24 CULTURE MEDIA 



Tube the "medium and sterilize, either in the Arnold on three successive days or 

 in the autoclave at 8-10 pounds' pressure for ten minutes. The tubes should be 

 cooled as quickly as possible in cold water after taking out of the sterilizer. 



AGAR GELATIN MEDIUM (NORTH). 



Lean chopped beef or veal, 500 grams. 



Agar, 10 grams. 



Gelatin, Gold label, 20 grams. 



Peptone, Witte's, 20 grams. 



Sodium chloride, 5 grams. 



Distilled water, q.s., 1000 c.c. 



Extract the chopped beef with 500 c.c. distilled water for 18 hours, strain through 

 muslin and combine the ingredients in the usual way. Adjust the reaction to the 

 neutral point, using phenolphthalein as indicator. 



North states that this medium is excellent for streptococci, pneumococci and 

 diphtheria bacilli because it is soft, moist, and can be used at 37 C. 



It is claimed to be of special value for carrying stock cultures. 



LITMUS MILK. 



Milk for media should be as fresh as possible. It should then be put in a 1000 

 c.c. Erlenmeyer flask, sterilized for fifteen minutes in the Arnold, and set over night 

 in the refrigerator. The next morning the milk beneath the cream should be 

 siphoned off. The short arm of the siphon should not reach the bottom of the flask 

 so as to avoid the sediment. Add sufficient litmus solution to this milk to give a 

 decided lilac tinge; tube and sterilize in the Arnold on three successive days. 



Litmus milk which apparently is as satisfactory as the above as regards nutritive 

 quality and cultural characteristics can be made from certain canned milks which 

 have not been condensed or sweetened and which do not contain chemical pre- 

 servatives. The "Natura" brand of milk is the one I have experimented with. 



Litmus Solution. A simple solution may be made by digesting the powdered 

 cubes repeatedly with hot water, mixing the extracts, and, after allowing them to 

 stand all night, decanting the solution from the inert sediment into a clean bottle. 



In litmus solution so made, however, a red dye is also present while calcium and 

 other salts are dissolved out. For bacteriological purposes a pure solution of the 

 blue dye should be used. This is called "azolitmin." It is freely soluble in water 

 but insoluble in alcohol. 



It can be conveniently prepared as follows: Weigh out 2 ounces of powdered 

 litmus; digest repeatedly with fresh quantities of hot water until all the coloring 

 matter is dissolved out; allow to settle, and decant off the fluid from the insoluble 

 powder. Add together the extracts, which should measure about a liter. Evap- 

 orate down the solution to a moderate bulk, then add a slight excess of acetic acid, 

 so as to convert all carbonates present into acetates. Continue the evaporation, 

 the later stages over a water bath, until the solution becomes pasty. Add 200 c.c. 

 of alcohol, and mix thoroughly. The alcohol precipitates the blue coloring matter, 

 while a red coloring matter, together with, the alkaline acetate present, remains in 



