EXAMINATION OF WATER, AIR, AND MILK 119 



meyer flask and contents are sterilized, the tube d is con- 

 nected, by means of the rubber hose e, with g, and the 

 plug at c is removed. By opening the pinchcock i, 5 

 liters of air are aspirated through the broth in flask a. 

 The flask is then disconnected, and i. c.c. is plated in 

 agar and i c.c. in gelatin. The former is incubated at 37", 

 and the latter kept at room temperature. After 48 hours 

 the colonies are counted, and the result is multiplied by 

 50. This then represents the amount of bacteria in 5 

 liters of air. 



EXERCISE III. BACTERIOLOGICAL STUDY OF MILK 



The method for determining the number of bacteria 

 in milk is fundamentally the same as for water, except 

 that dilutions must be carried higher, as milk generally 

 contains much larger numbers of bacteria. 



Sterilization and pasteurization oj milk. — Some of the 

 important germs in milk are saprophytes (which under 

 favorable circumstances produce disagreeable odors or 

 tastes), and such pathogens as the bacillus of tuberculosis 

 (which may be derived from the cow, or may be an acci- 

 dental contamination), the typhoid bacillus, the bacillus 

 of "summer complaint" in children (possibly identical 

 with the bacillus of epidemic dysentery), the germs of 

 cholera, diphtheria, and scarlet fever. All these, except 

 B. tuberculosis, flourish in milk at its ordinary tempera- 

 ture. 



None of the methods employed in sterilizing milk 

 render it sterile in the bacteriological sense of the word, 

 but by means commonly employed most of the non- 

 sporing pathogenic bacteria are destroyed, along with a 

 large number of saprophytes, thus rendering the milk 



