THE USE OF THE CULTURE MEDIA 47 



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. 



THE USE OF THE CULTURE MEDIA. 



The culture of bacteria is usually carried on in test-tubes 

 conveniently 6 x f in. These ought to be very thoroughly 

 washed and dripped, and their mouths plugged with plain 

 cotton wool. They are then sterilised for one hour at 170 C. 

 If the tubes be new, the glass, being usually packed in straw, 

 may be contaminated with the extremely resisting spores of 

 the b. subtilis. Cotton -wool plugs are universally used for 

 protecting the sterile contents of flasks and tubes from con- 

 tamination with the bacteria of the air. A medium thus 

 protected will remain sterile for years. Whenever a protecting 

 plug is removed for even a short time, the sterility of the 

 contents may be endangered. It is well to place the bouillon, 

 gelatin, and agar media in the test- tubes directly after filtration. 

 The media can then be sterilised in the test-tubes. 



In filling tubes, care must be taken to run the liquid down 

 the centre, so that none of it drops on the inside of the upper 

 part of the tube with which the cotton-wool plug will be in 

 contact, otherwise the latter will subsequently stick to 

 the glass and its removal will be difficult. In the case of 

 liquid media, test-tubes are filled about one-third full. With 

 the solid media the amount varies. In the case of gelatin 

 media, tubes filled one -third full and allowed to solidify 

 while standing upright, are those commonly used. With 



