BLOOD AGAR 



133 



Lactose Litmus Agar. Ordinary nutrient agar plus 2 per cent, 

 lactose and a sufficient quantity of litmus solution to give to the 

 medium a pale lavender color. 



Gelatin Agar. This is prepared like the ordinary nutrient gelatin. 

 It contains both gelatin and agar in the following proportions: If it 

 is to be incubated at 30 C., gelatin, 10 per cent.; agar, 0.5 per cent.; 

 if it is to be incubated at 37 C., gelatin, 12 per cent.; agar 0.75 per 

 cent. The gelatin and the agar are dissolved separately in propor- 

 tionate amounts of the nutrient bouillon. After separate filtration 

 and clearing they are mixed and sterilized by the fractional method, 

 as described for nutrient gelatin. 



Loe filer's Blood-serum Mixture. Prepare an ordinary nutrient 

 bouillon (veal will give better results than beef) and, before correcting 

 the reaction and sterilization, add 1 per cent, glucose. To each 100 

 c.c. of the sterile bouillon add 300 c.c. of blood serum, distribute the 

 mixture to sterile test-tubes and sterilize according to the method 

 given for blood serum. Coagulate the mixture in a slanting position. 



FIG. 69 



FIG. 70 



Wire baskets for agar or gelatin tubes. 



Serum Bouillon. Collect some ascitic, pleuritic, or hydrocele fluid 

 with aseptic precautions in a sterile flask. Mix the serous fluid with 

 twice its bulk of sterile nutrient bouillon. Distribute to sterile test- 

 tubes, sterilize by the fractional, discontinuous method for a week, 

 but do not finally raise the temperature to the coagulation point of 

 albumin. 



Blood Agar. Prepare nutrient agar by the usual method and keep 

 the tubes in the incubator for a few days to permit the condensed 

 water to evaporate. Obtain blood from a rabbit with aseptic pre- 

 cautions by exposing the jugular and drawing it into a sterile all- 

 glass syringe or a glass pipette. Squirt the blood into a small sterile 

 flask containing pieces of glass or glass pearls. Defibrinate the blood 

 by shaking, and pour a small amount of the fluid mixture of blood 

 serum and corpuscles into the dry agar tubes, which can be used at 

 once. In spite of all precautionary measures a certain number of 

 tubes will generally be contaminated. Pigeon's blood is also used. 



