CULTURE-MEDIA. 7 1 



CHAPTER III. 



CULTURE-MEDIA. 



CULTURE-MEDIA are substances in which bacteria are 

 artificially 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) 1 10 grams. 



Sodium chloride (common salt) 5 grams. 



Water i liter. 



The solid 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 inter- 

 vals with litmus-paper. The bouillon may now be filtered 

 through filter-paper. The filter-paper should be folded and 

 creased as is done by pharmacists; it is usually placed in a 

 glass funnel, and should be moistened with water before 

 using. After filtration the medium is to be placed in prop- 

 erly plugged tubes or flasks, and is to be sterilized once in 

 the autoclave, or in the steam sterilizer for fifteen minutes 



1 Commercial "peptones" are mixtures of albumose and a small 

 amount of peptone. 



