CULTURE MEDIA 



Beef bouillon for growing bacteria may be made with 2 tablespoonfuls of beef extract dissolved in 

 2 quarts of water. Tube in sterilized test tubes ; cork with cotton. 



Nutrient agar-agar is the best medium in which to grow bacteria. It may be prepared from the fol- 

 lowing materials : 1000 cubic centimeters of water, 10 grams of salt, 10 grams of peptone, 

 10 grams of beef extract, a little baking soda, and 10 grams of agar-agar. If agar-agar cannot 

 be obtained, use 100 grams of the best French gelatin. 



Dissolve the beef extract in the 1000 cubic centimeters of water. Cut the agar into pieces and 

 add with the salt and peptone. The mixture must then be heated in a double boiler to cause the 

 agar to dissolve. Next add enough baking soda to cause red litmus paper dipped in the mixture 

 to turn blue ; that is, the liquid should be faintly alkaline. The mass is then filtered within a 

 steam sterilizer by placing a glass funnel in the mouth of an Erlenmeyer flask and one or two 

 layers of absorbent cotton within the funnel as a filter. If the agar, flask, and funnel are kept 

 hot within the sterilizer, the liquid will readily pass through the cotton. A special hot-water funnel- 

 holder may be purchased to do away with the use of a sterilizer in filtering. After filtering, the 

 mouth of the flask should be closed with a plug of absorbent cotton. Then boil in a double 

 boiler 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. 



To prepare the nutrient agar-agar for use it may "be poured while hot into petri dishes which have 

 been previously sterilized with dry heat for several hours and then kept in a dry place free from 

 dust. It is well to sterilize the plates once or twice after they are coated, using a steam sterilizer. 



Test tubes partially filled with the nutrient jelly are also useful. Immediately after the hot jelly is 

 poured into the test tubes the latter should be plugged with absorbent cotton and then placed in 

 the steam sterilizer. 



Fehlings solution (so called in honor of its discoverer) can be purchased in the form of tablets or 



may be made, and when kept as two solutions will last indefinitely. 

 Add to 35 grams of copper sulfate (blue vitriol) 500 cubic centimeters of water. Put aside until it 



is completely dissolved. Call this Solution A. 

 To 1 60 grams of caustic soda and 173 grams of Rochelle salt add 500 cubic centimeters of water. 



Dilute to i liter. Call this Solution B. 

 For use, mix equal parts of A and B. 



lodin solution is made by simply adding a few crystals of the element iodin to 95 per cent alcohol ; 

 or, better, by taking I gram by weight of iodin crystals and | gram of iodide of potassium and 

 dissolving in water ; in either case dilute with water to a light brown color. 



Limewater can be made by shaking up a piece of quicklime the size of a walnut in about a pint of 

 water. Filter the limewater into bottles and it is ready for use. Close with a stopper smeared 

 with vaseline. 



Phenolphthalein. Dissolve I gram in 7 grams of grain alcohol ; then dilute with water. If the solution 

 appears milky, add a little more grain alcohol. 



Hay infusion for the growth of protozoa, normal salt solution, and other aids are described in the 

 " Manual of Suggestions for Teachers." 



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