NUTRITION OF BACTERIA. 33 



appropriated by the organism may exist in sufficient 

 quantities, and still the growth of the organism after a 

 very short time be entirely checked, owing to the pro- 

 duction during their growth of substances inhibitory to 

 their further development. Most conspicuous are the 

 changes produced by growing bacteria in the chemical 

 reaction of the media. Since the majority of them grow 

 best in media of a neutral or very slightly alkaline 

 reaction, 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 albumin 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 for 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 part, 

 for development occurs as well with as without it. It 

 was Pasteur who first demonstrated the existence of 

 species in the bacteria family 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 

 another group for the proper performance of whose 

 functions oxygen is essential ; these he called aerobic 

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



