276 PRACTICAL DAIRY BACTERIOLOGY 



small dots, each of which represents what is called a colony of 

 bacteria. They are supposed in all cases to be a mass of bac- 

 teria that have developed from a single individual in the orig- 

 inal milk which has been fixed in its position by the hardened 

 agar. If every bacterium in the original milk has thus grown 

 and produced a colony, it is only necessary to count the colonies 



FIG. 77 PETRI DISH 



in each agar plate to find the number of bacteria in 1/1,000 of 

 the original c.c. of milk product, which was the amount taken. 



Count the numbers on each plate; add them together and 

 divide by 4 to determine the average number. Multiply by 1,000 

 to give the number of bacteria per c.c. of the milk. 



For convenience in counting, the bottom of the plate may 

 be divided into small areas by marking with a wax pencil. If 

 the number is very great use a counting plate or counting cards. 

 These are divided into areas of i sq. centimeters. Place the 

 plate over the card and count the number of colonies on each 

 of several of the i cm. areas and obtain the average. Determine 

 by the card the number of such areas in the whole plate, and by 

 multiplying calculate the total number of colonies on the plate. 



The box shown in figure 78 will be found very convenient in 

 counting the colonies. It consists of a shielded lens, and a slate 



