300 MUTUAL INFLUENCES 



In the lactic fermentation of milk, Bad. lactis acidi combats all 

 other organisms by a rapid production of lactic acid. Though it is pres- 

 ent in fresh milk only in very small numbers, its rapid growth and the 

 formation of acid which will check and even kill most other bacteria 

 soon makes it the dominant organism in the flora of milk, and at the 

 time of curdling, 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 Bad. lactis 

 acidi. This metabiosis can be considered as such only from the stand- 

 point of the lactic organism. The protein bacteria are killed by the 

 acid formed by the rapidly growing lactic bacteria, ^rom the view- 

 point of the protein bacteria, the relation is antibiosis. Another illus- 

 tration 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 produced, 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. 



RELATIONS BETWEEN CELLS OF THE SAME SPECIES 



That cells of the same species will also influence each other, may well 

 be assumed. The simplest relation will be the competition for food. 

 This will be the case in nature more commonly than in laboratory media 

 which are, as a rule, so rich in nutrients that development ceases before 

 all food is used up. 



