108 BACTERIA IN MILK 



to gradually decrease and a few days later it was impossible 

 to make an accurate estimation of their number owing to the 

 overgrowth of acid producers. The inert group developed 

 well during the first period and, after a reduction at the tenth 

 day period, persisted to the end of the experiment. The sample 

 ultimately developed a proUfic growth of torulse. 



In considering the relative development of various groups 

 in milk, due regard must always be given to the two important 

 factors, viz., temperature and initial content, that determine 

 the results. 



The effect of temperature was carefully investigated by 

 Conn and Esten,^^ who plated out practically fresh milk usually 

 containing 20,000 bacteria per c.cm. on litmus lactose agar and 

 found that they were able to distinguish no less than 15 different 

 groups merely by their macroscopic appearance. They made 

 two series of experiments, the first at 37° C, 20° C, and 10° C. 

 and the second at 20° C, 10° C, and 1° C. The plating inter- 

 vals were: 



37° C. at 2 hour intervals 



20** C. at 6 hour intervals 



10° C. at 12 hour intervals 

 1° C. at 1 day intervals. 



The main conclusions, as summarised by Conn and Esten, 

 were: 



(1) The effect of variations of temperature upon the devel- 

 opment of different species of bacteria in milk is not always the 

 same under apparently identical conditions. In spite of such 

 variations, there seems to be clearly discernible a normal 

 development of bacteria associated with different tempera- 

 tures. 



(2) There is, in all cases, a certain period at the beginning 

 when there is no increase in the total number of bacteria. 

 During this period some species are multiplying whilst others 

 are apparently dying. The length of this period depends upon 

 the temperature. At 37° C. it is very short, while at 10° C. 

 it may last from six to eight days, since, at this temperature, 



