46 METHODS OF CULTIVATION OF BACTERIA 



tube may be used with a piece of sterile absorbent wool in its 

 bottom, on which the potato may rest. 



Glycerin potato, suitable for the growth of the tubercle 

 bacillus, may be prepared by covering the slices in the tubes 

 with 6 per cent solution of glycerin in water and steaming for 

 half an hour. The fluid is then poured off and the sterilisation 

 continued for another half hour. 



Potatoes ought not to be prepared long before being used, as 

 the surface is apt to become dry and 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 

 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 

 HC1 or NaOH solution (p. 34) and steaming for other fifteen 

 minutes. The water is then poured off and sterilisation 

 continued 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. 



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

 of the comparative reactions of the typhoid bacillus and the B. 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 ol' ordinary 15 per cent peptone gelatin which has not 

 been neutralised. Normal sodium hydrate solution is added till the 

 reaction is feebly acid to litmus, the whole boiled together, filtered, and 

 sterilised. Just before use potassium iodide is added so as to constitute 

 one per cent of the medium. 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 



