CULTURE MEDIA 95 



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 looo 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 iio°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 

 sides 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 maj^ be drawn 

 off with a siphon into tubes. These tubes are sterilized for the 

 first time in a slanting position , as the first sterilization coagulates 

 the serum. The coagulation may be done advantageously, as 

 advised by Councilman and Mallory, in the hot-air sterilizer at a 

 temperature below the boiling-point. After coagulation, sterilize 

 in the autoclave at 110° C. for 20 minutes. This serum makes an 

 opaque medium of a cream color. Blood-serum may be more 

 conveniently sterilized in the Koch serum inspissator (Fig. 36). 

 A clear blood-serum is to be obtained by sterilization at a tempera- 

 ture of 58° C. for one hour, on each of six days, if a fluid medium 

 is desired, or of 75° C. on each of four days if the serum is to be 

 solidified. In the latter case the tubes are to be placed in an in- 

 clined position. Opaque, coagulated blood-serum has most of 

 the advantages of the clear medium. Blood-serum may be se- 

 cured from small animals by collecting blood directly from the 

 vessels, and with proper technic may be obtained in a sterile 

 condition; and the serum may be separated and stored in a fluid, 

 state. Human blood-serum is sometimes obtained from the 

 placental blood. The preservation of blood-serum is sometimes 



