Salmonella enterica serovar Typhimurium SL1344, in which efflux pump genes (acrB, acrD, acrF, tolC) or regulatory genes thereof (marA, soxS, ramA) were inactivated, was grown in the presence of 240 antimicrobial and nonantimicrobial agents in the Biolog Phenotype MicroArray. Mutants lacking tolC, acrB, and ramA grew significantly worse than other mutants in the presence of 48 agents (some of which have not previously been identified as substrates of AcrAB-TolC) and particularly poorly in the presence of phenothiazines, which are human antipsychotics. MIC testing revealed that the phenothiazine chlorpromazine had antimicrobial activity and synergized with common antibiotics against different Salmonella serovars and SL1344. Chlorpromazine increased the intracellular accumulation of ethidium bromide, which was ablated in mutants lacking acrB, suggesting an interaction with AcrB. High-level but not low-level overexpression of ramA increased the expression of acrB; conferred resistance to chloramphenicol, tetracycline, nalidixic acid, and triclosan and organic solvent tolerance; and increased the amount of ethidium bromide accumulated. Chlorpromazine induced the modest overproduction of ramA but repressed acrB. These data suggest that phenothiazines are not efflux pump inhibitors but influence gene expression, including that of acrB, which confers the synergy with antimicrobials observed.
In the sequenced genome of Salmonella enterica serovar Typhimurium strain LT2, an open reading frame (STM0580) coding for a putative regulatory protein of the TetR family is found upstream of the ramA gene. Overexpression of ramA results in increased expression of the AcrAB efflux pump and, consequently, multidrug resistance (MDR) in several bacterial species. The inactivation of the putative regulatory protein gene upstream of ramA in a susceptible serovar Typhimurium strain resulted in an MDR phenotype with fourfold increases in the MICs of unrelated antibiotics, such as quinolones/fluoroquinolones, phenicols, and tetracycline. The inactivation of this gene also resulted in a fourfold increase in the expression of ramA and a fourfold increase in the expression of the AcrAB efflux pump. These results indicated that the gene encodes a local repressor of ramA and was thus named ramR. In contrast, the inactivation of marR, marA, soxR, and soxS did not affect the susceptibilities of the strain. In quinolone- or fluoroquinolone-resistant strains of serovar Typhimurium overexpressing AcrAB, several point mutations which resulted in amino acid changes or an in-frame shift were identified in ramR; in addition, mutations interrupting ramR with an IS1 element were identified in high-level fluoroquinolone-resistant serovar Typhimurium DT204 strains. One serovar Typhimurium DT104 isolate had a 2-nucleotide deletion in the putative RamR binding site found upstream of ramA. These mutations were confirmed to play a role in the MDR phenotype by complementing the isolates with an intact ramR gene or by inactivating their respective ramA gene. No mutations in the mar or sox region were found in the strains studied. In conclusion, mutations in ramR appear to play a major role in the upregulation of RamA and AcrAB and, consequently, in the efflux-mediated MDR phenotype of serovar Typhimurium.
The transcriptional activator RamA is involved in multidrug resistance (MDR) by increasing expression of the AcrAB-TolC RND-type efflux system in several pathogenic Enterobacteriaceae. In Salmonella enterica serovar Typhimurium (S. Typhimurium), ramA expression is negatively regulated at the local level by RamR, a transcriptional repressor of the TetR family. We here studied the DNA-binding activity of the RamR repressor with the ramA promoter (PramA). As determined by high-resolution footprinting, the 28-bp-long RamR binding site covers essential features of PramA, including the −10 conserved region, the transcriptional start site of ramA, and two 7-bp inverted repeats. Based on the RamR footprint and on electrophoretic mobility shift assays (EMSAs), we propose that RamR interacts with PramA as a dimer of dimers, in a fashion that is structurally similar to the QacR-DNA binding model. Surface plasmon resonance (SPR) measurements indicated that RamR has a 3-fold-lower affinity (KD [equilibrium dissociation constant] = 191 nM) for the 2-bp-deleted PramA of an MDR S. Typhimurium clinical isolate than for the wild-type PramA (KD = 66 nM). These results confirm the direct regulatory role of RamR in the repression of ramA transcription and precisely define how an alteration of its binding site can give rise to an MDR phenotype.
