ABSTRACT Acinetobacter baumannii has
greatly increased its degree of resistance to become multidrug resistant (MDR)
over the past 30 years and is on the red line of the most widely replicated
bacteria according to World Health Organization (WHO). The efflux pumps are the
main cause for the increasing antibiotic resistance of A. baumannii originated from nosocomial infection. The progressive
resistance of A. baumannii even on
the recent drugs (tigecycline and fosfomycin) reduces to very effective
antibiotic scale. With attention focused on MDR and pan-drug-resistant (PDR) in A. baumannii multiple works on efflux
pumps chemical inhibitor (NMP, PAβN,
omeprazole, verapamil, reserpine, CCCP) are still in progress. Certain
inhibitors from plants (Biricodar and
timcodar, Falvone, Mahonia, Dalea versicolor, Lycopus
europaeus, and Rosmarinus officinalis)
have the capability to have such compounds according to their very significant
synergistic effect with antibiotics. In this review we focused on the growth of
antibiotic resistance to explain the mechanism of efflux pumps into these
different super families and a comprehensive understanding of the extrusion,
regulation and physiology role of drug efflux pumps in the essential
development of anti-resistivity drugs. We recapitulated the evolution of the
work carried out in these fields during the last years and in the course of
elaboration, with the aim of increasing the chances of decreasing bacterial
resistivity to antibiotics.
Acinetobacter spp. was detected around the 20th century (1911) by famous bacteriologist Beijerinck  , but it was not until 1960 that A. baumannii was declared in hospital. A. baumannii belongs to the large family of non-fermentable gram-negative bacteria capable of harming patients in surgical intensive care  . During the 20 past years it has developed a capital importance and its classification among the nosocomial infections makes it a priority to all the public health organizations considering its increase and recurrence  . A. baumannii is much more present in humans and is the origins of multiple diseases like septic fever, pneumonitis pachymeningitis and other disease  . Over time it has gained its resistance through diverse modifications and is presently resistant to approximately all the various groups of antibiotics even the most widely used drugs (fluoroquinolones, macrolides, trimethoprim, b-lactams, tetracyclines, aminoglycosides, and chloramphenicol)  . The bacterial efflux operation causes the formation of toxins and rejects antibiotics from the cells, which confers a specific invulnerability to antibiotics. Multidrug resistant (MDR) efflux pumps are now present in almost all microorganisms, in which bacteria is one of the main causes of obstruction to action of drugs  ; several works have concluded that MDR is on origin of the decline progressive of drugs sensitization by bacterial mutation  that reduces largely the valid drug for cure. However using inhibitor components could restore bacterial susceptibility to antimicrobial agents. Efflux pumps inhibitors’ (EPIs) synthetic or natural component is the potential drugs for treatment of MDR or PDR A. baumannii. After describing the general mechanisms of efflux pumps systems in bacterial resistance, we will explain regulation and physiology role of drug efflux pumps in the essential development of anti-resistivity drugs and report the evolution of the work done during the recent years especially in EPIs.
2. Mechanisms of Bacterial Resistance to Antimicrobial Agents
The bacterial resistance to antibiotics has emerged in the face of several pathogenic agents, besides A. baumannii, albeit efforts to treat these pathogenic germs still progressing  . Bacterial pathogens that have shown resistivity to a single drug or to several agents are considered MDR bacterial. With all the efforts united to resolve the problem via the outgrowth of new line of antibiotics, the bacteria does not cease also to mutate quickly to acquire new mechanism of resistivity or to improve their resistance to antibiotics         . Many reports showed that bacterial resistance to antibiotics is believed that the pathogenic bacterium transfers certain genetic gene of resistance to drugs from one species to another and automatically acquires resistant phenotypes against the majority of the pre-existing antimicrobial agents  . This mechanism of opposition poses a critical problem to bacterial treatments. Another significant cause of this obstruction is the proximity of drugs to environment, agriculture and others, leading to the emergence and development of resistances. Clinically, a low or high or inappropriate use level of antibiotics will also imply in the increase in bacterial resistance  . Presently following many research there exist different mechanisms responsible for the bacterial resistance in addition 1) modification of drug target, 2) drug inactivation by enzymes, 3) modification of cell wall protein, and 4) activation of drug efflux system.
2.1. Alteration Drugs Target
Every drug has a specific target for destruction of bacteria, and as such when the target is changed bacteria can easily resist to antimicrobial agents, and these have been observed in quinolones (DNA gyrase variation or Qnr intercede purpose defense), aminoglycosides (16S rRNA methylation), and β-lactams (transformation in penicillin junction proteins). Reduced vulnerability to minocyclin and tigecyclin occurs via transformation in gene encoding S-adenosyl-L-methionine-dependent methyltransferase  . After undergone several transformation in gene lpx, pmrB and outer membrane induce the structure of lipopolysaccharide for causing polymyxin resistance    .
2.2. Drug inactivation by A. baumannii Enzyme
A. baumannii synthesizes aminoglycoside-modifying enzymes well as AAD, APH, AAC3, and AAC6' that are frequently encrypted by a quota of aba obstruction island-homogeneous gene cassettes including class 1 integrons   . A. baumannii also bring out a high quantity of antimicrobial -inactivating enzymes, which are encoded by plasmids and chromosome for the resistance of antibiotics developed from β-lactamine family  . These β-lactamases include, enzymes from 4 class: class A (CTX-M and VEB), class B (NDM, SIM, metallo-enyzmes, IMP, and VIM), class C (AmpC-type ADC enzymes) and class D or OXA (OXA-23, OXA-51, OXA-58, and OXA-66)    . For example, broad-spectrum TEM variants and either narrow-spectrum TEM enzymes (ambler class A); are mutually capable to hydrolyze approximately all β-lactams. In peculiarity, class_B and class_D as well as β-lactamas are implicated to hydrolyze carbapenems, a latest resort of antibiotics opposite several major pathogens    A. baumannii also raising the presence of some enzyme link to the drug resistance like ADP-ribosyltransferase (rifamycin), chloramphenicol acetyltransferase (chloramphenicol), and alteration enzyme TetX1 (tetracyclin)   .
2.3. Modification of Cell Wall Protein: Permeableness Barrier of OM
Target modification mechanisms, drug-specific inactivation, and efflux drugs crossways the cell membrane barriers play an essential role in influencing the sensitivity of Acinetobacter spp. to a wide range of antimicrobials. This trait is due to the non-appearance of conventional high permeability trimmers (porins of Enterobacteria spp.)  which has poor activity in Acinetobacter spp. and thus belong to the minor proteins  . First, matching who has correctly, study is A. baumannii and Pseudomonas aeruginosa (P. aeruginosa), OM also demonstrate very low permeability to cephalosporin’s    .
The Omp A protein monomer, the major protein of OM A. baumannii, has been shown analytically as the main nonspecific slow porin   which is identical to the slow pores OprF from P. aeruginosa and OmpA from E. coli  . Overexpressing OmpA gene in A baumannii would resulted in reduced sensitivity to chloranphenicol and aztreonam (8 times decrease MIC), both of which A. baumannii is intrinsically resistant   ; Nevertheless, there was only a moderate (≤2 times) effect on the colistine MIC values, tigecycline, and imipenem  , a common antibiotics in A. baumannii infection treatment   .
2.4. Drug Efflux Systems
During the recent years because of the poor OM absorbency drug efflux systems has become one of the most complicated mechanisms of bacterial resistance and has played an essential role on drug resistance specially to A. baumanii  . The indulgence to amikacine and levofloxacin is the result of a negative control of the gene and protein CarO 31 - 36 kDa  and up regulation of 14 genes at OM by varying the amount of physiological NaCl.
3. MDR Efflux Pumps: Structure and Regulation
The structure of the efflux system is comprised of 3 well-defined parts each playing a function in the drug efflux mechanism, including the outer membrane (OM)  , the internal membrane (IM) and the fusion protein at the intermediate level (MFP); Each part of the structure EP has a certain factor causing resistance to approximately every groups of antibacterial   . In reference to further research, it has been proven that there are five different families of efflux pumps present on A. baumannii:
・ resistance nodulation-division (RND) super family;
・ ATP binding cassette (ABC) transporters;
・ small multidrug resistance (SMR) family.
Recently other studies have reported a sixth efflux family named PACE (proteobacterial antimicrobial composite efflux) present in the A. baumannii  . However, because of inadequate data, we will concentrate much more on the first 5 families present in Figure 1 where a totally understanding of structure and regulation is not complete.
3.1. RND Efflux Pumps
The large family RND efflux is special of the rather complex compared to other family. it is very represented and has a special role in almost all major gram-negative bacteria and developing multiple resistance to antibiotic also call MDR such as A. baumannii, E. coli, and P. aeruginosa  . RND efflux on A. baumannii has
Figure 1. Structure of major families A. baumannii efflux pumps + PACE family a newly superfamily identify (adapted from  and  ).
mainly three gene (adeABC, adeIJK, adeFGH) and some special gene (adeDE, adeAA)  .
