36 LABORATORY GUIDE IN BACTERIOLOGY 



Litmus lactose gelatin and litmus dextrose gela- 

 tin. — Dissolve I per cent lactose or dextrose in sugar- 

 free gelatin, containing 13 per cent gelatin. The tubes 

 should contain 8 c.c. of the medium. After adding 

 I c.c. litmus solution and i c.c. of the material to be 

 plated the medium will contain 10 per cent gelatin 

 and remain solid. 



, Mannit agar. — Dissolve i per cent mannit in sugar- 

 free agar. 



EXERCISE 14. PREPARATION OF WHEY MEDIA 



Litmus whey (Petruschky, modified by Durham). — 

 Casein is precipitated from milk with rennet extract. 

 The whey is neutralized with 4 per cent citric acid 

 solution and heated on the water bath for half an hour. 

 It is then filtered, and litmus solution added until a 

 decided blue color is obtained. Sterilize in the auto- 

 clave. 



Whey gelatin. — ^Add 10 per cent gelatin to clarified 

 whey. 



Whey agar, — Add a few drops acetic acid to boiling 

 milk until the casein is precipitated. Neutralize, or 

 bring to a reaction of i per cent acid if desired, and 

 dissolve i per cent pepton, 2 per cent dextrose, and i . 5 

 per cent agar. Filter, tube, and sterilize in the 

 autoclave. 



EXERCISE 15. PREPARATION OF GLYCERIN MEDIA 



Glycerin broth. — ^Add 6 per cent pure glycerin to 

 ordinary broth. 



Glycerin agar. — ^Add 6 per cent pure glycerin to 

 nutrient agar. 



