50 INTERRELATIONSHIPS AMONG MICROORGANISMS 



THEORIES OF THE NATURE OF 

 ANTAGONISTIC ACTION 



The various theories proposed to explain the mechanism of antago- 

 nistic effects of microorganisms may be summarized under the follow- 

 ing processes: 



Exhaustion of nutrients 



Physicochemical changes in medium 



Pigment action 



Action at a distance 



Space antagonism 



Enzyme action, either directly by the antagonist or as a result of cell 



autolysis, under the influence of the antagonist 

 Production and liberation of antibiotic substances 



Pasteur (710) ascribed the antagonistic effect that aerobic bacteria 

 have upon the anthrax organism to the consumption of the oxygen by 

 the former} the unfavorable influence of normal blood upon the growth 

 of anthrax was believed to be due to competition for the oxygen by the 

 red blood corpuscles. Freudenreich (298) considered the antagonism 

 between Ps. aeruginosa and Bacillus anthracis as due to exhaustion of nu- 

 trients by the former. These studies were soon followed by numerous 

 other investigations in which the exhaustion of nutrients in the media 

 was believed to be responsible for the phenomenon of antagonism j the 

 onset of the stationary phase in bacterial growth was believed (579) to 

 belong here. Change in ^H of medium, exhaustion of nutrients, and 

 accumulation of toxic products were also found to be limiting factors. 



It thus became apparent, even in the early days of bacteriology, that 

 certain changes are produced by microbes in the medium in which they 

 grow which render it unfit for the growth of other organisms. It also 

 was soon recognized that the problem is more complicated than the 

 mere exhaustion of nutrients. The changes in relationship produced by 

 changes in surface tension, in oxidation-reduction potential, in reaction, 

 and in osmotic pressure were suggested as explanations. Among the 

 classical examples of the effect of reaction upon the growth of other 

 organisms is the acidification of milk by lactic acid bacteria. Metchnikov 

 emphasized the fact that Lactobacillus bulgaricus acts antagonistically 



