Silver nanoparticles (nano-Ag) exhibit antibacterial activities, which are influenced by their oxidation state, size, and dispersion in electrolytic solutions. Spherical nano-Ag (average diameter ~9 nm) were synthesized by borohydride reduction of Ag+ ions. Partially oxidized nano-Ag show antibacterial activity, while zero-valent nano-Ag do not. The level of chemisorbed Ag+ on the nanoparticle surface, as indicated by changes in surface plasmon resonance (SPR) absorption, correlates with antibacterial activity. Nano-Ag, like Ag+ in AgNO3 solution, are tolerated by Ag+-resistant bacteria. Antibacterial activity is size-dependent, with smaller particles showing higher activity per equivalent silver mass. Nano-Ag aggregate in high-electrolyte media, reducing antibacterial activity. However, complexation with albumin stabilizes nano-Ag against aggregation, preserving antibacterial activity. The antibacterial activity of nano-Ag depends on chemisorbed Ag+ and oxidized surfaces present in well-dispersed suspensions. Silver-resistant bacteria have developed mechanisms such as active efflux, periplasmic sequestration, and reduced outer-membrane porins to resist Ag+. Nano-Ag interact with bacterial membranes, causing proton motive force dissipation and cell death. Studies show that nano-Ag deliver antimicrobial activity through Ag+ interactions, similar to AgNO3 solution. The oxidation state of nano-Ag is highly sensitive to oxygen, leading to partial oxidation and chemisorption of Ag+. This study compares zero-valent and partially oxidized nano-Ag, investigating their antibacterial activities, size dependence, and dispersion in electrolytic solutions. Nano-Ag effects on Ag+-resistant bacteria are also reported. The results indicate that antibacterial activity of nano-Ag is dependent on chemisorbed Ag+ and oxidized surfaces.Silver nanoparticles (nano-Ag) exhibit antibacterial activities, which are influenced by their oxidation state, size, and dispersion in electrolytic solutions. Spherical nano-Ag (average diameter ~9 nm) were synthesized by borohydride reduction of Ag+ ions. Partially oxidized nano-Ag show antibacterial activity, while zero-valent nano-Ag do not. The level of chemisorbed Ag+ on the nanoparticle surface, as indicated by changes in surface plasmon resonance (SPR) absorption, correlates with antibacterial activity. Nano-Ag, like Ag+ in AgNO3 solution, are tolerated by Ag+-resistant bacteria. Antibacterial activity is size-dependent, with smaller particles showing higher activity per equivalent silver mass. Nano-Ag aggregate in high-electrolyte media, reducing antibacterial activity. However, complexation with albumin stabilizes nano-Ag against aggregation, preserving antibacterial activity. The antibacterial activity of nano-Ag depends on chemisorbed Ag+ and oxidized surfaces present in well-dispersed suspensions. Silver-resistant bacteria have developed mechanisms such as active efflux, periplasmic sequestration, and reduced outer-membrane porins to resist Ag+. Nano-Ag interact with bacterial membranes, causing proton motive force dissipation and cell death. Studies show that nano-Ag deliver antimicrobial activity through Ag+ interactions, similar to AgNO3 solution. The oxidation state of nano-Ag is highly sensitive to oxygen, leading to partial oxidation and chemisorption of Ag+. This study compares zero-valent and partially oxidized nano-Ag, investigating their antibacterial activities, size dependence, and dispersion in electrolytic solutions. Nano-Ag effects on Ag+-resistant bacteria are also reported. The results indicate that antibacterial activity of nano-Ag is dependent on chemisorbed Ag+ and oxidized surfaces.