TEMPERATURE AND BACTERIAL FLORA 109 



milk may, in six days, actually contain fewer bacteria than when 

 fresh. 



(3) After this preliminary period, there always follows a 

 multipUcation of bacteria; but the types that develop differ 

 so markedly, that samples of the same milk kept at different 

 temperatures are, at later periods, very different in their bac- 

 terial content, even though they contain the same number of 

 bacteria. 



(4) The development of the ordinary lactic species Bact. 

 lactis acidi (Str. lacticus), in practically all cases checks the 

 growth of other species of bacteria, and, finally, kills them, 

 since the bacteria regularly decrease in actual numbers after 

 the lactic bacteria have become very abundant. 



(5) In practically all samples of milk kept at 20° C, the 

 multipHcation of the Str. lacticus * begins quickly and pro- 

 gresses with great rapidity. They grow so rapidly that they 

 produce acid enough to curdle the milk in about forty hours, 

 the growth of other species being held in check. Milk when 

 curdled at this temperature shows a smooth acid curd, with no 

 gas bubbles. 



(6) A totally different result appears in milk kept at 37° C. 

 The results are somewhat more variable than at 20° C. Occa- 

 sionally the Str. lacticus grows vigorously at this temperature, 

 but the common result is a development of the B. lactis serogenes 

 type. It forms a curd full of gas bubbles. If B. coli communis 

 is in the milk, this also grows luxuriantly at 37° C. 



(7) In milk kept at 10° C, neither of the types of bacteria 

 seems to be favoured. The delay in growth lasts two to three 

 days, after which all types of bacteria appear to develop some- 

 what uniformly. Sometimes the lactic bacteria develop 

 abundantly, sometimes only slightly. The neutral bacteria 

 always grow rapidly, and the Uquefiers in many cases become 

 abundant. In time, the milk is apt to curdle, commonly with 



* Str. lacticus has been substituted for B. lactis acidi (Hueppe) in 

 order to avoid confusion with B. acidi lactici (Escherich). 



