270 GENERAL MICROBIOLOGY 



By what means may bacteria cause infection of the 

 udder? _ 



What is the significance of bacteria in the udder? As 

 to numbers and types? 



11. Give your results in full and point out any conclu- 

 sions and any practical applications possible. 



REFERENCES 



MARSHALL: Microbiology, pp. 297-299, 306-313. 

 WARD: Pure Milk and the Public Health (1909), pp. 1-7, 69, 86-88. 

 ROSENAU: The Milk Question (1912), pp. 71-74. 

 RUSSELL and HASTINGS: Outlines of Dairy Bacteriology (1910), 

 pp. 30-35, 75. 



EXERCISE 3. TO ILLUSTRATE EXTRANEOUS CON- 

 TAMINATION 



Apparatus. Forceps; scalpel or spatula; 1 seventeen 

 sterile Petri dishes; three 10 c.c. dilution flasks (for A and B) ; 

 fifteen sterile 1 c.c. pipettes; fifteen tubes of sterile litmus 

 lactose agar; soap; ordinary towel; two 1 qt. sterile basins; 

 one milk pail; two 1 liter flasks each containing 500 c.c. 

 sterile salt solution; one deep Petri dish; sterile glass rod. 



A. SCALES FROM COW'S SKIN (DEAD EPITHELIAL 



CELLS) 



Method. 1. With a sterile spatula scrape from the 

 skin of a cow's udder some scales, such as usually fall into 

 the milk, into a sterile Petri dish. 



2. Transfer by means of the sterile spatula some of these 

 to a 75 c.c. Erlenmeyer flask containing 10 c.c. of sterile 

 physiological salt solution. 



3. Shake thoroughly, then plate 1 c.c. of this suspension 

 in litmus lactose agar, 



