MEDIA FOR SEPARATING BACTERIAL GROUPS 47 



Milk as a Culture Medium. 



This is a convenient medium for observing the effects of 

 bacterial growth in changing the reaction, in coagulating the 

 soluble albumin, and in fermenting the lactose. It is prepared 

 as follows : Fresh milk is taken, preferably after having had the 

 cream " separated " by centrifugalisation, as is practised in the 

 best dairies, and is steamed for fifteen minutes in the Koch ; it 

 is then set aside in an ice chest or cool place over night to 

 facilitate further separation of cream. The milk is siphoned off 

 from beneath the cream. The reaction of fresh milk is alkaline. 

 If great accuracy is necessary, any required degree of reaction 

 may be obtained by the titration method. It is then placed in 

 tubes, and sterilised by methods B (2) or B (3). 



Bread Paste. 



This is useful for growing torulse, moulds, etc. Some 

 ordinary bread is cut into slices, and then dried in an oven till 

 it is so dry that it can be pounded to a fine powder in a mortar, 

 or rubbed down with the fingers and passed through a sieve. 

 Some 100 c.c. flasks are washed, dried, and sterilised, and a 

 layer of the powder half an inch thick placed on the bottom. 

 Distilled water, sufficient to cover the whole of it, is then run in 

 with a pipette held close to the surface of the bread, and, the 

 cotton-wool plugs being replaced, the flasks are sterilised in the 

 Koch's steriliser by method B (2). The reaction is slightly 

 acid. 



Media used for separating the Members of Bacterial Groups. 



A great number of media have been devised for use in 

 differentiating the members of the coli-typhoid and other 

 bacterial groups. The general feature of these media is that 

 they contain certain substances, often sugars, which tend to 

 bring out the special characters of the organism under investiga- 

 tion. Sometimes also substances are present which inhibit the 

 growth of bacteria other than those belonging to the group. 

 The following are the media which here deserve most attention : 



Hiss's Serum Water Media. These are composed of one part of ox's 

 serum and three parts of distilled water with 1 per cent, litmus ; various 

 sugars in a pure condition are added in the proportion of 1 per cent. 

 The development of acid by fermentation is shown by the alteration of 

 the colour and by coagulation of the medium. These media do not 

 coagulate at 100 C., and thus can be sterilised in the steam steriliser. 



