RELATION TO ENVIRONMENT CLASSIFICATION 35 



logical investigation of water, milk, manure, soil, or organic infu- 

 sions always reveals the presence of a large number of different 

 varieties within the same substance. If the food supply in such a 

 natural culture is at all limited in quantity, or the removal of waste 

 products is prohibited, it will usually be found that gradually the 

 numbers of varieties will diminish and a few species, or even only 

 one, will prevail. In the case of milk, for instance, after standing 

 for three or four days at a suitable temperature, two or three 

 varieties will be found to have taken the place of the twenty or 

 thirty, which may have been present originally. 



This behavior is due to the influence which various microorgan- 

 isms exert upon each other and is known as antagonism. Such antag- 

 onism probably depends upon the fact that the metabolic products of 

 the predominant species (the one or ones for whom the special cul- 

 tural conditions are most favorable) inhibit the growth of the less 

 vigorous varieties. Many examples, experimentally supported, of 

 such antagonism, can be given. Thus, the gonococcus is distinctly 

 inhibited by the soluble products of Bacillus pyocyaneus, 21 while in 

 the preesnce of pyogenic cocci it develops luxuriously, and the bacil- 

 lus of plague is completely inhibited when streptococci are present 

 in the culture. 22 



Mutual inhibition may also be due to the monopolizing of the 

 nutrition in the medium by the predominating species or to the 

 change in reaction produced by its growth. This last consideration 

 is probably the secret of the inhibitory effect exerted by acid-pro- 

 ducers upon bacteria of putrefaction, and has received practical the- 

 rapeutic application in MetchnikofPs lactic-acid bacillus therapy, 

 described elsewhere. 



When simultaneous presence of two species in the same environ- 

 ment favors the development of both, the condition is spoken of as 

 symbiosis. Such dependence is not so frequent as antagonism, but it 

 does occur. Examples of such a condition have been observed in 

 cultures containing diphtheria bacilli and streptococci 23 and have 

 been frequently observed in cultures containing both aerobic and 

 anaerobic bacteria, where the former favor the development of the 

 latter by monopolizing the supply of free oxygen. Symbiosis may 



21 Schafcr, Fortschr. d. Med., 5, 1896. 



22 Bitter, Rep. Egypt Plague Com., Cairo, 1897. 



23 Hilbert, Zeit. f . Hyg., xxix, 1895. 



