PROBLEM 96 117 



over the other. Any sheet-iron or tin box that will stand heating 

 red-hot may be used as a dry sterilizer. 



Methods. — Study the construction of the steam and dry 

 sterilizers. 



NOTE. — Sterilization means the raising of the temperature to such a degree of 

 heat as will kill all germs. 



Conclusion. — How is the sterilizer fitted to do its work? 



Yroblem 96 : How to prepare culture media. 



Method. — Beef bouillon which has been cleared and filtered 

 may be used for growing bacteria. 



Nutrient agar-agar l is the best medium in which to grow bacteria. 

 It may be prepared from the following materials : 1000 c.c. water, 

 10 g. salt, 10 g. peptone, 10 g. Liebig's beef extract, a little cooking 

 soda, and 10 g. agar-agar. If agar-agar cannot be obtained, use 

 100 g. of the best French gelatin. 



Dissolve the beef extract in the 1000 c.c. water. Cut the agar into 

 pieces and add with the salt and peptone. The mixture must 

 then be heated to cause the agar to dissolve, care being taken that 

 it does not burn. Enough cooking soda is added to cause red 

 litmus paper dipped in the mixture to turn blue, i.e., the liquid 

 should be faintly alkaline. Filtering hot agar should be carried 

 on within the steam sterilizer. A glass funnel should be put in the 

 mouth of an Erlenmeyer flask and one or two layers of absorbent 

 cotton placed within the funnel. If the agar, flask, and funnel are 

 kept hot within the sterilizer, the liquid will readily pass through 

 the cotton. After filtering, the mouth of the flask should be closed 

 with a plug of absorbent cotton. Then boil in the cooker for half an 

 hour. If the agar mixture is not clear, it should be filtered through 

 cotton a second time. If care has been taken, the nutrient solution 

 is now ready for use, and may be set aside as a stock solution. 



If it is desired to make a nutrient solution for molds, omit the 

 cooking soda and add a few drops of dilute hydrochloric acid; 

 because molds grow best in a slightly acid medium, while bacteria 

 thrive in a slightly alkaline medium. 



1 Agar-agar is a preparation from seaweed which gives an excellent vegetable 

 gelatin (a protein food) . 