Overexpression of ramA has been implicated in resistance to multiple drugs in several enterobacterial pathogens. In the present study, Salmonella Typhimurium strain LTL with constitutive expression of ramA was compared to its ramA-deletion mutant by employing both DNA microarrays and phenotype microarrays (PM). The mutant strain with the disruption of ramA showed differential expression of at least 33 genes involved in 11 functional groups. The study confirmed at the transcriptional level that the constitutive expression of ramA was directly associated with increased expression of multidrug efflux pump AcrAB-TolC and decreased expression of porin protein OmpF, thereby conferring multiple drug resistance phenotype. Compared to the parent strain constitutively expressing ramA, the ramA mutant had increased susceptibility to over 70 antimicrobials and toxic compounds. The PM analysis also uncovered that the ramA mutant was better in utilization of 10 carbon sources and 5 phosphorus sources. This study suggested that the constitutive expression of ramA locus regulate not only multidrug efflux pump and accessory genes but also genes involved in carbon metabolic pathways.
Understanding the impact of antimicrobial use on the emergence of resistant bacteria is imperative to prevent its emergence. For instance, activation of the AcrAB efflux pumps is responsible for the emergence of antimicrobial-resistant Salmonella strains. Here, we examined the expression levels of acrB and its multiple regulator genes (RamA, SoxS, MarA, and Rob) in 17 field isolates of S. Choleraesuis by using quantitative PCR methods. The expression of acrB increased in eight of the field isolates (P < 0.05). The expression of acrB was associated with that of ramA in one isolate, soxS in one isolate, and both these genes in six isolates. Thereafter, to examine the effect of selected antimicrobials (enrofloxacin, ampicillin, oxytetracycline, kanamycin, and spectinomycin) on the expression of acrB and its regulator genes, mutants derived from five isolates of S. Choleraesuis were selected by culture on antimicrobial-containing plates. The expression of acrB and ramA was higher in the mutants selected using enrofloxacin (3.3–6.3- and 24.5–37.7-fold, respectively), ampicillin (1.8–7.7- and 16.1–55.9-fold, respectively), oxytetracycline (1.7–3.3- and 3.2–31.1-fold, respectively), and kanamycin (1.6–2.2- and 5.6–26.4-fold, respectively), which are AcrAB substrates, than in each of the parental strains (P < 0.05). In contrast, in AcrAB substrate-selected mutants, the expression of soxS, marA, and rob remained similar to that in parental strains. Of the four antimicrobials, the level of ramA expression was significantly higher in the enrofloxacin- and ampicillin-selected mutants than in the oxytetracycline- and kanamycin-selected mutants (P < 0.05), whereas the expression levels of acrB and multiple regulator genes in spectinomycin-selected mutants were similar to those in each parental strain. These data suggest that exposure to antimicrobials that are AcrAB substrates enhance the activation of the AcrAB efflux pump via RamA, but not via SoxS, MarA, or Rob in S. Choleraesuis.
AcrAB efflux pump; antimicrobial resistance; RamA; Salmonella Choleraesuis; SoxS
The transcriptional activator RamA regulates production of the multidrug resistance efflux AcrAB–TolC system in several Enterobacteriaceae. This study investigated factors that lead to increased expression of ramA.
In order to monitor changes in ramA expression, the promoter region of ramA was fused to a gfp gene encoding an unstable green fluorescence protein (GFP) on the reporter plasmid, pMW82. The ramA reporter plasmid was transformed into Salmonella Typhimurium SL1344 and a ΔacrB mutant. The response of the reporter to subinhibitory concentrations of antibiotics, dyes, biocides, psychotropic agents and efflux inhibitors was measured during growth over a 5 h time period.
Our data revealed that the expression of ramA was increased in a ΔacrB mutant and also in the presence of the efflux inhibitors phenylalanine-arginine-β-naphthylamide, carbonyl cyanide m-chlorophenylhydrazone and 1-(1-naphthylmethyl)-piperazine. The phenothiazines chlorpromazine and thioridazine also increased ramA expression, triggering the greatest increase in GFP expression. However, inducers of Escherichia coli marA and soxS and 12 of 17 tested antibiotic substrates of AcrAB–TolC did not induce ramA expression.
