COMPARATIVE GROWTH OF BACTERIA. IO5 



bacteria originally present. If milk is kept at 70 it is com- 

 monly about twenty-four hours before the bacteria reach 

 10,000,000 per c.c. If the temperature is higher this num- 

 ber is reached more quickly, and if lower it takes longer. At 

 its close the lactic bacteria are numerous, perhaps 30% to 

 50% of the whole, and the other species are also abundant, 

 most of them being more numerous than at the start, but 

 always comprising smaller percentages, owing to the great 

 growth of the lactic bacteria. 



Second Period. The second period is characterized by the 

 extremely rapid development of lactic bacteria, in most 

 cases of the Bact. lactis acidi type. During the first period 

 these bacteria have been accommodating themselves to the 

 milk and increasing in vigor, so that by the beginning of the 

 second period they are actually capable of more rapid growth 

 than at the start. So vigorous are they, that not only do 

 their actual numbers increase rapidly, but they seem to exer- 

 cise a checking action upon other bacteria. Their percentage 

 increases, at first because they grow more rapidly than any 

 other species, but eventually because the other species 

 actually decrease in numbers. Some of the other types dis- 

 appear, while others remain alive but without further growth. 

 It commonly happens that the liquefiers are crowded into 

 the background and disappear, and although at the close 

 of the first period they may compose 20% to 50%, they 

 may not be found at all at the close of the second period, after 

 the lactic bacteria have had the opportunity to develop. By 

 the time the milk has distinctly soured, the lactic bacteria may 

 be 70% of the whole, and later, when it curdles, they fre- 

 quently comprise over 90%, sometimes 99% of all the bac- 

 teria present. In other words, the lactic bacteria, and par- 

 ticularly the first type including Bact. lactis acidi have 

 shown such exceptional vigor in the milk as quite to have 

 distanced all other species, and sometimes seem to destroy 

 many of them. By the time the milk has become curdled it 



