CHAPTER XI 



QUANTITATIVE BACTERIAL ANALYSIS OF MILK 



Significance of Bacterial Counts. — The bacterial count means 

 the numerical determination of the " growable bacteria" (i. e., 

 growable under the circumstances of the test as to medium, tem- 

 perature, humidity, darkness, presence or absence of oxygen, and 

 length of time allowed for growth) in the sample examined, ex- 

 pressed as " number of bacteria per c.c." 



Bacteria cannot be accurately counted directly under the 

 microscope; except when present in such immense numbers that 

 (Winslow; Slack) the merest estimate alone is required. Such a. 

 method is not practical for certified or even inspected milk, although 

 used with success for standards of 500,000; hence the count is 

 made by plating the original sample or dilutions therefrom, i. c. r 

 mixing them with melted nutrient jelly, pouring the mixture into a. 

 flat dish, and after cooling the jelly, allowing growth to take place. 

 The individual bacteria, held apart by the jelly, develop, producing 

 little aggregations of their descendants (colonies) ; and since each 

 bacterium produces but one such colony, a count of the colonies 

 indicates the number of the bacteria originally present. Thus, if 

 two hundred (growable) bacteria are present in the water, etc., 

 which is mixed with the melted jelly, 200 colonies, or one for each 

 bacterium, should grow in the plate. It has been found by experi- 

 ence and experiment that the ordinary " agar plate " will not support 



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