This study shows that expression of ramA is not induced by most substrates of the AcrAB–TolC efflux system, but is increased by mutational inactivation of acrB or when efflux is inhibited.
antibiotic resistance; efflux inhibitors; phenothiazines
Transcriptional regulators, such as SoxS, RamA, MarA, and Rob, which upregulate the AcrAB efflux pump, have been shown to be associated with multidrug resistance in clinically relevant Gram-negative bacteria. In addition to the multidrug resistance phenotype, these regulators have also been shown to play a role in the cellular metabolism and possibly the virulence potential of microbial cells. As such, the increased expression of these proteins is likely to cause pleiotropic phenotypes. Klebsiella pneumoniae is a major nosocomial pathogen which can express the SoxS, MarA, Rob, and RamA proteins, and the accompanying paper shows that the increased transcription of ramA is associated with tigecycline resistance (M. Veleba and T. Schneiders, Antimicrob. Agents Chemother. 56:4466–4467, 2012). Bioinformatic analyses of the available Klebsiella genome sequences show that an additional AraC-type regulator is encoded chromosomally. In this work, we characterize this novel AraC-type regulator, hereby called RarA (Regulator of antibiotic resistance A), which is encoded in K. pneumoniae, Enterobacter sp. 638, Serratia proteamaculans 568, and Enterobacter cloacae. We show that the overexpression of rarA results in a multidrug resistance phenotype which requires a functional AcrAB efflux pump but is independent of the other AraC regulators. Quantitative real-time PCR experiments show that rarA (MGH 78578 KPN_02968) and its neighboring efflux pump operon oqxAB (KPN_02969_02970) are consistently upregulated in clinical isolates collected from various geographical locations (Chile, Turkey, and Germany). Our results suggest that rarA overexpression upregulates the oqxAB efflux pump. Additionally, it appears that oqxR, encoding a GntR-type regulator adjacent to the oqxAB operon, is able to downregulate the expression of the oqxAB efflux pump, where OqxR complementation resulted in reductions to olaquindox MICs.
The transcriptomes of Salmonella enterica serovar Typhimurium SL1344 lacking a functional ramA or ramR or with plasmid-mediated high-level overexpression of ramA were compared to those of the wild-type parental strain. Inactivation of ramA led to increased expression of 14 SPI-1 genes and decreased expression of three SPI-2 genes, and it altered expression of ribosomal biosynthetic genes and several amino acid biosynthetic pathways. Furthermore, disruption of ramA led to decreased survival within RAW 264.7 mouse macrophages and attenuation within the BALB/c ByJ mouse model. Highly overexpressed ramA led to increased expression of genes encoding multidrug resistance (MDR) efflux pumps, including acrAB, acrEF, and tolC. Decreased expression of 34 Salmonella pathogenicity island (SPI) 1 and 2 genes, decreased SipC production, decreased adhesion to and survival within macrophages, and decreased colonization of Caenorhabditis elegans were also seen. Disruption of ramR led to the increased expression of ramA, acrAB, and tolC, but not to the same level as when ramA was overexpressed on a plasmid. Inactivation of ramR had a more limited effect on pathogenicity gene expression. In silico analysis of a suggested RamA-binding consensus sequence identified target genes, including ramR, acrA, tolC, sipABC, and ssrA. This study demonstrates that the regulation of a mechanism of MDR and expression of virulence genes show considerable overlap, and we postulate that such a mechanism is dependent on transcriptional activator concentration and promoter sensitivity. However, we have no evidence to support the hypothesis that increased MDR via RamA regulation of AcrAB-TolC gives rise to a hypervirulent strain.
Multidrug resistance (MDR) in Enterobacter aerogenes can be mediated by induction of MarA, which is triggered by certain antibiotics and phenolic compounds. In this study, we identified the gene encoding RamA, a 113-amino-acid regulatory protein belonging to the AraC-XylS transcriptional activator family, in the Enterobacter aerogenes ATCC 13048 type strain and in a clinical multiresistant isolate. Overexpression of RamA induced an MDR phenotype in drug-susceptible Escherichia coli JM109 and E. aerogenes ATCC 13048, as demonstrated by 2- to 16-fold-increased resistance to β-lactams, tetracycline, chloramphenicol, and quinolones, a decrease in porin production, and increased production of AcrA, a component of the AcrAB-TolC drug efflux pump. We show that RamA enhances the transcription of the marRAB operon but is also able to induce an MDR phenotype in a mar-deleted strain. We demonstrate here that RamA is a transcriptional activator of the Mar regulon and is also a self-governing activator of the MDR cascade.
