PMC:3266646 / 15478-25635 JSONTXT

{"project":"2_test","denotations":[{"id":"22303296-14706082-36966579","span":{"begin":163,"end":167},"obj":"14706082"},{"id":"22303296-16845433-36966580","span":{"begin":178,"end":182},"obj":"16845433"},{"id":"22303296-19560847-36966581","span":{"begin":194,"end":198},"obj":"19560847"},{"id":"22303296-21173183-36966582","span":{"begin":1681,"end":1685},"obj":"21173183"},{"id":"22303296-12456787-36966583","span":{"begin":1904,"end":1908},"obj":"12456787"},{"id":"22303296-21173183-36966584","span":{"begin":2098,"end":2102},"obj":"21173183"},{"id":"22303296-21078928-36966585","span":{"begin":2871,"end":2875},"obj":"21078928"},{"id":"22303296-21173187-36966586","span":{"begin":2894,"end":2898},"obj":"21173187"},{"id":"22303296-19207745-36966587","span":{"begin":3117,"end":3121},"obj":"19207745"},{"id":"22303296-11601985-36966588","span":{"begin":3488,"end":3492},"obj":"11601985"},{"id":"22303296-8595853-36966589","span":{"begin":3761,"end":3765},"obj":"8595853"},{"id":"22303296-10648517-36966590","span":{"begin":3788,"end":3792},"obj":"10648517"},{"id":"22303296-17114320-36966591","span":{"begin":4256,"end":4260},"obj":"17114320"},{"id":"22303296-9765578-36966592","span":{"begin":4729,"end":4733},"obj":"9765578"},{"id":"22303296-15995185-36966593","span":{"begin":4792,"end":4796},"obj":"15995185"},{"id":"22303296-11929524-36966594","span":{"begin":4852,"end":4856},"obj":"11929524"},{"id":"22303296-16845433-36966595","span":{"begin":5115,"end":5119},"obj":"16845433"},{"id":"22303296-16959966-36966596","span":{"begin":5559,"end":5563},"obj":"16959966"},{"id":"22303296-8905098-36966597","span":{"begin":7455,"end":7459},"obj":"8905098"},{"id":"22303296-9420057-36966598","span":{"begin":7475,"end":7479},"obj":"9420057"},{"id":"22303296-17015663-36966599","span":{"begin":7677,"end":7681},"obj":"17015663"},{"id":"22303296-19207745-36966600","span":{"begin":8148,"end":8152},"obj":"19207745"},{"id":"22303296-12407121-36966602","span":{"begin":9043,"end":9047},"obj":"12407121"}],"text":"Efflux pumps\nThe role of efflux pumps in conferring antibiotic resistance and multi-drug resistances in bacteria has been extensively studied and reviewed (Poole, 2004; Piddock, 2006; Martinez, 2009; Nikaido and Pages, 2011). Efflux systems conferring drug resistance typically belong to five main families: the ATP-binding cassette (ABC) transporter, the major facilitator superfamily (MFS), the small multi-drug resistance (SMR), the multi-drug and toxic-compound extrusion (MATE), and the resistance nodulation division (RND) families, the latter present only in Gram-negative bacteria and chromosomally located. The structural genes for these systems can be located on transferable genetic elements and constitute the main acquired mechanisms for drug resistance (e.g., the Tet and the CmlA/FloR efflux systems families for tetracycline and chloramphenicol resistance, respectively). However, bacteria are intrinsically provided with chromosomally encoded efflux systems that are believed to participate in the cell homeostasy, by extruding endo and/or exogenous toxic compounds, heavy metals, virulence factors, quorum sensing signal, etc. In Gram-negative bacteria, RND systems exhibit a wide substrate spectrum, which usually includes drugs of different classes. Nikaido and Pages (2011) have recently reviewed the role of these efflux pumps in a wide range of pathogenic and opportunistic bacterial species such as E. coli, Klebsiella pneumoniae, Enterobacter spp., P. aeruginosa, Acinetobacter baumannii, and the emergent opportunistic Stenotrophomonas maltophilia. Typically, the expression of RND efflux pumps is finely regulated by a dedicated regulator (Coyne et al., 2011). A more complex regulation network, linking efflux to membrane permeability and other cellular functions, is likely to occur in these bacteria, as described for the mar regulon in E. coli (reviewed by Grkovic et al., 2002). Some RND efflux genes are not expressed in absence of inducing signal, whereas others exhibit a basal level of expression, and therefore contribute to intrinsic resistance (Coyne et al., 2011). Point mutations in a regulator or in the promoter sequence of RND efflux genes can be responsible for their over-expression and, in turn, for enhanced resistance. Similarly, the acquisition of an insertion sequence, carrying a strong and constitutive promoter, upstream of the regulator or the promoter sequence of RND efflux genes, can also mediate their over-expression and cause drug resistance. These systems have been mostly studied in the context of antibiotic resistance; therefore only little information concerning the natural and physiological mechanisms inducing the expression of RND efflux genes exist. Recently, studies have identified the role of oxidative and nitrosative stress in the activation of MexXY and MexEF–OprN, respectively (Fetar et al., 2011; Fraud and Poole, 2011). These stress signals are likely to occur in the environment and might represent natural inducers of the efflux systems expression. The natural role of efflux systems has been extensively reviewed by Martinez et al. (2009) who concluded that the intrinsic role of efflux in the bacterial physiology has lead to the conservation of the genes coding for efflux pumps among species of the same genus. For example, if the over-expression of mdfA confers MDR to E. coli, a basal expression is involved in the Na+(K+)/H+ antiport, that allows the pH homeostasis of the cell (Lewinson and Bibi, 2001). Efflux pumps conferring resistance to antibiotics, such as the AcrAB–TolC from Salmonella spp. has also been shown to efflux bile salts, therefore conferring a selective advantage which allowed colonizing and surviving in human or animal intestines (Lacroix et al., 1996). Mosqueda and Ramos (2000) described the contribution of efflux pumps in the cellular extrusion of toluene, an organic solvent, in Pseudomonas putida. This species, able to grow on the liquid interface of water and toluene and to survive in highly contaminated environments, extrudes the