Location of drug efflux pumps and pathways of drug influx and efflux across the OM and IM in Gram-negative bacteria. Location of drug efflux pumps and pathways of drug influx and efflux across the OM and IM in Gram-negative bacteria. The influx of drugs (shown as pills) through the OM occurs in one or more of the following three pathways: porin channels (e.g., OmpF of E. coli and OprF of P. aeruginosa), specific protein channels (e.g., CarO of A. baumannii and OprD of P. aeruginosa for carbapenems), and the LPS-containing asymmetric lipid bilayer region. After their entry into the periplasmic space, the drug molecules can further penetrate the IM via diffusion. However, these drugs can be extruded out of the cell by efflux transporters, which exist as either single-component pumps (“singlet”; e.g., Tet pumps) or multicomponent pumps (e.g., AcrAB-TolC and MexAB-OprM tripartite efflux systems that each typically contain a pump, an OM channel protein [OMP], and an accessory membrane fusion protein [MFP]). While the singlet pumps may take up the drug from the cytosol and the periplasm and function with porins or other types of protein channels to make the efflux process effective, the multicomponent exporters capture their substrates from the periplasm and the IM and directly pump them into the medium. The competition between the influx and efflux processes ultimately determines the steady state of drug molecules in bacterial cells. With the lipophilic drug molecules that cross the OM slowly or the hydrophilic drugs that penetrate the A. baumannii/P. aeruginosa low-permeability porins (i.e., “slow porins”), the efflux mechanism become very effective, thus being able to yield MDR. In contrast, with the less hydrophobic and smaller drug molecules that can rapidly penetrate, for example, E. coli porins, efflux is not effective to counteract drug influx, thus hardly decreasing the concentrations of the drug in the cell. Xian-Zhi Li et al. Clin. Microbiol. Rev. 2015; doi:10.1128/CMR.00117-14