STUDY OF MICROORGANISMS 47 



an organism, attention is directed chiefly to the chemical 

 changes that it produces in the substances in which it is 

 growing. /. e. t to the biochemical properties of the organism. 

 Culture media an- prepared containing definite chemical 

 substances, and the products formed therefrom are deter- 

 mined. The points to which attention is chiefly directed 

 are the fermentation of various carbohydrates with the for- 

 mation of acids, alcohols, and gases: as well as the action 

 of the organism on different proteins, e. g., casein and iiela- 

 tin. The ability of the organism to produce disease in 

 plants and animals may also be studied. 



The final criterion in the identification of any organism 

 is the determination of its ability to cause a specific effect. 

 Thus the identification of the anthrax bacillus rests on the 

 production, in animals known to be susceptible to anthrax, 

 of a disease that has the characteristics of anthrax. An 

 organism may have the same morphology, the same cul- 

 tural characteristics, and the same physiological character- 

 istics, as determined by the usual laboratory tests, as the 

 true anthrax bacillus; but, unless it produces death in 

 guinea-pigs, with certain definite changes in the tissues, it 

 can not be identified as the anthrax organism. The identi- 

 fication of a pure culture of bacteria is a difficult task', in 

 the solution of which all the tests that science has devised 

 may be used. 



