Detection of antibiotic-producing Streptomyces inhabiting forest soils

A modified agar overlay technique was developed for detecting antibiotic production by Streptomyces isolates and compared with two routinely used overlay procedures and the shake flask-disk assay method. Two types of inhibitory patterns could be discerned in the modified overlay method, and one of these was always exhibited by Streptomyces isolates which produced extractable antimicrobial substances. In a preliminary screening of 600 isolates, higher numbers of antibiotic-producing streptomyces were detected with the modified overlay procedure than with the streak or pour overlay method and the shake flask assay. The modified overlay distinguished between antibiotic production and other antagonistic effects. The inhibitory agents extracted after 6 days of culture growth on the modified overlay agar were of a higher concentration and more stable over longer periods of time than those recovered from shake flasks. It was observed that acidoduric streptomyces produced a greater proportion of antibiotics against gram-negative bacteria than did neutrophilic cultures, and this effect was greatest with the isolates from the most acidic soils. The neutrophiles were more active against gram-positive bacteria and, upon antibiotic classification, it appeared that the neutrophilic and acidoduric cultures from various soils produced different antimicrobial compounds.