306 BACTERIOLOGY. 



The Culture of Anaerobic Bacteria. 



Obligative anaerobic bacteria, those which grow only 

 in the absence of oxygen, require special conditions for cul- 

 tivation. Their growth is favored by the addition of 1 to 2 

 per cent, of glucose to the nutrient medium, such as gela- 

 tin, bouillon or agar. Freshly prepared media are, as a 

 rule, best adapted for cultural purposes. 



The numerous methods which have been proposed for 

 obtaining growths of anaerobic bacteria can be classified 

 under the following heads: 



1. Exclusion of oxygen. 4. Displacement of air. 



2. Exhaustion of air. 5. Cultures apparently in air. 



3. Absorption of oxygen. 6. Microbic association. 



Exclusion of oxygen. Several different methods can be 

 grouped under this head. Thus, the earliest method em- 

 ployed was to cover the liquid with a layer of oil. Although 

 not perfect, yet this method of Pasteur can at times be em- 

 ployed to advantage. Later on, Koch endeavored to obtain 

 colonies under anaerobic conditions by placing a thin mica 

 sheet on the surface of a gelatin plate. Better results are 

 obtained by covering the gelatin or agar plate with another 

 sterile glass plate (Sanfelice). 



The well-known method of Liborius, culture in deep lay- 

 ers of gelatin or agar, depends upon the exclusion of air. 

 The method is simple and very convenient. The tubes 

 should contain glucose agar or gelatin, and the medium 

 should be about 2 inches deep. Stab cultures are made in 

 the usual manner. Growth develops along the line of in- 

 oculation in the lower two-thirds of the medium, while the 

 upper third (about ^ inch), serves to exclude the air. In 

 order to insure complete exclusion of oxygen, the contents 

 of an ordinary agar or gelatin tube can be liquefied and 



