ANTAGONISTIC INTERRELATIONSHIPS +5 



Various special types of competition, for example, competition between 

 Strains of root-nodule bacteria (Rhizobium), whereby one strain 

 checks completely the multiplication of other strains, even outside the 

 plant, the dominant strain then becoming responsible for all the 

 nodules produced (679). 



These phenomena of competition are found not only in natural sub- 

 strates, such as soil and water, but also in artificial media. When several 

 microbes are growing in the same culture medium, some will be re- 

 pressed in course of time whereas others will survive and take their 

 place. This is due to the fact that these microbes compete for the use of 

 the same nutrients or that conditions, such as reaction, oxygen supply, 

 and temperature, are more favorable to some organisms than to others. 

 Another phenomenon may also be involved, that some organisms may 

 produce toxic substances that repress the growth of others. In artificial 

 media, slowly growing tubercle bacteria, diphtheria organisms, and 

 others will be repressed by the rapidly growing saprophytes. Under 

 aerobic conditions, aerobic bacteria and fungi will repress yeasts and 

 anaerobic bacteria, whereas under anaerobic conditions the reverse will 

 take place. An alkaline reaction will favor the development of bacteria, 

 an acid reaction will favor the growth of fungi. 



ANTAGONISTIC INTERRELATIONSHIPS 



When two or more organisms live together, one may become antag- 

 onistic to the others. The composition of the medium and the conditions 

 of growth influence the nature and the action of the antagonist j the 

 metabolism and cell structure of the antagonized organism may be 

 modified or the cell itself may be destroyed (184). In urine, for 

 example, staphylococci may become antagonistic to E. coli or vice versa, 

 depending on the initial numbers of the two groups, on the formation 

 of metabolic products, or on the exhaustion of nutrients (246). The 

 toxic substances produced by the antagonists comprise a variety of com- 

 pounds, ranging from simple organic acids and alcohols to highly com- 

 plex bodies of protein or polypeptide nature. 



Various types of antagonism are recognized. Nakhimovskaia (670) 





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