DUNHAM'S PEPTONE SOLUTION 43 



12. Counterpoise and make up any loss in weight with 

 boiling distilled water. 



13. Filter boiling hot through plaited filter paper just 

 previously washed with boiling water. Pass the filtrate 

 through the same paper until clear. 



14. Fill 60 to 70 sterile test tubes, using approximately 

 8 c.c. of the medium for each tube. 



15. Heat in flowing steam twenty minutes on three suc- 

 cessive days. 



16. At the end of the final heating, place the tubes of 

 agar in an inclined position to solidify (do not allow the 

 medium to touch the plug) so that a large surface is pre- 

 sented for the cultivation of microorganisms. These are 

 called agar slants. 



Note. If agar tubes are to be used only for agar slants, less of the 

 medium is needed in the tube than when they are to be used for plating. 



17. To sterilize a large flask of agar, heat for thirty 

 minutes on four successive days. 



EXERCISE 10. PREPARATION OF DUNHAM'S PEPTONE 

 SOLUTION 



Dunham's solution is utilized for determining the 

 power of microorganisms to produce indol, ammonia or 

 nitrites from peptone, which properties are character- 

 istic of certain species. 



Apparatus. 1000 c.c. of tap water; 10 gms. peptone, 

 Witte's; 5 gms. salt; large burner; large funnel; plaited 

 filter paper; filling funnel; sterile test tubes; apparatus 

 for steam sterilization. 



Method. 1. Mix 1% peptone and 0.5% salt to a 

 smooth paste with a measured (small) amount of water. 



2. Dilute to 1000 c.c. with tap water. 



3. Counterpoise and note the weight. 



4. Boil ten minutes over a free flame; counterpoise and 

 make up any loss in weight with distilled water. 