The relationship between efflux system overexpression and cross-resistance to cefoxitin, quinolones, and chloramphenicol has recently been reported in Klebsiella pneumoniae. In 3 previously published clinical isolates and 17 in vitro mutants selected with cefoxitin or fluoroquinolones, mutations in the potential regulator genes of the AcrAB efflux pump (acrR, ramR, ramA, marR, marA, soxR, soxS, and rob) were searched, and their impacts on efflux-related antibiotic cross-resistance were assessed. All mutants but 1, and 2 clinical isolates, overexpressed acrB. No mutation was detected in the regulator genes studied among the clinical isolates and 8 of the mutants. For the 9 remaining mutants, a mutation was found in the ramR gene in 8 of them and in the soxR gene in the last one, resulting in overexpression of ramA and soxS, respectively. Transformation of the ramR mutants and the soxR mutant with the wild-type ramR and soxR genes, respectively, abolished overexpression of acrB and ramA in the ramR mutants and of soxS in the soxR mutant, as well as antibiotic cross-resistance. Resistance due to efflux system overexpression was demonstrated for 4 new antibiotics: cefuroxime, cefotaxime, ceftazidime, and ertapenem. This study shows that the ramR and soxR genes control the expression of efflux systems in K. pneumoniae and suggests the existence of efflux pumps other than AcrAB and of other loci involved in the regulation of AcrAB expression.
It has been proposed that lack of a functional efflux system(s) will lead to a lower frequency of selection of resistance to fluoroquinolones and other antibiotics. We constructed five strains of Salmonella enterica serovar Typhimurium SL1344 that lacked efflux gene components of resistance nodulation cell division pumps (acrB, acrD, acrF, acrBacrF, and tolC) plus three strains that lack genes that effect efflux gene expression (marA, soxS, and ramA) and a hypermutable strain (mutS::aph). Strains were exposed to ciprofloxacin at 2× the MIC in agar, in the presence and absence of Phe-Arg-β-naphthylamide, an efflux pump inhibitor. Mutants were selected from all strains except those lacking acrB, tolC, or acrBacrF. For strains from which mutants were selected, there were no significant differences between the frequencies of resistance. Except for mutants of the ramA::aph strain, two phenotypes arose: resistance to quinolones only and multiple antibiotic resistance (MAR). ramA::aph mutants were resistant to quinolones only, suggesting a role for ramA in MAR in S. enterica serovar Typhimurium. Phe-Arg-β-naphthylamide (20 μg/ml) had no effect on the frequencies of resistance or ciprofloxacin MICs. In conclusion, functional AcrB and TolC in S. enterica serovar Typhimurium are important for the selection of ciprofloxacin-resistant mutants.
Control of membrane permeability is a key step in regulating the intracellular concentration of antibiotics. Efflux pumps confer innate resistance to a wide range of toxic compounds such as antibiotics, dyes, detergents, and disinfectants in members of the Enterobacteriaceae. The AcrAB-TolC efflux pump is involved in multidrug resistance in Enterobacter cloacae. However, the underlying mechanism that regulates the system in this microorganism remains unknown. In Escherichia coli, the transcription of acrAB is upregulated under global stress conditions by proteins such as MarA, SoxS, and Rob. In the present study, two clinical isolates of E. cloacae, EcDC64 (a multidrug-resistant strain overexpressing the AcrAB-TolC efflux pump) and Jc194 (a strain with a basal AcrAB-TolC expression level), were used to determine whether similar global stress responses operate in E. cloacae and also to establish the molecular mechanisms underlying this response. A decrease in susceptibility to erythromycin, tetracycline, telithromycin, ciprofloxacin, and chloramphenicol was observed in clinical isolate Jc194 and, to a lesser extent in EcDC64, in the presence of salicylate, decanoate, tetracycline, and paraquat. Increased expression of the acrAB promoter in the presence of the above-described conditions was observed by flow cytometry and reverse transcription-PCR, by using a reporter fusion protein (green fluorescent protein). The expression level of the AcrAB promoter decreased in E. cloacae EcDC64 derivates deficient in SoxS, RobA, and RamA. Accordingly, the expression level of the AcrAB promoter was higher in E. cloacae Jc194 strains overproducing SoxS, RobA, and RamA. Overall, the data showed that SoxS, RobA, and RamA regulators were associated with the upregulation of acrAB, thus conferring antimicrobial resistance as well as a stress response in E. cloacae. In summary, the regulatory proteins SoxS, RobA, and RamA were cloned and sequenced for the first time in this species. The involvement of these proteins in conferring antimicrobial resistance through upregulation of acrAB was demonstrated in E. cloacae.
