CHAPTEE X 



BACTERIAL ESTIMATIONS TO DETERMINE THE GENERAL 

 CONTAMINATION OF MILK 



IN this chapter certain bacterial estimations are considered 

 which have for their object the measurement of the degree of 

 bacterial contamination of milk, and more particularly that 

 part of it which is manurial in origin. The significance of 

 such determinations is more fully considered in Chapter XIV. 



I. ESTIMATION OF THE NUMBER OF BACTERIA 



The estimation of the total number of bacteria in milk is- 

 impossible with present-day methods, and it becomes necessary 

 to select an arbitrary basis of enumeration. There are no 

 known nutrient media and no known conditions of growth 

 which will allow all the bacteria in milk to develop. All 

 that can be said is that some media and some conditions are 

 more favourable to the growth of a larger number of the 

 bacteria in milk than others, and by their employment a, 

 higher count is obtained than by the use of less favourable 

 media and conditions. Since all bacterial enumerations are 

 relative, and since not all bacteria in milk are of undesirable 

 significance, there is no particular reason why a medium 

 giving the highest counts should, of necessity, be preferred ta . 

 one allowing fewer bacteria to develop. 



The bacteria which we need to estimate in milk are those 

 whose addition is a measure of its undesirable pollution, and 

 an enumeration of the total number of bacteria in milk (even if 

 it were possible) is of itself a secondary matter. If this point 

 of view is accepted, it is possible to obtain guiding principles- 



177 N 



