A number of procedures may be used to greatly facilitate the above methods of isolation by taking advantage of the different physiological properties of different organisms in a mixture such as ability to form spores, different resistance to antiseptics, special food requirements, and pathogenic properties. (_a_) If material contains resistant spores, it may be _heated to temperatures high_ enough to kill all of the organisms except the spores (80° for half an hour, for example) and then plated out. Or (_b_) _an antiseptic which restrains the growth_ of some organisms and not others may be placed in the culture media (carbolic acid, various anilin dyes, (p. 162), 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 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 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. CHAPTER XIX. STUDY OF INDIVIDUAL BACTERIA--STAINING.