PHYSIOLOGY OF MICROORGANISMS 53 



oxygon from a combined source, the oxyhemaglobin of the 

 blood. 



The bacteria that can grow only in the presence of rela- 

 tively large anionnls of oxygen are termed aerobic. In 

 li<|iii(ls the growth oi' such organisms will he confined to the 

 surface. The organisms that can grow through a wide 

 range with respect to the amount of oxygen, from a total 

 absence to that of atmospheric air, are called facultdtive. 

 If the organism grows better in the absence of free oxygen 

 than in its presence, it is termed a facultative aerobe. If 

 the reverse is true, the term facultative anaerobe is used. 

 The facultative bacteria make up the greater part of the 

 known species. 



The cultivation of anaerobic bacteria is accomplished by 

 replacing the air of the culture container with hydrogen, 

 nitrogen, or carbon-dioxide, by absorbing the oxygen of the 

 air with some substance such as an alkaline solution of 

 pyrogallic acid. Most of them can be grown in appropriate 

 culture media without the artificial exclusion of air. It 

 seems probable that in the soil, and in other places in na- 

 ture, they can grow in the presence of larger amounts of 

 oxygen than will permit their growth in the laboratory. 



The yeasts are facultative. It seems, however, that, 

 while growth may go on for a period in. the absence of oxy- 

 gen, it can not continue for an indefinite time, as is ap- 

 parently true with the anaerobic bacteria. 



The molds are all aerobic, a fact that becomes of great 

 importance in food preservation. 



Reaction. The true chemical reaction, the relative num- 

 ber of hydrogen ions as compared to hydroxyl ions, is a 

 factor of great importance in influencing the growth of 

 microorganisms. The bacteria, as a rule, grow best in neu- 

 tral materials. The yeasts will grow in neutral substances, 



