Neonicotinoid thiacloprid transformation by the N2-fixing bacterium Microvirga flocculans CGMCC 1.16731 and toxicity of the amide metabolite

Abstract Thiacloprid is a widely-used neonicotinoid insecticide, but its enzymatic conversion and the toxicity of the amide metabolite are poorly understood. Here, a N2-fixing bacterium, Microvirga flocculans CGMCC 1.16731, was reported to metabolize thiacloprid via hydration and hydroxylation to thiacloprid amide and 4-hydroxy thiacloprid respectively. M. flocculans transformed 90.5% of 0.63 mmol/L thiacloprid in 30 h with a half-life of 9.0 h. In soil, the bacterium transformed 92.4% of 80 μmol/kg soil thiacloprid in 9 d. A cobalt-type nitrile hydratase (NHase) composed of an α-subunit (TnhA) and a β-subunit (TnhB) converted thiacloprid to thiacloprid amide. Co-expression of activator (TnhC) with NHase could improve the TnhA solubility and therefore enhanced 4-folds higher NHase activity. The NHase produced recombinantly in Escherichia coli transformed 97% of 0.76 mmol/L thiacloprid in 10 min. M. flocculans NHase had a Km value of 0.63 mmol/L and Vmax of 10.2 μmol/min/mg toward thiacloprid. Thiacloprid amide has higher toxic effect on growth of M. flocculans than thiacloprid, whereas lower toxic on the aquatic invertebrate Daphnia magna. Both thiacloprid and thiacloprid amide inhibited tnhA transcription. This increases our understanding of the enzymatic mechanism of environmental fate of thiacloprid and toxicity of its amide metabolite toward soil microbes and aquatic organisms.