46 METHODS OF CULTIVATION OF BACTERIA 



FIG. 9. Cylinder of 

 potato cut obliquely. 



Potatoes as Culture Material. 



Potatoes are best used as slices in tubes, according to the 

 method introduced by Ehrlich. A large, long potato is well 

 washed and scrubbed, and peeled with a 

 clean knife. A cylinder is then bored 

 from its interior with an apple corer or a 

 large cork borer, and is cut obliquely, 

 as in Fig. 9. Two wedges are thus 

 obtained, each of which is placed broad 

 end downward in a test-tube of special 

 form (see Fig. 10). In the ,wide part 



at the bottom of this tube is placed a piece of cotton wool, 

 which catches any condensation water which may form. The 

 wedge rests on the constriction above this bulbous portion. 

 The tubes, washed, dried, and with cotton wool in the bottom 

 and in the mouth, are sterilised before the slices of potato are 

 introduced. After the latter are inserted, the tubes are 

 sterilised in the Koch steam steriliser for one hour, or in the 

 autoclave for fifteen minutes, at 115 C. An ordinary test-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 steeping the potato slices for some 

 time in a known quantity of distilled water, and 

 then estimating the reaction of the water by FlG> 

 phenol-phthalein. The required degree of acidity 

 or alkalinity is obtained by adding the necessary ube ' 

 quantity of HC1 or NaOH solution (p. 35), 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 their sterilisation has been successful. 



10. 

 E h r 1 i c h ' s 

 con- 



