AcrAB-NodT overexpression drives envelope destabilization and metal sensitivity in Caulobacter vibrioides.

Multidrug efflux pumps are widely studied for the protection they offer bacteria against antibiotic stress, yet the physiological consequences of their chronic overexpression are far less understood. In Caulobacter vibrioides, the resistance-nodulation-division efflux system AcrAB-NodT is a principal mediator of xenobiotic extrusion, regulated under normal conditions by the TetR-type repressor TipR. When this regulatory control is lost and the pump is constitutively overexpressed, the consequences extend well beyond resistance. Sustained AcrAB-NodT activity consumes proton motive force at an elevated rate, diverts cellular resources away from envelope biogenesis, and introduces structural strain through the dense assembly of large trans-envelope complexes. Because NodT differs from classical TolC in its conformational flexibility, its excessive incorporation into the outer membrane generates local discontinuities that weaken lipopolysaccharide organization and compromise the selective permeability barrier. These structural failures carry a direct and measurable cost: cells become significantly more susceptible to transition metals, including copper, zinc, nickel, and cadmium. A destabilized outer membrane allows uncontrolled metal influx, while depleted proton motive force impairs the dedicated metal efflux systems that would otherwise correct the imbalance. The result is intracellular metal accumulation that drives oxidative stress, protein misfolding, and growth inhibition. Envelope stress response pathways, particularly σE and Cpx signaling, engage in an attempt to restore homeostasis, but under chronic overexpression conditions their capacity is repeatedly exceeded. This review integrates structural, regulatory, and physiological evidence to illuminate how efflux hyperactivity and envelope fragility are mechanistically coupled in C. vibrioides, and why this coupling matters for understanding the ecological and evolutionary constraints that govern multidrug resistance in environmental bacteria.