ANTAGONISTIC I'HOlMlliTIKS 



211 



to the i);it lioticii. the (l(\<2;i'(>(> of scnhhiiiii; on 

 tlio test potatoes was reduced Iroiii 100 per 

 cent to nil. 



(loss tound, ho\\(>\'er, thai the i;('neral soil 

 niicrofioi-a ratlier than eei'tain specific or- 

 «>;anisnis ha<l a conlrollinji; elTect upon tlie 

 development of seal); inoculation of soil 

 with .S'. pracco.r alone '^iwv neji,"ati\'e results. 

 Sanford and Connack also wow unal)le to 

 obtain control of potato seal) hy tiie simul- 

 taneous inoculation with «S'. scabies- and S. 

 praccox of sti^am-sterilized soils or of natural 

 soil (Muiched with green plant materials; 

 these organisms were found to be perfectly 

 compatible on potato-glucose agar as well 

 as in a steam-sterilized soil. It was suggested 

 that the control of scab on potatoes ob- 

 tained by Millard was possibly due not to 

 the direct action of «S'. praccox but to certain 

 other luidetermined microorganisms favored 

 by the presence of the green manure or by 

 other undetermined conditions. 



In their studies on the decomposition of 

 plant residues by pure and mixed cultures 

 of microorganisms, Waksman and Hutchings 



showed that act inoniycetes exert an antago- 

 nistic elTect upon the acli\-ities of other 

 microorganisms. Thus the ground work was 

 laid foi' the systematic study of the antago- 

 nistic properties of act inoniycetes and their 

 ability to pi'oduce antibiotic substances. 



Screeiiiiij:; Programs 



In search for microbes that ha\-e the ca- 

 pacity to produce antibiotics, two jjroce- 

 dures have been generally followed. One is 

 based upon the obser\'ation of bacterial 

 plates that had become infected with a cul- 

 ture from the outside, usually dust, whereby 

 the growth of the contaminant was sur- 

 rounded by clear zones in which bacterial 

 growth was inhibited. This was true of nu- 

 merous observations made in microbiological 

 laboratories, the most famous of which is 

 that of Fleming on production of penicillin 

 by a mold. The second method consists in 

 isolating \arious organisms from natiu'al 

 substrates and testing them, by the agar 

 streak method, for their ability to inhibit 

 the growth of other bacteria and fungi. These 



Table 51 



Isolation of (int(iyonislic aciinomycetes from various substrates 

 (Waksman, Horning, Welsch, and Woodruff) 



The organisms in group I were the most active antagonists, those in groups II and III had more 

 limited antagonistic properties, and those in group IV showed no antibacterial effects with the meth- 

 ods used. 



