GROWTH OF BACTERIA IN NORMAL MILK. 6 1 



in the milk kept at 13° would be less, but the figures in the 

 first column of Table 25 are very surprising. There was from 

 the sixteenth hour to the twenty-eighth hour, a period of 

 twelve hours, a decrease of 40,000; in the next twelve hours 

 an increase of about 11,000; and in milk which was fifty-two 

 hours old the number of bacteria was not quite twice as great 

 as it was in milk sixteen hours old. In other words, there was 

 practically no increase in total numbers of bacteria during the 

 period of nearly forty hours covered by the experiment. This 

 being the case, it was evident that we might expect to find 

 a considerable variation in the proportions of the different 

 species, for it is quite likely that some species would be found 

 to be favored by such low temperatures and would develop, 

 while others would be unfavorably affected. This expectation 

 is entirely borne out by the results, as shown in the tables 

 on pages 59 and 60. The variations shown in the different 

 species are as follows: 



2. The common B. acidi ladici apparently failed to develop 

 at 13°. Indeed, at first it rapidly decreased in numbers. At 

 sixteen hours there were present about 30,000, 21 per cent, of 

 the whole. There was no further development of this species; 

 in fact, the number fell off in the next twelve hours; and in 

 milk fifty-two hours old it was only 8,000, a little over a fourth 

 of what it was originally, and the proportion only about 3 per 

 cent. 



3. While the temperature of 13° was manifestly unfavor- 

 able to the development of B. acidi ladici, it was on the other 

 hand favorable to the development of the Streptococcus group, 

 which rose from 16 per cent, to 28 per cent. If this is com- 

 pared with the development at 20° it will be seen that the 

 Streptococcus group has been favored by the low temperature 

 more than by the high temperature, since in the sample at 

 20° there were only about 14 per cent, of these organisms. 



4. At 13° there is seen an unusual development of Sarcina 

 I. This micro-organism, which was present in the milk kept 

 at 20° in a proportion of only about 2 per cent., increased 

 extraordinarily in the sample kept at 13°, appearing in the 

 last test as high as 44 per cent. The large number given in 

 the test made at twenty-eight hours, 55 per cent., is probably 

 an error, due perhaps to a failure to differentiate Sarcina and 

 the Streptococcus; the other figures are probably reliable. 



