X. NUTRIENT MEDIA. 



IN order that the life and growth of bacteria may 

 be accurately observed in the laboratory, it is neces- 

 sary 



1. To isolate individual members of the different 

 varieties of micro-organisms. 



2. To cultivate organisms, thus isolated, apart from 

 other associated or contaminating bacteria i. e., in 

 pure culture. 



For the successful achievement of these objects it is 

 necessary to provide nutriment in a form suited to the 

 needs of the particular bacterium or bacteria under 

 observation, and in a general way it may be said that 

 the nutrient materials should approximate as closely 

 as possible, in composition and character, to the natural 

 pabulum of the organism. 



The general requirements of bacteria as to their 

 food-supply have already been indicated (page 142) 

 and many combinations of proteid and of carbohy- 

 drate have been devised, from time to time, on those 

 lines. These, together with various vegetable tissues, 

 physiological or pathological fluid secretions, etc., are 

 collectively spoken of as nutrient media or culture media. 



The greater number of these media are primarily 

 fluid, but, on account of the rapidity with which bac- 

 terial growth diffuses itself through a liquid, it is im- 

 possible to study therein the characteristics of indi- 

 vidual organisms. Many such media are, therefore, 

 subsequently rendered solid by the addition of sub- 

 stances like gelatine or agar, in varying proportions, 

 the proportions of such added material being generally 

 mentioned when referring to the media; e. g. t 10 per 

 cent, gelatine, 2 per cent. agar. Gelatine is employed 



146 



