Methods of Studying Bacteria. 



15 



six hours at the proper temperature, tiny spots will begin 

 to appear on the surface, or in the depth of the culture 

 medium. These patches are called colonies and are com- 

 posed of an almost infinite number of individual germs, 

 the result of the continued growth of a single organism 



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

 in a sample of milk. Each mass represents a bacterial growth (colony) derived 

 from 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 bacterium; c, and d t solid forms. 



that was in the drop of milk which was firmly held in 

 place when the gelatin solidified. The number of these 

 colonies represents approximately the number of germs 

 that were present in the milk drop. If the plate is not 

 too thickly sown with these germs, the colonies will con- 

 tinue to grow and increase in size, and as they do, minute 

 differences will begin to appear. These differences may be 

 in the color, the contour and the texture of the colony, or 



