18 Agricultural Bacteriology. 



seen by the naked eye is soon obtained. If, for exam- 

 ple, the number of bacteria in a sample of water is to be 

 determined, the conditions mentioned above are ob- 

 tained in the following way. A definite amount of 

 water is intimately mixed with a small amount of beef- 

 broth to which gelatin has been added. This furnishes 

 a medium which is solid at ordinary temperatures, but 

 which can be easily melted, and by cooling changed 

 back to a solid again. If the gelatin is at once cooled 

 after the water has been mixed with it, the bacterial 

 cells will be held in place in the now solid medium. The 

 gelatin is then placed under favorable conditions for 

 the growth of the bacteria. The cells begin to increase 

 in number, and as their progeny can not move away, the 

 resulting mass of cells soon becomes large enough to be 

 recognized by the unaided eye. In liquid media, such 

 as beef -broth, the bacteria are not held in place and the 

 liquid becomes uniformly turbid because of the distri- 

 bution of the bacteria in it. 



Each of the masses of growth, technically called a 

 "colony," is the progeny of a single cell. Thus, if the 

 colonies are counted, the result will be the number of 

 bacterial cells present in the substance at the time the 

 cultures were made. In order to make the counting of 

 the colonies easy the mixture of gelatin and water is 

 placed in a shallow glass dish so as to form a thin layer of 

 solid gelatin on the bottom of the dish. It is protected 

 from the bacteria of the air by a glass cover. 



It is essential if one is to determine how many bac- 

 teria are present in the amount of water used, that each 

 of the colonies on the culture plate is the result of the 

 growth of an organism present in the water. This ne- 

 cessitates that the food medium, the glass dish and 



