BACTERIA AS ANTAGONISTS 81 



ing to a different inhibition mechanism. Mixed cultures showed greater 

 activity than pure cultures, either because the latter lose their antibiotic 

 property when grown for a long time on artificial media or because 

 mixed cultures comprise two or more species with a greater combined 

 action. The antagonistic substances produced by these bacteria were ac- 

 tive at 37° C, whereas at ice-chest temperature the action was delayed 

 so that the pathogen had an opportunity to develop. This was believed 

 to offer a possible explanation for the fact that when water supplies 

 become contaminated in cold weather, their power of producing infec- 

 tion is retained for a longer time than when the contamination takes 

 place in warm weather. 



Frost concluded that the phenomenon of antagonism results in 

 checking the growth of E. tyfhosa as well as in killing the pathogen. 

 Evidence that antagonistic substances exist in an active state in the soil 

 or in water appeared to be lacking j rather, the results suggested that 

 formation of such substances depends on the actual development of 

 specific antagonistic organisms. Changes in environment, such as tem- 

 perature, oxygen supply and reaction of the medium, and nature and 

 concentration of nutrients, were believed to have little or no influence 

 on the production of the antibiotic substances j these were produced 

 under conditions favoring growth of the antagonists. 



The activity of the influenza organism was found (993) to be largely 

 dependent on the presence of accompanying bacteria. Some of these, 

 especially micrococci, are favorable to the growth of this organism 

 whereas others, such as Ps. aeruginosa and B. subtilis, are injurious. 



According to Lewis (525), luxuriant growth of Ps. fluorescens in 

 manured soil and in protein solution containing B. cereus is due to an- 

 tagonistic action of the former organism against the latter. The former 

 also inhibits the growth of B. anthracis, Bacillus megatherium^ V . 

 commay Chromobacterium violaceum, and Rhodococcus. Other species 

 of the genera Bacillus, Eberthella, Sarcina, Neisseria, and Phytomonas 

 are somewhat more resistant to the action of Ps. -fluorescens. Salmonella 

 species are less sensitive, whereas E. coli, A. aerogenes, and S. marces- 

 cens are highly resistant. Ps. fuorescens produces a thermostable sub- 

 stance which is toxic to all bacteria except the green fluorescent forms 



