232 MILK AND THE PUBLIC HEALTH chap. 



As regards the characters considered to be essential to 

 constitute B. eoli, considerable divergence is noticeable between 

 different workers. The problem is less acute than is the 

 case when determining the significance of these organisms in 

 water, since in the latter allied organisms may be derived from 

 soil or other comparatively harmless sources. In milk, on the 

 other hand, B. coli and the closely allied forms all, as far as 

 we know, equally indicate outside manurial pollution. It is 

 also possible that certain varieties of B. coli, for example those 

 which ferment or do not ferment dulcite, saccharose, etc., are 

 of greater significance as manurial indicators than other 

 strains, but this has not yet been established. 



This estimation, as regards samples taken at the byre, is 

 a very direct and valuable method of measuring the degree and 

 extent of manurial contamination. 



The value of the enumeration for chance samples of ordin- 

 ary vended milk involves matters of great complexity, and it 

 is a problem of great difficulty to correctly gauge the sig- 

 nificance of their presence. If it were a practicable and univer- 

 sally adopted procedure to thoroughly cool milk after collection, 

 and then to maintain it until sold at a temperature so low that 

 B. coli will not multiply in it, any standards framed for B. 

 coli in byre milk would be equally applicable to milk as sold. 

 Under present conditions this is by no means the case, and it 

 becomes a very complicated problem to say what number of 

 B. coli may be allowed in vended milk samples as sold under 

 present-day conditions. 



The solution of the problem turns upon the rate of multipli- 

 cation oiB. coli in milk under varying conditions of temperature 

 and bacterial content. A number of experiments dealing with 

 this matter have been considered in Chapter IV., but certain 

 theoretical considerations bearing upon the question of definite 

 standards may be conveniently discussed here. 



The actual number of B. coli in milk samples will depend 

 upon five factors : 



(a) The initial number added at the time of milkino- 



(&) The temperature at which the milk has been kept. 



(c) The time since milking. 



{d) The other kinds of bacteria present, and their relative 

 proportion in the sample. 



