Protein complexes that move compounds, generally toxic, out of bacterial cells
Protein TolC, the outer membrane component of a tripartite efflux pump inEscherichia coli.AcrB, the other component of pump,Escherichia coli.
Anefflux pump is anactive transporter in cells that moves out unwanted material. Efflux pumps are an important component inbacteria, particularly in their ability to remove antibiotics.[1][2] The efflux process can also involve the movement of heavy metals, organic pollutants, plant-produced compounds, quorum sensing signals, bacterial metabolites, andneurotransmitters. All microorganisms, with a few exceptions, have highly conserved DNA sequences in their genome that encode efflux pumps. Efflux pumps actively move substances out of a microorganism, in a process known asactive efflux, which is a vital part ofxenobiotic metabolism. This active efflux mechanism is responsible for various types of resistance to bacterial pathogens within bacterial species, the most concerning being antibiotic resistance, as microorganisms can have adapted efflux pumps to divert toxins out of the cytoplasm and into extracellular media.[3]
Efflux systems function via an energy-dependent mechanism (active transport) to pump out unwanted toxic substances through specific efflux pumps. Some efflux systems are drug-specific, whereas others can accommodate multiple drugs using small multidrug resistance (SMR) transporters.[4][5]
Bacterial efflux pumps are classified into five major superfamilies, based on theiramino acidsequence and the energy source used to export their substrates:
Of these, only the ABC superfamily is a primary transporter, the rest beingsecondary transporters utilizingproton or sodiumgradient as a source of energy. Whereas MFS dominates inGram positive bacteria, the RND family was once thought to be unique to Gram negative bacteria. They have since been found in all majorkingdoms.[7]
Efflux pumps generally consist of anouter membrane efflux protein, a middle periplasmic protein, an inner membrane protein, and a transmembrane duct. The transmembrane duct is located in the outer membrane of the cell. The duct is also bound to two other proteins: a periplasmic membrane protein and an integral membrane transporter. The periplasmic membrane protein and the inner membrane protein of the system are coupled to control the opening and closing of the duct (channel). When a toxin binds to this inner membrane protein, the inner membrane protein gives rise to a biochemical cascade that transmits signals to the periplasmic membrane protein and outer membrane protein to open the channel and move the toxin out of the cell. This mechanism uses an energy-dependent, protein-protein interaction that is generated by the transfer of the toxin for an H+ ion by the inner membrane transporter.[8]The fully assembledin vitro andin vivo structures of AcrAB-TolC pump have been solved by cryoEM and cryoET.[9][10]
Although antibiotics are the most clinically important substrates of efflux systems, most efflux pumps probably have other natural physiological functions. Examples include:
TheE. coli AcrAB efflux system, which has a physiologic role of pumping out bile acids and fatty acids to lower their toxicity.[11]
The AcrAB–TolC system inE. coli is suspected to have a role in the transport of the calcium-channel components in theE. coli membrane.[13]
The MtrCDE system plays a protective role by providing resistance to faecal lipids in rectal isolates ofNeisseria gonorrhoeae.[14]
The AcrAB efflux system ofErwinia amylovora is important for this organism'svirulence, plant (host) colonization, and resistance to plant toxins.[15]
The MexXY component of the MexXY-OprM multidrug efflux system ofP. aeruginosa is inducible by antibiotics that target ribosomes via the PA5471 gene product.[16]
Efflux pumps have also been shown to play a role inbiofilm formation. However, the substrates for such pumps, and whether changes in their efflux activity affect biofilm formation directly or indirectly, remain to be determined.[17]
The ability of efflux systems to recognize a large number of compounds other than their natural substrates is probably because substrate recognition is based onphysicochemical properties, such ashydrophobicity,aromaticity andionizable character rather than on defined chemical properties, as in classicalenzyme-substrate orligand-receptor recognition. Because most antibiotics areamphiphilic molecules - possessing both hydrophilic and hydrophobic characters - they are easily recognized by many efflux pumps.[citation needed]
The impact of efflux mechanisms onantimicrobial resistance is large; this is usually attributed to the following:
Thegenetic elements encoding efflux pumps may be encoded onchromosomes and/orplasmids, thus contributing to both intrinsic (natural) and acquired resistance, respectively. As an intrinsic mechanism of resistance, efflux pump genes can survive a hostile environment (for example, in the presence of antibiotics), which allows for the selection ofmutants that over-express these genes. Being located on transportable genetic elements such asplasmids ortransposons is also advantageous for the microorganisms as it allows for the easy spread of efflux genes between distant species.[18]
Antibiotics can act as inducers and regulators of theexpression of some efflux pumps.[16]
Expression of several efflux pumps in a given bacterial species may lead to a broad spectrum of resistance when considering the shared substrates of some multi-drug efflux pumps, where one efflux pump may confer resistance to a wide range of antimicrobials.[3]
In eukaryotic cells, the existence of efflux pumps has been known since the discovery ofP-glycoprotein in 1976 by Juliano andLing.[19] Efflux pumps are one of the major causes of anticancer drug resistance in eukaryotic cells. They include monocarboxylate transporters (MCTs),multiple drug resistance proteins (MDRs)- also referred to as P-glycoprotein, multidrug resistance-associated proteins (MRPs), peptide transporters (PEPTs), and Na+ phosphate transporters (NPTs). These transporters are distributed along particular portions of the renal proximal tubule, intestine, liver, blood–brain barrier, and other portions of the brain.
Several trials are currently being conducted to develop drugs that can be co-administered with antibiotics to act as inhibitors for the efflux-mediated extrusion of antibiotics. As yet, no efflux inhibitor has been approved for therapeutic use, but some are being used to determine the prevalence of efflux pumps in clinical isolates and incell biology research.Verapamil, for example, is used to blockP-glycoprotein-mediated efflux of DNA-bindingfluorophores, thereby facilitating fluorescent cell sorting for DNA content. Variousnatural products have been shown to inhibit bacterial efflux pumps, including the carotenoidscapsanthin andcapsorubin,[20] the flavonoidsrotenone andchrysin[20] and the alkaloidlysergol.[21] Somenanoparticles, for example,zinc oxide, also inhibit bacterial efflux pumps.[22]
^Juliano RL, Ling V (November 1976). "A surface glycoprotein modulating drug permeability in Chinese hamster ovary cell mutants".Biochimica et Biophysica Acta (BBA) - Biomembranes.455 (1):152–62.doi:10.1016/0005-2736(76)90160-7.PMID990323.
^abMolnár J, Engi H, Hohmann J, Molnár P, Deli J, Wesolowska O, Michalak K, Wang Q (2010). "Reversal of multidrug resistance by natural substances from plants".Current Topics in Medicinal Chemistry.10 (17):1757–68.doi:10.2174/156802610792928103.PMID20645919.
