176 PRACTICAL DAIRY BACTERIOLOGY 



firmed by the test of the plating. If, however, the number of 

 bacteria in a single microscopic field is not more than 20 to 30, 

 the plating is unnecessary for determining that the number of 

 bacteria is within a 500,000 per c.c. standard. 



Significance of the Milk Standard. When we consider the 

 real significance of determining the number, we are forced to 

 conclude that it is less than could be desired, and indeed it be- 

 comes questionable whether as now done its real value is worth 

 its cost. The number of bacteria in any sample of milk depends 

 upon three factors and not upon one. It is true that the original 

 filth or cleanliness of the dairy, together with the care in pro- 

 ducing the milk, is a factor contributing thereto, but the age 

 of the milk and the temperature at which it is kept are, also, 

 important. Indeed, as we have seen, the age and temperature 

 are of more significance in determining the number of bacteria 

 in milk than is the question of the original cleanly or filthy 

 conditions. It is true that, even if fresh samples of milk are 

 examined, the number of bacteria in the different samples will 

 vary. Milk produced under good conditions may have as few 

 as from 5,000 to 10,000 per c.c., while that produced under poor 

 conditions may have from 100,000 to 200,000, or in some cases, 

 perhaps, half a million per c.c. In other words, the cleanliness 

 of the original conditions of milk production will determine 

 variations in the number of bacteria in the milk when strictly 

 fresh, ranging from a few thousand to a few hundred thousand. 

 But, on the other hand, age and temperature run these few 

 thousands up into millions, and it is not uncommon to find the 

 number of bacteria rising as high as from 10,000,000 to 20,- 

 000,000 per c.c. Hence, the high numbers present in our city 

 milk are dependent upon the age of the milk and the temper- 

 ature at which it has been kept rather than upon the cleanliness 

 of the conditions in which the milk is produced. Roughly 

 speaking we may say that variations in number below 200,000 

 or 300,000 are due primarily to differences in the cleanliness 



