52 METHODS OF CULTIVATION OF BACTERIA. 



of sterile absorbent wool in its bottom, on which the 

 potato may rest. 



The use of the potato as a medium is very important, as 

 in certain cases the growths of bacteria on it are very charac- 

 teristic. Potatoes ought not to be prepared long before 

 being used, as the surface is apt to become dry and dis- 

 coloured. It is well to take the reaction of the potato with 

 litmus before sterilisation, as this varies ; normally in young 

 potatoes it is weakly acid. Sometimes it is necessary to 

 make it alkaline, which may be done by steeping for a few 

 minutes in a very weak soda solution. Potatoes before 

 being inoculated ought always to be incubated at 37 C. 

 for a night, to make sure that their sterilisation has been 

 successful. 



Bread Paste. 



This is useful for growing torulae, 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. If new, these ought to be 

 carefully washed and dripped, their mouths plugged with 

 pledgets of plain cotton wool and sterilised 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 bacillus subtilis. Tubes which have been in 

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



