Chapter 3 



The Search for Antibiotics: 

 Screening Programs 



The detection of antagonists capable of 

 producing antibiotics was at first a matter 

 of chance. It was by chance that one of the 

 bacterial culture plates of Fleming was con- 

 taminated by the now famous Penicillimn. 

 Since then, investigators have tried in many 

 ways to be more systematic in their methods 

 of detecting antibiotic-producers. 



Basic Screening Procedures 



We have already discussed Dubos' en- 

 richment of soil samples with pathogens in 

 an attempt to increase the chances of de- 

 tecting antibiotic-producers. This method 

 would be logical if antibiotics were lytic 

 enzymes which would ha^'e the property of 

 making the pathogen available as food for 

 the antagonist. Since this is not the case, the 

 soil enrichment method has remained one of 

 historical interest with no practical im- 

 portance, as demonstrated in 1946 b}^ Waks- 

 man and Schatz. 



A second approach comprised efforts to 

 induce antagonistic properties in a non- 

 antagonist. Basically the method consisted 

 in confronthig a pathogen with another mi- 

 croorganism in a medium poor in nutrients, 

 in the hope that the microorganism would 

 become antagonistic toward the pathogen. 

 Successful results have been reported by 

 Schiller (1952) in Russia and by Davide 

 (1949) in Sweden. ]\Iany other investigators, 

 including us, have tried this method without 

 success. 



By far the most successful method in the 

 search for antibiotics has consisted in test- 

 ing the antagonistic properties of large num- 

 bers of microorganisms in vitro. The general 

 procedure can be modified in a number of 

 ways. Briefly, the method comprises the fol- 

 lowing steps: (1) The substrate to be studied 

 is plated out on media which permit the 

 growth of actinomycetes. (2) The various 

 actinomycetes are isolated in pure cultures, 

 usually on slants of media favorable for 

 abundant growth of these organisms. (3) 

 Each actinomycete culture is inoculated in 

 Petri dishes containing agar media consid- 

 ered favorable for the production of antibi- 

 otics; the inoculation is usually made as a 

 broad streak so that incubation yields a rib- 

 bon of growth of even width. (4) After 

 growth of the actinomycete, at what is con- 

 sidered a favorable temperature (25-30°C) 

 for a favorable length of time (.3 to 7 days), 

 test organisms, against which antagonists are 

 sought, are streaked at a right angle to the 

 actinomycetic ribbon. (5) After incubation 

 of the test organisms under optimal condi- 

 tions for their growth, the antagonistic po- 

 tentialities of the actinomycetes are esti- 

 mated by the width of the inhibition zone. 

 Such cross-streak tests are illustrated in 

 Chapter 15 of Volume L 



There is, of course, no ideal medium which 

 permits the plating out of a natural sub- 

 strate with the resulting growth of all the 

 actinomycetes pi'esent in the substrate and 



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