NUTRITION OF BACTERIA. 33 



count must also be taken of the products of growth of 

 the organisms in these substances. Nitrogen and carbon 

 compounds in the proper form to be appropriated by 

 bacteria may exist in sufficient quantities, and still their 

 growth may be checked after a very short time by the 

 accumulation of products of nutrition that are inhibitory 

 to their further development. Most conspicuous are the 

 changes that growing bacteria produce in the chemical 

 reaction of the media. Since the majority of them grow 

 best in media of a neutral or very slightly alkaline reac- 

 tion, any excessive production of alkalinity or acidity, 

 as a product of growth, arrests development, and no 

 evidence of life or further multiplication can be detected 

 until this deviation from the neutral reaction has been 

 corrected. 



Most favorable for the development of bacteria are 

 neutral or very slightly alkaline solutions of proteid 

 materials in one form or another. 



Of considerable importance and interest in the study 

 of the nutritive changes of bacteria is the difference in 

 their relation to oxygen. With certain forms oxygen 

 is essential to the proper performance of their func- 

 tions, while with another group no evidence of life can 

 be detected under the access of oxygen, and in a third 

 group 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 bacteria which not only grow and 

 multiply and perform definite physiological functions 

 without the aid of oxygen, but to the existence of which 

 oxygen is positively harmful. To these he gave the 

 name anaerobic bacteria, in contradistinction to the 

 aerobic group, for the proper performance of whose 



