CULTURE MATERIALS EMPLOYED. 57 



second dilution they were, as a rule, fewer in number 

 and widely separated, so that the individuals of each 

 species were in no way prevented by the proximity of its 

 neighbors from growing in its own typical way. There 

 was then no difficulty in picking out the colonies result- 

 ing from the growth of the different individual bacteria. 



Such, then, are the principles upon which Koch's 

 method for the isolation of bacteria by means of solid 

 media is based. 



The fundamental part of the media employed is the 

 bouillon, which contains all the elements necessary for 

 the nutrition of most bacteria, the gelatin being em- 

 ployed simply for the purpose of rendering the bouillon 

 solid. The medium on which the organisms are growing 

 is, therefore, simply solidified bouillon, or beef tea. 



In practice, two forms of gelatin are employed the 

 one an animal or bone gelatin, the ordinary table gelatin of 

 good quality ; and the other a vegetable gelatin, known 

 as agar-agar, or Japanese gelatin, which is obtained 

 from a group of algae growing in the sea along the coast 

 of Japan, where it is employed as an article of diet by 

 the natives. 



Aside from these differences in origin of the two forms 

 of gelatin employed, their behavior toward heat and 

 toward bacteria renders them of different application in 

 the bacteriological work. The animal gelatin liquefies at 

 a much lower temperature, and likewise solidifies at a very 

 much lower temperature 1 , than does the agar-agar. Ordi- 

 nary gelatin liquefies at about 24 C., and becomes solid 

 at from 8-10 C. It may be employed for those organ- 

 isms which do not require a higher temperature for their 

 development than 22 C. Agar-agar, on the other hand, 

 i Iocs not liquefy until the temperature has reached al>out 



