CULTURES IN SOLID MEDIA. 



The characters of growth in agar-agar jelly are not so varied, 

 but this medium possesses the advantage of not liquefying at a tem- 

 perature of 35 to 38 C., which is required for the development of 

 certain pathogenic bacteria. Variations in mode of growth are 

 also manifested in nutrient agar similar to those referred to as pro- 

 duced by non-liquefying bacteria in flesh-peptone-gelatin. These 

 relate to the surface growth and to growth along the line of punc- 

 ture. One character not heretofore mentioned consists in the for- 

 mation of gas bubbles in stab cultures either in gelatin or agar. 



Colonies. If we melt the gelatin or agar in a test tube, pour 

 the liquid medium into a shallow glass dish previously sterilized, 



FIG. 42. 



and allow it to cool while properly protected by a glass cover, we 

 will have a broad surface of sterile nutrient material. If now we ex- 

 pose it to the air for ten or fifteen minutes, and again cover it and 

 put it aside for two or three days at a favorable temperature, we can 

 scarcely fail to have a number of colonies upon the surface of the 

 culture medium, which have been developed from atmospheric germs 

 which were deposited upon it during the exposure. Each of these 

 colonies, as a rule, is developed from a single bacterium or spore, 

 and consequently the little mass, visible to the naked eye, which we 

 call a colony, is a pure culture of a particular species. In this ex- 

 periment we are more apt to have colonies of mould fungi than of 

 bacteria, but the principle is the same, viz. , that a colony developed 

 from a single germ is a pure culture. By touching our platinum 



