BLOOD SERUM. 47 



gelatine. It is, for instance, never liquefied, whereas some 

 organisms, by their growth, liquefy gelatine and others do 

 not, a fact of prime importance. 



Sometimes to the above media are added colouring 

 matters, changes in which may give information as to the 

 nature of the action of bacteria. Thus the production of 

 acids can be detected by adding a few drops of a watery 

 solution of litmus (French, tournesol) to the ordinary 

 bouillon or gelatine. In the case of the B. diphtherias 

 ordinary bouillon may be used. Sanarelli used a 2 per 

 cent lactose gelatine tinted blue with a little litmus, to 

 distinguish the acid-producing bacillus coli communis, from 

 the typhoid bacillus. 



Agar smeared with Blood. This method was introduced 

 by Pfeiffer for growing the influenza bacillus, and it has 

 been used for the organisms which are not easily grown on 

 the ordinary media, e.g. the gonococcus and the pneumo- 

 coccus. Human blood or the blood of animals may be 

 used. "Sloped tubes" (vide p. 54) of agar are employed 

 (glycerine agar is not so suitable). Purify a finger first with 

 i-iooo corrosive sublimate, dry, and then wash with 

 absolute alcohol to remove the sublimate. Allow the 

 alcohol to evaporate. Prick with a needle sterilised by 

 heat, and, catching a drop of blood in the loop of a sterile 

 platinum wire (vide p. 55), smear it on the surface of the 

 agar. The excess of the blood runs down and leaves a 

 film on the surface. Cover the tubes with india-rubber 

 caps, and incubate them for one to two days at 38 C. before 

 use, to make certain that they are sterile. 



Blood Serum. 



Koch introduced this medium, and it is prepar 

 follows : Plug the mouth of a tall cylindrical glass 

 (say a 1000 c.c. measure) with cotton wool, and sterilise by 

 steaming it in a Koch's steriliser for one and a half hours. 

 Take it to the place where a horse, ox, or sheep is to be killed. 

 When the artery or vein of the animal is opened, allow the 



