CULTURES, AND THEIR STUDY. 



209 



it is seen to touch the colony and take part of its con- 

 tents away. In this maneuvre the wire must not touch 

 the objective, the glass, or anything except the colony. 

 Having secured the adhesion of a few bacteria to the 

 sterile wire, the pure culture is made by introducing 

 them into a sterile culture-medium. 



If the pure culture is to be made in bouillon, the tube 

 is held obliquely, so that when the cotton plug is cau- 

 tiously removed no germs can fall in from the air. The 

 plug is removed by a twisting movement. The wire, with- 

 out being allowed to touch the mouth or sides of the 

 tube, is plunged into its 

 contents and stirred about 

 until the bacteria are de- 

 tached, and is then re- 



FlGS. 31, 32. — The various appearances of colonies of bacteria under the 

 microscope : a, colony of Bacillus muscoides (Liborius) ; b, colony of Bacillus 

 anthracis (Fliigge). 



moved and the plug replaced. The wire should be im- 

 mediately sterilized by heating to incandescence, lest the 

 bacteria be pathogenic and capable of doing subsequent 

 harm. 



If the culture is to be made in gelatin, a different 

 method is employed. The tube is either held horizon- 

 tally, or, as is perhaps better, inverted ; the cotton plug 



14 



