REPORT OF THE DEPARTMENT OF 

 BACTERIOLOGY. 



A COMPARISON OF THE MICROSCOPICAL 



METHOD AND THE PLATE METHOD OF 



COUNTING BACTERIA IN MILK.* 



JAMES D. BREW. 

 SUMMARY, 

 i. There is little relationship between the results obtained by the 

 plate method and the direct microscopic method of counting bacteria 

 in milk, when used for determining the number of bacteria in single 

 samples of fresh, unpasteurized milk. There is, however, a relation- 

 ship between the two counts when series of samples are examined. 

 The count obtained by the microscope is almost invariably much 

 higher and is probably the more accurate of the two. 



2. The relative differences between the two counts are greater 

 where the bacteria are few in number. In samples of milk showing 

 plate counts of less than 10,000 per cubic centimeter, the count by the 

 microscope shows approximately 44 times as many individual 

 bacteria; or 17 times as many when the clumps and isolated bacteria 

 are counted as units, individual bacteria in the clumps not 

 being counted. In those samples which give a plate count 

 of about 1,000,000 per cubic centimeter, the count made with the 

 microscope shows approximately 5 times as many individual bac- 

 teria; or when the isolated bacteria and clumps of bacteria are 

 counted as units the number of these units is slightly less than 

 the number of colonies given by the plate method. 



3. The difference between the plate count and the total number 

 of individual bacteria according to the microscopic count is greater 

 than the difference between the plate count and the microscopic 

 count when the isolated bacteria and clumps are counted as indi- 

 vidual objects. This is undoubtedly due to the fact that a colony 

 on an agar plate develops either from a clump of bacteria or from a 

 single bacterium. 



4. In raw market milk practically all of the bacteria are alive 

 and are adapted to growth on lactose agar incubated at 21 degrees 

 C. when there are 1,000,000 or more per cubic centimeter. 



5. The microscopic method of counting bacteria in milk has these 

 decided practical advantages: The number of bacteria can be 

 shown in a given sample of milk within a very few minutes. The 

 apparatus required is less expensive than that required for the 



•Reprint of Bulletin No. 373, February; for Popular Edition see p. 893. 



[79] 



