ASSOCIATIVE INTERRELATIONSHIPS 41 



tolerant forms), as well as the physical conditions of the substrate as 

 a whole. 



The presence and abundance of organisms that produce substances having 

 a favorable and stimulating or an injurious and toxic effect upon 

 other organisms, or that may compete for the available nutrients. 

 The equilibrium in the microbiological population in a natural me- 

 dium such as soil or water may be upset by the introduction of spe- 

 cific nutrients, as well as by treatment with chemical and physical 

 agents whereby certain organisms are destroyed and others stimu- 

 lated. 



The presence of specific microorganisms in a natural medium may be con- 

 siderably influenced by the presence of certain parasitic or phagocytic 

 agents. The role of protozoa in controlling bacterial activities by 

 consuming the cells of the bacteria has been a subject of much specu- 

 lation. The presence of bacteria, fungi, and nematodes capable of 

 destroying insects is of great importance in human economy. Many 

 other relationships, such as the presence of phages against specific 

 organisms, are often found greatly to influence the nature and com- 

 position of a specific population. 



ASSOCIATIVE INTERRELATIONSHIPS 



Numerous instances of associative interrelationships among micro- 

 organisms are found in nature. These may be grouped as follows: 



Preparation or modification of the substrate by one organism whereby it 

 is rendered more favorable or more readily available for the growth 

 of another organism. As an illustration one may cite the breakdown 

 of cellulose by specific bacteria, thereby making the particular en- 

 ergy source available to noncellulose-decomposing organisms, in- 

 cluding not only certain bacteria and fungi but also higher forms of 

 life such as ruminant animals (herbivores) and insects (termites, 

 cockroaches), which carry an extensive cellulose-decomposing micro- 

 biological population in their digestive systems. Another illustration 

 is the breakdown of complex proteins by proteolytic bacteria, result- 

 ing in the formation of amino acids and polypeptides, which form 

 f favorable substrates for peptolytic bacteria. The ammonia liberated 



from proteins and amino acids supplies a source of energy for nitrify- 



