48 METHODS OF CULTIVATION OF BACTERIA. 



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 

 steaming the potato slices for a quarter of an hour in a known 

 quantity of distilled water and then estimating the reaction of 

 the water by phenol-phthaleine. The required degree of acidity 

 or alkalinity is obtained by adding the necessary quantity of 

 HCl or NaOH solution (p. 37) and steaming for other fifteen 

 minutes. The water is then poured off and steirilisation con- 

 tinued for another half-hour. Potatoes before being inoculated 

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

 that their sterilisation has been successful. This is necessary 

 only if sterilised in the Koch steamer. 



Eisner's Medium. — This is one of the media introduced in the study of 

 the comparative reactions of the typhoid bacillus and the bacillus coli. The 

 preparation is as follows : 500 grammes potato are grated up in a litre of water, 

 allowed to stand over night, then strained, and added to an equal quantity of 

 ordinary 15 per cent peptone gelatin which has not been neutralised. Normal 

 sodium hydrate solution is added till the reaction is feebly acid, the whole 

 boiled together, filtered, and sterilised. Just before use potassium iodide is 

 added so as to constitute one per cent of the amount used. Moore has used 

 a similar agar preparation. Here 500 grammes potato are scraped up in one 

 litre of water, allowed to stand for three hours, strained, and put aside over 

 night. The clear fluid is poured off, made up to one litre, rendered slightly 

 alkaline, 20 grammes agar are added, and the whole is treated as in making 

 ordinary agar. The medium is distributed in test-tubes — 10 c.c. to each — 

 and immediately before use, to each is added .5 c.c. of a solution of 10 grammes 

 potassium iodide to 50 c.c. water. 



Milk as a Culture Medium. 



This is a convenient medium for observing the effects of 

 bacterial growth in changing the reaction, 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, as is practised in the 

 best dairies, 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. The reaction of fresh milk is alkahne. 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). 



