Efflux Pumps

The Flow Equilibrium Model#

Nies (2025) proposed that efflux pumps, not importers, are the primary determinants of intracellular metal speciation.^{[1]Nies 2025 — A Flow Equilibrium Model Controlling Cytoplasmic Transition Metal Cation Pools and Preventing Mis-MetalationDietrich H Nies, Julie A Maupin-Furlow · 2025Open reference 1 ↓}

Metal importers lack specificity—most divalent transition metals are ~0.75 A diameter and enter through the same channels. Metalloregulator control efflux pump expression, adjusting export rates to maintain correct metal concentrations. Metals flow continuously through the cell: import into the Labile Metal Pool, protein binding or efflux.

The balance between import and efflux determines the steady-state labile pool concentration.

Direct Cross-Resistance Examples#

CzcCBA (RND family): Expels cobalt, zinc, and cadmium—but its broad substrate range also includes certain antibiotics. Metal exposure selects for CzcCBA overexpression, conferring antibiotic resistance without any antibiotic exposure.

AcrAB-TolC (RND family): The major multidrug efflux system in Gram-negative bacteria. Copper stress activates the MarR repressor, which induces AcrAB-TolC expression, creating broad-spectrum antibiotic resistance via a metal intermediate.^{[3]Co-Selection of Resistance to Antibiotics, Biocides and Heavy Metals, and Its Relevance to Foodborne PathogensWales AD, Davies RH · 2015Open reference 3 ↓}

TetL: Transports both tetracycline and cobalt. MexAB-OprM: The Pseudomonas aeruginosa multidrug efflux pump that also exports metals; linked to imipenem resistance through co-regulatory circuits with metal efflux operons.

Relevance to Co-Selection#

Efflux pumps are the dominant mechanism of the cross-resistance arm of Co-Selection.^{[2]Baker-Austin 2006 — Co-selection of Antibiotic and Metal ResistanceBaker-Austin C, Wright MS, Stepanauskas R et al. · 2006Open reference 2 ↓}

Unlike co-resistance (where distinct resistance genes ride the same mobile element), cross-resistance requires no genetic linkage—a single pump does double duty. This means cross-resistance can arise in any bacterium with broad-substrate efflux pumps, without horizontal gene transfer.

Reduced outer membrane permeability (another cross-resistance mechanism selected by metals) further reduces antibiotic entry, compounding efflux-mediated resistance.

Efflux Pump Inhibitors (EPIs)#

Blocking efflux could simultaneously resensitize bacteria to both metals and antibiotics. PAβN (phenylalanine-arginine beta-naphthylamide) inhibits RND efflux pumps. Combining EPIs with antibiotics reverses resistance in some clinical isolates.

The challenge: EPIs must be non-toxic to host cells and specific enough to avoid disrupting essential mammalian transporters.

Antimicrobial Metal Strategies#

Antimicrobial-metals exploit efflux pump limitations. Silver, copper, and zinc can overwhelm efflux capacity at sufficient concentrations. Zinc ionophores (e.g., PBT2) bypass the pump entirely by importing zinc faster than efflux can export it.

Combining metals with antibiotics can saturate efflux pumps with metal substrates, reducing antibiotic export capacity.