BACTERIAL RELATION TO OXYGEN. 27 



it. In the case of certain of the chromogenic forms 

 the presence or absence of oxygen has a very decided 

 effect upon the production of the pigments by which 

 they are characterized. 



For the normal development of bacteria it is not only 

 essential that the sources from which they can obtain the 

 necessary nutritive elements should exist, but account 

 must also be taken of the products of growth of the 

 organism in these substances. Nitrogen and carbon 

 compounds in the proper form to be taken up and appro- 

 priated by the organism may exist in sufficient quanti- 

 ties, and still the growth of the organism after a very 

 short time be entirely checked, owing to the production 

 during their growth of substances inhibitory to their fur- 

 ther development. Most conspicuous are the changes 

 produced by the growing bacteria in the reaction of the 

 media. Since the majority of these bodies grow best in 

 media of a neutral or very slightly alkaline reaction, any 

 excessive production of alkalinity or acidity, as a pro- 

 duct 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. It was Pasteur who first 

 demonstrated the existence of species in the bacteria 

 family which not only grow and multiply and per- 

 form definite physiological functions without the aid 

 of oxygen, but to the existence of which oxygen is 

 positively harmful. To these he gave the name of 



