Microbial Equilibiiuin 



271 



producing luiinan and animal diseases, and possibly als(j fungi and 

 bacteria causing plant diseases. A nunlber of theories have been 

 proposed at xarious times in an effort to explain the mechanism of 



Fig. 105. Antagonistic effect of one fungus, Pseiideurotium zonatum (center), 

 upon another, Trichoderma lignorum (from Goidanich et al.). 



antagonism of one organism to another. These theories may be sum- 

 marized as follows: 



1. Exhaustion of available nutrients in the medium or substrate. 



2. Physicochemical changes, produced by growing one organism 

 in a certain medium, which affect the activities of another. These 

 include changes in osmotic pressure, surface tension, oxidation-reduc- 

 tion potential, and reaction. 



3. Certain types of reactions, such as radiation effects, which may 

 be designated as action at a distance. 



4. Space antagonism or competition for available space in a given 

 medium. 



5. Production of specific enzymes, either by the antagonist itself 

 or as a result of autolysis of the antagonized cells, which have the 

 capacity of lysing or dissolving the cells of other organisms. 



