260 BACTERIOLOGICAL ANALYSIS OF MILK. 



upon the culture medium they grow and multiply, but, unable 

 to move through the solidified medium, the descendants of 

 each bacterium remain together in a mass and, in the course 

 of two days, become abundant enough to produce a spot 

 which can be seen with the naked eye. The plate, therefore, 

 becomes dotted over with little points of various size and 

 shapes known as bacteria colonies. It is only necessary, 

 therefore, to count the number of colonies on one of these 

 plates and we know approximately the number of bacteria 

 that were present in one one-hundredth of a cubic centimeter 

 of the original milk; and multiplying the number by one 

 hundred we get the number of bacteria per cubic centimeter 

 of the milk. In counting these bacteria on the plate it is some- 

 times necessary to use various devices for dividing the plate 

 into areas. If the number is small they can be counted 

 without difficulty, but if the number of colonies on the plate 

 is large it is more convenient to place underneath the plate 

 a piece of black paper with white lines ruled upon it, dividing 

 the plate into a series of sections of equal size. Such cards 

 for aiding the counting can be obtained from dealers in bac- 

 teriological material, and a quantity of them should be at 

 hand in every laboratory to assist in the counting. If the 

 numbers are not very great the counting may be done without 

 the aid of these slips, by simply marking the under side of 

 the petri dishes with a waxed pencil, and thus dividing the 

 plate into a series of sections which may be counted individ- 

 ually. The actual determination of the number of bacteria 

 on these plates is not difficult, though it requires a little 

 practice. 



The number thus obtained represents approximately the 

 number of bacteria in a cubic centimeter of the original milk, 

 but the number is only an approximate one. Different 

 tests of the sample of milk will show considerable irregulari- 

 ties, and it is for this reason that six petri dishes have been 

 made. Each of the six should be counted and the average 



