Nanoparticles (NP) of Ag, CuO and ZnO have antimicrobial activity. NP’s release into the environment may be harmful to the efficiency of beneficial microbes such as those that improve plant growth or degrade pollutants. Antimicrobial effects of Ag, CuO and ZnO NP were examined using a Lux biosensor, as well as by assays of culturability. The biosensor strain PP_0588, constructed in Pseudomonas putida KT2440 harbored a plasmid bearing the luxAB reporter genes. Strain KT2440 colonizes plant roots and is derived from an isolate with bioremediation potential. Loss in light output was observed with each NP. Loss in culturability parallel loss in light output only for Ag and CuO NP. With nano-ZnO, loss in light output correlated with temporary stasis in growth. Treatment at 108 cells/ml showed toxicity at 0.2-0.3, 7-10 and 7-10 mg/L for nano-Ag, nano-CuO and nano-ZnO, respectively. A mixture of nano-Ag and nano-CuO was more effective than either NP alone. Thus, the NP examined had deleterious effect on the environmental pseudomonad examined.