THE NUTRITION OF BACTERIA 



11 



ti. Plug tubes with cotton and sterilize in the Arnold sterilizer 

 for twenty -five minutes on three consecutive days. 



Caution. Long exposure to high temperature may alter the 

 gelatin so that it will not solidify when cool. 



Exercise 10. Preparation of Potatoes for Cultures 



Potatoes have been for a long time used in bacteriological 

 work. At the present time they are used chiefly for test-tube 

 cultures, the former use of potato 

 slices for isolating bacteria having 

 been displaced by the use of gelatin 

 or agar media in Petri dishes. 



1. Select large, ripe potatoes and 

 scrub them in water. Remove the 

 eyes. Cut plugs from the potatoes 

 with the special cutter or with a 

 large cork-borer. The pieces should 

 be dropped at once into cold water 

 and washed several times, if they 

 are to be kept white. Divide the 

 cylinders with a diagonal cut and 

 trim if necessary to fit tubes. 



2. The pieces of potato should 

 be put into large test tubes con- 

 taining a few drops of water. Some 

 device is necessary to keep the 

 potato plugs from standing in the 

 water. Roux test tubes accomplish 

 this by means of a constriction near 



the bottom. Ordinary test tubes J *> or dinary tube, with cotton wad 



supporting the plug 



may be used, however, by first 



putting in glass beads or a wad of absorbent cotton (Fig. 8). 



3. After plugging the tubes with cotton, sterilize them on 

 four consecutive days in the Arnold sterilizer, or once in the 

 autoclave at 115C. The potato bacillus (Bacillus mesentericus 

 vulyatun*) is often very difficult to kill, and the potato media 

 should be watched for a week to make sure that they are sterile. 



FIG. 8. Potato tubes 



A, Roux tube, with constriction to 

 hold the plug out of the water ; 



