This study explores the potential antimicrobial mechanisms of commercial silver nanoparticles (Ag NPs) in the environmental bacterium, Pseudomonas chlororaphis O6. The 10 nm size NPs aggregated in water, as demonstrated by atomic force microscopy. Solubility of the NPs at 10 mg/L was 0.28 mg/L (pH 6) and 2.3 mg/L (pH 7); release from 10 mg/L bulk Ag was below detection. The NPs eliminated cell culturability at 3 mg/L, whereas no effect was observed at 10 mg/L bulk Ag. Zeta potential measurements revealed that the NPs were negatively charged; unlike Ag ions, their addition to the negatively charged cells did not change cell charge at pH 6, but showed a trend to reduce cell charge at pH 7. Isolated extracellular polymeric substances (EPS) from PcO6 was polydisperse, with negative charge that was neutralized by Ag ions, but not by the NPs. Addition of EPS eliminated Ag NP's toxicity in cells lacking EPS. Intracellular accumulation of (OH)-O-center dot was not detected in NP-treated cells; however, the use of scavengers suggested the NPs caused extracellular H2O2 production. No evidence was found for loss of membrane integrity upon treatment with the NPs. Our findings indicate that growth of environmental bacteria could be impaired by Ag NPs, depending on the extent of EPS production. (C) 2011 Elsevier B.V. All rights reserved.