The increasing incidence of antimicrobial resistance and antibiotic toxicity highlights the urgent need for novel antimicrobial agents that are biocompatible and environmentally sustainable. Phlorotannins, a unique class of phenolic secondary metabolites biosynthesized exclusively by brown seaweeds, have emerged as promising bioactive compounds. The present study investigated the antimicrobial potential of phlorotannins extracted from Stoechospermum marginatum against seafood-borne human pathogenic bacteria, including Staphylococcus aureus ATCC BAA 976, Bacillus subtilis ATCC 9372, Listeria monocytogenes ATCC 19111, Escherichia coli ATCC 4157, Escherichia coli AS-EC114 and Klebsiella pneumoniae AS-KP53. Phlorotannins exhibited significant antibacterial activity, with inhibition zones ranging from 16.7 ± 0.33 mm to 21.2 ± 0.44 mm and minimum inhibitory concentrations (MICs) between 32 and 128 µg mL⁻¹. Gram-positive bacteria were more susceptible than Gram-negative strains. Time-kill assays demonstrated concentration- and time-dependent bactericidal effects, with complete inhibition achieved within 6-8 hours of exposure. Furthermore, synergistic interactions were observed when phlorotannins were combined with conventional antibiotics-trimethoprim, imipenem, ciprofloxacin, and cefepime-against multidrug-resistant E. coli AS-EC114 and K. pneumoniae AS-KP53, yielding mean fractional inhibitory concentration indices between 0.16 and 0.31. Scanning electron microscopy revealed substantial structural damage to bacterial cell walls, corroborating the antimicrobial effects. These findings indicate that phlorotannins derived from S. marginatum represent a potent, renewable, and environmentally benign source of natural antimicrobial agents with potential applications in combating antibiotic-resistant pathogens.
Keywords: Seaweed, Stoechospermum marginatum, Phlorotannins, Antimicrobial activity, Minimum inhibitory concentration, Time-kill assay, Synergy.