MEAT EXTRACT PEPTONE GELATINE 41 



gelatine, to which 5 grm. extract of meat, 5 grm. grape- 

 sugar, and 80 grm. peptone have been added. As soon 

 as the gelatine is dissolved in water the other ingre- 

 dients are mixed in and the whole boiled, after which the 

 solution is filtered off by means of the hot-water funnel 

 and neutralised. Should clouding by any chance occur, 

 recourse must be had to those measures described in speak- 

 ing of the preparation of peptone bouillon gelatine. When 

 this has been sterilised — which must be done with par- 

 ticular care, as the extract of meat contains many germs — 

 a culture-medium is obtained which can in most cases be 

 used exactly like the preceding. The finished gelatine is 

 stained a brownish colour, owing to pigments derived from 

 the meat extract. 



Both these modes of preparation yield gelatines which 

 are most extensively used in all bacteriological researches. 

 Attempts are now made to alter the culture-media by 

 adding various substances to the gelatine, such as grape- 

 sugar (up to 2 per cent.) and dextrine or glycerine (4 to 6 

 per cent.). By means of these different admixtures, the 

 nutritive value of the gelatine for certain micro-organisms 

 is said to be increased. 



In summer the gelatine has a tendency to Hquefy, and 

 its strength must accordingly be increased from 10 to 15 

 per cent. ; while for cultivating anaerobes a 7^ per cent, 

 gelatine is required. 



Additions to nutrient gelatine. — A modification deserving 

 of special notice is litnms gelatine, which is prepared by 

 mixing a tolerably concentrated solution of blue litmus 

 with the gelatine, thus obtaining the substance to which 

 this name is given. Its importance lies in the fact that the 

 acids or alkalies formed by micro-organisms in their growth 

 can thus be qualitatively demonstrated. 



It is advisable to add the most widely different sub- 



