The development and spread of antibiotic resistance in bacteria is a universal threat to both humans and animals that is generally not preventable, but can nevertheless be controlled and must be tackled in the most effective ways possible. To explore how the problem of antibiotic resistance might best be addressed, a group of thirty scientists from academia and industry gathered at the Banbury Conference Centre in Cold Spring Harbor, New York, May 16-18, 2011. From these discussions emerged a priority list of steps that need to be taken to resolve this global crisis.

Multidrug efflux pumps cause serious problems in cancer chemotherapy and treatment of bacterial infections. Yet high-resolution structures of ligand transporter complexes have previously been unavailable. We obtained x-ray crystallographic structures of the trimeric AcrB pump from Escherichia coli with four structurally diverse ligands. The structures show that three molecules of ligands bind simultaneously to the extremely large central cavity of 5000 cubic angstroms, primarily by hydrophobic, aromatic stacking and van der Waals interactions. Each ligand uses a slightly different subset of AcrB residues for binding. The bound ligand molecules often interact with each other, stabilizing the binding.

AcrAB is a constitutively expressed, major multidrug efflux system of Escherichia coli. We have purified the cytoplasmic membrane component, AcrB, to near homogeneity, and reconstituted the protein into proteoliposomes. In the presence of ⌬pH (outside acid), the protein catalyzed the extrusion of fluorescent phospholipids, which were then trapped by protein-free acceptor vesicles. Known substrates of AcrAB, such as bile acids, erythromycin, and cloxacillin, inhibited this activity. Addition of various drugs to AcrB-containing proteoliposomes, in the presence of ⌬pH (inside acid) resulted in proton efflux, suggesting that AcrB is a proton antiporter. Interestingly, fluorescent lipid extrusion was accelerated strongly by the periplasmic protein AcrA in the presence of Mg 2؉ , and at pH 5.0 AcrA alone produced a slow mixing of lipids of different vesicles, without causing the mixing of intravesicular material. These results suggest that AcrA brings two membranes together, and under certain conditions may even cause the fusion of at least the outer leaflets of the membranes, contributing to the ability of the AcrAB-TolC system to pump drugs out directly into the medium.

A set of multidrug efflux systems enables Gram‐negative bacteria to survive in a hostile environment. This review focuses on the structural features and the mechanism of major efflux pumps of Gram‐negative bacteria, which expel from the cells a remarkably broad range of antimicrobial compounds and produce the characteristic intrinsic resistance of these bacteria to antibiotics, detergents, dyes and organic solvents. Each efflux pump consists of three components: the inner membrane transporter, the outer membrane channel and the periplasmic lipoprotein. Similar to the multidrug transporters from eukaryotic cells and Gram‐positive bacteria, the inner membrane transporters from Gram‐negative bacteria recognize and expel their substrates often from within the phospholipid bilayer. This efflux occurs without drug accumulation in the periplasm, implying that substrates are pumped out across the two membranes directly into the medium. Recent data suggest that the molecular mechanism of the drug extrusion across a two‐membrane envelope of Gram‐negative bacteria may involve the formation of the membrane adhesion sites between the inner and the outer membranes. The periplasmic components of these pumps are proposed to cause a close membrane apposition as the complexes are assembled for the transport.