Silver nanoparticles have antibacterial activity. However, the nano particles are unstable and easily form aggregates, which decreases their antibacterial activity. To improve the dispersion stability of silver nanoparticles in aqueous media and to increase their effectiveness as antibacterial agents, we coated triangular plate-like silver nanoparticles (silver nanoplates, Ag NPLs) with one or two layers of gold atoms (Ag@AulL NPLs and Ag@Au2L NPLs, respectively). These gold coatings improved the dispersion stability in aqueous media with high salt concentrations. Ag@AulL NPLs showed stronger antibacterial activity on pathogenic bacteria than Ag NPLs and Ag@Au2L NPLs. Furthermore, the Ag@AulL NPLs decreased the number of bacteria in RAW 264.7 cells. The Ag@AulL NPLs displayed no cytotoxicity towards RAW 264.7 cells and could be used as antibacterial agents for intracellular bacterial infections.