PART II. 



BACTERIA IN RELATION TO MILK. 



CHAPTER IV. 

 CONTAMINATION OF MILK. 



29. Milk as a food for bacteria. The fact that 

 milk so readily undergoes decomposition changes shows 

 that it is well suited for the nourishment of bacterial 

 life. Its high content in organic matter, and the dilu- 

 tion of the same in a watery medium makes it an excel- 

 lent food for germ as well as mammalian life. Its dif- 

 ferent constituents, however, possess different nutritive 

 values. Of most importance are the nitrogen-containing 

 compounds. 



The albumen which is in solution is readily available. 

 Casein can not be appropriated on account of its insol- 

 uble nature, unless it is first rendered soluble, a pro- 

 cess which occurs with those bacteria that secrete en- 

 zymes that act on proteids. 



Of the constituents of the milk that belong to the car- 

 bon-containing compounds, only one can be utilized by 

 bacteria. The fat possesses but little food value for these 

 organisms, because it cannot be decomposed by them. 

 The milk sugar, however, is an admirable food for many 

 species, more especially those that are known as the lac- 

 tic acid producing or natural milk bacteria. 



The bacterial cell contains so little mineral matter that 

 the requirements of the cell for its growth are very lim- 

 ited, yet the mineral elements of the milk are needed for 

 the growth of any protoplasm, and are used by the bac- 

 teria in the formation of new cell matter. 



[24] 



