CLASSIFICATION 37 



Nitrifying bacteria. These are the organisms which 

 take nitrogen out of the air for plant food. 



Aerobic and anaerobic bacteria. Bacteria are also 

 named according to their gaseous environmental require- 

 ments. Certain bacteria will not grow in the presence of 

 oxygen (atmosphere), and consequently they must be culti- 

 vated in a medium from which the air has been excluded, or 

 in the presence of some neutral gas such as hydrogen. While 

 certain bacteria, like those of symptomatic anthrax, tetanus, 

 and malignant oedema, require the absence of oxygen, 

 others, like Bacillus subtilis, will not multiply without it. 

 There are, however, a large number of bacteria which are 

 able to multiply independently of the presence or absence of 

 this element. In reference to oxygen requirements bacteria 

 are grouped as follows : 



Obligatory aerobic bacteria require oxygen. 



Obligatory anaerobic bacteria require the absence of 

 oxygen. 



Facultative aerobic bacteria grow best in the absence of 

 oxygen, but will grow in the presence of air. 



Facultative anaerobic bacteria grow best in the pres- 

 ence of oxygen, but will grow in its absence. 



Bacteria are also known by special names according to 

 the temperature best adapted for their multiplication. These 

 are : 



Psychrophilic bacteria. These are bacteria that re- 

 quire a low temperature (15-20 degrees C.) for their multi- 

 plication. 



Mesaphilic bacteria. These are bacteria that require a 

 medium temperature (15-37 degrees C.) for their multipli- 

 cation. Several saprophytic and nearly all known patho- 

 genic bacteria are in this group. 



Thermophilic bacteria. These are bacteria that will 

 multiply in a high temperature (40-60 degrees C.). 



