Associative Effects 265 



about hy tlu-in. Tlu' probltMu was considered nearly solved, once a 

 pure culture was obtained. On the other' hand, the investigator who 

 worked with the soil population as a whole usually polluted it to 

 such an extent b\- the addition of an excess of a single type of 

 material that any natural relationship among microorganisms was 

 thereb)' erased. The information gained from studies of "pure" and 

 "mixed" cultures was patched together to fit the complex natural 

 processes occurring in soils or in water basins, with the result that 

 often a "crazy quilt" arrangement resulted rather than a clear pic- 

 ture of the natural processes. This was due entirely to failure to 

 recognize that many of the processes carried out by microorganisms, 

 and often the very existence of these organisms, are greatly modi- 

 fied in the natural environment as compared with their growth in 

 pine cultures and in the test tube. 



The study of the associative and antagonistic interrelationships 

 among microorganisms, especially the marked interest that has re- 

 cently been centered upon the production by these organisms of 

 antibiotic substances, has resulted in the accumulation of many 

 facts that permit more systematic generalization dealing with the 

 subject under consideration. 



Associative Effects 



The associative influences among microorganisms living in the soil 

 are numerous. They may be classified briefly as follows: 



1. Effect of aerobic organisms upon the growth of anaerobes. 

 The aerobes consume the free oxygen in the soil atmosphere, thus 

 creating conditions that are favorable for the growth of organisms 

 not requiring oxygen, the anaerobes. 



2. Preparation of an essential nutrient or substrate by one organ- 

 ism for the growth of another. This type of relationship is very 

 common in the soil. Nitrite-forming bacteria oxidize ammonia to 

 nitrite, thus producing a substrate which is required for the activi- 

 ties of the nitrate-forming bacteria, since the latter are not able to 

 use any other source of energy. Proteolytic bacteria hydrolyze pro- 

 teins to amino acids, thereby producing substances which are essen- 

 tial for the activities of peptolytic bacteria or of organisms that 

 cannot attack native proteins. Cellulose-decomposing bacteria give 

 rise to organic acids and other intermediary products essential for 

 the activities of various organisms which themselves cannot attack 

 cellulose. 



