56 METHODS OF CULTIVATION OF BACTERIA. 



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 ob- 

 tained 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 | in. If new, these ought to be 

 carefully washed and dripped, and their mouths are plugged 

 with pledgets of plain cotton wool. They are then sterilised 

 for one hour at 170 C. The reason is that the glass, being 

 usually packed in straw, is covered with the extremely resist- 

 ing spores of the bacillus subtilis. Tubes which have been in 

 use are merely well washed, dried thoroughly, and plugged. 

 Cotton-wool plugs are universally used for protecting the 

 sterile contents of flasks and tubes from contamination 

 with the bacteria of the air. The contained air passes 

 through the plug during sterilisation ; what passes back on 

 cooling is filtered free of germs by the wool. A medium 

 thus protected will remain sterile for years. Whenever a 

 protecting plug is removed j for even a short time, the 

 sterility of the contents may be endangered. It is well to 



