Cytotoxic Effect Associated with Overexpression of QNR Proteins in Escherichia coli

Objective: The objective was to evaluate the cytotoxic effect associated with overexpression of multiple Qnr-like plasmid-mediated quinolone resistance (PMQR) mechanisms in Escherichia coli. Methods: Coding regions of different PMQR genes (qnrA1, qnrB1, qnrC, qnrD1, qnrS1, and qepA2) and efsqnr were cloned into pET29a(+) vector and overexpressed in E. coli BL21. E. coli BL21 with and without an empty pET29a(+) vector were used as controls. The cytotoxic effect associated with PMQR mechanism overexpression was determined by transmission electron microscopy and viability assays. Results: Overexpressed qnr genes produced loss of bacterial viability in the range of 77-97% compared with the controls, comparable with loss of viability associated with EfsQnr overexpression (97%). No loss of viability was observed in E. coli overexpressing QepA2. In transmission electron microscopy assays, signs of cytotoxicity were observed in E. coli cells overexpressing EfsQnr and Qnr proteins (30-45% of the bacterial population showed morphological changes). Morphological changes were observed in less than 5% of bacterial populations from the control strains and E. coli overexpressing QepA2. Conclusions: Overexpression of qnr genes produces a cytotoxic cellular and structural effect in E. coli, the magnitude of which varies depending on the family of Qnr proteins.