The antibacterial polyketides of Aspergillus terreus OUCMDZ- 1925 from Yellow River delta

The Yellow River delta formed from the convergence of the Yellow River mouth and the Bohai Sea possesses a unique wetland environment different from both the sea and the land. This special ecosystem is rich in salt-and alkali-tolerant microorganisms. By means of an integrated chemical and antibacterial screening method, a salt-tolerant fungal strain, Aspergillus terreus OUCMDZ-1925 was isolated and identified from the internal organs of Chelon haematocheilus from the Yellow River estuary, Dongying, Shandong, China. The metabolites of A. terreus OUCMDZ-1925 in solid state fermentation (rice medium) showed antibacterial effects on Bacillus subtilis. Chemical investigation resulted in the isolation and identification of (+)-butyrolactone I (1) with high yield (2.5g/kg), along with the other four minor polyketides, butyrolactone IV (2), butyrolactone II (3), butyrolactone III (4) and methyl 2,4-dihydroxy-3,5,6-trimethylbenzoate (5). Compounds 1–3 and 5 showed strong antibacterial effects on Staphylococcus aureus (ATCC 51650) with MIC values of 0.31μg/mL (positive control, ciprofloxacin, MIC 0.16μg/mL). The moderate antibacterial activities were first observed for compounds 1 and 2 against Enterobacter aerogenes (ATCC 13408), and for compounds 1 and 3 against Bacillus subtilis (ATCC 93151) with MIC values of 6.25μg/mL and 12.50μg/mL, respectively (positive control, ciprofloxacin, MIC 1.25μg/mL and 5.0μg/mL, respectively). In addition, compounds 1–5 and 4 displayed weak antibacterial effects on Escherichia coli (ATCC 25922) and S. aureus (ATCC 51650), respectively.