298 PRACTICAL DAIRY BACTERIOLOGY 



the sample at 70, diluted 10,000 times. Allow all plates to grow 

 at room temperature until the colonies are well developed. 

 Determine the total number of bacteria in each sample, and the 

 number and percentage of each kind of bacteria that can be 

 clearly distinguished by its colony. Compare the numbers and 

 percentages of the different species at the different temperatures. 



*No. 35. Bacteria of the Air. Place one dozen tubes of steril- 

 ized milk with cotton plugs removed in various places around 

 the barn, dairy, house and laboratory. Leave undisturbed for 

 6 hours. Then replace the plugs, carry all to laboratory, and 

 place at about 20. Watch for several days and notice whether 

 all appear to undergo the same kind of fermentation. Do any 

 of them sour normally? Do any remain unchanged? 



No. 36. Types of Bacteria from the Air. After the tubes in 

 No. 35 show signs of fermentation make a gelatin plate from 

 each, using a loopful of the milk in a water blank and inocu- 

 lating gelatin tubes with a small loopful of the water blank 

 dilution. After proper growth examine the colonies in the dif- 

 ferent plates and compare with each other. What does the ex- 

 periment teach? If time permits, the colonies may be isolated, 

 purified and further studied, as in No. 28. 



*No. 37. Dust Plates. Pour 12 tubes of melted gelatin into 

 Petri dishes; cover and allow to harden. 



a. During the milking of a cow hold one of the above gelatin 

 plates under the udder of the cow, close to the milk pail ; remove 

 the cover so as to allow any dust particles to fall on the 

 gelatin, for half a minute. Replace the cover and carry to the 

 laboratory. 



b. Place 3 of the flasks at different places in a cow barn 

 several hours after feeding. Remove the covers for 3 minutes. 

 Replace covers and carry to laboratory. 



c. Repeat b ten minutes after feeding the cows with hay. 



d. Repeat c after feeding with other foods. 



e. Expose one plate the same length of time out of doors. 