The adeABC operon was newly discovered on the antibacterial agents of fluoroquinolones and aminoglycosides in the efflux system RND and divided into 3 part: adeB on inner membrane efflux transporters, adeA on membrane fusion proteins, and adeC on external membrane proteins  . AdeB has the largest representation on A. baumannii strains (80%), adeA and adeC has 42%, and 40% respectively  . The gene adeABC have almost the same structure that genes MexAB-OprM for P. aeruginosa and genes AcrAB-TolC for E. coli   . Because of this high proportion of adeB gene compare to the others, its inactivation would dramatically cause sensitization to antimicrobial drugs in the hospital for A. baumannii  . The increasing concentration of MIC would be beneficial to important drug classes like aminoglycosides, tetracyclines-tigecycline, β-lactams, fluoroquinolones, macrolides, trimethoprim, and chloramphenicol  . Despite the advancement of research, rifampicin, flusidic acid and sometimes colistin remain resistant to isolate A. baumannii. Single last chances of fight against A. baumannii isolates are tigecycline but show a hard resistance to adeABC and also it presents a high resistance efflux. The MIC levels of tigecycline remain a clinical problem   . Remarkably, about 20% of adeC was found to be involved in tigecycline resistance tests in A. baumannii demonstrating that in the adeABC gene, adeAB can keep walking without adeC except on  . The adeC plays a much more an almost negligible role in RND efflux system.
The two components adeR and adeS are responsible for the regulation of the expression system of adeABC  . They are also called protein kinases and are found on both sides of adeABC in different trajectory. AdeRS, plays a determining role in increasing resistance of adeABC. Some result shows that a dysfunction of adeR and or adeS will increase the resistance of tigecyclin, chloramphenicol, minocyline, erythromycine, cefotaxime, tetracycline, fluoroquinolones, and trimethoprim   ; as well increase the sensitization of amino-glycosides of A. baumannii isolate. Recently an intense sight of carbapenem resistance was discovered in A. baumannii isolate from the adeABC system such as class D carbapenemases, meropenem, and imipenem, and it remains a serious concern in clinical therapy   . The ISAba1 insertion produced by the adeS mutation confers resistance over expression to tegecycline.
The second largest pump of the RND family’s adeIJK also comprises of adeI, adeJ, adeK genes which occur on the three parts of the pump efflux structure respectively. AdeIJK was described initially in the years 2008   with the A. baumannii clinical strains fluctuating between 86% and 100% in a presence of the predominant gene adeJ. With various reported a MIC dimness of adeIJK mainly the resistance of A. baumannii to β-lactamines, lincosamides, fluoroquinolones, chloramphenicol, trimethoprime, and fusidic acid has been noticed  . The selection of the majority gene adeJ, will lead to an amplification in the sensitivity of chloramphenicol, macrolides, lincosamides, tetracyclines and quinolones and β-lactams   . The regulation of adeIJK is less complex than that of adeABC, but at about 750 - 850 kbp of adeIJK operon there is a regulator adeN belonging to the class of tetR  . The presence of this regulator adeN and mutation in different media led to an increase the resistance to antimicrobial drugs (ertapenem, aztreonam, tigecycline, meropenem, and minocycline) in A. baumannii  . Several studies have shown that the threshold of expression of adeIJK is lower than that of ABC, which indicated that the level of toxicity of adeIJK in the patient is well regulated    . It has been detected that adeIJK and adeABC have some similarity as the efflux of the same antibacterial drugs (fluoroquinolones, tetracyclines and chloramphenicol)  from A. baumannii and properties comparable to P. aeruginosa mexAB-OprM. Studies on production and regulation adeABC and adeIJK resulted in the formation of biofilms  .
Outstanding variation of adeABC and adeIJK, has induced the discovery of adeFGH operon sometime after adeIJK identification. The presence of adeFGH in the genus A. baumannii through exposure to certain antibacterial agents (norfloxacin)   and is also a true source of multidrug. The genes of the adeFGH operon, the adeG is the most representative of more than 80% of the others  . AdeFGH has also become popular in the species of A. baumannii due to its severe resistance to fluoro-quinolones, tetracyclines, tigecycline, chloramphenicol, trimethoprim, sulfamethoxazole and moderate resistance to erythromycin, rifampicin and aminoglycosides,  and also β-lactams. AdeFGH is regulated by LysR (LTTR), also called adeL. The adeL mutation will conduct to the adeFGH level rise. adeXYZ has also been found in A. baumannii genospecies 3 and has the same structure and positioning of MFP, OM, IM with propositions 80% (adeX), 89% (adeY) and 87% (adeZ)    . AdeXYZ and adeDE are also regularly present in A. baumannii GDG3 as opposed to GDG2 for ade IJK and adeABC. The engagement of adeXYZ in the resistance mechanism is not entirely described. Some research showed that the suppression or perturbation of adeFGH or of another in the RND system does not greatly modify the sensitivity to the antibiotics   .
It has been found that the A. baumannii (GDG3) gene and specific resistance to certain antibiotics (ceftazidime, tetracycline, amikacin, ciprofloxacin, erythromycin, rifampin, meropenem, chloramphenicol)   , is due to adeDE gene in A. baumannii chromosome, and have resistance to imipenem. Unlike to other efflux system (adeABC, adeIJK), the adeDE gene does not have an outer membrane   . Previous studies have demonstrated inconsistency between adeDE and adeABC-adeIJK due to the presence of adeABC-adeIJK/inter 1-negative adeS in some isolates for the detection of adeE  .
3.5. MFS Efflux Pumps
MFS is the subsequent most studied efflux mechanism in species A. baumannii have identified some genes cmlA, tet(A/B), craA, and floR as the most present and appertain to the superfamily MFS  . Plural research has explained a particularity of resistance caused by tetA and tetB  . These two genes are not involved in resistance to tigecycline, yet tetA leads to a tetracycline resistance while tetB induce the resistance to tetracyclin and minocyclin  . In A. baumannii isolate resistant to tetracycline, the overexpression rate of tetA is 30% - 45% whereas tetB is 32% - 72%   . The cmlA gene of MFS is resistant to certain β-lactams, chloramphenicole, fluoroquinolones, tetracycline and rifampicin. craA is particularly resistant to chloranphenicol, imipenems, quinolones, aminoglycosides, and tetracyclines  . The MFS energy source is proton motive force (H+) facilitating H+ motive force inhibition to increase the sensitization of antimicrobial drugs  . Horizontal transmission was discovered by the association of two tetB-tetR genes in plasmid and the ISCR2 element of the MDR isolate. It has been reported that floR gene and cmlA gene were associate with abaR gene in A. baumannii chromosome    .
3.6. MATE Efflux Pumps
The first and most frequent gene of the MATE family present in A. baumannii is the adeM gene. It represents between 63% - 100% in MDR of A. baumannii   . AdeM protein contains about 447 amino acids and multiform hydrophobic regions. The antimicrobial drugs resistant due to adeM gene are not related to adeABC and totally known. In certain studies it was noted that adeM is not associated with the resistance of β-lactams, or cephalosporin   . But it could have an implication of resistance in that of amino glycosides, trimethoprim, fluoroquinolones, erythrocin, and chloramphenicol. The MATE family is powered by double reservoir of energy PMF (motive force of the proton) and sodium ion gradient Na+  . This high energy source could be a particular reasons why adeM gene is seen as an important target for elaboration of efflux pumps inhibitory antibiotics that could help restores A. baumannii sensitization   . The recapitulate of Efflux pumps family in A. baumannii and Antimicrobial drug target was listed in Table 1.
3.7. SMR Efflux Pumps
AdeS gene was characterized in 2009 initially in the efflux pump system by increasing the MIC (5 - 6 times) on an E. coli strain for the resistance of novobiocin and erythromycin. AdeS gene is the main efflux pump of SMR family
Table 1. Efflux pumps families in A. baumannii and antimicrobial drug.
present in A. baumannii. This gene adeS belongs to a particular resistance for fluoroquinolones, novobiocin, erythromycin, detergents (benzalkonium chloride), chloramphenicol, and dyes    . AdeS is identical at 52% to emrE (E. coli) found in the genome A. baumannii genome. AdeS is composed of about 108 acid amines. Because of its constant need for energy (H+), the suppression of this energetic source would restore susceptibility to drugs on MDR A. baumannii    .
