76 PRACTICAL DAIRY BACTERIOLOGY 



should always be considered in drawing any conclusion as to 

 the numbers of bacteria in milk. In ordinary municipal testing 

 of milk this feature is too frequently lost sight of, or, at least, 

 not properly considered. 



A single illustration will suffice to show the effect of the dif- 

 ferent temperatures upon the numbers of bacteria. A sample 

 of milk was examined for bacteria when fresh, and was found 

 to -contain 6,525 per c.c. It was then divided into two lots, one 

 preserved at 50 and the other at 70. 



GROWTH OF BACTERIA AT 50 AND 70 



Fresh milk 6,525 per c.c. 



25 hours old at 50 6,425 " " 



25 hours old at 70 . . ... . 6,275,000 " " 



The difference in this case is very striking. In these two 

 samples of the same milk one showed no more bacteria at the 

 end of 24 hours than at the beginning, while the other had 

 increased 1,000 fold. It is not common to find quite such a 

 marked difference, but in all cases the difference in the number 

 of bacteria in two samples thus tested is very great. It is im- 

 portant to note that these two samples of the same milk would 

 receive very different verdicts from any municipal examina- 

 tion. One would be passed as an exceptionally good milk, and 

 the other condemned as a very bad milk. But if the bacteria 

 in the fresh milk were the common harmless species, liable to 

 be found in milk, the sample which at the end of 24 hours con- 

 tained 6,000,000 would be just as wholesome as the one that 

 contained 6,500. In other words, the ordinary examination of 

 milk, which condemns milk containing more than a certain 

 number, fails to reach the desired end. It does not distinguish 

 between good milk, in which the harmless lactic bacteria have 

 become quite abundant, because of a few extra degrees of tem- 

 perature, and unwholesome samples of milk, in which the num- 



