Antibacterial activity and mechanisms of depolymerized fucoidans isolated from Laminaria japonica

•The antibacterial activity of fucoidans from Laminaria japonica was evaluated.•The unprocessed fucoidans did not exhibit obvious antibacterial activity.•The depolymerized fucoidans exhibited excellent antibacterial activity.•Low molecular weight and strong anionic property improve antibacterial activity.•Depolymerized fucoidans inhibit bacterial growth by destroying membrane integrity. Fucoidans, sulfated polysaccharides in brown algae, were depolymerized though high-pressure hydrolysis, and their antibacterial activity, structural properties, and antibacterial mechanisms were investigated in this work. The fucoidans from Laminaria japonica show no antibacterial activity before depolymerization; however, their depolymerized products can effectively (p