64 



DAIRY BACTERIOLOGY 



may thus be infected with streptococci not only direct from the 

 manure, but also from the bedding and the dust from the fodder. 

 The milk pails are equally important as sources of infection, for 

 they will as a rule be impregnated with lactic acid bacteria. These 

 bacteria may also be introduced by flies. 



The bacteriological state of milk will be determined at the 

 outset according to the degree of care with which it has been 

 handled, and the nature of the bedding and the feed. If, how- 

 ever, milk is kept for any length of time, the temperature at which 

 it is kept will play an all-important part as it is the factor which 

 determines which groups of microorganisms shall develop in 

 preference to other groups. Not only the various groups of 

 bacteria, but also the milk itself must take part in the struggle for 

 existence, for milk contains bactericidal substances which, however, 

 are gradually weakened in their action, presumably by the bacteria 

 themselves ; only at low temperatures, at which bacterial develop- 

 ment is inhibited to a considerable extent, can these substances 

 retain their activity for any length of time, and at these tem- 

 peratures it is found that the number of organisms decreases at 

 first instead of increasing. In the following table are given the 

 numbers of organisms after twenty-four and forty-eight hours in 

 the same milk kept in sterile flasks at different temperatures. The 

 milk originally contained 84,000 organisms per cubic centimetre, 

 of which 2,000 were liquefying. 



* The milk was clotted. 



It will be seen that at C. the number of microorganisms was 

 lowered in twenty-four hours from 84,000 to 52,000 owing to the 

 action of the bactericidal constituents of the milk. At the same 

 time the number of liquefying bacteria had increased, and the 



