298 MUTUAL INFLUENCES 



development of both organisms. The association of lactic bacteria and 

 Oidium lactis in milk is, for a certain period at least, a symbiosis. The 

 bacterium will produce only a certain amount of acid, and then it can 

 grow no more because the acid is too strong; the mold will destroy the 

 acid and thus gives the bacterium a chance for continued activity. The 

 bacterium produces the acid which the mold likes; the mold in turn 

 removes the excess acid which otherwise would check the bacterial 

 activity. 



True symbiosis is more common in the relation of microorganisms 

 with higher plants and animals. The standard example in the plant 

 kingdom is Ps. radicicola in the nodules of legumes, feeding on carbo- 

 hydrates provided by the plant and furnishing the plant nitrogen from 

 the air which the plant cannot assimilate directly. The typical exam- 

 ple in the animal kingdom is B. coli in the intestine of animals, being 

 nourished by the food of the animal and rendering the food more easily 

 digestible. 



METABIOSIS 



Metabiosis may be considered a one-sided symbiosis; two organisms 

 live together, but only one is benefited, the other remains uninfluenced 

 or later may be injured by the association; the latter case is the most 

 common. In this relation, one usually prepares the food for the other. 

 It has previously been mentioned that the metabolic products of one 

 species serve as food for another species, thus breaking up the various 

 organic compounds step by step to smaller and simpler molecules. 

 Quite commonly, each step is accomplished by a different species of 

 microorganism. Consequently, metabiosis is a very common occurrence 

 among microorganisms. 



The classical example is the two nitrifying bacteria: the nitrate bac- 

 terium is unable to oxidize ammonia, and depends entirely upon the ni- 

 trite bacterium to oxidize the ammonia to nitrite; then, and only then, 

 can the nitrite bacterium grow. 



The relation between yeasts and acetic bacteria is also very well 

 known. The yeast ferments the sugar to alcohol, and then the acetic 

 organisms oxidize the alcohol to acetic acid. The yeast is in no way 

 helped by the acetic bacteria, while these could not form acetic acid 

 from sugar readily. These bacteria depend upon the action of the 

 alcohol-forming yeast. Other cases of metabiosis are found in the 



