Frost—Bacteriological Control of Public Milk Supplies. 1343 
It has no doubt been recognized before that the colonies on 
plates from highly contaminated milks appear before those on 
plates from the better grades of milk, but no cognizance of it 
seems to have been taken of the possibility of using this as a 
means of judging the quality of a milk. The following data 
illustrate the facts in the case: 
Table VI.—RATE OF GROWTH OF MILK COLONIES ON AGAR 
AT 21° C. 
Number and character of Upper figures give number of bacteria per c.c. 
milk. Lower figures give percentage of total bacteria per c.c. 
1st day 
2nd day 
4th day 
5th day 
6th day 
143 Raw milk................. 
4,000 
89,000 
113,000 
120,000 
3*# 
74% 
94% 
100% 
144 Raw milk. 
175,000 
189,000 
204,000 
210,000 
83% 
90% 
97% 
100% 
146 Pasteurized.... 
0 
0 
14,000 
18,000 
35,000 
0% 
0 % 
40% 
51% 
100% 
147 Inspected (very good 
1 
milk)..... 
0 
10,000 
1 21,000 
24,000 
32,000 
0% 
31% 
| 65% 
75% 
100% 
If these figures are plotted the following graph is obtained. 
Fig. 11. A similar set is shown in a graphical form in Fig. 12. 
Since it would be too much of a burden to make daily counts 
over the greater part of a week, it seemed worth while to deter¬ 
mine whether two counts would not be sufficient. Forty-eight 
hours seems to be the crucial time, and the following graph, con¬ 
structed on counts made at the end of 48 hours and four days, 
gives the necessary data. (See Fig. 12.) 
From these results presented in Table YI. and graphs, Figs. 
11 and 12, it appears that it is possible to predicate the general 
characteristics of a milk by determining the rate of growth of 
its colonies on agar plates at 21° C. This phenomenon can be 
well brought out then by the following procedure: 
Agar plates are inoculated with a suitable dilution of a milk 
of unknown character and incubated at room temperature, about 
21° C. They should be counted daily, or at least after two and 
five days. The figures representing the percentage of colonies 
found on the second and ether intermediate clays are then de¬ 
termined by considering the fifth day as giving the total. In 
this wav it is ascertained whether or not the percentage on the 
second or other intermediate days is greater or less than 50 %. 