3.8. ABC Efflux Pumps
ATP binding cassette (ABC) of super family efflux pumps are recognized to be censurable for multidrug-resistance due of P-glycoprotein (ABCB1)  . ABC proteins are including in the cytoplasm (inner) membrane of germ, and membranes in eukaryotes. In the human body, ABC proteins encodes for 49 proteins, a particular fraction has been distinguished in function and biochemistry terms to others   . They have been organized into 7 sub-families established on phylo-genetic examination. P-glycoprotein (ABCB1) contains 170 kDa trans-membrane glycoprotein and practically the most at largely studied transporters that promote cancer cells to develop drug resistance. Unlike the other family of efflux pumps, ABC family is powered by hydrolysis energy sources of ATP (ADP + Pi) which gives cellular resistance to large number of drug molecules   . The ABC proteins functionally contain two areas for substrate transport and 2 areas of NBD (nucleotide binding) with ATP hydrolyse in the process. ABC family is recognized in A. baumannii to have resistance to erythromycin and gramicidin, but it is very present on cancer cells  .
3.9. PACE Efflux Pumps
The proteo-bacterial antimicrobial compound efflux family (PACE) is uncommon of the newest families of efflux pumps identified in the latest 15 years  . PACE family described on plural gram-negative bacteria like E. coli, K. pneumoniae, Vibrio parahaemolyticus, Salmonella enterica, P. aeruginosa, Enterobactre cloacae, and serovar Typhi    . Its homologous aceI gene found to be resistant to chlorhexidine and its overexpression also lead to resistance to dequalinium, benzalkonium chloride, and acriflavine. In A. baumannii, aceI also induces the resistance of chlorhexidine and oxidants  . The aceI gene of PACE family could be the 6th group of MDR efflux pumps  .
3.10. Mechanisms of Transporters in the Efflux Pump Systems
According to structural and bioenergetics characteristics, carriers could be separated into two major groups   , 1) transporters that hydrolyze ATP as an energy source; they are also summons ABC transporters (ATP binding cassette)  , and 2) transporters that use the proton H+ (and/or Na+ sodium MATE family) for energy source  . Transporter of proton is the main common conveyance present in gram-negative bacteria especially in multidrug resistant. The mechanisms of transporter expression and regulation in bacterial present a complex structure with different variable which are still understudy   .
The First Transporters: ATP binding cassette Transporters. The mammalian P-glycoprotein (P-gp, MDR1) is one of the particularly studied ABC transporters, and their utilization in chemotherapy has shown that their expression confers resistance to cytotoxic compounds  . Ubiquitous ABC transporters have many different functions in transport including drugs, metabolites and the flow of toxins  . ABC transporters particularly constitutes, of two hydrophobic transmembrane domains and two cytoplasmic domains which binding ATP  .
The Second Transporters in MDR bacterial efflux pumps system are represented in four families: RND, MFS, MATE, and SMR family   . The MFS protein have 12 or 14 transmembrane segments (TMS) coming from two separate congregate and they are also responsible for transport of drugs, sugars, and intermediate metabolites  . Membrane proteins of the SMR family are engaged in the activity of lipophilic cationic drugs in A. baumannii  . These are the slightest known drug efflux proteins, with just 4 TMS predicted. They can function as either hetero- or homo-oligomeric complexes. Unlike pumps MFS families, RND and SMR, which act as anti-proton/anti-drug, the 12-TMS collapse pumps MATE family afresh recognize  are mainly anti-drugs Na+  . RND efflux systems presented a 3D structure of proteins of tripartite which is not totally understood by the configuration of these systems   . It was also noted that RND pumps of A. baumannii and gram-negative a tripartite system. RND efflux protein are combined of 12 TMS including 2 large periplasmic which provide specify substrate   .
4. Mechanisms of Efflux Pumps Inhibitors (EPI)
In fighting bacterial resistance which has increased, it would be useful to employ inhibitors of resistance efflux pumps to restore the fundamental action of antibiotic. Efflux pumps are the newest bacterial resistance mechanism allowing resistance to almost all antibiotics  . Some molecules (chemical or natural) have the capacity to act specifically on the efflux system to restore the action of antibiotics and commonly called efflux pumps inhibitors (EPIs)  . In the A. baumannii species several chemical inhibitors have already been tested  . Only certain inhibitor has shown conclusive results but remains difficult to apply in clinical due to high levels of toxicity for the human organism   . To discover adequate EPI, different strategies can be considered depending on the cause gene or the level of cellular resistance  . Given the enormous variety of drugs, it would be cost-effective and economical to focus more on the classes of antibiotics that could have a serious impact on A. baumannii with respect to pharmacokinetics and toxicity. Finally, the screening of banks or chemical compounds emitted by biodiversity may allow the identification of performing compounds, which could be further enhanced by experiments with structure-activity relationships  . The mechanisms implicated in inhibition of efflux pump systems are not clearly understood   but it has been suggested that inhibition of efflux pump performance in A. baumannii may be completed by different channel, Figure 2 present the various target of EPI such as 1) Change regulatory steps of efflux pumps expression; 2) Inhibit the practical construction of the multi-component pump; 3) Obstruct the outer membrane ways (adeC, adeK) with a plug; 4) Disintegrate the energy resource of efflux, direct-specific or indirect-general via a destruction of energy mechanisms of the bacterial transporters; 5) Apply a non-antibiotic molecule to the affinity sites of the efflux pump for competitive or no inhibition; 6) Modify the chemical structure of useful antibiotics in order to reduce its relationship for efflux identification and limiting sites or to obstruct the efflux transport.
PAβN and 1-(1-naphthylmethyl)-piperazine (NMP) are commonly used for the mechanism of inhibition efflux pump, they were tested in combination with different antimicrobial drugs facing A. baumannii   . At MIC values ≥ 400 μg/ml (PAβN) and 200 ≥ 400 μg/ml (NMP) we observed intense antibacterial activity in the behavior of these two agents. The work done by Pannek S et al. on these two EPI reveals a reversal of the resistance phenotype or a limitation in the sensitization of bacterial cells with a low concentration at 25 μg/ml  . When MIC decrease eight-fold some antimicrobial drugs like levofloxacin, chloramphenicol, linezolid, ciprofloxacin, clarithromycin, tetracycline, and rifampicin, restores sensitivity to drugs with a density 100 μg/ml of the two EPIs,
Figure 2. Various targets for inhibition of complex efflux pump (adapted from  and  ).
which means that one or both EPIs have a affirmative effect   . It was also noticed that PAβN and NMP, either at 100 μg/ml, restored the susceptibility on tigecycline (double-reduction of MIC) and fluoroquinolone (decrease MIC 2 - 16 times)   .
In other studies, PAβN at 10 μg/ml decreased predominantly MIC concentrations of trimethoprim, clindamycin and chloramphenicol   . Twice a time on clinical isolates, whereas PAβN at 20 μg/ml reduced nicidixic acid MIC to 16-fold but showed little effect on sensitiveness to ciprofloxacin  . At 100 μg/ml PAβN are also sensible minocycline activity by decreasing ≥ 04-fold MIC values  . Delightful, one study has propose a contradictory effect of NMP at 64 μg/ml on susceptibility to tetracyclines (i.e., increased susceptibility to minocycline, tetracycline, doxycycline) and tigecycline (reduced susceptibility)    . Presumably, the EPIs have powerful effect on resistivity reversal with molecules that have relatively acute MIC values such as clindamycin, chloramphenicol, linezolid rifampicin, trimethoprim clarithromycin    . Moreover, another study also examine the effect of phenothiazines, omeprazole (prochlorperazine, chlorpromazine, and promazine), verapamil and reserpine, on susceptibility cells with phenothiazines being the only emissary capable to re-establish sensibility to some antibiotics (≥8 time MIC decrease)   .
Recently some research demontrated the collision on colistin susceptibility of colistin-susceptible and colistin-resistant bacteria gram(-) including A. baumannii by using the effect of CCCP (carbonyl cyanide m-chlorophenyl hydrazone), NMP, PAβN, omeprazole, verapamil, reserpine   . The expression status of any drug efflux pump was not evaluate, and only carbonyl-cyanide m -chlorophenyl hydrazone (CCCP) was reveal to particularly offers influence on reversing colistine resistant for A. baumannii. Nevertheless, proton channel suchlike CCCP act on dislocation of proton motive energy crossways the cytoplasm membrane and do not active on pump perse  . Efficacy of EPI carbonyl-cyanide m-chlorophenyl hydrazone (CCCP) on colistin resistance is exotics   . Serum agents, N-tert-butyl-2-(1-tert-butyltetrazol-5-yl) sulfanylacetamide and (E)-4-(4-chlorobenzylidene) amino) benzenesulfonamide were combined to find accumulation and potentiating the improvement of the minocycline activity of several antimicrobials opposite A. baumannii   .
