BEST METHOD FOR ESTIMATING BACTERIA IN MILK 495 



at least in part, to agglutination. The germicidal action of milk is 

 specific. This action in milk and blood serum resemble each other 

 in some particulars, but blood serum acts more quickly and much 

 more powerfully than milk. Heating milk above 80 C. destroys its 

 germicidal properties. The effect of lesser degrees of heat varies 

 with the microorganism. Thus the restraining action for Bacillus 

 lactis aerogenes is weakened by first heating the milk at 55 C. and 

 almost destroyed at 60 C." 



While raw milk, therefore, shows a diminution of bacteria during 

 the first hours after collection, a rapid multiplication occurs later, 

 particularly if the milk has not at once been cooled and kept at a low 

 temperature permanently until used for consumption. If this has 

 been done the number of bacteria does not increase much for thirty- 

 six hours, but milk, according to Park, contains many species of 

 bacteria which will multiply even at 39 F., i. e., at a temperature not 

 much above the freezing point of water. Park found that a specimen 

 of milk containing originally 3000 bacteria per cubic centimeter, kept 

 at 32 F., showed a decrease of microorganisms after seven days; 

 this was also true in a specimen containing originally 30,000 per cubic 

 centimeter. At 39 F. the counts, after seven days, showed 4 and 38 

 million respectively; at 42 F., 11 and 120 million; at 50 F., after four 

 days, 12 and 300 million, and at 60 F., after two days, 28 and 163 

 million. These figures and those ascertained by various observers, 

 including Conn, Esten, Harrison, and others, demonstrate the great 

 influence of higher temperatures upon bacterial multiplication in 

 milk. Since milk shipped to cities by farmers and dairies is frequently 

 insufficiently cooled, market milk often shows a high count even in 

 samples which have been collected with reasonable care and which 

 do not show much dirt contamination. Rosenau found in the milk 

 of Washington, D. C., an average of 22,134,000 bacteria per cubic 

 centimeter during the summer of 1906. Commenting upon these 

 figures he says : 



"So far as numbers are concerned they need not greatly alarm 

 us, for we know that disease is due to agencies and conditions other 

 than merely the presence of enormous numbers of bacteria. By 

 universal consent, however, milk containing excessive numbers of 

 bacteria is unsuitable for infant feeding. . . . As we grow older 

 it seems that the gastro-intestinal mucous membrane becomes com- 

 paratively immune or resistant to bacterial action. . . . The 

 number of bacteria in milk is not so important from a public health 

 standpoint as the kind and nature of the bacterial products. But 

 with cleanliness and the liberal use of ice, the total number of bacteria 

 can be kept down, and this affords a mode of protection against the 

 dangerous species and their toxic products. Milk containing few 

 bacteria will contain proportionately few or no harmful varieties." 



Best Method for Estimating Bacteria in Milk. The most approved 

 method of estimating bacteria in milk consists in inoculating a suitable 



