Efflux (microbiology)

Active efflux is a mechanism responsible for moving compounds, like neurotransmitters, toxic substances, and antibiotics, out of the cell; this is considered to be a vital part of xenobiotic metabolism. This mechanism is important in medicine as it can contribute to bacterial antibiotic resistance.

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 may accommodate multiple drugs, and thus contribute to bacterial multidrug resistance (MDR).

Bacteria

Bacterial efflux pumps

Efflux pumps are proteinaceous transporters localized in the cytoplasmic membrane of all kinds of cells. They are active transporters, meaning that they require a source of chemical energy to perform their function. Some are primary active transporters utilizing adenosine triphosphate hydrolysis as a source of energy, whereas others are secondary active transporters (uniporters, symporters, or antiporters) in which transport is coupled to an electrochemical potential difference created by pumping hydrogen or sodium ions from or to the outside of the cell.
Bacterial efflux transporters are classified into five major superfamilies, based on their amino acid sequence and the energy source used to export their substrates:

  1. The major facilitator superfamily (MFS)
  2. The ATP-binding cassette superfamily (ABC)
  3. The small multidrug resistance family (SMR)
  4. The resistance-nodulation-cell division superfamily (RND)
  5. The Multi antimicrobial extrusion protein family (MATE).

Of these, only the ABC superfamily are primary transporters, the rest being secondary transporters utilizing proton or sodium gradient as a source of energy. Whereas MFS dominates in Gram positive bacteria, the RND family was once thought to be unique to Gram negative bacteria. They have since been found in all major Kingdoms.

Function

Although antibiotics are the most clinically important substrates of efflux systems, it is probable that most efflux pumps have other natural physiological functions. Examples include:

The ability of efflux systems to recognize a large number of compounds other than their natural substrates is probably because substrate recognition is based on physicochemical properties, such as hydrophobicity, aromaticity and ionizable character rather than on defined chemical properties, as in classical enzyme-substrate or ligand-receptor recognition. Because most antibiotics are amphiphilic molecules - possessing both hydrophilic and hydrophobic characters - they are easily recognized by many efflux pumps.

Impact on antimicrobial resistance

The impact of efflux mechanisms on antimicrobial resistance is large; this is usually attributed to the following:

Eukaryotes

In eukaryotic cells, the existence of efflux pumps has been known since the discovery of P-glycoprotein in 1976 by Juliano and Ling. 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 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.

Efflux inhibitors

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 in cell biology research. Verapamil, for example, is used to block P-glycoprotein-mediated efflux of DNA-binding fluorophores, thereby facilitating fluorescent cell sorting for DNA content. Various natural products have been shown to inhibit bacterial efflux pumps including the carotenoids capsanthin and capsorubin,[2] the flavonoids rotenone and chrysin,[2] and the alkaloid lysergol.[3] Some nanoparticles, for example zinc oxide, also inhibit bacterial efflux pumps.[4]

See also

References

  1. 1 2 Morita, Y.; Sobel, M.L.; Poole, K. (2006). "Antibiotic inducibility of the MexXY multidrug efflux system of Pseudomonas aeruginosa: involvement of the antibiotic-inducible PA5471 gene product.". Antimicrobial Agents and Chemotherapy 188 (5): 1847–55. doi:10.1128/JB.188.5.1847-1855.2006. PMC: 1426571. PMID 16484195.
  2. 1 2 Molnár, 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): 1757–1768. doi:10.2174/156802610792928103. PMID 20645919.
  3. Cushnie, T.P.; Cushnie, B.; Lamb, A.J. (2014). "Alkaloids: An overview of their antibacterial, antibiotic-enhancing and antivirulence activities". International Journal of Antimicrobial Agents 44 (5): 377–386. doi:10.1016/j.ijantimicag.2014.06.001. PMID 25130096.
  4. Banoee, M.; Seif, S.; Nazari, Z. E.; Jafari-Fesharaki, P.; Shahverdi, H. R.; ; Moballegh, A.; Moghaddam, K. M.; Shahverdi, A. R. (2010). "ZnO nanoparticles enhanced antibacterial activity of ciprofloxacin against Staphylococcus aureus and Escherichia coli". Journal of Biomedical Materials Research Part B: Applied Biomaterials 93 (2): 557–61. doi:10.1002/jbm.b.31615. PMID 20225250.
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