The perfect results of EPIs could stimulate the action of new antimicrobial drugs. The compound, 3-(phenylsulfonyl)-2-pyrazinecarbonitrile, is an agent developed fronting resistant nosocomial pathogens   . The combine of PAβN can decrease this MIC value by four time of A. baumannii at MIC is 64 μg/ml   . Another lately kibdelomycin natural antibiotic was found, exhibits a broad-spectrum effect with the MIC 90 value of 0.125 facing A. baumannii  , this agent appears to be a distressed substratum of efflux pumps. Finally, any agents that can traverse the OM of A. baumannii are expected to counter the activity of the efflux pumps in augmentation the drug ingress to their targets   .
In this regard, many plant extracts of EPI (in addition steroidal alkaloids conessine) are clever to break down the OM barrier to exert a synergistic efficacy on the amelioration of the activity of divers antimicrobials facing A. baumannii    . Natural efflux pump inhibitors (plant extracts): Biricodar and timcodar, Falvone, Berberis, Mahonia, Dalea versicolor, Lycopus europaeus, Rosmarinus officinalis, are the most common use against bacteria  . The analytical results of the natural inhibitor Rosmarinus officinalis and Lycopus europaeus have shown great efficacy on efflux pumps to restore the sensitivity of antibiotics against MDR strains of A. baumannii and P. aeruginosa  . The natural extract Geranium coespitosum, Punica granatum and Euphorbiaceae can inhibit the potentiating activity of strains MDR Staphylococcus aureus to restore the sensibility of erythromycin, fluoroquinolone, gentamicin, ampicillin, tetracycline, chloramphenicol   . Extracts of Berberis aetnensis coming from volcano region can reduce the resistance of ciprofloxacin for P. aeruginosa, S. aureus, and E. coli   . The natural inhibitors Mellisa officinalis, Daucus carota, Levisticum officinale, Glycyrrhiza glabra, has demonstrated a great activity facing S. tyhimuriun and K. pneumoniae by restore the sensibility of tetracycline, chloramphenicol and fluoroquinolones   .
Drug efflux mechanisms are serious global problems for the fight of nosocomial infections including A. baumannii in clinic. RND families are the greatly complex and resist numerous types of antimicrobial drugs. Despite of the development and use of chemical molecules (NMP, PAβN, omeprazole, verapamil, reserpine, CCCP) as an EP inhibitor, many research having present results that are approximately conclusive in vitro always face elevated degree of toxicity to the physical body if it is applied in clinic. Hence the importance for future research focuses more on natural inhibitor extract from plants (Berberis, Mahonia, Dalea versicolor, Lycopus europaeus, Rosmarinus officinalis). The development of these new type inhibitors could constitute a better and effective voice to resolve definitively the bacterial MDR problem (including A. baumannii). Therefore, control pharmacokinetic, pharmaco-dynamic complete and combined will give high efficacy and acceptable degree of toxicity.
Cite this paper
Temgoua, F. and Wu, L. (2019) Mechanisms Efflux Pumps of Acinetobacter baumannii (MDR): Increasing Resistance to Antibiotics. Journal of Biosciences and Medicines, 7, 48-70. doi: 10.4236/jbm.2019.71006.
 Levy, S.B. (1995) Antimicrobial Resistance: A Global Perspective. Antimicrobial Resistance, 9, 1-13. https://doi.org/10.1007/978-1-4757-9203-4_1
 Adams-Haduch, J.M., Paterson, D.L., Sidjabat, H.E., Pasculle, A.W., Potoski, B.A., Muto C.A., Harrison, L.H. and Doi, Y. (2008) Genetic Basis of Multidrug Resistance in Acinetobacter baumannii Clinical Isolates at a Tertiary Medical Center in Pennsylvania. Antimicrobial Agents and Chemotherapy, 52, 3837-3843. https://doi.org/10.1128/AAC.00570-08
 Bouvet, P. and Grimont, P. (1987) Identification and Biotyping of Clinical Isolates of Acinetobacter. Annales de l’Institut Pasteur/Microbiologie, 138, 569-578. https://doi.org/10.1016/0769-2609(87)90042-1
 El-Tahawy, T.A. (2004) The Crisis of Antibiotic-Resistance in Bacteria. Saudi Medical Journal, 25, 837-842.
 Blair, J.M., Webber, M.A., Baylay, A.J., Ogbolu, D.O. and Piddock, L.J. (2015) Molecular Mechanisms of Antibiotic Resistance. Nature Reviews Microbiology, 13, 42. https://doi.org/10.1038/nrmicro3380
 Pagès, J.M. and Amaral, L. (2009) Mechanisms of Drug Efflux and Strategies to Combat Them: Challenging the Efflux Pump of Gram-Negative Bacteria. Biochimica et Biophysica Acta, 1794, 826-833. https://doi.org/10.1016/j.bbapap.2008.12.011
 Kanamori, H., Parobek, C.M., Weber, D.J., Van Duin, D., Rutala, W.A., Cairns, B.A. and Juliano, J.J. (2016) Next-Generation Sequencing and Comparative Analysis of Sequential Outbreaks Caused by Multidrug-Resistant Acinetobacter baumannii at a Large Academic Burn Center. Antimicrobial Agents and Chemotherapy, 60, 1249-1257. https://doi.org/10.1128/AAC.02014-15
 Zhao, S.Y., Jiang, D.Y., Xu, P.C., Zhang, Y.K., Shi, H.F., Cao, H.L. and Wu, Q. (2015) An Investigation of Drug-Resistant Acinetobacter baumannii Infections in a Comprehensive Hospital of East China. Annals of Clinical Microbiology and Antimicrobials, 14, 7. https://doi.org/10.1186/s12941-015-0066-4
 Morris, A., Kellner, J.D. and Low, D.E. (1998) The Superbugs: Evolution, Dissemination and Fitness. Current Opinion in Microbiology, 1, 524-529. https://doi.org/10.1016/S1369-5274(98)80084-2
 Levy, S.B. (2005) Antibiotic Resistance—The Problem Intensifies. Advanced Drug Delivery Reviews, 57, 1446-1450. https://doi.org/10.1016/j.addr.2005.04.001
 Maki, D.G., Safdar, N. and Ebert, S.C. (2007) Prevalence, Consequences, and Solutions. Pharmacotherapy, 27, 1123-1134. https://doi.org/10.1592/phco.27.10part2.121S
 Croft, A.C., D'Antoni, A.V. and Terzulli, S.L. (2007) Update on the Antibacterial Resistance Crisis. Medical Science Monitor, 13, RA103-RA118.
 French, G. (2010) The Continuing Crisis in Anti-biotic Resistance. International Journal of Antimicrobial Agents, 36, S3-S7. https://doi.org/10.1016/S0924-8579(10)70003-0
 Webber, M. and Piddock, L. (2003) The Importance of Efflux Pumps in Bacterial Antibiotic Resistance. Journal of Antimicrobial Chemotherapy, 51, 9-11. https://doi.org/10.1093/jac/dkg050
 Aujla, S.J., Chan, Y.R., Zheng, M., Fei, M., Askew, D.J., Pociask, D.A., Reinhart, T.A., McAllister, F., Edeal, J. and Gaus, K. (2008) IL-22 Mediates Mucosal Host Defense against Gram-Negative Bacterial Pneumonia. Nature Medicine, 14, 275-281. https://doi.org/10.1038/nm1710
 Chen, Q., Li, X., Zhou, H., Jiang, Y., Chen, Y., Hua, X. and Yu, Y. (2013) Decreased Susceptibility to Tigecycline in Acinetobacter baumannii Mediated by a Mutation in Trm Encoding SAM-Dependent Methyltransferase. Journal of Antimicrobial Chemotherapy, 69, 72-76. https://doi.org/10.1093/jac/dkt319
 Beceiro, A., Moreno, A., Fernández, N., Vallejo, J.A., Aranda, J., Adler, B., Harper, M., Boyce, J.D. and Bou, G. (2014) Biological Cost of Different Mechanisms of Colistin Resistance and Their Impact on Virulence in Acinetobacter baumannii. Antimicrobial Agents and Chemotheraphy, 58, 518-526. https://doi.org/10.1128/AAC.01597-13
 Kim, Y., Bae, I.K., Jeong, S.H., Yong, D. and Lee, K. (2015) In Vivo Selection of Pan-Drug Resistant Acinetobacter baumannii during Antibiotic Treatment. Yonsei Medical Journal, 56, 928-934. https://doi.org/10.3349/ymj.2015.56.4.928
 Chin, C.Y., Gregg, K.A., Napier, B.A., Ernst, R.K. and Weiss, D.S. (2015) A PmrB-Regulated Deacetylase Required for Lipid A Modification and Polymyxin Resistance in Acinetobacter baumannii. Antimicrobial Agents and Chemotheraphy, 59, 7911-7914. https://doi.org/10.1128/AAC.00515-15
 Li, X.Z., Plésiat, P. and Nikaido, H. (2015) The Challenge of Efflux-Mediated Antibiotic Resistance in Gram-Negative Bacteria. Clinical Microbiology Reviews, 28, 337-418. https://doi.org/10.1128/CMR.00117-14
 Zou, L., Meng, J., McDermott, P.F., Wang, F., Yang, Q., Cao, G., Hoffmann, M. and Zhao, S. (2014) Presence of Disinfectant Resistance Genes in Escherichia coli Isolated from Retail Meats in the USA. Journal of Antimicrobial Chemotherapy, 69, 2644-2649. https://doi.org/10.1093/jac/dku197
 Sun, J., Deng, Z. and Yan, A. (2014) Bacterial Multidrug Efflux Pumps: Mechanisms, Physiology and Pharmacological Exploitations. Bio-chemical and Biophysical Research Communications, 453, 254-267. https://doi.org/10.1016/j.bbrc.2014.05.090
 Traub, W.H. and Bauer, D. (2000) Surveillance of Nosocomial Cross-Infections Due to Three Acinetobacter Genospecies (Acinetobacter baumannii, Genospecies 3 and Genospecies 13) during a 10-Year Observation Period: Serotyping, Macrorestriction Analysis of Genomic DNA and Antibiotic Susceptibilities. Chemotherapy, 46, 282-292. https://doi.org/10.1159/000007300
 Silbergeld, E.K., Graham, J. and Price, L.B. (2008) Industrial Food Animal Production, Antimicrobial Resistance, and Human Health. Annual Review of Public Health, 29, 151-169. https://doi.org/10.1146/annurev.publhealth.29.020907.090904
 Chancey, S.T., Zahner, D. and Stephens, D.S. (2012) Acquired Inducible Antimicrobial Resistance in Gram-Positive Bacteria. Future Microbiology, 7, 959-978. https://doi.org/10.2217/fmb.12.63
 Chang, G., Szewczyk, P. and He, X. (2013) Structures of Multidrug Efflux Pumps from the MFS, SMR, MATE and ABC Transporter Families. Microbial Efflux Pumps: Current Research, 21, 886-887.
