NUTRITION OF BACTERIA. 41 



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 oxy- 

 gen 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 oxy- 

 gen appears to play but an unimportant r6le, for de- 

 velopment occurs as well with as without it. It was 

 Pasteur who first demonstrated the existence of partic- 

 ular species of bacteria which not only grow and multi- 

 ply and perform definite physiological functions with- 

 out the aid of free oxygen, but to the existence of which 

 it is positively harmful. To these he gave the name 

 anaerobio bacteria, in contradistinction to the aerobic 

 group, for the proper performance of whose functions 

 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 faaidtative 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. 



Though the multiplication of the facultative varieties 