Tigecycline resistance in Klebsiella pneumoniae results from ramA upregulation that causes the overexpression of the efflux pump, AcrAB-TolC. Tigecycline mutants, derived from Ecl8ΔramA, can exhibit a multidrug resistance phenotype due to increased transcription of the marA, rarA, acrAB, and oqxAB genes. These findings support the idea that tigecycline or multidrug resistance in K. pneumoniae, first, is not solely dependent on the ramA gene, and second, can arise via alternative regulatory pathways in K. pneumoniae.
High-level fluoroquinolone (FQ) resistance in Salmonella enterica serovar Typhimurium phage type DT204 has been previously shown to be essentially due to both multiple target gene mutations and active efflux by the AcrAB-TolC efflux system. In this study we show that in intermediatly resistant acrB-inactivated serovar Typhimurium DT204 mutants, high-level resistance to FQs can be restored on in vitro selection with FQs. In each FQ- resistant mutant selected from serovar Typhimurium DT204 acrB mutant strains, an insertion sequence (IS1 or IS10) was found integrated upstream of the acrEF operon, coding for AcrEF, an efflux pump highly homologous to AcrAB. In one of the strains, transposition of IS1 caused partial deletion of acrS, the putative local repressor gene of the acrEF operon. Sequence analysis showed that both IS1 and IS10 elements contain putative promoter sequences that might alter the expression of adjacent acrEF genes. Indeed, reverse transcription-PCR experiments showed an 8- to 10-fold increase in expression of acrF in these insertional mutants, relative to their respective parental strain, which correlated well with the resistance levels observed to FQs and other unrelated drugs. It is noteworthy that AcrEF did not contribute to the intrinsic drug resistance of serovar Typhimurium, since acrF deletion in wild-type strains did not result in any increase in drug susceptibility. Moreover, deletion of acrS did not cause any acrF overexpression or any decrease in drug susceptibility, suggesting that acrEF overexpression is mediated solely by the IS1 and IS10 promoter sequences and not by inactivity of AcrS. Southern blot experiments showed that the number of chromosomal IS1 and IS10 elements in the serovar Typhimurium DT204 genome was about 5 and 15 respectively. None were detected in epidemic serovar Typhimurium DT104 strains or in the serovar Typhimurium reference strain LT2. Carrying IS1 and/or IS10 elements in their chromosome may thus be a selective advantage for serovar Typhimurium DT204 strains as opposed to DT104 strains for which no high-level FQ resistance nor insertional mutations were found. Taken together, the results of the present study indicate that the IS1- or IS10- activated AcrEF efflux pump may relay AcrAB in serovar Typhimurium, and underline the importance of transposable elements in the acquisition of FQ and multidrug resistance.
Nosocomial isolates of Klebsiella pneumoniae resistant to all commonly used antimicrobial agents have emerged in many regions of the world. It is unknown if efflux systems contribute to the multidrug resistance phenotype.
The expression of genes encoding the efflux pump AcrAB and the global regulators MarA, SoxS and RamA were examined and correlated with antimicrobial resistance.