 Deng, M., Zhu, M.H., Li, J.J., Bi, S., Sheng, Z.K., Hu, F.S., Zhang, J.J., Chen, W., Xue, X.W. and Sheng, J.F. (2014) Molecular Epidemiology and Mechanisms of Tigecycline Resistance in Clinical Isolates of Acinetobacter baumannii from a Chinese University Hospital. Anti-microbial Agents and Chemotheraphy, 58, 297-303. https://doi.org/10.1128/AAC.01727-13
 Abdallah, M., Olafisoye, O., Cortes, C., Urban, C., Landman, D. and Quale, J. (2015) Activity of Eravacycline against Enterobacteriaceae and Acinetobacter baumannii, Including Multidrug-Resistant Isolates, from New York City. Antimicrobial Agents and Chemotheraphy, 59, 1802-1805. https://doi.org/10.1128/AAC.04809-14
 Wallace, L., Daugherty, S.C., Nagaraj, S., Johnson, J.K., Harris, A.D. and Rasko, D.A. (2016) Use of Comparative Genomics to Characterize the Diversity of Acinetobacter baumannii Surveillance Isolates in a Health Care Institution. Antimicrobial Agents and Chemotheraphy, 60, 5933-5941. https://doi.org/10.1128/AAC.00477-16
 Nikaido, H. (2003) Molecular Basis of Bacterial Outer Membrane Permeability Revisited. Microbiology and Molecular Biology Reviews, 67, 593-656. https://doi.org/10.1128/MMBR.67.4.593-656.2003
 Bahl, C.D., Hvorecny, K.L., Bridges, A.A., Ballok, A.E., Bomberger, J.M., Cady, K.C., O’Toole, G.A. and Madden, D.R. (2014) Signature Motifs Identify an Acinetobacter Cif Virulence Factor with Epoxide Hydrolase Activity. Journal of Biological Chemistry, 289, 7460-7469. https://doi.org/10.1074/jbc.M113.518092
 Dreier, J. and Ruggerone, P. (2015) Interaction of Antibacterial Compounds with RND Efflux Pumps in Pseudomonas aeruginosa. Frontiers in Microbiology, 6, 345-349. https://doi.org/10.3389/fmicb.2015.00660
 Al Naiemi, N., Duim, B., Savelkoul, P.H., Spanjaard, L., De Jonge, E., Bart, A., Vandenbroucke-Grauls, C.M. and de Jong, M.D. (2005) Widespread Transfer of Resistance Genes between Bacterial Species in an Intensive Care Unit: Implications for Hospital Epidemiology. Journal of Clinical Microbiology, 43, 4862-4864. https://doi.org/10.1128/JCM.43.9.4862-4864.2005
 Aleksic, V., Mimica-Dukic, N., Simin, N., Nedeljkovic, N.S. and Knezevic, P. (2014) Synergistic Effect of Myrtus Communis L. essential Oils and Conventional Antibiotics against Multi-Drug Re-sistant Acinetobacter baumannii Wound Isolates. Phytomedicine, 21, 1666-1674. https://doi.org/10.1016/j.phymed.2014.08.013
 Blair, J.M., Richmond, G.E. and Piddock, L.J. (2014) Multidrug Efflux Pumps in Gram-Negative Bacteria and Their Role in Antibiotic Resistance. Future Microbiology, 9, 1165-1177. https://doi.org/10.2217/fmb.14.66
 He, X., Lu, F., Yuan, F., Jiang, D., Zhao, P., Zhu, J., Cheng, H., Cao, J. and Lu, G. (2015) Biofilm Formation Caused by Clinical Acinetobacter baumannii Isolates Is Associated with Overexpression of the AdeFGH Efflux Pump. Antimicrobial Agents and Chemotheraphy, 59, 4817-4825. https://doi.org/10.1128/AAC.00877-15
 Smani, Y., Fàbrega, A., Roca, I., Sánchez-Encinales, V., Vila, J. and Pachón, J. (2014) Role of OmpA in the Multidrug Resistance Phenotype of Acinetobacter baumannii. Antimicrobial Agents and Chemotheraphy, 58, 1806-1808. https://doi.org/10.1128/AAC.02101-13
 D’Costa, V.M., Griffiths, E. and Wright, G.D. (2007) Expanding the Soil Antibiotic Resistome: Exploring Environmental Diversity. Current Opinion in Microbiology, 10, 481-489. https://doi.org/10.1016/j.mib.2007.08.009
 Hood, M.I., Jacobs, A.C., Sayood, K., Dunman, P.M. and Skaar, E.P. (2010) Acinetobacter baumannii Increases Tolerance to Antibiotics in Response to Monovalent Cations. Antimicrobial Agents and Chemotheraphy, 54, 1029-1041. https://doi.org/10.1128/AAC.00963-09
 Yonehara, R., Yamashita, E. and Nakagawa, A. (2016) Crystal Structures of OprN and OprJ, Outer Membrane Factors of Multidrug Tripartite Efflux Pumps of Pseudomonas aeruginosa. Proteins: Structure, Function, and Bioinformatics, 84, 759-769. https://doi.org/10.1002/prot.25022
 Penwell, W.F., Shapiro, A.B., Giacobbe, R.A., Gu, R.F., Gao, N., Thresher, J., McLaughlin, R.E., Huband, M.D., DeJonge, B.L. and Ehmann, D.E. (2015) Molecular Mechanisms of Sulbactam Antibacterial Activity and Resistance Determinants in Acinetobacter baumannii. Antimicrobial Agents and Chemotheraphy, 59, 1680-1689. https://doi.org/10.1128/AAC.04808-14
 Wright, G.D. (2011) Molecular Mechanisms of Antibiotic Resistance. Chemical Communications, 47, 4055-4061. https://doi.org/10.1039/c0cc05111j
 Hassan, K.A., Jackson, S.M., Penesyan, A., Patching, S.G., Tetu, S.G., Eijkelkamp, B.A., Brown, M.H., Henderson, P.J. and Paulsen, I.T. (2013) Transcriptomic and Biochemical Analyses Identify a Family of Chlorhexidine Efflux Proteins. Proceedings of the National Academy of Sciences of the United States of America, 110, 20254-20259. https://doi.org/10.1073/pnas.1317052110
 Chitsaz, M. and Brown, M.H. (2017) The Role Played by Drug Efflux Pumps in Bacterial Multidrug Resistance. Essays in Biochemistry, 61, 127-139. https://doi.org/10.1042/EBC20160064
 Kumar, S., Mukherjee, M.M. and Varela, M.F. (2013) Modulation of Bacterial Multidrug Resistance Efflux Pumps of the Major Facilitator Superfamily. International Journal of Bacteriology, 20, 41.
