| Microbial biocontrol agents are an alternative to traditional chemical fungicides used in agriculture to combat plant pathogenic fungi. Streptomyces violaceusniger YCED-9 was isolated from soil and selected for its ability to antagonize many fungal strains both in vitro and in vivo. The goal of this work was to study the production and chemical characteristics of the antifungal compounds produced by YCED-9, and to investigate their role in plant diseases caused by fungi. Using different liquid media formulations, several indicator strains and standard protocols of extraction, solvent fractionation and liquid chromatography, we purified, characterized and identified three polyketide antibiotics produced by YCED-9 in submerged fermentation. An antibacterial active antibiotic (ABA) with fungistatic activity was produced in an empirically optimized medium. Its production was monitored by bioactivity of YCED culture broths against Bacillus subtilis. Extracts and purification fractions were analyzed. Mass spectrometry, NMR and IR analysis of the purified white powder, and its comparison with a standard, confirmed its identity as nigericin sodium salt (MW = 746). AFA (Anti-Fusarium), a complex of macrocyclic lactone antibiotics was produced only in a rich complex medium. Its production was dissociated from that of nigericin. The main extracellular component of AFA was purified by HPLC and identified as guanidylfungin A (MW = 1129.7) after elemental and spectral analysis. A potent fungicidal, AFA is active against most fungal strains with the exception of the oomycetes. A third compound, the benzoquinone geldanamycin (MW = 560) was abundantly produced in many different media formulations, and was the only metabolite active against Pythium spp. The hydrolases chitinase and {dollar}beta{dollar}-1,3-glucanase were produced in liquid cultures of YCED-9 supplemented with either chitin, laminarin or fungal cell walls. The in vivo antagonism of YCED-9 towards fungal plant pathogens of turfgrass, was studied with a microassay based on pre and post-emergence seedling diseases. These assays and greenhouse experiments confirmed that YCED-9 prevents turfgrass diseases caused by Rhizoctonia solani and S clerotinia homeocarpa. The biocontrol potential of YCED-9 can be partially attributed to the production of antibiotics in natural substrates. YCED-9 produces nigericin in sterile soil and grass rhizosphere. All three antibiotics are produced after colonization of lignocellulose in dry grass and thatch. |
