Envelope destabilization by AcrAB₂NodT overexpression links antibiotic resistance to metal sensitivity in Caulobacter vibrioides.

Antibiotic (AB) resistance often arises through mutations that alter AB targets, reduce membrane permeability, or increase efflux activity, yet its physiological costs remain poorly understood. In Caulobacter vibrioides, a ΔtipR mutant that constitutively upregulates the RND efflux pump AcrAB₂NodT exhibits heightened sensitivity to copper (Cu), revealing a trade-off linked to excessive pump expression. Deleting acrAB₂nodT or impairing its transport activity restored cell envelope integrity and, ultimately, Cu resistance, uncoupling the effects of pump abundance from active efflux. The ΔtipR mutant also showed increased sensitivity to other metals, including zinc, nickel, and cadmium, indicating a broader vulnerability to metal stress. Mechanistically, pump overexpression depleted the proton motive force and lowered ATP levels, key processes for metal resistance. These findings uncover the physiological cost of efflux upregulation and suggest a potential therapeutic vulnerability: targeting the metabolic burden of resistance mechanisms could improve antibiotic efficacy by exploiting fitness trade-offs.