DIVISION IV. 



MUTUAL INFLUENCES. 



INTRODUCTION. 



The biological relations of microorganisms are of the greatest im- 

 portance in nature. Pure cultures in nature are very rare and of exceptional 

 occurrence; they are hardly ever found except in certain diseases of man, 

 animals and plants. Generally, nature works with mixed cultures. All 

 natural fermentations, decompositions and putrefractions are accom- 

 plished by a number of different species among which perhaps one domi- 

 nates, but is influenced by the rest. The study of the mutual relations of 

 microorganisms is in the very first stage as yet; practically all laboratory 

 work is done with pure cultures. The experiences obtained with pure 

 cultures are not sufficient to explain all microbial activity in nature. 



There are many possibilities of mutual influence between different 

 organisms. Generally three main cases are distinguished: symbiosis, 

 where two organisms profit by the combination; metabiosis, where one 

 profits by the other's action without benefiting the other in return, and 

 antibiosis, where one organism injures the other. These cases cannot be 

 separated strictly. The relations are not always constant through the 

 entire development of the cultures; an originally beneficial influence 

 may change to an injurious one in a few days. Many terms have been 

 coined to designate all these various possibilities, but in order to avoid 

 this multiplicity of more or less indefinite names for the various relations, 

 the general term "association" has come into use, especially when the 

 relationship is not well understood. 



SYMBIOSIS. 



Symbiosis is not very common among microorganisms, and it is 

 difficult to find examples where true symbiosis exists through the entire 

 development of both organisms. The association of lactic bacteria and 

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



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