CULTURE-MEDIA. 59 



CHAPTER III. 

 CULTURE-MEDIA. 



Culture-media are substances in which bacteria are artifi- 

 cially cultivated. The number of such substances is very large, 

 different materials being suited to different purposes and to 

 different kinds of bacteria. The most important ones are 

 nutrient bouillon or beef-tea, nutrient gelatin, and nutrient 

 agar-agar. The two last have a jelly-like consistency, owing to 

 the addition of a gelatinizing substance, but otherwise are of 

 the same composition as bouillon. 



Nutrient Bouillon. 



Beef -extract (such as Liebig's) 3 grams. 



Peptone, pure (Witte's)* 10 grams. 



Sodium chloride (common salt) 5 grams. 



Water i liter. 



The soHd ingredients are dissolved in water, and the mixture 

 is boiled for a few minutes. It is made neutral or very faintly 

 alkaline by the addition of a solution of sodium hydroxide, drop 

 by drop, the reaction being tested at intervals with litmus-paper. 

 As soon as the proper reaction is reached, it is filtered through 

 filter-paper. ' The filter-paper should be folded and creased as is 

 done by pharmacists ; it is in the usual manner placed in a glass 

 funnel, and should be moistened with water before using. 

 After filtration the medium is to be placed in properly plugged 

 tubes or flasks, and is to be steriHzed once in the autoclave, or 



* Commercial " peptones " are mixtures of albmnose and a small amount 

 of peptone. 



