44 BACTERIOLOGY. 



THEIR RELATION TO OXYGEN. Of considerable 

 importance and interest in the study of the nutritive 

 changes of bacteria is the difference in their relation to 

 oxygen. For certain species free oxygen is essential to 

 the proper performance of their functions; in another 

 group no evidence of life can be detected under its 

 access ; while in a third group free oxygen appears to 

 play but an unimportant role, for development occurs 

 as well with as without it. It was Pasteur who first 

 demonstrated the existence of particular species of bac- 

 teria which not only grow and multiply and perform 

 definite physiological functions without the aid of free 

 oxygen, but to the existence of which it is positively 

 harmful. To these he gave the name anaerobic bac- 

 teria, in contradistinction to the aerobic group, for the 

 proper performance of whose functions free oxygen is 

 essential. In addition to these there is a third group, 

 for the maintenance of whose existence the absence or 

 presence of uncombined oxygen is apparently of no 

 moment development progresses as well with as with- 

 out it ; the members of this group comprise the class 

 known as facultative in their relation to this gas. It is 

 to this third group, the facultative, that the majority of 

 bacteria belong. Since all growing bacteria, anaerobic 

 as well as aerobic, generate carbonic acid in the course 

 of their development, it is evident that oxygen must 

 in reality be obtained by them from some source, and 

 must be regarded as essential to their life-processes; 

 but the manner in which it is appropriated by them 

 varies, the aerobic species taking it from the air as free 

 oxygen, while the anaerobic species, not possessed of 

 this power, obtain it through the decomposition of more 

 or less stable oxygen-containing compounds. 



