24 Dairy Bacteriology. 



cells, the result of the continued growth of a single or- 

 ganism that was in the drop of milk and which was 

 firmly held in place when the gelatin solidified. The 

 number of these colonies represents approximately the 

 number of living bacteria that were present in the 

 amount of milk added to the tube of gelatin. If the 

 plate is not too thickly sown with the bacteria, the col- 

 onies will continue 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 the manner in which it acts 

 toward gelatin. 



In order to make sure that the number of colonies is 

 not so numerous as to prevent counting and further 

 study of their characteristics, a series of plate cultures 

 is usually made in which varying amounts of milk are 

 added to the tubes of gelatine. This is attained by add- 

 ing a definite amount of the milk or other substance 

 to be examined to a measured amount of sterile water, 

 e. g., one cubic centimeter of milk to ninety -nine cubic 

 centimeters of water. One cubic centimeter of this mix- 

 ture may be used for the inoculation of the plate culture. 

 This dilution may be carried on to any desired extent; 

 in the examination of many dairy products, it^is neces- 

 sary to use very minute quantities of material, often 

 only one one-millionth of a cubic centimeter. 



To study further the peculiarities of the different bac- 

 teria, small portions of the individual colonies are trans- 

 ferred to tubes of sterile culture-media. In order to do 

 this the colony is touched with a piece of platinum wire; 

 the min" ;e amount of growth that adheres to the wire 

 is sufficic. t to seed the tube of fresh culture-medium. 

 The inoculating needle must always be sterilized before 

 use by passing it through a gas flame. 



