86 BACTERIA AS ANTAGONISTS 



fhosa can be antagonized by a number of different soil bacteria, of 

 which Ps. fluorescens exhibits the strongest effect. He observed that al- 

 though P. vulgaris acted more rapidly, the active substance did not dif- 

 fuse to so great a distance in the medium, thus pointing to a different in- 

 hibition mechanism. Mixed cultures showed greater activity than pure 

 cultures, either because the latter lost their antibiotic property when 

 grown for a long time on artificial media or because mixed cultures com- 

 prise two or more species with a greater combined action. The antago- 

 nistic substances produced by these bacteria were active at 37° C, where- 

 as at ice-chest temperature the action was delayed so that the pathogen 

 had an opportunity to develop. This was believed to offer a possible ex- 

 planation for the fact that when water supplies become contaminated 

 in cold weather, their power of producing infection 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. ty fhosa 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 ( 1 02 5 ) 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 {S^^)-, 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. Ps. fuo- 

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

 comma, Chrom^obacterium, 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- 



