98 Report of the Department of Bacteriology of the 



as previously described. Here it is plainly evident that a much 

 wider discrepancy exists between the two counts where the bacterial 

 content of the milk is low. Thus the ratios show that when the plate 

 count averages less than 10,000 colonies per cubic centimeter the total 

 number of individual bacteria seen with the microscope is approxi- 

 mately 44 times as great. The relative difference between the two 

 counts constantly becomes less as the numbers of bacteria increase. 

 So great is the decrease in the difference that, when the number of 

 colonies growing on the agar plates approaches 1,000,000 per cubic 

 centimeter, the total number of individual bacteria by the micro- 

 scopic count is only approximately 5 times as great. The relative 

 difference between the two counts appears to grow less as the counts 

 grow still higher but too few samples of this sort were examined 

 to warrant a positive statement. 



The ratios which exist between the plate count and the micro- 

 scopic count when each isolated bacterium and each clump are 

 counted as individual objects are similar to those noted above, 

 except that the differences between the two counts are much less. 

 When the microscopic count is made in this way the number of 

 bacteria per cubic centimeter is only about 16.8 times as great as 

 the plate count when this averages less than 10,000 per cubic centi- 

 meter. The relative difference between the two counts likewise 

 rapidly diminishes as the number of bacteria increases so that the 

 plate count is slightly larger when milk containing approximately 

 1,000,000 bacteria per cubic centimeter is examined. This con- 

 dition is probably explained by the fact that the colonies on solid 

 nutrient media originate either from single bacteria or from clumps. 

 The mechanical breaking up of the clumps in diluting the milk to 

 prepare the petri plates causes the plate count to be slightly greater 

 than the microscopic count under these circumstances. See Table X. 



The two striking points which should be emphasized are (1) that 

 as the numbers of bacteria in raw market milk increase the relative 

 differences between the counts by the two methods decrease, and (2) 

 that after the bacteria in raw market milk have increased to a 

 certain number practically all of them grow on nutrient media 

 when incubated at ordinary temperatures. In those cases where all 

 of the bacteria grow, there can be no dead bacteria present. 



The wide discrepancy between the two methods of counting when 

 the numbers of bacteria are low is probably explained as follows: 



