THE PREPARATION OF CULTURE MEDIA 13. 



of glycerin before the jelly is finally sterilised a most 

 useful culture medium is obtained. This material has. 

 the advantage of remaining moist for a much longer 

 time than the gelatine peptone, and, whilst most 

 organisms thrive especially well upon it, some, like 

 the bacillus tuberculosis, refuse to grow on gelatine 

 without its addition. Other materials, such as grape 

 sugar, mannite, acetic acid, have also been added in 

 different proportions for special purposes (see also- 

 pp. 63-73 in Chapter Til. on the Examination of Water 

 for Bacteria). 



The following gelatine preparations may be more 

 especially referred to : — 



Gelatinised milk-serum. — One litre of fresh milk is- 

 warmed to 60° or 70° C, 70 to 100 grms. of gelatine 

 are then added and dissolved. A few minutes' boiling 

 will serve to precipitate the casein, and it is then 

 passed through a fine muslin strainer. The fluid is 

 allowed to stand for about twenty minutes at the tem- 

 perature of the body, in order to allow the fat to come 

 to the surface, after which it is allowed to cool and 

 the layer of cream is carefully removed. To the result- 

 ing slightly opalescent fluid 1 per cent, of albumen 

 peptone is added, and the whole is neutralised, boiled, 

 filtered, and sterilised in the same way as nutrient 

 gelatine.^ 



Wort gelatine and Wort agar. — These culture mate- 

 rials are useful for the growth of those organisms which, 

 like yeasts and moulds, are favoured by an acid me- 

 dium. To ordinary malt wort 10 per cent, of gelatine is 

 added and the mixture heated in the steam steriliser for 

 some time, and then filtered without being neutralised. 

 The acidity which the medium thus acquires is rather 

 greater than that of the wort before the addition of 



^ Hueppe, Die Methoden d. BaMerienforsclmng, 1891, p. 248. 



