Bacterial Multiplication 143 



000,000 square microns), and since each field of the microscope 

 has an area of 21,106 square microns, it would be necessary to 

 examine 4738 different fields of the microscope (21,106 divided 

 into 100,000,000) to see the entire area of milk spread out on the 

 slide. Since bacterial counts are expressed "per milliliter" of liquid 

 being examined, and only 0.01 ml. of milk was placed on the slide, 

 it would be necessary to count the organisms in one hundred such 

 areas of milk, or 4738 X 100, before a full milliliter would be ex- 

 amined microscopically. In round numbers the Microscope Factor 

 in the above problem would be 475,000 (4738 X 100). For every 

 cell, or clump of cells, seen in an average microscope field there 

 should be about 475,000 similar cells or clumps of cells, in the 

 entire milliliter of milk. 



Obviously, examining that number of fields is impractical. But 

 experience has shown that by counting the average number of or- 

 ganisms, or clumps of organisms, in thirty, fort}', or fifty fields, a 

 fairly representative count can be obtained for the entire smear. 

 The lower the bacterial count, the greater is the number of fields 

 that should be examined for increased accuracy. If the smear is 

 to be representative of the entire batch of milk, uniform, adequate 

 mixing of the milk samples is imperative. 



Because individuals have different ideas about what is meant by 

 adequate mixing, it has become necessary to standardize this proc- 

 ess for greater accuracy in determining bacterial counts. If one 

 person shook a milk sample ten times in a half-hearted fashion, 

 and someone else vigorously shook that same sample twenty times, 

 one would not expect to get the same results. Bacteria tend to 

 grow in clumps, and the more these aggregates are broken up by 

 shaking, the more individual organisms or smaller clumps can be 

 expected. The standard shaking technic calls for twenty-five 

 strokes (up and down constituting one stroke), in the space of 

 one foot, in an arc of ninety degrees (a quarter of a circle), in ex- 

 actly seven seconds. As specific as this description of shaking is, 

 variations do occur between individuals. Ask a dozen persons to 

 hold their hands exactly one foot apart, and the chances are that 

 you would get ten or a dozen different distances. The concept of 



