BACTERIOLOGICAL TECHNIC 29 



3. Immerse the pieces in running water for 24 hours. 



4. Trim the pieces of potato so they slide down 

 the tube and insert one half-cylinder into each potato 

 tube. The wide end rests on the constriction of the 

 tube. Pour a small amount of water into it, and 

 fill about one-half of the part of the tube below the 

 constriction. Large culture tubes without constric- 

 tions may be used. A small amount of cotton should 

 be pushed to the bottom of these and the cotton soaked 

 with water. The potato then rests on the cotton 

 (Figs. 16, 17). 



5. Sterilize in the autoclave for 8 to 10 minutes 

 at 120 C., or in the arnold for three consecutive days. 



NOTE. Potatoes usually harbor a spore-bearing bacillus, the 

 spores of which are highly resistant. Therefore a longer expo- 

 sure iii the autoclave is necessary to insure sterilization. 



SUBSTITUTE FOR POTATO 



1. Dissolve 15 grams agar in 600 c.c. water and 

 filter. 



2. Dissolve the following salts in 200 c.c. water: 



Asparagin 5 grams 



K 2 HPO 4 2 grams 



Na 2 HPO 4 2 grams 



MgSO 4 2 grams 



CaCl 2 2 grams 



Ammonium lactate 2 grams 



3. Add the solution of salts to the hot agar. 



4. Dissolve 10 grams pep ton. 



5. Adjust the reaction to the neutral point with 

 phenolphthalein.. 



6. Suspend 30 grams washed starch in water. 



7. Add the starch suspension slowly with constant 

 stirring to the previous mixture. 



