مقالات پذیرفته شده در هفتمین کنگره بین المللی زیست پزشکی
Function of Efflux Pumps in Gram-Negative Bacteria
Function of Efflux Pumps in Gram-Negative Bacteria
Roozbeh Yalfani,1,*
1. Department of Nursing, Faculty of Medical Sciences, Islamic Azad University,Varamin-Pishva branch, Tehran, Iran
Introduction: As indicated by recent kinetic modeling studies and experiments, the interplay between slow uptake and active efflux appears to be finely tuned. Therefore, even small changes in one or both factors can dramatically increase intracellular drug concentrations, rendering the bacteria susceptible to antibiotic treatment. This results in various possible approaches such as (i) the optimization of drugs for better influx and/or efflux avoidance, (ii) the permeabilization of the OM by additional chemosensitizers, or (iii) the inhibition of multidrug efflux pumps. Synergistic approaches between (ii) membrane permeabilizers and (iii) efflux pump inhibitors (EPIs) can also be used to sensitize Gram-negative bacteria to antibiotics.
Methods: In addition to possible acquired resistance mechanisms, Gram-negative bacteria already have a high intrinsic resistance to most clinical antibiotics, a property that can essentially be attributed to the combination of an additional outer membrane (OM) and the presence of powerful multidrug efflux pumps. The highly asymmetric OM of Gram-negative bacteria, which is formed by lipopolysaccharides (LPS) on the outer leaflet and phospholipids on its inner leaflet, represents a significant permeability barrier, particularly for hydrophobic compounds such as bile salts, disinfectants, and most antimicrobials. Consequently, the OM reduces the uptake of antibiotics. However, as a passive barrier alone, it cannot influence the resulting intracellular equilibrium concentrations. Multidrug efflux transporters actively counteract influx across the outer (and inner) membrane. As a result, many antibiotics reach only sublethal concentrations at their sites of action within the bacterium. Not surprisingly, multidrug efflux pumps have been found overexpressed in many clinical isolates.
Results: Efflux pumps are bacterial transport proteins which are involved in extrusion of substrates from the cellular interior to the external environment. These substrates are often antibiotics, imparting the efflux pump expressing bacteria antibiotic resistant phenotype. From the first drug-resistant efflux pump discovered in the 1990s, the development in molecular microbiology has led to the characterization of many efflux pumps in Gram-positive bacteria (GPB) including methicillin-resistant Staphylococcus aureus (MRSA), Streptococcus pneumoniae, Clostridium difficile, Enterococcus spp. and Listeria monocytogenes and Gram-negative bacteria (GNB) such as cinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae, Stenotrophomonas altophilia, Campylobacter jejuni, Pseudomonas aeruginosa, Neisseria gonorrhoeae, Vibrio cholerae and Salmonella spp. Since these transport substrates against a concentration gradient, these efflux pumps are energy dependent. Based on the mechanism by which these derive this energy, the efflux pumps are broadly classified into two categories. The primary efflux pumps draw energy from active hydrolysis of ATP, whereas the secondary efflux pumps draw energy from chemical gradients formed by either protons or ions such as sodium. Efflux pumps have been categorized into five superfamilies, include (i) the ATP-binding cassette (ABC) family, (ii) the small multidrug resistance family, (iii) the major facilitator superfamily, (iv) the resistance-nodulation-division (RND) family, and (v) the multidrug and toxic compound extrusion family.
Conclusion: Here we give a brief explanation about RND. Members of the tripartite Resistance Nodulation cell Division (RND) superfamily are the major multidrug efflux pumps in Gram-negative bacteria. Tripartite RND efflux pumps consist of membrane fusion proteins (MFPs, also known as periplasmic adaptor proteins, PAPs), an RND core component, and an outer membrane factor (OMF), which together form an elongated complex that connects both the inner and outer bacterial membrane. The RND core component is present in the inner membrane (IM) as a homo- or heterotrimer. RND proteins recognize drug substrates and energize the drug efflux at the expense of the proton motive force (PMF). The MFPs build a hexameric ring on top of the RND proteins. In the active complex, this MFP forms a tubular structure, which connects to the open OMF porin in the outer membrane. As a result, substrates can be taken up by the RND core from the periplasm (or the outer leaflet of the IM) and removed from the cell by extrusion through the long MFP-OMF conduit across the OM. Some pathogenic Gram-negative bacteria contain multiple clinically relevant tripartite RND efflux pumps with partially overlapping substrate specificities such as MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXY-OprM from Pseudomonas aeruginosa or AdeABC, AdeFGH, and AdeIJK from Acinetobacter baumannii that are either constitutively expressed (MexAB and AdeIJK), induced by stress, or overexpressed due to mutation. In other Gram-negatives such as Escherichia coli (AcrAB-TolC), Salmonella enterica (AcrAB-TolC), Klebsiella pneumoniae (AcrAB-TolC), Campylobacter spp. (CmeABC), and Neisseria gonorrhoeae (MtrCDE), single RND-tripartite systems appear to be dominant. It is critical that we look for novel strategies to combat the threat of antibacterial resistance. One potential strategy is to target the regulation of bacterial resistance mechanisms and another strategy is to target the regulation of gene expression.