Antibiotics Against Plant Disease: VIII. Screening for Nonpolyenic Antifungal Antibiotics Produced by Streptomycetes.

In a survey of Streptomyces species, methods were designed and followed that would specifically select strains capable of producing heat-stable, nonpolyenic, antifungal antibiotics. Of 500 strains grown in shaken flasks, 240 of the culture liquors contained active factors as demonstrated by paper-disc assay against Mucor ramannianus. Culture filtrates and mycelial extracts of the active strains were examined by ultraviolet spectrophotometry; 166 were nonpolyenic as determined by absorption spectra. Heat-stability tests of the nonpolyenic antibiotics over a broad pH range revealed that 15 were stable under all test conditions, 70 moderately stable, and 81 unstable. Culture liquors containing stable, nonpolyenic antifungal agents were chromatographed with eight solvent systems in an attempt to identify the antibiotics. The producing cultures were studied by cross-antagonism tests to discover similarities with producers of known antibacterial antibiotics. Two of the antibiotics produced by promising strains were identified as cycloheximide and musarin. Six antibiotics, presumably new, were detected.