Twenty isolates belonged to the two important clones representing KPC-possessing strains endemic to our region. Virtually all of these isolates had negligible or absent expression of the genes, and resistance to fluoroquinolones and aminoglycosides could be explained by alternative mechanisms. All of these isolates were susceptible to tigecycline. A group of 14 heterogeneous isolates was also examined. There was a correlation between expression of marA with expression of soxS. Only expression of soxS was significantly correlated with expression of acrB. With a background substitution in GyrA, increased expression of acrB and marA appeared to contribute to fluoroquinolone resistance in some isolates. A correlation was noted between expression of soxS and ramA (but not marA and acrB) and tigecycline MICs. Following in vitro exposure to tigecycline, resistance occurred in association with a marked increase in marA and acrB expression in isolates lacking expression of soxS and ramA.
While laboratory-derived tigecycline resistance was associated with increased acrB expression, the variation in tigecycline MICs in clinical isolates was associated only with selected regulator genes. It appears that other mechanisms beyond activation of the acrAB system mediate tigecycline resistance.
efflux; tigecycline; multidrug-resistant
The mechanisms by which RND pumps contribute to pathogenicity are currently not understood. Using the AcrAB-TolC system as a paradigm multidrug-resistant efflux pump and Salmonella enterica serovar Typhimurium as a model pathogen, we have demonstrated that AcrA, AcrB, and TolC are each required for efficient adhesion to and invasion of epithelial cells and macrophages by Salmonella in vitro. In addition, AcrB and TolC are necessary for Salmonella to colonize poultry. Mutants lacking acrA, acrB, or tolC showed differential expression of major operons and proteins involved in pathogenesis. These included chemotaxis and motility genes, including cheWY and flgLMK and 14 Salmonella pathogenicity island (SPI)-1-encoded type III secretion system genes, including sopE, and associated effector proteins. Reverse transcription-PCR confirmed these data for identical mutants in two other S. Typhimurium backgrounds. Western blotting showed reduced production of SipA, SipB, and SipC. The absence of AcrB or TolC also caused widespread repression of chemotaxis and motility genes in these mutants, and for acrB::aph, this was associated with decreased motility. For mutants lacking a functional acrA or acrB gene, the nap and nir operons were repressed, and both mutants grew poorly in anaerobic conditions. All phenotypes were restored to that of the wild type by trans-complementation with the wild-type allele of the respective inactivated gene. These data explain how mutants lacking a component of AcrAB-TolC are attenuated and that this phenotype is a result of decreased expression of numerous genes encoding proteins involved in pathogenicity. The link between antibiotic resistance and pathogenicity establishes the AcrAB-TolC system as fundamental to the biology of Salmonella.
The AcrAB-TolC efflux pump is involved in maintaining intrinsic organic solvent tolerance in Escherichia coli. Mutations in regulatory genes such as marR, soxR, and acrR are known to increase the expression level of the AcrAB-TolC pump. To identify these mutations in organic solvent tolerant E. coli, eight cyclohexane-tolerant E. coli JA300 mutants were isolated and examined by DNA sequencing for mutations in marR, soxR, and acrR. Every mutant carried a mutation in either marR or acrR. Among all mutants, strain CH7 carrying a nonsense mutation in marR (named marR109) and an insertion of IS5 in acrR, exhibited the highest organic solvent-tolerance levels. To clarify the involvement of these mutations in improving organic solvent tolerance, they were introduced into the E. coli JA300 chromosome by site-directed mutagenesis using λ red-mediated homologous recombination. Consequently, JA300 mutants carrying acrR::IS5, marR109, or both were constructed and named JA300 acrRIS, JA300 marR, or JA300 acrRIS marR, respectively. The organic solvent tolerance levels of these mutants were increased in the following order: JA300 < JA300 acrRIS < JA300 marR < JA300 acrRIS marR. JA300 acrRIS marR formed colonies on an agar plate overlaid with cyclohexane and p-xylene (6:4 vol/vol mixture). The organic solvent-tolerance level and AcrAB-TolC efflux pump-expression level in JA300 acrRIS marR were similar to those in CH7. Thus, it was shown that the synergistic effects of mutations in only two regulatory genes, acrR and marR, can significantly increase organic solvent tolerance in E. coli.
