NUTRIENT GELATIN AND AGAR 73 



of 100 grm. of the best " gold label " gelatin (Coignet's) 

 per litre. After the addition of the egg, steam for an hour 

 and then 61ter through two thicknesses of filter -paper in 

 a hot- water funnel (this is best, but it may be done in the 

 steamer at a low temperature, e.g. 35 C.). Fill into 

 test-tubes and sterilise. After the third steaming the 

 tubes are allowed to solidify, either in the upright or 

 oblique position, according as they are required for stab 

 or surface cultivation. It may also be kept in bulk for 

 plate cultures, etc. 



In hot summer weather 15 or even 20 per cent, of gelatin (150 

 grm. or 200 grm. to the litre) are necessary for the product to 

 remain solid, as nutrient gelatin melts at 24 C. or a little under. 

 Prolonged boiling diminishes and ultimately destroys the gela- 

 tinising power of gelatin, so the less it is heated the better. It 

 must not be autoclaved. 



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). To a 

 litre of unhopped beer- wort add 100 grm. of gelatin, 

 dissolve by heating, clarify, and filter, as in the case of 

 ordinary gelatin. It is not neutralised. Beer-wort agar 

 may be similarly prepared, using 40-50 grm. of agar, 

 instead of gelatin, per litre. 



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. It is prepared in the same manner as nutrient 

 beef- or Lemco-broth with the addition of 15 grm. (i.e. \\ 

 per cent.) of powdered agar to the litre. Heat over 

 the water-bath or in the steamer until dissolved (an 



