ISOLATION OF BACTERIA IN PURE CULTURE 183 



in the culture media (carbolic acid, various anilin dyes, 

 excess acid, or alkali, ox bile, etc.), when the more resistant 

 organisms grow on the final plates, the others not. (c) 

 Special food substances (various carbohydrates) from which 

 the organism desired forms special products (acids, alde- 

 hydes) that may be shown on the plates by various indica- 

 tors, is one of the commonest means. Or media in which 

 certain organisms thrive and others not, so that the former 

 soon " crowd out" the latter (unsterilized milk for lactic acid 

 bacteria, inorganic media in soil bacteriology), may be used. A 

 combination of the general methods (b) and (c) is much used 

 in the separation of the organisms of the "intestinal group" 

 in human practice, {d) The inoculation of a susceptible ani- 

 mal with a mixture suspected to contain a given pathogenic 

 bacterium frequently results in the development of the 

 latter in pure culture in the body of an animal, from which 

 it may be readily recovered. In all of the above methods 

 (except Barber's) the first "pure culture" obtained should 

 be "purified" by replating in a series of dilution plates to 

 make sure that it is pure. 