Escherichia coli; Organic solvent tolerance; MarR; AcrR; AcrAB-TolC; Efflux pump
A study examining the influence of TolC on AcrA, AcrR, and MarR1 mutants indicates that functional TolC is required for the operation of the AcrAB efflux system and for the expression of the Mar phenotype. That the effect of TolC on the AcrAB pump is not regulatory in nature is shown by studies measuring the influence of a tolC::Tn10 insertion mutation on the expression of an acrA::lacZ reporter fusion. These results are compatible with the hypothesis that TolC is a component of the AcrAB efflux complex.
MarA and its homologue, RamA, have been implicated in multidrug resistance (MDR). RamA overexpression in Salmonella enterica serovar Typhimurium and Escherichia coli conferred MDR independently of marA. Inactivation of ramA did not affect the antibiotic susceptibilities of wild-type S. enterica serovar Typhimurium or 15 unrelated clinical MDR isolates. Thus, ramA overexpression is not a common MDR mechanism in Salmonella.
Multidrug-resistant Salmonella enterica serovar Typhimurium definitive phage type 104 (DT104) strains harbor a genomic island, called Salmonella genomic island 1 (SGI1), which contains an antibiotic resistance gene cluster conferring resistance to ampicillin, chloramphenicol, florfenicol, streptomycin, sulfonamides, and tetracyclines. They may be additionally resistant to quinolones. Among the antibiotic resistance genes there are two, i.e., floR and tet(G), which code for efflux pumps of the major facilitator superfamily with 12 transmembrane segments that confer resistance to chloramphenicol-florfenicol and the tetracyclines, respectively. In the present study we determined, by constructing acrB and tolC mutants, the role of the AcrAB-TolC multidrug efflux system in the multidrug resistance of several DT104 strains displaying additional quinolone resistance or not displaying quinolone resistance. This study shows that the quinolone resistance and the decreased fluoroquinolone susceptibilities of the strains are highly dependent on the AcrAB-TolC efflux system and that single mutations in the quinolone resistance-determining region of gyrA are of little relevance in mediating this resistance. Overproduction of the AcrAB efflux pump, as determined by Western blotting with an anti-AcrA polyclonal antibody, appeared to be the major mechanism of resistance to quinolones. Moreover, chloramphenicol-florfenicol and tetracycline resistance also appeared to be highly dependent on the presence of AcrAB-TolC, since the introduction of mutations in the respective acrB and tolC genes resulted in a susceptible or intermediate resistance phenotype, according to clinical MIC breakpoints, despite the presence of the FloR and Tet(G) efflux pumps. Resistance to other antibiotics, ampicillin, streptomycin, and sulfonamides, was not affected in the acrB and tolC mutants of DT104 strains harboring SGI1. Therefore, AcrAB-TolC appears to direct efflux-mediated resistance to quinolones, chloramphenicol-florfenicol, and tetracyclines in multidrug-resistant S. enterica serovar Typhimurium DT104 strains.
The AcrAB multidrug efflux pump, which belongs to the resistance-nodulation-division (RND) family, recognizes and extrudes a wide range of antibiotics and chemotherapeutic agents, and causes the intrinsic antibiotic resistance in Escherichia coli. The expression of AcrAB is controlled by the transcriptional regulator AcrR, whose open reading frame is located 141-base-pair upstream of the acrAB operon. To understand the structural basis of AcrR regulation, we have determined the crystal structure of AcrR, to 2.55 Å resolution, revealing a dimeric two-domain molecule with an entirely helical architecture similar to members of the TetR family of transcriptional regulators. Each monomer of AcrR forms a multi-entrance pocket of 350 cubic angstroms in the ligand-binding domain. The ligand-binding pocket is surrounded with mostly hydrophobic residues. In addition, a completely buried negatively charged glutamate, expected to be critical for drug binding, is located at the center of the binding pocket. The crystal structure provides novel insight into the mechanisms of ligand binding and AcrR regulation.
