XIV BACTERIAL POLLUTION OF MILK 231 



examined after only a short period from milking, or perhaps 

 taken at a cooler time of the year. 



While this objection applies to some of the other bacterial 

 tests for pollution, it is particularly operative for this estima- 

 tion, since ordinary milk is so bacterially complex. The 

 experiments recorded in Chapter IV. show that there is 

 no regularity in the multiplication of bacteria in milk, even 

 under known conditions of time and temperature. It is not 

 in any way possible to gauge the amount of external pollution 

 which has been added to milk from the estimation of the 

 number of bacteria in chance samples of vended milk. 



Estimation of the Number of Bacillus coli and allied 



Organisms 



In seeking for a reliable measure of the degree of manurial 

 pollution of milk, attention is naturally directed to the group 

 containing B. coli and allied organisms. The writer, in his 

 book The Bade^^iological Examination of Water - Supplies 

 (1906), defined the conditions of a perfect bacterial indicator 

 as the following : 



1. It should be abundant in the substances for which its 

 presence serves as an indicator. 



2. It should be absent, or at least relatively absent, from 

 all other sources. 



3. It should be easily isolated and numerically estimated. 



4. Its characteristics should be definite and not liable to 

 variation, whereby its distinctive characters might be impaired. 



B. coli, as an indicator of the manurial contamination of 

 milk, fulfils these conditions very adequately, as far as samples 

 collected at the cowsheds are concerned. 



B. coli is extremely abundant in manure, while it is 

 absent from sources not subject to manurial pollution. 



It has been shown in earlier chapters that B. coli and 

 other glucose fermenters are not found in milk drawn direct 

 from the teat, and that they are absent in milk samples 

 collected under conditions of great cleanliness. 



B. coli and allied organisms can be readily isolated and 

 numerically determined, while their characters can be defined 

 with sufficient accuracy. 



