Upregulation of the AcrAB2NodT efflux pump confers antibiotic resistance at the cost of collateral metal sensitivity

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 ∆tipR mutant that constitutively upregulates the RND efflux pump AcrAB2nodT exhibits heightened sensitivity to copper (Cu), revealing a tradeoff linked to excessive pump expression. Deleting acrAB2nodT or impairing its transport activity restored Cu resistance and cell envelope integrity, uncoupling the effects of pump abundance from active efflux. The ∆tipR 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, lowered ATP levels, and impaired motility 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 tradeoffs.