ADVANTAGES OF A CARBOHYDRATE MEDIUM 



483 



The data given in this table indicate that lactose agar is of 

 considerably more value than plain agar for the quantitative 

 bacterial analysis of milk. Of the eighteen samples examined, 

 fifteen gave higher counts on lactose agar; with one sample 

 there was no difference, while two samples gave counts which 

 showed a negligible advantage in favor of plain agar. If we con- 

 sider the experimental error as about 20 per cent, and discard 

 all differences of less than that amount, it will be seen that four- 

 teen, or 78 per cent, gave increased counts on the lactose agar, 

 while none of the samples showed a similar increase in favor of 

 plain agar. Taking all eighteen samples we find an average in- 

 crease of 43 per cent in the counts obtained with lactose agar. 



The results reported in table 1 are all from samples of raw 

 milk. In table 2 will be found the data obtained from six sam- 



TABLE 2 



Number of bacteria on plain and lactose agars 



Pasteurized Milk 



pies of pasteurized milk. Three of the samples were pasteur- 

 ized by the holder method at 140°C. for twenty minutes, while 

 the other three were treated by the flash method at 180°C. The 

 milks were plated within an hour after pasteurization. The re- 

 sults are very striking and demonstrate the superiority of lac- 

 tose agar over plain agar for the examination of freshly pas- 

 teurized milk. 



THE SIZE OF COLONIES ON PLAIN AND LACTOSE AGARS 



Although the difference in the counts obtained is sufficient 

 reason for advocating the use of a carbohydrate medium for the 



