Assessing bioavailability and genotoxicity of heavy metals and metallic nanoparticles simultaneously using dual-sensing Escherichia coli whole-cell bioreporters

Genetically engineered bacterial strains called whole-cell bioreporters (WCBs) generated by fusing the promoter region of stress-responsive genes and reporter genes have been widely used as biosensors to detect toxic materials in the environment. In this study, we report a dual-sensing WCB harboring recAp::egfp and zntAp::mcherry to measure the genotoxicity and bioavailability of heavy metals and metallic nanoparticles (NPs) simultaneously. Since the dual-sensing WCB harbored recAp::egfp and zntAp::mcherry, the genotoxicity and bioavailability of heavy metals and metallic NPs that activate ZntR would be assessed by measuring the fluorescence signal of enhanced green fluorescent protein (eGFP) and mCherry, respectively. Among the tested heavy metal(loid)s, only Cr induced both eGFP and mCherry expression, and some of them only induced mCherry, thereby suggesting that Cr is genotoxic. In case of the tested metallic NPs, Ti2O NPs, ZnO NPs, and Au NPs showed weak inhibitory effects on growth, but the eGFP was not induced. It was inferred that the tested NPs were not genotoxic and the inhibitory effects would not be related to direct DNA damage pathways. In addition, it was observed that ZnO NPs induced mCherry expression, indicating that the Zn ion was dissolved from the NPs. Although the dual-sensing WCB described here was limited to ZnO NPs, WCBs would be an alternative tool to investigate the dissolution of metallic NPs when the corresponding metal ion sensing systems were available.