196 ISOLATION OF BACTERIA IN PURE CULTURE 



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

 (page 148), 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, aldehydes) that may be shown on the plates by 

 various indicators 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 bacteri- 

 ology) may be used. A combination of the general methods 

 (6) and (c) is much used in the separation of the organisms 

 of the "intestinal group" in human practice, (d) The 

 inoculation of a susceptible animal 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. 



