Toxicity Response of a Beneficial Soil Bacterium Used as a Biosensor to Nanoparticles of Silver and Copper and Zinc Oxides.

Nanoparticles (NP) of Ag, copper oxide and zinc oxide are used medically, and in food safety and agriculture as antimicrobials. However, soil microbes that have beneficial properties in nutrient cycling and plant performance could be adversely affected by release of NP into the environment. Our research focuses on the effect of NPs of Ag, Cu oxides and Zn oxides on the beneficial, plant-colonizing soil bacterium, Pseudomonas putida KT2440. Pseudomonads are common soil microbes that function in plant health and carbon cycling. This isolate was engineered as a biosensor by transfer of a plasmid containing luxAB genes that endow light production. A toxic response is observed as a loss in light output relative to the control. Toxicity was observed for the NPs of Ag, Cu oxide and Zn oxide and for the free metal ions when exposed at doses between 0.1 to 10 mg/L. The threshold for response to NP Ag and Ag ion was the same at 0.2 mg/l. The cells were more sensitive by a factor of ten to Cu and Zn ions, with threshold concentrations of 0.5 mg/L. Loss in light output correlated with non-culturability for NP-Ag and Cu ions, reduced culturability for nano-CuO and bacteriostasis for nano-ZnO and Zn ions at a 10 mg/L dose. Toxicity did not correlate with induced cell lysis. Field flow fraction measurements showed that these commercially prepared CuP and ZnO NPs agglomerated, forming particles with a mean size of 390 nm, although particles of about 5 nm also were present. Removal of the agglomerated particles of CuO by filtration showed that filtrates retained toxicity. Our current studies are determining whether this activity resides with metal ions released from the NPs or with the nanosized complexes.