Fungal cell-wall lytic enzymes, antifungal metabolite(s) production, and characterization from Streptomyces exfoliatus MT9 for controlling fruit-rotting fungi.

Abstract

An antifungal actinomycete strain MT9 was isolated from Loktak Lake, Manipur, India and its cultural characteristics, fatty acid methyl ester, 16S rRNA gene analysis suggests that strain MT9 is identical to Streptomyces exfoliatus. Strain MT9 displayed strong and broad-spectrum antagonism towards several fruit-rotting fungi by mycelial growth suppression. Crude fungal cell-wall lytic enzymes, i.e., chitinase, β-1,3-glucanase, and protease produced by S. exfoliatus MT9 were optimally active at pH 8.0 and 50 °C, pH 5.0 and 60 °C, pH 9.0 and 70 °C, respectively. All three mycolytic enzymes had good stability over a wide pH range of 5.0-10.0, with protease being more thermostable than both chitinase and β-1,3-glucanase. Interestingly zymogram analysis revealed that S. exfoliatus MT9 secretes six distinct chitinase isoenzymes with approximate molecular weights of 9.42, 13.93, 27.87, 36.43, 54.95, 103.27 kDa, six active protease isoenzymes with apparent molecular weights of 12.45, 30.20, 37.45, 46.32, 52.46, 131.46 kDa, and an active band of 119.39 kDa as β-1,3-glucanase enzyme. Extracellular fluid and its organic solvent extracts also exhibited inhibitory activity to various fruit-rotting fungi. The MIC value of n-butanol extract was 2-25 µg/ml against tested fruit-rotting fungi. Antifungal secondary metabolite(s) was found to be polyene in nature. To the best of our knowledge, this is the first report on extracellular production of fungal cell-wall lytic enzymes and antifungal metabolites by bioactive S. exfoliatus MT9 under submerged fermentation.