Escherichia coli (E.coli) and Staphylococcus aureus (S.aureus) are the most typical pathogenic bacteria with a significantly high risk ofbio-contamination,widely existing in hospital and public places.Recent studies on antibacterial materials and the related mechanisms have attracted more interests of researchers.However,the antibacterial behavior of materials is usually evaluated separately on the single bacterial strain,which is far from the practical condition.Actually,the interaction between the polymicrobial communities can promote the growing profile of bacteria,which may weaken the antibacterial effect of materials.In this work,a 420 copper-bearing martensitic stainless steel (420CuSS) was studied with respect to its antibacterial activity and the underlying mechanism in a co-culturing infection model using both E.coli and S.aureus.Observed via plating and counting colony forming units (CFU),Cu releasing,and material characterization,420CuSS was proved to present excellent antibacterial performance against the mixed bacteria with an approximately 99.4 ％ of antibacterial rate.In addition,420CuSS could effectively inhibit the biofilm formation on its surfaces,resulting from a synergistic antibacterial effect of Cu ions,Fe ions,reactive oxygen species (ROS),and proton consumption of bacteria.