STANDARD AGAR FOR ESTIMATION OF BACTERIA IN MILK 533 



10 cc. quantity of the standard agar so enriched the latter that 

 certain organisms could multiply to the extent of producing vis- 

 ible colonies while these organisms could not do so when only 

 0.001 cc. of milk was added. In other words, the addition to, 

 or elimination from, a tube of agar of 0.009 cc. of milk was suf- 

 ficient to change very materially the growth supporting qualities 

 of the medium. It appeared then that to keep the medium of 

 constant composition in all dilutions it was only necessary to 

 supply to the 1-1000 dilution and all higher dilutions enough 

 milk to make up the deficiency. Ths was done approximately by 

 making the higher dilutions in sterile 1-100 milk instead of in 

 sterile water. Table 2 gives the results of a preliminary test to 

 determine the efficiency of this improvement. In this initial test 

 the 1-100 dilution was plated out in duplicate, but only one 

 plate was made from each of the two 1-1000 dilutions. In a 

 second test, the results of which are given in table 3, all three 

 dilutions were plated out in duplicate. For convenience the 

 columns showing the counts on the 1-100 plates are marked "C" 

 and those showing the counts on the plates made from the 1-1000 

 dilutions in sterile water and sterile 1-100 milk are marked ^'Mi" 

 and ''M2" respectively. The samples employed in these experi- 

 ments were not selected samples. They were picked at random 

 from those brought in for examination. . 



Several important points are noticed on examination of the 

 figures in tables 2 and 3. In the first place it is observed that 

 in many samples the M2 count is considerably higher than the 

 Ml count and that the M2 counts check better with the C counts 

 than do the Mi figures. This establishes the validity of the 

 reasoning which was the basis of the experiment. The added 

 milk enhances the growth supporting qualities of the medimn. 

 It is interesting also to note that in the majority of cases in 

 which these differences appear the samples are those of pasteur- 

 ized milk. In going through our records to get the figures 

 given in table 1, samples were selected which best presented the 

 inconsistencies mentioned. It was not noticed until afterwards 

 that every sample was one of pasteurized milk. 



