GROWTH OF BACTERIA IN NORMAL MILK. 5 1 



that had not been iced contained i ,000,000,000 per cubic centi- 

 meter. This result is quite unexpected, and, although in strict 

 accordance with previous and subsequent experiments, its sig- 

 nificance is at the present time unknown. That the souring 

 of milk is due to the growth of bacteria has been abundantly 

 demonstrated, but why it is that at the time of curdling a 

 sample of milk that has been previously iced contained only 

 about one-third as many bacteria as a similar sample of milk 

 that had not been iced is a question to which we can at present 

 give no answer. 



3. The icing of the milk appears to have an influence upon 

 the species of bacteria that develop. As may be seen from the 

 tables the iced milk contained up to the thirty-eighth hour a 

 larger variety of bacteria than was present in milk that had 

 not been iced. This may have been due, however, to the fact 

 that the dilution was comparatively greater in the case of iced 

 milk than in the case of the non-iced milk and, therefore, that 

 a careful differentiation of species was more easily made. 



4. The species B. acidi ladici II. was present in this sample 

 of milk in considerable abundance and, as shown by the tables, 

 increased more rapidly in the milk that had not been iced than 

 it did in the iced milk. In the former there were 27 per cent, 

 at the time of curdling, in the latter only 4.5 per cent. The 

 icing thus apparently checked the development of this partic- 

 ular species. 



5. The icing apparently had no influence upon B. aerogenes, 

 which continued slowly to develop in numbers in both samples 

 of milk, but in no case became over i per cent, of the whole. 



6. The Streptococcus group developed in absolute numbers 

 and in proportionate numbers for thirty-eight hours and after- 

 wards decreased. The increase in numbers and proportions 

 for the first period of the experiment is in accordance with the 

 results given on previous pages. The decrease in the numbers 

 in the later periods is interesting and suggestive. It will be seen 

 further that the numbers of streptococci in this experiment 

 were large, in one case being no less than 61 per cent. This 

 large number was noticed in both the iced and the non-iced 

 milk. In the milk that had not been iced, however, the num- 

 bers then fell off rapidly, and in the iced milk they also fell off 



