20 



Dairy Bacteriology. 



patches are called colonies and are composed of infinite 

 numbers of individual germs, the result of the continued 

 growth of the single organism that was in the drop of milk 

 which was firmly held in place when the gelatin solidi- 

 fied. The number of these colonies represent in gen- 

 eral 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 continue to grow and increase in 



FIG. 3. Profile view of gelatin plate culture. Shaded part represents the 

 gelatin medium in the covered glass dish; on the surface, different bacteria 

 are developing; b, is a liquefying form that dissolves the gelatin while c and d 

 grow on surface only and do not render gelatin soluble. 



size, and as they do, minute differences will begin to ap- 

 pear. These differences may be in the color, the con- 

 tour and the texture of the colony, or the manner in 

 which it acts toward gelatin. (See fig. 3.) In order 

 to make sure that the seeding is not too copious so as to 

 interfere with continued study, an attenuation is usually 

 made. This consists in taking a drop of the infected 

 gelatin in the first tube, and transferring it by means of 

 a sterile needle into another tube of sterile media. Usu- 

 ally this operation is repeated again so that these culture 

 plates are made with different amounts of seed with the 

 expectation that in at least one plate the seeding will not 

 be so thick as to prevent further study. 



To further study the peculiarities of different germs, 

 the separate colonies are transferred to other sterile tubes 



