RELATION TO ENVIRONMENT 131 



the density of culture medium may have its tonic effect on the 

 bacteria. Certain bacteria that are accustomed to a con- 

 siderable concentration will be affected by a diminution in the 

 degree of concentration* and some forms will move towards or 

 away from a different concentration from that to which they 

 are accustomed. 



Oxygen. It is a general law of physiology that all life 

 must breathe. In 1861 Pasteur discovered a microorganism 

 which grew best in the absence of the free oxygen of the air, 

 and since that time we have come to divide bacteria into two 

 classes, so far as their relation to the free oxygen of the air 

 is concerned. One class, known as the aerobes, lives in the 

 free oxygen of the air and are comparable, in their oxygen re- 

 quirements, to ordinary forms of life. The other class, known 

 as the anaerobes, grows best in the absence of free oxygen, and 

 some of them grow only when free oxygen is removed. It is 

 not to be understood, however, that these anaerobic micro- 

 organisms are an exception to the general rule that all life 

 must breathe. These forms do breathe. The difference is the 

 source of their oxygen. The anaerobes take their oxygen 

 from organic compounds rather than from the free oxygen of 

 the air. Some of the bacteria can grow under both aerobic 

 and anaerobic conditions ; some grow best, for instance, under 

 aerobic conditions, but have the faculty of growing under 

 anaerobic conditions. Some bacteria grow best under anae- 

 robic conditions, but have the faculty of growing under aerobic 

 conditions. So that in reality we have four classes of bacteria 



