PUBLIC PROBLEMS 



and care of the milk production ; whereas variations in numbers 

 from half a million upwards to several millions must be attrib- 

 uted to differences in the age and temperature of the milk. 

 Furthermore, where we have very high numbers the reason 

 lies practically always in a too warm temperature rather than 

 any other factor. But these high numbers of bacteria usually 

 consist of the harmless lactic acid type, and the fact that the 

 numbers have run up to 20,000,000 may not detract in the 

 slightest degree from the wholesomeness of the milk. High 

 numbers of bacteria, therefore, do not necessarily mean un- 

 wholesome milk; while, on the other hand, milk may have a 

 small number of bacteria and yet contain enough distinct 

 specific disease germs to be decidedly dangerous. 



One other feature connected with the numerical standard 

 is to be noticed. The method used for determining the numbers 

 by no means gives them all. As stated on page 175, the number 

 is determined by diluting 1,000 times, plating upon agar and 

 incubating at 98. This really gives only a small part of the 

 bacteria present. If the plates are made on gelatin and in- 

 cubated at 70, much higher numbers will be found. For ex- 

 ample, a sample of milk diluted 1,000 times, plated on agar and 

 incubated at 37, gave 3,000,000 bacteria. The same milk diluted 

 10,000 times, plated at the same time on gelatin and incubated 

 at 20, gave 30,000,000. Usually there will not be found quite 

 so large a difference, but there is always a smaller number 

 found by the ordinarily adopted method than by using gelatin 

 and incubating at lower temperatures. Hence, a numerical de- 

 termination never gives the actual number of bacteria in the 

 milk, but only of the numbers that grow upon agar and at 

 temperatures of 98. 



Objections to a Numerical Standard. For these reasons it 

 becomes a serious question as to whether a milk standard is of 

 much value. There are several objections which arise against 

 its adoption. 



