12 EXPERIMENTAL DAIRY BACTERIOLOGY 



gelatin or agar are detected, or the least cloudiness in bouillon 

 appears, the medium is not sterile and must be immediately 

 resterilized. As far as possible media that are not to be used 

 at once should be stored in an ice box to prevent evapora- 

 tion. In case of media that are to be held in storage for a 

 considerable period, or incubated for some time at blood heat, 

 the containers may be sealed by dipping the lower part of 

 the cotton plugs in melted paraffin, or by placing a cap of tin 

 foil over the cotton plug before sterilizing. 



In each basket of culture medium should be placed a slip 

 of paper stating the kind of medium, its reaction, the date of 

 manufacture, and manner of sterilization. As far as conven- 

 ient, culture media should be made in considerable quantities, 

 so as to increase uniformity of conditions in culture work. 



Preparation of broth. Nutrient broth is prepared as fol- 

 lows: Infuse 500 grams (18 ounces) of chopped lean beef in 

 1000 cc. of distilled water at refrigerator temperature for 24 

 hours. This should be done in a vessel of glass or enameled 

 iron. Strain the infusion through cheese cloth or cotton flan- 

 nel, pressing out the liquid until the meat appears dry. Place 

 this infusion in a weighed media cooker, and add water to 

 bring the weight of the infusion to 1000 grams. Add 1 per 

 cent peptone (10 grams per liter) and warm gently, not above 

 50 C. on the water bath, until the peptone is dissolved. Ti- 

 trate two portions of 5 cc. each, and bring the reaction to + 1 

 per cent by the addition of N/l NaOH. Titrate another por- 

 tion of 5 cc. It will usually be found that the calculated 

 amount of N/l NaOH is not sufficient to bring the reaction 

 to the desired point, due to the fact that the amount of 

 N/20 NaOH necessary to neutralize 5 cc. of the medium 

 diluted with 45 cc. of water does not exactly correspond 

 with the amount of N/l NaOH necessary to neutralize the 



