130 BACTERIOLOGY 



the usual way. When sterilization is complete enough 

 sterilized litmus tincture should be added to each tube to 

 give a decided, though not very intense, blue color. This 

 must be done carefully, to avoid contamination of the tubes 

 during manipulation. It is better not to add the litmus 

 tincture before sterilizing the tubes, as its color-character- 

 istics are altered by contact with organic matters under the 

 influence of heat. This medium is used for both test-tube 

 and plate cultivation, just as is ordinary agar-agar and 

 gelatin. 



LOFFLER'S BLOOD-SERUM MIXTURE. 



Loffler's blood-serum mixture consists of one part of 

 neutral meat-infusion bouillon, containing 1 per cent, of 

 grape-sugar, and three parts of blood-serum. This mixture 

 is placed in test-tubes, sterilized, and solidified in exactly 

 the way given for blood-serum. It requires for its solidi- 

 fication a somewhat higher temperature and a longer ex- 

 posure to this temperature than does blood-serum to which 

 no bouillon has been added. (See also the Councilman- 

 Mallory method.) 



THE SERUM-WATER MEDIUM OF HISS. 



A medium which has been found very serviceable in the 

 differentiation between closely related bacteria is prepared 

 by mixing one part of blood-serum (either horse or bovine) 

 and three parts of distilled water. This is neutralized, and 

 heated in a water-bath or an Arnold steam sterilizer until 

 it becomes opalescent. A 5 per cent, aqueous solution of 

 litmus is then added in the proportion of 1 per cent. Any 

 one of the carbohydrates, as dextrose, lactose, saccharose, 



