98 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



Certain bacteria when growing upon this medium form red 

 colonies because the fuchsin turns red as a result of the 

 development of aldehyde and acid. Other species of bac- 

 teria grown upon this medium do not change the color 

 at all. 



Production of Acetyl Methyl Carbinol. — This compound 

 is produced by certain bacteria growing in the presence of 

 carbohydrates. It is recognized by the addition of strong 

 alkali such as sodium hydrate or potassium hydrate. When 

 allowed to stand for a few hours an eosin pink or red color 

 will develop, particularly near the surface, providing there 

 is some peptone present. This is frequently called the 

 Voges-Proskauer reaction, after the men who first noted 

 it. 



Determination of Oxygen Relationships. — Bacteria are 

 frequently divided into two groups, those which require 

 free oxygen of the air for their development and those which 

 will grow without. The first are termed aerobes, the second 

 anaerohes. Those bacteria which can grow either in the 

 presence or the absence of free oxygen are termed facultor 

 tive anaerobes, and those which will not grow in the pres- 

 ence of free oxygen are termed oiligate anaerobes. 



It is frequently necessary in the laboratory to determine 

 accurately the oxygen preferences of a microorganism. 

 The bacteria which will grow only upon the surface of a 

 medium, but never grow in the closed arm of a fermenta- 

 tion tube under any conditions, are the obligate aerobes. 

 For the most part these are organisms which are actual 

 oxidizers, changing carbonaceous materials to carbon di- 

 oxide and water, for example. The facultative anaerobes 

 are those which will grow in contact with air, but, at least 

 under certain conditions, will grow in the absence of free 

 atmospheric oxygen. Most of the facultative organisms 

 are obligate aerobes on certain media and facultative on 

 others. Many bacteria, for example, which usually require 



