FERMENTS, ENZYMES, TOXINS AND PTOMAINS 53 



from the body, under aseptic precaution, and kept at suit- 

 able conditions of moisture and temperature they may 

 ultimately become completely liquefied as a result of the 

 digestive action of hydrolysing enzymes contained within 

 them. 



Their Relation to Oxygen. — Of primary 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 develop- 

 ment occurs as well with as without it. It was Pasteur who 

 first demonstrated the existence of particular species of 

 bacteria 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 bacteria, in contra- 

 distinction 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 without 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 



