CULTURE-MEDIA. 73 



The coagulation of milk, which is brought about by certain 

 bacteria, is a very valuable differential point. Litmus milk is 

 prepared as above and has added to it i per cent, of azolitmin 

 before sterilizing. This indicates 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. 



Nitrate Broth. — Dissolve i gram of peptone in looo c.c. 

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

 This solution is distributed into test-tube, 10 c.c. in each tube. 



Broth for the Indol Test. — Standard broth described above 

 may be used for this test provided it contains no muscle-sugar, 

 the muscle-sugar having been removed by cultivating B. coli in 

 the beef infusion for twenty-four hours previous to its prepara- 

 tion. Or the following solution may be employed for the test: 



Dunham's solution is valuable to test the development of 

 indol by bacteria (see Part II., Chapter II.). 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); 

 alakaline solutions give a clear rose-color which disappears in 

 the presence of acids. 



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 

 side of the jar with a glass rod. It may be left on the ice for 

 from twenty-four to forty-eight hours. At the end of that time 

 the serum will have separated from the clot and may be drawn 

 off with a siphon or pipette into tubes. The tubes containing 

 the serum should be placed in a slanting position, as nearly 



