CHAPTER 3 



INTERRELATIONSHIPS AMONG MICROORGAN- 

 ISMS IN MIXED POPULATIONS 



It must not be forgotten that there are extremes in another di- 

 rectiony where one of the two associated organisms is injuring 

 the other, as exemflified by m,any farasites, but these cases I 

 leave out of account here. This state of affairs has been termed 

 antibiosis. — H. M. Ward. 



SYMBIOSIS AND ANTIBIOSIS 



Microbes grow and bring about many metabolic reactions in natural 

 substrates, such as soils and water basins, in a manner quite different 

 from those in pure cultures where they are not influenced by the 

 growth of other organisms. In artificial and natural media, whether 

 these be synthetic materials, complex organic mashes and infusions used 

 for the preparation of industrially essential products, or the bodies of 

 plants and animals, pure cultures of microbes are free from the asso- 

 ciative and competitive effects of other microbes found in natural sub- 

 strates. In mixed populations, a number of reactions that do not com- 

 monly take place in pure cultures are involved. Even in the case of 

 mixed infections, a pathogen may be preceded or followed by one or 

 more saprophytes, whereby the processes of destruction brought about 

 in the living animal or plant body are alleviated or hastened. In the 

 mixed populations found in natural substrates, the ecological relation- 

 ships are largely responsible for many of the essential differences in 

 the behavior and metabolism of the microbes, as compared with the 

 same organisms growing in pure culture. 



Almost all microorganisms inhabiting a natural milieu, such as soil 

 or water, are subject to numerous antagonistic as well as associative, or 

 even symbiotic, interrelations. Every organism is influenced, directly 

 or indirectly, by one or more of the other constituent members of the 

 complex population. These influences were at first visualized as due 

 primarily to competition for nutrients {S"^^)- This was well expressed 



