THE GROWTH OF BACTERIA IN MILK 79 



which give ptomaine poisoning and possibly summer complaint 

 and cholera infantum. 



At 60. If the milk is warmed to from 60 to 70, the 

 growth of bacteria is very much more rapid. It is extremely 

 important to notice the type of bacteria specially favored by this 

 temperature is the normal, harmless, useful lactic acid bacteria 

 and Bact. lactis acidi. If milk is kept constantly at 60, the 

 lactic acid bacteria multiply far more rapidly than any other 

 species. Examinations of milk at intervals of an hour or two 

 show a constantly increasing proportion of this type. At the 

 outset the number of lactic acid bacteria is comparatively small, 

 frequently less than I per cent, and seldom as high as 10 per 

 cent. ; but they constantly increase hour after hour, and when 

 the milk is 24 hours old, the lactic acid bacteria are apt to com- 

 prise 50 per cent, of the whole. In another day, just about the 

 time the milk is ready to sour, the proportion of this type is 

 commonly over 95 per cent, and sometimes 100 per cent. In 

 other words, if milk is preserved in a temperature from 60 

 to 70, the lactic acid bacteria far outrun other species and soon 

 take their place. The acid which they develop seems to be 

 injurious to the other species, and these rapidly disappear as 

 the lactic organisms increase in numbers. As a rule, therefore, 

 milk n'hich sours at a temperature of 70 or thereabouts will 

 be found to contain nearly a pure culture of the common lactic 

 acid type, the other organisms that were, so abundant at the 

 outset having disappeared. 



For this reason we understand why lactic acid bacteria are 

 regarded as a dairy bacteria par excellence. Although this 

 species is not present in fresh milk in great numbers and con- 

 stitutes only a small proportion of the whole, yet by the time 

 the milk is 24 hours old, at ordinary temperatures, they have 

 so outnumbered the others that they constitute the larger pro- 

 portion of the bacteria present. 



