184 MUTUAL INFLUENCES. 



it is often difficult to find any other organisms besides the lactic bacteria. 

 In the preceding chapter was mentioned the metabiosis of certain protein- 

 digesting bacteria with Bact. lactis acidi. This metabiosis can be con- 

 sidered as such only from the standpoint of the lactic organism. The 

 protein bacterium is killed by the acid formed by the rapidly growing 

 lactic bacteria. From the viewpoint of the protein bacteria, the relation is 

 antibiosis. Another illustration of antibiosis is the acetic fermentation. 

 The formation of acetic acid prevents the development of all bacteria and 

 of most yeasts and molds. 



In all these cases, the deciding agent is a well-known chemical com- 

 pound. In other combinations, the principle is unknown. Bact. lactis acidi 

 will check the growth of B. subtilis not only in milk where it forms acid, 

 but also in sugar-free broth where acid production is impossible. Acetic 

 bacteria act upon the yeast cells not only by means of the acetic acid pro- 

 duced, but also by some other, unknown agent, since vinegar is more 

 injurious than the corresponding amount of pure acetic acid in water. 

 A very remarkable organism is Ps. pyocyanea; it secretes a substance, 

 pyocyanase, which will kill and dissolve the cells of other bacteria rapidly. 



Parasitism, which would be classified under antibiosis, has not been 

 found to exist among bacteria or yeasts; but we know of cases where one 

 mold grows on the other; this is especially true with the largest represen- 

 tatives of the mucor family, which are often attacked and sometimes 

 killed by smaller fungi. 



