40 Bacteria in Relation to Country Life 



development. They are designated as anaerobes, or 

 obligate anaerobes. The obligate anaerobes in the soil 

 include, among others, the cellulose ferments, the nitrp- 

 gen-fixing butyric ferments, and the lock-jaw bacillus. 



Organisms that will develop preferably with an 

 unhindered access of air, but also will grow when it is 

 excluded, are designated as facultative anaerobes. In 

 other words, they are both aerobes and facultative 

 anaerobes. A number of denitrifying bacteria are 

 prominent members of this class. It has been shown 

 that they will not destroy the nitrate when freely sup- 

 plied with oxygen, but that they will derive the latter 

 from the nitrate when atmospheric oxygen is excluded. 



Still other organisms are designated as facultative 

 aerobes, that is, they will grow, preferably, when oxy- 

 gen is excluded, but will exist also when it is present. 



Aerobes and anaerobes are found together in water, 

 soil, and elsewhere in nature, and there is no doubt 

 that the first of these facilitate the development of the 

 others by using up the oxygen in the medium. Such 

 relations evidently exist between aerobic and anaerobic 

 water and soil bacteria and account for the abundant 

 presence of anaerobic nitrogen-fixing bacteria even 

 in open and well- ventilated soils. 



Action of sunlight. — Direct sunshine exerts a more 

 or less destructive action on living cells. The destructive 

 action of direct sunlight is readily observable also in 

 the case of bacteria, sunshine being regarded as one 

 of the most potent forces in nature in the destruction 

 of pathogenic and non-pathogenic germs. A liquid 

 or solid culture of bacteria exposed in thin layers to 