Multidrug resistance (MDR) in clinical isolates of Escherichia coli can be associated with overexpression of marA, a transcription factor that upregulates multidrug efflux and downregulates membrane permeability. Using random transposome mutagenesis, we found that many chromosomal genes and environmental stimuli affected MarA-mediated antibiotic resistance. Seven genes affected resistance mediated by MarA in an antibiotic-specific way; these were mostly genes encoding unrelated enzymes, transporters, and unknown proteins. Other genes affected MarA-mediated resistance to all antibiotics tested. These genes were acrA, acrB, and tolC (which encode the major MarA-regulated multidrug efflux pump AcrAB-TolC), crp, cyaA, hns, and pcnB (four genes involved in global regulation of gene expression), and the unknown gene damX. The last five genes affected MarA-mediated MDR by altering marA expression or MarA function specifically on acrA. These findings demonstrate that MarA-mediated MDR is regulated at multiple levels by different genes and stimuli, which makes it both complex and fine-tuned and interconnects it with global cell regulation and metabolism. Such a regulation could contribute to the adaptation and spread of MDR strains and may be targeted to treat antibiotic-resistant E. coli and related pathogens.
Active efflux pump is a primary fluoroquinolone resistant mechanism of clinical isolates of Salmonella enterica serovar Typhimurium. RamA is an essential element in producing multidrug resistant (MDR) S.enterica serovar Typhimurium. The aim of the present study was to elucidate the roles of RamA on the development of ciprofloxacin, the first choice for the treatment of salmonellosis, resistance in S. enterica serovar Typhimurium. Spontaneous mutants were selected via several passages of S. enterica serovar Typhimurium CVCC541 susceptible strain (ST) on M-H agar with increasing concentrations of ciprofloxacin (CIP). Accumulation of ciprofloxacin was tested by the modified fluorometric method. The expression levels of MDR efflux pumps were determined by real time RT-PCR. In ST and its spontaneous mutants, the ramA gene was inactivated by insertion of the kan gene and compensated on a recombinant plasmid pGEXΦ(gst-ramA). The mutant prevention concentration (MPC) and mutant frequencies of ciprofloxacin against ST and a spontaneous mutant in the presence, absence and overexpression of RamA were tested. Four spontaneous mutants (SI1-SI4) were obtained. The SI1 (CIP MICs, 0.1 mg/L) without any target site mutation in its quinolone resistant determining regions (QRDRs) and SI3 (CIP MICs, 16 mg/L) harboring the Ser83→Phe mutation in its QRDR of GyrA strains exhibited reduced susceptibility and resistance to multidrugs, respectively. In SI1, RamA was the main factor that controlled the susceptibility to ciprofloxacin by activating MdtK as well as increasing the expression level of acrAB. In SI3, RamA played predominant role in ciprofloxacin resistance via increasing the expression level of acrAB. Likewise, the deficiency of RamA decreased the MPCs and mutant frequencies of ST and SI2 to ciprofloxacin. In conclusion, the expression of RamA promoted the development of ciprofloxacin resistant mutants of S. enterica serovar Typhimurium. The inhibition of RamA could decrease the appearance of the ciprofloxacin resistant mutants.
RamA is a transcription factor involved in regulating multidrug resistance in Salmonella enterica serovar Typhimurium SL1344. Green fluorescent protein (GFP) reporter fusions were exploited to investigate the regulation of RamA expression by RamR. We show that RamR represses the ramA promoter by binding to a palindromic sequence and describe a superrepressor RamR mutant that binds to the ramA promoter sequence more efficiently, thus exhibiting a ramA inactivated phenotype.
AcrAB-TolC is the major multidrug efflux system in Enterobacteriaceae recognizing structurally unrelated molecules including antibiotics, dyes, and detergents. Additionally, in Escherichia coli it mediates resistance to bile salts. In the plant pathogen Erwinia amylovora AcrAB-TolC is required for virulence and phytoalexin resistance. Exchange analysis of AcrAB-TolC was conducted by complementing mutants of both species defective in acrB or tolC with alleles from either species. The acrB and tolC mutants exhibited increased susceptibility profiles for 24 different antibiotics. All mutants were complemented with acrAB or tolC, respectively, regardless of the taxonomic origin of the alleles. Importantly, complementation of E. amylovora mutants with respective E. coli genes restored virulence on apple plants. It was concluded that AcrAB and TolC of both species could interact and that these interactions did not yield in altered functions despite the divergent ecological niches, to which E. coli and E. amylovora have adopted.
Erwinia amylovora; Escherichia coli; TolC; AcrAB; multidrug efflux