Antibacterial activity of seawater-derived Bacillus subtilis strain SC2 and its bioactive fraction

The increasing prevalence of multidrug-resistant (MDR) bacterial pathogens highlights the need to explore microorganisms from under-investigated environments as potential sources of antibacterial metabolites. In the present study, marine Bacillus isolates were recovered from seawater and screened for antibacterial activity against three MDR Escherichia coli isolates. Twenty isolates were obtained, of which eight exhibited antibacterial activity and were subjected to secondary screening following solvent extraction. Among the tested extracts, the chloroform extract of isolate MBCP-8 showed the strongest inhibitory activity against all three MDR E. coli strains. Based on 16 S rRNA gene sequencing and phylogenetic analysis, MBCP-8 was identified as Bacillus subtilis strain SC2 and deposited in GenBank under accession number PP854588. The active chloroform extract was further fractionated by silica-gel column chromatography, and Fraction 3 exhibited the highest antibacterial activity. Broth microdilution testing was subsequently performed to determine the minimum inhibitory concentration (MIC) of Fraction 3. UPLC-UV-QTOF-MS profiling revealed two prominent molecular features tentatively annotated as discodermolide and 2-undecyl-4(1 H)-quinolinone N-oxide based on mass spectral/database matching. However, these annotations require confirmation using authentic standards and orthogonal structural techniques, including high-resolution MS/MS and 1D/2D NMR spectroscopy, before definitive compound identification. Overall, the findings demonstrate the antibacterial potential of seawater-derived B. subtilis SC2 and provide a basis for further purification, structural validation, mechanistic investigation, and evaluation of its metabolites against MDR pathogens.