which lived in the 

 absence of oxygen were designated as anaerobic. The 

 " vibrion butyrique " was the first known anaerobic germ. 

 In 1877, Pasteur demonstrated that an experimental disease, 

 "septicemie" or, as it is more commonly known, malignant 

 edema, was due to an anaerobic bacillus. Since then about 

 fifty anaerobic bacteria have been described. 



An examination of the various anaerobic bacteria will 

 show that some are absolutely dependant upon the total 

 exclusion of oxygen. The presence of a small amount of 

 air will promptly stop the growth of such organisms. 

 These remarkable forms can be designated as obligative 

 anaerobes since they are obliged to live under strictly 

 anaerobic conditions. On the other hand, there are aerobic 

 bacteria which live only in the presence of an abundance 

 of oxygen. As soon as the amount of air, or oxygen is 

 diminished growth stops. They are obliged to live in the 

 presence of air and, for this reason, they are termed obliga- 

 tive aerobes. 



The obligative aerobes and the obligative anaerobes con- 

 stitute the two extremes as far as oxygen requirements are 

 concerned. Most of the bacteria show an adaptability to 

 one or the other of these conditions, and as a result the 

 two obligative groups are connected by a series of- species 

 which can, more or less readily, accommodate themselves 

 to the presence or absence of oxygen. A given organism 

 which thrives best in the presence of air may grow, though 

 perhaps less abundantly, in the absence of oxygen. Such 



