g 2 BACTERIOLOGY 



The tubes are plugged, and sterilized in the autoclave at 110 C. 

 for 30 minutes. Potato is best when freshly prepared; it is likely 

 to become dry and discolored with keeping. 



Milk. Milk fresh as possible is placed in a covered jar, 

 steamed for fifteen minutes, and then kept on ice for twenty- 

 four hours. At the end of that time the middle portion is re- 

 moved by means of a siphon. The upper and lower layers must 

 not be taken; the upper part contains cream, and the lower part 

 particles of dirt, both of which are to be avoided. About 7 to 10 

 c.c. are to be run into each test tube. The tube is plugged with 

 cotton, and sterilized in the autoclave at 110 C. for 30 minutes. 



The coagulation of milk, which is accomplished by certain 

 bacteria, is a very valuable differential point. A little litmus 

 tincture may be added to the tubes of milk before sterilizing, 

 until they acquire a blue color, to indicate whether or not acids 

 are formed by the bacteria which are afterward cultivated in 

 the milk. 



Dunham's Peptone Solution. 



Peptone 10 grams. 



Sodium chloride 5 grams. 



Water i liter. 



Boil, filter, sterilize in the usual manner. 



Dunham's solution is valuable to- test the development of 

 indol by bacteria (see Part II., Chapter VIII.). The develop- 

 ment of acids may be detected after the addition of 2 per cent of 

 rosolic acid solution (0.5 per cent solution in alcohol); alkaline 

 solutions give a clear rose-color which disappears in the presence 

 of acids. 



Nitrate Broth. Dissolve i gram of peptone in 1000 c.c. of 

 distilled water, and add 2 grams of nitrite-free potassium nitrate. 

 Fill into test-tubes, 10 c.c. in each, and sterilize in the autoclave 

 at 110 C. for 15 minutes. 



Blood -serum. The blood of the ox or cow may be obtained 

 easily at the abattoir. It should be collected in a clean jar. 

 When it has coagulated, the clot should be separated from the 



