DETERMINATION OF BACTERIA IN MILK 577 



exception of the certified milk the counts were very much higher, 

 the approximate percentage increase over the extract-agar count 

 ranging from 3 to 75,000 per cent. The colonies were very much 

 larger and could be readily counted. Particular care was taken 

 with the count of the colonies on the standard extract agar and 

 we believe our counts are higher than would have been the case if 

 the plates had been counted in the average laboratory, because 

 when there was a question of doubt about a colony it was always 

 included. This was done to avoid favoring the media in which 

 we were interested. The greatest differences in count were 

 usually found in pasteurized milk. 



As to the counts on milk-powder agar and infusion agar (1910 

 standard) it will be noted that of the 36 samples of milk plated, 

 in 22 cases the counts were higher on the milk-powder agar. 

 In some cases they were practically the same or within the limits 

 of error, but in othei cases they were distinctly higher. 



It seems unnecessary to discuss the results further, for a study 

 of the figures in the table will show clearly the remarkable increase 

 in count obtained on milk-powder agar over that from the 

 standard extract agar. 



If the bacteria were not in the milk they would not have been 

 on the milk-powder agar plates; and if one is determining the 

 number of bacteria in a sample of milk one must, if possible, 

 use a medium on which they will grow. 



The fact that the medium shows high counts is not its only 

 merit. It is possible with this medium to obtain a direct count 

 of the number of peptonizing bacteria, of strong acid-forming, 

 and of weak acid-forming bacteria in a sample of milk. The 

 medium has been purposely arranged so as to be slightly cloudy 

 after sterilization. After forty-eight hours' incubation, colonies 

 of strong acid-forming bacteria have a white cloud about the 

 colony, while the peptonizers have a clear zone, after the plate 

 has been flowed with acid. Even though the plates are white, 

 due to the precipitation of the dissolved casein through acid 

 formation, it is easy to count the colonies. The colonies of 

 weak acid-forming bacteria can be determined by adding enough 

 brom-cresol purple indicator to cover the plate. This should 



