Over one hundred pure cultures of filamentous soil bacteria (Actinomycetes) were isolated from 15 independently selected soil samples. The isolated microorganisms were screened for the production of antimetabolites against 4 test strains of bacteria: Pseudomonas aeruginosa, Staphylococcus epidermidis, Escherichia coli, and a tetracycline-resistant E. coli strain containing the plasmid pBR322. Approximately 20% (18) of the isolates produced antibiotics detected by the screening protocol. Included among these were broad and narrow spectrum antibiotics, and a novel antimetabolite that inhibited only the tetracycline-resistant E. coli strain in the presence of tetracycline. The latter antimetabolite showed no independent inhibition of E. coli when tetracycline was not present. The antimetabolite-producing strains, identified as members of the genus Streptomyces, were grown on different culture media for varying lengths of time to optimize antibiotic production. Chemical extraction, purification, and characterization of several of the antimetabolites is in progress.

Victoria C. Heverly  (R. W. Morris, Ph.D. and R. T. St.John, Ph.D.),
Biology Department, Widener University,  Chester, PA 19013.

      Antibacterial antibiotics produced by strains of Streptomyces isolated from soil samples were characterized using a modified Kirby Bauer antibiotic sensitivity screening procedure. Frozen spore suspensions of both narrow and broad spectrum antibiotic producing strains were inoculated into liquid cultures and incubated with vigorous aeration for two weeks at 28 C.  The resulting culture medium contained soluble antimetabolites that inhibited the growth of the same profile of  bacteria as the original isolates did on agar medium. The antimetabolites were purified by ion exchange and Sephadex column chromatography. The column fractions were assayed for antibiotic activity by transferring fractions to sterile filter discs employed in the Kirby Bauer  assay. Chromatographic data for one strain (Ald13) indicated antibiotic activity in multiple fractions of low molecular weight and anionic properties.  These data suggest that the strain may produce more than one molecular species with antimicrobic activity. Further purification and characterization studies utilizing  high performance liquid chromatography and nuclear magnetic resonance are planned.