CHAPTER XXII 

 BACTERIA CONTINUED : LABORATORY METHODS 



Apparatus and material. To grow bacteria in the labora- 

 tory the following apparatus and material are necessary : a 

 steam sterilizer, hot-air sterilizer, two platinum needles 1 ; test 

 tubes, Petri dishes, absorbent cotton, litmus paper, sheet gela- 

 tin, agar-agar, extract of beef, potatoes, caustic soda solution, 

 and hydrochloric acid. 



Gelatin medium. Dissolve in 1000 cubic centimeters of 

 distilled water 10 grams of peptone, 5 grams of common salt, 

 2^- grams of beef extract, and 100 grams of sheet gelatin, and 

 place in the steam sterilizer until dissolved. 2 



Let the mixture cool to 55 C. (you can hold it in your 

 hand) and add a teaspoonful of albumen dissolved in cold 

 water, or the whites of two eggs. Boil until the liquid looks 

 clear. 3 Line a funnel with wet absorbent cotton or with filter 

 paper designed for gelatin or agar-agar filtration. Pour the 

 gelatin mixture into the funnel and catch in a sterilized 

 flask. Place in a steam sterilizer. If the funnel is kept 

 thoroughly warm, the gelatin will pass through the filter in 

 about an hour. Test the gelatin with litmus paper. It will 

 be found to be acid. Add a weak solution of caustic soda to 

 it, drop by drop, until blue litmus paper does not change 



1 Cut platinum wire (No. 27) into two-inch lengths. Fuse one end of each 

 into a glass rod, and bend the free end of one of the needles thus made into 

 a small loop, to be used in measuring drops in liquid cultures. 



2 A portable sheet-iron oven and an ordinary steam cooker may be used 

 if necessary. 



3 A fact that must be borne in mind in preparing gelatin is that its gelat- 

 inizing power is injured by prolonged heating during the process of prepa- 

 ration or sterilization, and is lost immediately when heated to 140 C, 



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