CHAPTER VII 



METHODS OF STUDYING PHYSIOLOGIC CHARACTERS 



MANY species of microorganisms are differentiated from 

 each other on the basis of differences in their physiologic 

 characters. For example, certain closely related disease- 

 producing bacteria are differentiated because of their abil- 

 ity to produce acid or gas from certain sugars. A study of 

 these characters teaches something of the parts which these 

 microorganisms play in nature in bringing about chemical 

 changes. Only a few of the most important of these physi- 

 ologic reactions and characters are here discussed, many 

 others will be noted in subsequent chapters. Those here 

 given are the ones most frequently used in the differentia- 

 tion of species. 



A study of physiologic characters of bacteria is largely 

 a study of specific chemical changes brought about by 

 these microorganisms. The characters most frequently used 

 in the laboratory are changes in hydrogen ion concentration, 

 determination of acid production, alcohol production, oxy- 

 gen relationship, gas production, reduction changes, pro- 

 duction of indol, digestion of starch and other insoluble car- 

 bohydrates, digestion of proteins, nitrogen fixation, oxida- 

 tion of ammonia to nitrites, and oxidation of nitrites to 

 nitrates. 



Determination of Changes in Hydrogen Ion Concentra- 

 tion. Many bacteria when grown in nutrient solutions, 

 particularly those containing carbohydrates, bring about 

 marked changes in hydrogen ion concentration. The 

 amount of such change accomplished by a given organism 

 depends upon several factors, the most important being 

 the following: first, the amount of acid produced, second 



92 



