Graduate Thesis Or Dissertation
 

Diffusible antimicrobial agents produced by Streptomyces from acidic soils : a modified overlay method for detecting antimicrobial activityand partial characterization of the antimicrobial agents

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  • A modified agar overlay technique for the preliminary screening of antibiotic producing Streptomyces spp. has been developed and compared to the routinely used shake flask method and the unmodified agar overlay technique. The percentage of isolates exhibiting antimicrobial activities was found to be greater in the modified overlay technique. Three types of inhibition were normally associated with the modified overlay technique. The first type included those plates of Streptomyces cultures with zones of inhibition not extending past the edges of the colonies of the Streptomyces (inhibition was just over the antibiotic producing organism). The second type showed inhibition zones which did extend past the edges of the Streptomyces colonies but did not show a steady and progressive increase of the zone of inhibition with time. The third type were those plates of Streptomyces isolates which demonstrated inhibition zones between the antibiotic producing organisms and the test organisms with the zones of inhibition increasing steadily and markedly with the time of assay. Antibiotics could not be extracted from types 1 and 2 but could always be extracted from type 3. It was shown that the antibiotics extracted after 2 days of culture growth from the modified overlay plates were of a much higher concentration than those from the shake flask cultures or unmodified overlays extracted at 2 days also. Antibiotics of crude concentrates from the cultivation medium of 38 cultures isolated from three different acidic soils were classified by a systematic paper chromatographic analysis. There were 51 antibiotics which could be classified according to this scheme, and no new classes or subclasses were found.
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