BACTERIOLOGICAL TECHNIQUE 23 



funnel into a sterile flask, the neck of which is packed 

 with cotton wool. Such a medium is known as 6.P.B., 

 gelatine peptone bouillon, 10 to 15 per cent., according to 

 the gelatine added. A medium of this composition will fur- 

 nish nitrogen and carbon from the albumen and peptone; 

 the necessary salts are also present in the meat extract. 



Instead of using actual minced beef ' bouillon,' it is often 

 more convenient to make up a medium directly with Liebig's 

 Extract of Meat. The following formula has been found 

 satisfactory for occasional investigation in a Sewage Works 

 Laboratory. Ingredients : — 



The meat extract, peptone, salt and water are boiled for 

 a quarter of an hour, and the gelatine gradually added as 

 it dissolves. The whole is allowed to cool (to 50° C. 

 approx.) and neutralised with about 30 c.c. of a 4 per cent, 

 solution of caustic soda (NaOH). The white of an egg is 

 mixed with an equal volume of water and added to the 

 neutralised liquid. The mixture is placed in the steam bath 

 for one hour and 1'5 grammes soda crystals added. 



After a further forty minutes in the steam bath the 

 Hquid is filtered through a hot water filter as described.^ 



The melted medium is carefully poured, preferably from 

 a separating funnel, into a series of sterile test tubes (cf. 

 Fig. 5 b) ; about 10 c.c. are added to each test tube, care 



' For very exact work, e.g. differentiation of species, etc., very careful 

 neutralisation of the media is necessary, for the details of which special 

 text books should be consulted. It may be mentioned that the alkalinity 

 or acidity of a medium is often expressed in the number of c.c. of normal 

 acid or soda required for neutralisation, a — sign being used to denot? 

 alkalinity and a + sign to denote acidity. 



