BON domain-containing protein-mediated co-selection of antibiotic and heavy metal resistance in bacteria

The widespread antibiotic resistance of bacteria has become one of the most severe threats to public health. However, the mechanisms that allow microbial acquisition of resistance are still poorly understood. In the present study, a novel BON domain-containing protein was heterologously expressed in Escherichia coli. It functions as an efflux pump-like to confer resistance to various antibiotics, especially for ceftazidime, with a >32-fold increase in minimum inhibitory concentration (MIC). The fluorescence spectroscopy experiment indicated that BON protein could interact with several metal ions, such as copper and silver, which has been associated with the induced co-regulation of antibiotic and heavy metal resistance in bacteria. Furthermore, the BON protein was demonstrated to spontaneously self-assemble into a trimer and generate a central pore-like architecture for antibiotic transporting. A WXG motif as a molecular switch is essential for forming the transmembrane oligomeric pores and controls the interaction between BON protein and cell membrane. Based on these findings, a mechanism termed "one-in, one-out", was proposed for the first time. The present study provides new insights into the structure and function of BON protein and a previously unidentified antibiotic resistance mechanism, filling the knowledge gap in understanding BON protein-mediated intrinsic antibiotic resistance.