 Allen, H.K., Donato, J., Wang, H.H., Cloud-Hansen, K.A., Davies, J. and Handelsman, J. (2010) Call of the Wild: Antibiotic Resistance Genes in Natural Environments. Nature Reviews Microbiology, 8, 251. https://doi.org/10.1038/nrmicro2312
 Angoti, G., Bandehpour, M., Goudarzi, H., Hajizadeh, M., Zarringhalam Moghaddam, M. and Kouchaki, A. (2016) Detection of Efflux Pump Genes (adeA, adeB, adeC and abeM) in Acinetobacter baumannii Isolated from Hospitalize Patients, North-West of Iran. Infection, Epidemiology and Medicine, 2, 8-11. https://doi.org/10.18869/modares.iem.2.4.8
 Yoon, E.J., Courvalin, P. and Grillot-Courvalin, C. (2013) RND-Type Efflux Pumps in Multidrug-Resistant Clinical Isolates of Acinetobacter baumannii: Major Role for AdeABC Overexpression and AdeRS Mutations. Antimicrobial Agents and Chemotheraphy, 57, 2989-2995. https://doi.org/10.1128/AAC.02556-12
 Zarrilli, R., Pournaras, S., Giannouli, M. and Tsakris, A. (2013) Global Evolution of Multidrug-Resistant Acinetobacter baumannii Clonal Lineages. International Journal of Antimicrobial Agents, 41, 11-19. https://doi.org/10.1016/j.ijantimicag.2012.09.008
 Yoon, E.J., Chabane, Y.N., Goussard, S., Snesrud, E., Courvalin, P., Dé, E. and Grillot-Courvalin, C. (2015) Contribution of Resistance-Nodulation-Cell Division Efflux Systems to Antibiotic Resistance and Biofilm Formation in Acinetobacter baumannii. MBio, 6, e00309-15. https://doi.org/10.1128/mBio.00309-15
 Venter, H., Mowla, R., Ohene-Agyei, T. and Ma, S. (2015) RND-Type Drug Efflux Pumps from Gram-Negative Bacteria: Molecular Mechanism and Inhibition. Frontiers in Microbiology, 6, 377. https://doi.org/10.3389/fmicb.2015.00377
 Sugawara, E. and Nikaido, H. (2014) Properties of AdeABC and AdeIJK Efflux Systems of Acinetobacter baumannii Compared with Those of the AcrAB-TolC System of Escherichia coli. Antimicrobial Agents and Chemotheraphy, 58, 7250-7257. https://doi.org/10.1128/AAC.03728-14
 Wright, G.D. (2007) The Anti-biotic Resistome: The Nexus of Chemical and Genetic Diversity. Nature Reviews Microbiology, 5, 175-186. https://doi.org/10.1038/nrmicro1614
 Erickson, K.E., Madinger, N.E. and Chatterjee, A. (2017) Draft Genome Sequences of Clinical Isolates of Multidrug-Resistant Acinetobacter baumannii. Genome Announcement, 5, e01547-16. https://doi.org/10.1128/genomeA.01547-16
 Dal, T., Aksu, B., Pagès, J.M. and Over-Hasdemir, U. (2013) Expression of the adeB Gene and Responsiveness to 1-(1-Naphthylmethyl)-piperazine and Phenylalanyl-arginyl-β-naphthylamide in Clinical Isolates of Acinetobacter baumannii. Journal of Antimicrobial Chemotherapy, 68, 1200-1202. https://doi.org/10.1093/jac/dks511
 Bratu, S., Landman, D., Martin, D.A., Georgescu, C. and Quale, J. (2008) Correlation of Antimicrobial Resistance with β-Lactamases, the OmpA-Like Porin, and Efflux Pumps in Clinical Isolates of Acinetobacter baumannii Endemic to New York City. Antimicrobial Agents and Chemotheraphy, 52, 2999-3005. https://doi.org/10.1128/AAC.01684-07
 Xing, L., Barnie, P.A., Su, Z. and Xu, H. (2014) Development of Efflux Pumps and Inhibitors (EPIs) in A. baumanii. Clinical Microbiology, 3, 135.
 Abbott, I., Cerqueira, G.M., Bhuiyan, S. and Peleg, A.Y. (2013) Carbapenem Resistance in Acinetobacter baumannii: Laboratory Challenges, Mechanistic Insights and Therapeutic Strategies. Expert Review of Anti-Infective Therapy, 11, 395-409. https://doi.org/10.1586/eri.13.21
 Asai, S., Umezawa, K., Iwashita, H., Ohshima, T., Ohashi, M., Sasaki, M., Hayashi, H., Matsui, M., Shibayama, K. and Inokuchi, S. (2014) An Outbreak of blaOXA-51-like- and blaOXA-66-Positive Acinetobacter baumannii ST208 in the Emergency Intensive Care Unit. Journal of Medical Microbiology, 63, 1517-1523. https://doi.org/10.1099/jmm.0.077503-0
 Hernando-Amado, S., Blanco, P., Alcalde-Rico, M., Corona, F., Reales-Calderón, J.A., Sánchez, M.B. and Martínez, J.L. (2016) Multidrug Efflux Pumps as Main Players in Intrinsic and Acquired Resistance to Antimicrobials. Drug Resistance Updates, 28, 13-27. https://doi.org/10.1016/j.drup.2016.06.007
 Schindler, B.D., Frempong-Manso, E., DeMarco, C.E., Kosmidis, C., Matta, V., Seo, S.M. and Kaatz, G.W. (2015) Analyses of Multidrug Efflux Pump-Like Proteins Encoded on the Staphylococcus aureus Chromosome. Antimicrobial Agents and Chemotheraphy, 59, 747-748. https://doi.org/10.1128/AAC.04678-14
 Hu, F.S., Zhang, J.J., Chen, W., Xue, X.W., Deng, M., Zhu, M.H., Li, J.J., Bi, S. and Sheng, Z.K. (2014) Molecular Epidemiology and Mechanisms of Tigecycline Resistance in Clinical Isolates of Acinetobacter baumannii from a Chinese University Hospital. Antimicrobial Agents and Chemotheraphy, 58, 297-303. https://doi.org/10.1128/AAC.01727-13
 Vargiu, A.V., Pos, K.M., Poole, K. and Nikaido, H. (2016) Bad Bugs in the 21st Century: Resistance Mediated by Multi-Drug Efflux Pumps in Gram-Negative Bacte-ria. Frontiers in Microbiology, 7, 833. https://doi.org/10.3389/fmicb.2016.00833
 Li, J., Li, B., Wendlandt, S., Schwarz, S., Wang, Y., Wu, C., Ma, Z. and Shen, J. (2013) Identification of a Novel vga (E) Gene Variant That Confers Resistance to Pleuromutilins, Lincosamides and Streptogramin A Antibiotics in Staphylococci of Porcine Origin. Journal of Antimicrobial Chemotherapy, 69, 919-923. https://doi.org/10.1093/jac/dkt482
 Du, D., Zhao, N.R., Voss, J.E., Klimont, E.T., Venter, H., Chiu, W. and Luisi, B.F. (2014) Structure of the AcrAB-TolC Multidrug Efflux Pump. Nature, 509, 512. https://doi.org/10.1038/nature13205
 Yan, N. (2013) Structural Advances for the Major Facilitator Superfamily (MFS) Transporters. Trends in Biochemical Sciences, 38, 151-159. https://doi.org/10.1016/j.tibs.2013.01.003
 Vilacoba, E., Almuzara, M., Gulone, L., Traglia, G.M., Figueroa, S.A., Sly, G., Fernández, A., Centrón, D. and Ramírez, M.S. (2013) Emergence and Spread of Plasmid-Borne tet (B): ISCR2 in Minocy-cline-Resistant Acinetobacter baumannii Isolates. Antimicrobial Agents and Chemotheraphy, 57, 651-654. https://doi.org/10.1128/AAC.01751-12
 Yan, N. (2015) Structural Biology of the Major Facilitator Superfamily Transporters. Annual Review of Biophysics and Biomolecular Structure, 44, 257-283. https://doi.org/10.1146/annurev-biophys-060414-033901
 Si, H., Zhang, W.J., Chu, S., Wang, X.M., Dai, L., Hua, X., Dong, Z., Schwarz, S. and Liu, S. (2015) Novel Plasmid-Borne Multidrug Resistance Gene Cluster Including Lsa (E) from a Line-zolid-Resistant Enterococcus faecium Isolate of Swine Origin. Antimicrobial Agents and Chemotheraphy, 59, 7113-7116. https://doi.org/10.1128/AAC.01394-15
 Bellmann-Sickert, K., Stone, T.A., Poulsen, B.E. and Deber, C.M. (2015) Efflux by Small Multidrug Resistance Proteins Is Inhibited by Membrane-Interactive Helix-Stapled Peptides. Journal of Biological Chemistry, 290, 1752-1759. https://doi.org/10.1074/jbc.M114.616185
 Blanco, P., Hernando-Amado, S., Rea-les-Calderon, J.A., Corona, F., Lira, F., Alcalde-Rico, M., Bernardini, A., Sanchez, M.B. and Martinez, J.L. (2016) Bacterial Multidrug Efflux Pumps: Much More than Antibiotic Resistance Determinants. Microorganisms, 4, 14. https://doi.org/10.3390/microorganisms4010014
 Ruzin, A., Keeney, D. and Bradford, P.A. (2007) AdeABC Multidrug Efflux Pump Is Associated with Decreased Susceptibility to Tigecycline in Acinetobacter calcoaceticus-Acinetobacter baumannii Complex. Journal of Antimicrobial Chemotherapy, 59, 1001-1004. https://doi.org/10.1093/jac/dkm058
 Lin, M.F., Lin, Y.Y., Tu, C.C. and Lan, C.Y. (2017) Distribution of Different Efflux Pump Genes in Clinical Isolates of Multidrug-Resistant Aci-netobacter baumannii and Their Correlation with Antimicrobial Resistance. Journal of Microbiology Immunology and Infection, 50, 224-231. https://doi.org/10.1016/j.jmii.2015.04.004
 Ling, B.D., Zhang, L. and Li, X.Z. (2016) Antimicrobial Resistance and Drug Efflux Pumps in Acinetobacter, in Efflux-Mediated Antimicrobial Resistance in Bacteria. Efflux-Mediated Antimicrobial Resistance in Bacteria, 329-358. https://doi.org/10.1007/978-3-319-39658-3_13
 Nie, L., Grell, E., Malviya, V.N., Xie, H., Wang, J. and Michel, H. (2016) Identification of the High-Affinity Substrate-Binding Site of the Multidrug and Toxic Compound Extrusion (MATE) Family Transporter from Pseudomonas stutzeri. Journal of Biological Chemistry, 291, 15503-15514. https://doi.org/10.1074/jbc.M116.728618
 Lloris-Garcerá, P., Seppala, S., Slusky, J.S., Rapp, M. and von Heijne, G. (2014) Why Have Small Multidrug Resistance Proteins Not Evolved into Fused, Internally Duplicated Structures? Journal of Biological Chemistry, 426, 2246-2254.