solvent by the TtgABC pump. The genes coding for this RND efflux pump usually exhibit a basal expression level but are induced by the presence of toluene in the medium. In water sediment, Groh et al. (2007) demonstrated that a MexF-like pump from Shewanella oneidensis, further than contributing to resistance to tetracycline and chloramphenicol, confers an increased fitness in anoxic environments. The underlying mechanism is unclear but could involve the extrusion of toxic compounds. A well documented role, for some efflux pumps, is their involvement in the cell to cell communication. This function has been demonstrated for MexAB–OprM in P. aeruginosa (Evans et al., 1998), BpeAB–OprB in Burkholderia pseudomallei (Chan and Chua, 2005) and AcrAB–TolC in Enterobacteriaceae (Rahmati et al., 2002). These RND pumps, further than extruding homoserine lactones, are also able to confer MDR. Moreover, several reports have shown that efflux pumps, notably from the RND family, are involved in mechanisms leading to bacterial virulence. For example, Piddock (2006) highlighted the crucial role of efflux pumps in extruding abiotic substances such as flavonoids during plant colonization and in establishing virulence. In antibiotic producing bacteria, efflux pumps play a crucial role as a self defense mechanism by extruding the bioactive secondary metabolites. For instance, an efflux-mediated self-resistance has been developed in the oxytetracycline-producing Streptomyces rimosus (Petkovic et al., 2006). Bacteria living in the same habitat, being exposed to the produced antibiotics, could either adapt their intrinsic mechanisms, e.g., by the over-expression of an efflux pump, or acquire by horizontal gene transfer the resistance mechanism from the producers. The first option would require a point mutation to over-express a pre-existing efflux system able to pump out the toxic compound, whereas the second pathway would involve the mobilization and transfer of the gene coding for the self-protecting mechanism. Thus, efflux pumps had an ecological role much before they conferred drug resistances in clinics, as they constitute a selective advantage in presence of competing microorganisms. The massive usage of these drugs has further selected optimized mechanisms and enhanced their spread. The role of mobile and mobilizing genetic elements, such as insertion sequences, integrons, transposons, and plasmids, were critical for a successful and rapid spread. Nikaido and Pages (2011) have observed that the rise of resistance due to efflux pumps mechanisms in clinics is tightly linked to the sub-inhibitory concentration of the antibiotics during clinical therapies. Consequently, the appearance of this kind of resistance favors the emergence of other mechanisms such as reduced membrane permeability to drugs, increase of point mutation in the drug target genes or activation of enzymatic resistance mechanisms. It would be of interest to investigate this aspect of resistance development in environmental habitats, where the concentration of antibiotics varies dependent on the degree of pollution and where other selective forces are present. Especially, heavy metals, naturally present in the soil, and solvents produced as consequences of metabolic activities, have been demonstrated to be substrates of several efflux pumps conferring multi-drug resistance (Silver and Phung, 1996; Moken et al., 1997). Concerning heavy metals, pumps have the additional role to defend bacteria from a toxic excess and to maintain the proper intra-cellular concentration for co-factors and enzymes (Teitzel et al., 2006). The presence of these compounds in freshwater could therefore select for the over-expression of an intrinsic efflux pump. Some heavy metals efflux genes, notably from the SMR family, are located on R plasmids containing antibiotic resistance genes, and heavy metals may favor the co-selection of these two features. In the environment, maintenance and propagation of antibiotic resistance genes might have been promoted by heavy metals selection (Martinez et al., 2009). Moreover, a causal relationship between pollution of the water environment by antibiotics or other pollutants agents and the selection of bacteria expressing or over-expressing efflux pumps appears conceivable. Hernandez et al. (2011a) have recently demonstrated in vitro how triclosan, a detergent antibiotic used in cosmetic, binds the regulator SmeT of the SmeDEF pump in S. maltophilia, leading to the over-expression of the pump and consequent multi-drug resistance. This observation is of major concern since S. maltophilia is an aquatic species that can be responsible for nosocomial infection.\nUntil now, it remains unclear how the efflux pumps contribute to the emergence of resistant bacteria in the environment. It has been demonstrated that an efflux pump over-expression could be coupled with a reduced bacterial fitness. However, this is not a general rule. Sanchez et al. (2002) investigated the fitness of two P. aeruginosa mutants over-expressing the MexAB–OprM and MexCD–OprJ efflux pumps, both conferring multi-drug resistance. The authors demonstrated in vitro that the MexAB–OprM over-expressing mutant showed a significantly decreased survival in water compared to the wild type strain, while no significant differences were observed for the second efflux pump mutant. In addition, the production of biofilm in both mutants was not affected if not promoted in the MexCD–OprJ mutant. Production of biofilm implies a higher probability of survival in natural water ecosystem and would thus constitute a beneficial characteristic. Selection in polluted environments of opportunistic species such as P. aeruginosa, S. maltophilia, or A. baumannii, over-expressing efflux systems could contribute to the spread of these bacteria and their introduction into clinics. It would be interesting to focus on the above described mechanisms also in water environments, to gain a better understanding of their physiological function and their role in the emergence of bacterial drug resistance."}