COLOURED MEDIA 57 



filtered through paper. The filtrate is kept in a flask plugged with wool. To one- 

 half of this liquid sulphuric acid is added until the colour is nearly red, and the 

 other half then added to it ; a sensitive indicator is thus obtained. The solution 

 is distributed in tubes which are plugged and sterilized at 115 C. 



Litmus-lactose-gelatin. 



1. Prepare and sterilize a number of tubes of gelatin in the ordinary way 

 (p. 39), adding (at stage 4) 2 to 4 per cent, of lactose. 



2. Prepare a number of tubes of sterilized litmus solution. 



3. Just before use, melt the lactose-gelatin in a water bath and to each 

 tube add with a sterile pipette sufficient sterile litmus solution to impart a 

 distinctly blue colour to the medium. 



Never sterilize a medium after adding litmus : the subsequent heating is liable 

 to discharge the blue colour. 



Litmus-glucose-gelatin, litmus-mannite-gelatin, etc., and various litinus- 

 agars are prepared in a similar manner. 



Barsiekow's medium. 



Prepare separately the two following solutions and sterilize them : 



A. Sodium chloride, ...... 0*5 gram. 



Nutrose, ------- 1 



Water, - - - 75 grams. 



B. The carbohydrate (lactose, mannite, etc.), - - I gram. 

 Water, - ...... 25 grams. 



Litmus solution, ...... Q.S. to give an amethyst tint 



to the solution. 



After cooling, mix the two solutions and distribute in tubes. 



Litmus milk. 



Add a sufficient quantity of sterile litmus solution to sterile milk. [It is 

 highly important that the reaction of the milk should be neutral. Milk 

 bought in shops is often acid, in which case a sufficiency of sodium carbonate 

 must be added to neutralize the medium.] 



Noeggerath's medium. 



1. Mix in the following proportions saturated aqueous solutions of the dyes 

 mentioned : 



Methyl-blue, - 2 c.c. 



Gentian-violet, ... 4 



Methyl-green, 1 



Chrysoidine, - 4 

 Fuchsin, ........ 3,, 



2. Add 200 c.c. distilled water. 



3. The mixture now has a neutral greyish- blue tint : leave it to stand for a 

 fortnight, then if the colour has altered bring it back to the neutral tint by the 

 addition of any colour that is required. Sterilize at 100 C. 



4. Immediately before use add 7 to 10 drops of the sterile mixture to a tube of 

 agar or gelatin previously melted in the water bath. 



In place of the above mixture, Gasser prefers to add 20 drops of a saturated 

 aqueous solution of fuchsin to each tube of melted agar. 



These media were recommended by their authors for the diagnosis of the typhoid 

 bacillus, but have now fallen into disuse (vide the typhoid bacillus). 



