48 METHODS OF CULTIVATION OF BACTERIA 



discoloured. 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. The reaction of the potato may 

 be more accurately estimated by steeping a given weight of 

 potato slices for some time in a known quantity of distilled 

 water, and then estimating ,the reaction of the water by phenol- 

 phthalein. The required degree of acidity or alkalinity is 

 obtained by adding the necessary quantity of HC1 or NaOH 

 solution (p. 34), and steeping again. The water is then poured 

 off and the potatoes placed in tubes. Potatoes before being 

 inoculated ought always to be incubated at 37° C. for a night, 

 to make sure that the sterilisation has been successful. 



Milk as a Culture Medium. 



This is a convenient medium for observing the effects of 

 bacterial growth, 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, 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 and placed in sterile test-tubes. 

 A little litmus, sufficient to tint the milk, is often added before 

 final sterilisation to show change in reaction produced by 

 bacterial growth — litmus milk. 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 torute, 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. 