 Tanaka, Y., Hipolito, C.J., Maturana, A.D., Ito, K., Kuroda, T., Higuchi, T., Katoh, T., Kato, H.E., Hattori, M. and Kumazaki, K. (2013) Structural Basis for the Drug Extrusion Mechanism by a MATE Multi-drug Transporter. Nature, 496, 247-251. https://doi.org/10.1038/nature12014
 Yang, H., Huang, L., Barnie, P.A., Su, Z., Mi, Z., Chen, J., Aparna, V., Kumar, D. and Xu, H. (2015) Characterization and Distribution of Drug Resistance Associated β-Lactamase, Membrane Porin and Efflux Pump Genes in MDR A. baumannii Isolated from Zhenjiang, China. International Journal of Clinical and Experimental Medicine, 8, 15393.
 Yang, Y.S., Lee, Y., Tseng, K.C., Huang, W.C., Chuang, M.F., Kuo, S.C., Lauderdale, T.L.Y. and Chen, T.L. (2016) In Vivo and in Vitro Efficacy of Minocycline-Based Combination Therapy for Minocycline-Resistant Acinetobacter baumannii. Antimicrobial Agents and Chemotheraphy, 60, 4047-4054. https://doi.org/10.1128/AAC.02994-15
 Higgins, C.F. (2001) ABC Transporters: Physiology, Structure and Mechanism—An Overview. Research in Microbiology, 152, 205-210. https://doi.org/10.1016/S0923-2508(01)01193-7
 Lewinson, O. and Livnat-Levanon, N. (2017) Mechanism of Action of ABC Importers: Conservation, Divergence, and Physiological Adaptations. Journal of Biological Chemistry, 429, 606-619.
 Locher, K.P. (2016) Mechanistic Diversity in ATP-Binding Cassette (ABC) Transporters. Nature Structural & Molecular Biology, 23, 487-494. https://doi.org/10.1038/nsmb.3216
 López, M., álvarez-Fraga, L., Gato, E., Blasco, L., Poza, M., Fernández-García, L., Bou, G. and Tomás, M. (2016) Genome Sequence of a Clinical Strain of Acinetobacter baumannii Belonging to the ST79/PFGE-HUI-1 Clone Lacking the AdeABC (Resistance-Nodulation-Cell Division-Type) Efflux Pump. Genome Announcement, 4, e00962-e00916.
 Spengler, G., Kincses, A., Gajdács, M. and Amaral, L. (2017) New Roads Leading to Old Destinations: Efflux Pumps as Targets to Reverse Multidrug Resistance in Bacteria. Molecules, 22, 468. https://doi.org/10.3390/molecules22030468
 Hassan, K.A., Liu, Q., Henderson, P.J. and Paulsen, L.T. (2015) Homologs of the Acinetobacter baumannii AceI Transporter Represent a New Family of Bacterial Multidrug Efflux Systems. MBio, 6, e01982-e01984. https://doi.org/10.1128/mBio.01982-14
 Fuangthong, M., Julotok, M., Chintana, W., Kuhn, K., Rittiroongrad, S., Vattanaviboon, P. and Mongkolsuk, S. (2010) Exposure of Acinetobacter baylyi ADP1 to the Biocide Chlorhexidine Leads to Acquired Resistance to the Biocide Itself and to Oxidants. Journal of Antimicrobial Chemotherapy, 66, 319-322. https://doi.org/10.1093/jac/dkq435
 Wilkens, S. (2015) Structure and Mechanism of ABC Transporters. Prime Reports, 7, 14. https://doi.org/10.12703/P7-14
 Chen, Z., Shi, T., Zhang, L., Zhu, P., Deng, M., Huang, C., Hu, T., Jiang, L. and Li, J. (2016) Mammalian Drug Efflux Transporters of the ATP Binding Cassette (ABC) Family in Multidrug Resistance: A Review of the Past Decade. Cancer Letters, 370, 153-164. https://doi.org/10.1016/j.canlet.2015.10.010
 Leonard, G.D., Fojo, T. and Bates, S.E. (2003) The Role of ABC Transporters in Clinical Practice. Oncologist, 8, 411-424. https://doi.org/10.1634/theoncologist.8-5-411
 Kathawala, R.J., Gupta, P., Ashby, C.R. and Chen, Z.S. (2015) The Modulation of ABC Transporter-Mediated Multidrug Resistance in Cancer: A Review of the Past Decade. Drug Resistance Updates, 18, 1-17. https://doi.org/10.1016/j.drup.2014.11.002
 Choi, Y.H. and Yu, A.M. (2014) ABC Transporters in Multidrug Resistance and Pharmacokinetics, and Strategies for Drug Development. Current Pharmaceutical Design, 20, 793-807. https://doi.org/10.2174/138161282005140214165212
 Li, W., Zhang, H., Assaraf, Y.G., Zhao, K., Xu, X., Xie, J., Yang, D.H. and Chen, Z.S. (2016) Overcoming ABC Transporter-Mediated Multidrug Resistance: Molecular Mechanisms and Novel Therapeutic Drug Strategies. Current Pharmaceutical Design, 27, 14-29.
 Massey, P.R., Fojo, T. and Bates, S.E. (2014) ABC Transporters: Involvement in Multidrug Resistance and Drug Disposition, in Handbook of Anticancer. Pharmacokinetics and Pharmacodynamics, 6, 373-400. https://doi.org/10.1007/978-1-4614-9135-4_20
 Kumar, R. and Pooja Patial, S. (2016) A Review on Efflux Pump Inhibitors of Gram-Positive and Gram-Negative Bacteria from Plant Sources. Current Microbiology and Applied Sciences, 5, 837-855. https://doi.org/10.20546/ijcmas.2016.506.092
 Cheesman, M.J., Ilanko, A., Blonk, B. and Cock, I.E. (2017) Developing New Antimicrobial Therapies: Are Synergistic Combinations of Plant Extracts/Compounds with Conventional Antibiotics the Solution? Pharmacognosy Reviews, 11, 57. https://doi.org/10.4103/phrev.phrev_21_17
 Lomovskaya, O. and Watkins, W. (2001) Inhibition of Efflux Pumps as a Novel Approach to Combat Drug Resistance in Bacteria. Journal of Molecular Microbiology and Biotechnology, 3, 225-236.
