Ethanol extracts of Xanthium sibiricum leaf and three biological antimicrobial agents(ε-poly lysine,nisin and natamycin) were investigated for their single and combined antibacterial activities against six common pathogenic microorganisms.Results showed that the combined treatments revealed remarkable synergistic and/or additive antibacterial activities against these six pathogens.Among them,the ethanol extracts of Xanthium sibiricum leaf and natamycin exhibited the strongest synergistic interaction in inhibiting Penicillium expansum(FICI = 0.375).Furthermore,this mechanism of antibacterial synergism might be attributed to their ability to enhance the permeability of cell membrane,and thereby facilitate the penetration and effective accumulation of natamycin in the bacteria,then cause the rupture of cell membrane,finally lead to the death of bacteria.