POTATO AS A CULTURE MEDIUM. 



47 



Fig. 12. — Cylinder of 

 potato cut obliquely. 



longer, or in the autoclave for a quarter of an hour. When 

 cold, each is grasped between the left thumb and forefinger 

 (which have been sterilised with sublimate) and cut through the 

 middle with a sterile knife. It is best to have the cover of the 

 jar raised by an assistant, and to perform the cutting beneath it. 

 Each half is put in one of the watch-glasses, the cut surfaces, 

 which are then ready for inoculation with a bacterial growth, 

 being uppermost. Smaller jars, each of which holds half of a 

 potato, are also used in the same way and are very convenient. 



{b) By Slices in Tubes. — This method, introduced by Ehrlich, 

 is the best means of utilising potatoes as a medium. 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. 12. Two wedges 

 are thus obtained, and to preserve their 

 white appearance as much as possible these slices are placed 

 in running water for 12 to 18 hours, then deposited broad end 

 down in a test-tube of special form (see Fig. 13). 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 intro- 

 duced. After the latter are inserted, the tubes are 

 steriHsed in the Koch steam steriliser for one hour, 

 or in the autoclave for five ipinutes at i atmos- 

 phere pressure and 120° 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 

 iich's° t^ubr con- glycerin in water and steaming for half an hour, 

 taining piece of 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 



