THE USE OF CULTURE MEDIA. 49 



Added value is given to the usefulness of milk by the addi- 

 tion to each litre of 150 c.c. of a watery tincture of litmus 

 (Merck's extract of litmus, 6 grammes; water, 1000 c.c), whilst 

 the retention of a small percentage of cream confers a distinct 

 advantage. 



Bread Paste. 



This is useful for growing torulae, moulds, etc. Some ordi- 

 nary 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. 



Beerwort. 



Beerwort, and beerwort to which 10 per cent of gelatin has 

 been added, are also used for the study of these higher organ- 

 isms. One takes ordinary hopped beerwort and places it in, say, 

 a litre flask, autoclaves it for five minutes, cools it thoroughly, and 

 filters ; by this means all resinous matters are precipitated and 

 removed, and the wort remains permanently clear. It is tubed, 

 and sterilised in the autoclave for seven minutes, or, as usual, in 

 the Koch or Arnold apparatus. 



The Use of the Culture Media. 



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

 conveniently 6 x f 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 sterilized for one hour at 

 170° C. The reason is that the glass, being usually packed in 

 straw, is covered with the extremely resisting spores of the ba- 

 cillus 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. A 

 medium thus protected will remain sterile for years. Whenever 

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



