Methods of Studying Bacteria. 



19 



27. Methods of isolation. Suppose for instance one 

 wishes to isolate the different varieties of bacteria found 

 in milk. The method of procedure is as follows: Sterile 

 gelatin in glass tubes is melted and cooled down so as to 

 be barely warm. To this gelatin which is germ-free a 

 drop of milk is added. The gelatin is then gently shaken 

 so as to thoroughly distribute the milk particles, and 

 poured out into a sterile flat glass dish and quickly cov- 



b--- 



FIG. 2. A gelatin plate culture showing appearance of different organisms 

 from a sample of milk. Each mass represents a bacterial growth (colony) de- 

 rived froia a single cell. Different forms react differently toward the gelatin, 

 some liquefying the same, others growing in a restricted mass, a, represents a 

 colony of the ordinary bread mold; b, a liquefying organism; c, and d, solid 

 forms. 



ered. This is allowed to stand on a cool surface until 

 the gelatin hardens. After the culture plate has been 

 left for twenty- four to thirty- six hours at the proper 

 temperature, tiny spots will begin to appear on the sur- 

 face, or in the depth of the culture medium. These 



