58 A MANUAL OF BACTERIOLOGY 



Glucose gelatin. Ordinary gelatin with the addition of 

 1 to 2 per cent, of grape sugar. 



Beer-wort gelatin. This is one of the best culture media 

 for yeasts and some of the fungi (e.g. ringworm). Procure 

 from the brewery some beer-wort, preferably unhopped, 

 and add to every litre 100 grm. of gelatin. Dissolve, 

 clarify, and filter, as in the case of ordinary gelatin. It 

 is not neutralised. 



Nutrient agar-agar. This is one of our most valuable 

 culture media, and has the advantage over nutrient 

 gelatin that it remains solid at blood-heat. 



Agar is a carbohydrate substance of high melting-point 

 and considerable gelatinising power, obtained from 

 Eastern seaweeds. The powdered form is now generally 

 used. Add 15 grm. (i.e. 1J per cent.) of powdered agar 

 to 1 litre of acid beef-broth, together with 10 grm. of 

 peptone and 5 grm. of common salt in a large glass flask, 

 place in the water-bath until dissolved (half an hour to 

 one hour), and then render alkaline as for peptone beef- 

 broth ; allow it to cool to 50 0., and add the white of 

 an egg. Return to the steamer for an hour and a half, 

 then filter through an agar filter-paper (" papier Chardin ") 

 in a hot- water funnel or in the steamer. By this treatment 

 a litre of agar should pass through the filter in two to three 

 hours. If it does not come through clear, add another 

 white of egg and repeat the process. 



If an autoclave is available, a quicker and better method 

 is, after neutralising and adding the white of an egg, to 

 place in the autoclave with a small beaker inverted over 

 the mouth of the flask, and heat to 134 C. (two atmospheres 

 pressure) for half an hour. Turn the gas out, and allow 

 to cool without opening the stopcock. When cool, open, 

 and filter through the special agar filter-paper in a hot- 

 water funnel ; the agar will pass through in about ten 

 minutes or a quarter of an hour. Fill into test-tubes and 



