l82 MUTUAL INFLUENCES. 



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 carbohydrates pro- 

 vided by the plant and furnishing the plant nitrogen from the air which 

 the plant cannot assimilate directly. The typical example 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 pre- 

 viously 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. Conse- 

 quently, 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 nitrate 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 association of lactic bacteria 

 with certain protein destroying organisms. The lactic bacteria often 

 develop much better if the protein bacteria grow together with them or 

 have grown previously in milk. Metabiosis does not require the growth 



