270 Associative and Antagonistic Effects 



Microbial Equilibrium 



The numerous microorganisms inhabiting the soil are Hving largely 

 in a state of mutual equilibrium. Any modification of this equi- 

 librium results in a number of changes in the nature and abundance 

 of the microbiological population. The numerous interrelationships 

 among these organisms permit not only an understanding of their 

 specific ecological nature under a certain set of conditions, but also 

 a better understanding of the metabolic products resulting from 

 the activities of this population. Since the complex nature of this 

 population does not permit its treatment as a whole, certain relation- 

 ships among different organisms may be isolated and examined sep- 

 arately. Attention may be directed, for example, to the relations 

 between the non-spore-forming bacteria in the soil and the spore- 

 formers, of the actinomycetes and the bacteria, of some fungi and 

 other fungi, of bacteria and fungi, of nonpathogenic organisms and 

 pathogens, and of protozoa and bacteria. 



Conn and Bright found that, when Bacillus cereus and Pseudo- 

 monas fliiorescens were inoculated simultaneously into sterile manured 

 soil, the former failed to develop, whereas the latter grew abun- 

 dantly. Lewis reported that Ps. fliiorescens repressed the growth of 

 B. mycoides and of other spore-forming bacteria and micrococci; 

 however, Aerobacter aerogenes and Serratia marcescens were highly 

 resistant; fungi were not inhibited; yeasts were inhibited only to a 

 limited extent, and actinomycetes were more sensitive. Lewis also 

 confirmed results of other investigators that the production of bac- 

 tericidal and inhibitory substances by bacteria depends on the 

 amount of available oxygen; these substances were found to be 

 thermostable and were adsorbed by charcoal and by soil. 



Greig-Smith demonstrated that various actinomycetes are capable 

 of producing substances toxic to bacteria. The fact that actinomy- 

 cetes grow rather slowly suggested to him the possibility that they 

 comprise the factor limiting bacterial development in the soil. Cer- 

 tain actinomycetes were later found to be antagonistic to S. pyogenes 

 and to spore-forming bacteria, but not to Ps. aeruginosa. The latter, 

 because of its capacity to produce pyocyanase, was believed to be 

 capable of vaccinating the substrate against the growth of other 

 microorganisms. 



The antagonistic activities of microorganisms have received par- 

 ticular attention as potential agents for suppressing the growth and 

 even for destroying bacteria and other microorganisms capable of 



