EFFECT OF CHEMICAL ENVIRONMENT 135 



do not grow in the entire absence of oxygen. In a suitable 

 medium such organisms will develop in a definite layer 

 some distance below the surface. Still other bacteria are 

 termed facultative. These organisms apparently will grow 

 either in the presence of atmospheric oxygen or in its 

 absence. In practically every instance, however, they grow 

 in the absence of oxygen only when there is nitrate or some 

 sugar or other available carbohydrate present. The Bacterium 

 coli, for example, will grow only in the open arm of the fer- 

 mentation tube in broth, but when sugar is added it will 

 grow equally well in the closed arm. It is possible that the 

 oxygen relationships of some microorganisms are more 

 complex than indicated. It is stated that the organism of 

 contagious abortion in cattle has two oxygen optimums, it 

 will grow either in decreased oxygen concentration or in 

 greatly increased oxygen concentration, but not well in the 

 presence of atmospheric concentration except after long 

 continued cultivation. 



The fact that many organisms will not grow in the 

 presence of free oxygen makes it necessary to use special 

 methods of cultivation in the laboratory. Media in which 

 anaerobic bacteria are to be cultivated may be boiled 

 just before use in order to drive out the oxygen. After 

 inoculation they may be covered by a layer of oil or paraffin, 

 or the air may be displaced in closed tubes by some other 

 gas. Most frequently hydrogen is used or oxygen is 

 absorbed by use of some reagent such as sodium pyrogal- 

 late, leaving an atmosphere of nitrogen. In some cases 

 a vacuum may be used satisfactorily. 



Effect of Kinds of Nutrients Present. — Bj careful ex- 

 perimentation it is usually possible to find that combina- 

 tion of nutrient materials which will allow of the most 

 rapid development of a particular microorganism. The 

 substances needed and the proportions vary greatly with 