 Gholami, M., Hashemi, A., Hakemi-Vala, M., Goudarzi, H. and Hallajzadeh, M. (2015) Efflux Pump Inhibitor Phenylalanine-Arginine β-Naphthylamide Effect on the Minimum Inhibitory Concentration of Imipenem in Acinetobacter baumannii Strains Isolated from Hospitalized Patients in Shahid Motahari Burn Hospital, Tehran, Iran. Jundishapur Journal of Microbiology, 8, e19048. https://doi.org/10.5812/jjm.19048
 Ardebili, A., Talebi, M., Azimi, L. and Lari, A.R. (2014) Effect of Efflux Pump Inhibitor Carbonyl Cyanide 3-Chlorophenylhydrazone on the Minimum Inhibitory Concentration of Ciprofloxacin in Aci-netobacter baumannii Clinical Isolates. Jundishapur Journal of Microbiology, 7, e8691. https://doi.org/10.5812/jjm.8691
 Blanchard, C., Barnett, P., Perlmutter, J. and Dunman, P.M. (2014) Identification of Acinetobacter baumannii Serum-Associated Antibiotic Efflux Pump Inhibitors. Antimicrobial Agents and Chemotheraphy, 58, 6360-6370. https://doi.org/10.1128/AAC.03535-14
 Roy, S. and Basu, S. (2016) Correlation of β-Lactam Resistance with Over Expression of Efflux Pumps among Neonatal Septicaemic Isolates of Acinetobacter baumannii from India. Journal of Infectious Diseases, 45, 69.
 Pannek, S., Higgins, P.G., Steinke, P., Jonas, D., Akova, M., Bohnert, J.A., Seifert, H. and Kern, W.V. (2006) Multidrug Efflux Inhibition in Acinetobacter baumannii: Comparison between 1-(1-naphthylmethyl)-piperazine and phenyl-arginine-β naphthylamide. Journal of Antimicrobial Chemotherapy, 57, 970-974. https://doi.org/10.1093/jac/dkl081
 Li, X.Z., Elkins, C.A. and Zgurskaya, H.I. (2016) Efflux-Mediated Antimicrobial Resistance in Bacteria: Mechanisms. Regulation and Clinical Implications, 66, 324-326.
 Bohnert, J.A. and Kern, W.V. (2016) Antimicrobial Drug Efflux Pump Inhibitors, in Efflux-Mediated. Antimicrobial Resistance in Bacteria, 6, 755-795. https://doi.org/10.1007/978-3-319-39658-3_29
 Ni, W., Li, Y., Guan, J., Zhao, J., Cui, J., Wang, R. and Liu, Y. (2016) Effects of Efflux Pump Inhibitors on Colistin Resistance in Multidrug-Resistant Gram-Negative Bacteria. Antimicrobial Agents and Chemotheraphy, 60, 3215-3218. https://doi.org/10.1128/AAC.00248-16
 Mawabo, I.K., Noumedem, J.A., Kuiate, J.R. and Kuete, V. (2015) Tetracycline Improved the Efficiency of Other Antimicrobials against Gram-Negative Multi-drug-Resistant Bacteria. Journal of Infection and Public Health, 8, 226-233. https://doi.org/10.1016/j.jiph.2014.09.001
 Siriyong, T., Chusri, S., Srimanote, P., Tipmanee, V. and Voravuthikunchai, S.P. (2016) Holarrhena Antidysenterica Extract and Its Steroidal Alkaloid, Conessine, as Resistance-Modifying Agents against Extensively Drug-Resistant Acinetobacter baumannii. Microbial Drug Resistance, 22, 273-282. https://doi.org/10.1089/mdr.2015.0194
 Jamshidi, S., Sutton, J.M. and Rahman, K.M. (2017) Computational Study Reveals the Molecular Mechanism of the Interaction between the Efflux Inhibitor PAβN and the AdeB Transporter from Acinetobacter baumannii. ACS Omega, 2, 3002-3016. https://doi.org/10.1021/acsomega.7b00131
 Ahmed, S.S., Alp, E., Hopman, J. and Voss, A. (2016) Global Epidemiology on Colistin Resistant Acinetobacter baumannii. Journal of Infectious Diseases, 6, 41-45.
 Cortez-Cordova, J. and Kumar, A. (2011) Activity of the Efflux Pump Inhibitor Phenylalanine-Arginine β-Naphthylamide against the AdeFGH Pump of Acinetobacter baumannii. International Journal of Antimicrobial Agents, 37, 420-424. https://doi.org/10.1016/j.ijantimicag.2011.01.006
 Lopes, B. and Amyes, S. (2013) Insertion Sequence Disruption of adeR and Ciprofloxacin Resistance Caused by Efflux Pumps and gyrA and parC Mutations in Acinetobacter baumannii. International Journal of Antimicrobial Agents, 41, 117-121. https://doi.org/10.1016/j.ijantimicag.2012.08.012
 Rhee, J.Y., Choi, J.Y. and Ko, K.S. (2016) Efflux Pump Inhibitor Carbonyl Cyanidem-chlorophenylhydrazone (CCCP) Enhances Bacteriostatic Activity of Trimethoprim-Sulfamethoxazole against Clinical Stenotrophomonas maltophilia Isolates from Korea. Journal Bacteriology and Virology, 46, 185-192. https://doi.org/10.4167/jbv.2016.46.4.185
 Mahmood, H.Y., Jamshidi, S., Mark Sutton, J. and Rahman, K.M. (2016) Current Advances in Developing Inhibitors of Bacterial Multidrug Efflux Pumps. Current Medicinal Chemistry, 23, 1062-1081. https://doi.org/10.2174/0929867323666160304150522
 Mowla, R., Wang, Y., Ma, S. and Venter, H. (2017) Kinetic Analysis of the Inhibition of the Drug Efflux Protein AcrB Using Surface Plasmon Resonance. Biochimica et Biophysica Acta, 33, 1234-1236.
 Pule, C.M., Sampson, S.L., Warren, R.M., Black, P.A., van Helden, P.D., Victor, T.C. and Louw, G.E. (2015) Efflux Pump Inhibitors: Targeting Mycobacterial Efflux Systems to Enhance TB Therapy. Journal of Antimicrobial Chemotherapy, 71, 17-26. https://doi.org/10.1093/jac/dkv316
 Opperman, T.J. and Nguyen, S.T. (2015) Recent Advances toward a Molecular Mechanism of Efflux Pump Inhibition. Frontiers in Microbiology, 6, 421. https://doi.org/10.3389/fmicb.2015.00421
 Rana, T., Singh, S., Kaur, N., Pathania, K. and Farooq, U. (2014) A Review on Efflux Pump Inhibitors of Medically Important Bacteria from Plant Sources. IJPSRR, 26, 101-111.
 Mullié, C., Bouharkat, B., Guiheneuf, R., Serra, C., Touil-Meddah, A.T. and Sonnet, P. (2016) Efflux Pumps in Acinetobacter baumannii: Role in Antibiotic Resistance and Interest of Efflux Pump Inhibitors as Additional Therapeutic Weapons. Antimicrobial Research, 6, 572-583.
 Borges, A., Abreu, A.C., Dias, C., Saavedra, M.J., Borges, F. and Simoes, M. (2016) New Perspectives on the Use of Phytochemicals as an Emergent Strategy to Control Bacterial Infections Including Biofilms. Molecules, 21, 877. https://doi.org/10.3390/molecules21070877
 Saviuc, C., Gheorghe, I., Coban, S., Drumea, V., Banu, O., Bezirtzoglou, E. and Lazar, V. (2016) Rosmarinus officinalis Essential Oil and Eucalyptol Act as Efflux Pumps Inhibitors and Increase Ciprofloxacin Efficiency against Pseudomonas Aeruginosa and Acinetobacter baumannii MDR Strains. Romanian Biotechnological Letters, 21, 11783.
 Mangiaterra, G., Laudadio, E., Cometti, M., Mobbili, G., Minnelli, C., Massaccesi, L., Citterio, B., Biavasco, F. and Galeazzi, R. (2017) Inhibitors of Multidrug Efflux Pumps of Pseudomonas aeruginosa from Natural Sources: An in Silico High-Throughput Virtual Screening and in Vitro Validation. Medicinal Chemistry Research, 26, 414-430. https://doi.org/10.1007/s00044-016-1761-1
 Prasch, S. and Bucar, F. (2015) Plant Derived Inhibitors of Bacterial Efflux Pumps: An Update. Phytochemistry Reviews, 14, 961-974. https://doi.org/10.1007/s11101-015-9436-y
 Sati, S., Takuli, P., Kumar, P. and Khulbe, K. (2015) Antibacterial Activity of Three Medicinal Plants of Kumaun Himalaya against Some Pathogenic Bacteria. International Journal of Pharmacy and Pharmaceutical Sciences, 16, 1361-1368.
 Bonesi, M., Loizzo, M.R., Conforti, F., Passalacqua, N.G., Saab, A., Menichini, F. and Tundis, R. (2013) Berberis aetnensis and B. libanotica: A Comparative Study on the Chemical Composition, Inhibitory Effect on Key Enzymes Linked to Alzheimer’s Disease and Antioxidant Activity. Journal of Pharmacy and Pharmacology, 65, 1726-1735. https://doi.org/10.1111/jphp.12172