As I expect you know, it's a general characteristic of metals that they form positive ions when exposed to oxygen. This is the oxidiation process that causes iron to rust. It's slower with silver (the most familiar antimicrobial nanoparticle), but Ag+ ions will form on any silver surface exposed to air. What's more, it turns out bacterial cell walls are negatively charged, so they stick to silver nanoparticles (presumably displacing any stabilizing groups that may be present in the silver nanoparticle preparation). This binding process may disrupt the cell wall, or allow toxic silver ions or reactive oxygen species to enter the cell. You can read more about these processes here:
