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 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.
Salmonella enterica serovar Typhimurium has at least nine
multidrug efflux pumps. Among these pumps, AcrAB is effective in generating
drug resistance and has wide substrate specificity. Here we report that
indole, bile, and an Escherichia coli conditioned medium induced the
AcrAB pump in Salmonella through a specific regulator, RamA. The
RamA-binding sites were located in the upstream regions of acrAB and
tolC. RamA was required for indole induction of acrAB. Other
regulators of acrAB such as MarA, SoxS, Rob, SdiA, and AcrR did not
contribute to acrAB induction by indole in Salmonella.
Indole activated ramA transcription, and overproduction of RamA
caused increased acrAB expression. In contrast, induction of
ramA was not required for induction of acrAB by bile. Cholic
acid binds to RamA, and we suggest that bile acts by altering pre-existing
RamA. This points to two different AcrAB regulatory modes through RamA. Our
results suggest that RamA controls the Salmonella AcrAB-TolC
multidrug efflux system through dual regulatory modes in response to
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.
We experimentally identified and characterized 97 novel, non-protein-coding RNA candidates (npcRNAs) from the human pathogen Salmonella enterica serovar Typhi (hereafter referred to as S. typhi). Three were specific to S. typhi, 22 were restricted to Salmonella species and 33 were differentially expressed during S. typhi growth. We also identified Salmonella Pathogenicity Island-derived npcRNAs that might be involved in regulatory mechanisms of virulence, antibiotic resistance and pathogenic specificity of S. typhi. An in-depth characterization of S. typhi StyR-3 npcRNA showed that it specifically interacts with RamR, the transcriptional repressor of the ramA gene, which is involved in the multidrug resistance (MDR) of Salmonella. StyR-3 interfered with RamR–DNA binding activity and thus potentially plays a role in regulating ramA gene expression, resulting in the MDR phenotype. Our study also revealed a large number of cis-encoded antisense npcRNA candidates, supporting previous observations of global sense–antisense regulatory networks in bacteria. Finally, at least six of the npcRNA candidates interacted with the S. typhi Hfq protein, supporting an important role of Hfq in npcRNA networks. This study points to novel functional npcRNA candidates potentially involved in various regulatory roles including the pathogenicity of S. typhi.
RamA regulates the AcrAB-TolC multidrug efflux system. Using Salmonella Typhimurium, we investigated the stability of RamA and its impact on antibiotic resistance.
To detect RamA, we introduced ramA::3XFLAG::aph into plasmid pACYC184 and transformed this into Salmonella Typhimurium SL1344ramA::cat and lon::aph mutants. An N-terminus-deleted mutant [pACYC184ramA(Δ2-21)::3XFLAG::aph] in which the first 20 amino acids of RamA were deleted was also constructed. To determine the abundance and half-life of FLAG-tagged RamA, we induced RamA with chlorpromazine (50 mg/L) and carried out western blotting using anti-FLAG antibody. Susceptibility to antibiotics and phenotypic characterization of the lon mutant was also carried out.
We show that on removal of chlorpromazine, a known inducer of ramA, the abundance of RamA decreased to pre-induced levels. However, in cells lacking functional Lon, we found that the RamA protein was not degraded. We also demonstrated that the 21 amino acid residues of the RamA N-terminus are required for recognition by the Lon protease. Antimicrobial susceptibility and phenotypic tests showed that the lon mutant was more susceptible to fluoroquinolone antibiotics, was filamentous when observed by microscopy and grew poorly, but showed no difference in motility or the ability to form a biofilm. There was also no difference in the ability of the lon mutant to invade human intestinal cells (INT-407).
In summary, we show that the ATP-dependent Lon protease plays an important role in regulating the expression of RamA and therefore multidrug resistance via AcrAB-TolC in Salmonella Typhimurium.
Salmonella; transcription factors; proteolysis
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
The central carbon metabolism genes in Corynebacterium glutamicum are under the control of a transcriptional regulatory network composed of several global regulators. It is known that the promoter region of ramA, encoding one of these regulators, interacts with its gene product, RamA, as well as with the two other regulators, GlxR and SugR, in vitro and/or in vivo. Although RamA has been confirmed to repress its own expression, the roles of GlxR and SugR in ramA expression have remained unclear. In this study, we examined the effects of GlxR binding site inactivation on expression of the ramA promoter-lacZ fusion in the genetic background of single and double deletion mutants of sugR and ramA. In the wild-type background, the ramA promoter activity was reduced to undetectable levels by the introduction of mutations into the GlxR binding site but increased by sugR deletion, indicating that GlxR and SugR function as the transcriptional activator and repressor, respectively. The marked repression of ramA promoter activity by the GlxR binding site mutations was largely compensated for by deletions of sugR and/or ramA. Furthermore, ramA promoter activity in the ramA-sugR double mutant was comparable to that in the ramA mutant but was significantly higher than that in the sugR mutant. Taken together, it is likely that the level of ramA expression is dynamically balanced by GlxR-dependent activation and repression by RamA along with SugR in response to perturbation of extracellular and/or intracellular conditions. These findings add multiple regulatory loops to the transcriptional regulatory network model in C. glutamicum.
SugR, RamA, GlxR, GntR1, and a MarR-type transcriptional regulator bind to the promoter region of the gapA gene encoding glyceraldehyde-3-phosphate dehydrogenase (GAPDH), essential for glycolysis in Corynebacterium glutamicum. We previously showed that SugR, a transcriptional repressor of phosphotransferase system genes for the sugar transport system, is involved in the downregulation of gapA expression in the absence of sugar. In this study, the role of RamA in the expression of the gapA gene was examined. Comparing the gapA expression and GAPDH activity of a ramA mutant with those of the wild type revealed that RamA is involved in upregulation of gapA expression in glucose-grown cells. DNase I footprint analyses and electrophoretic mobility shift assays revealed that RamA binds with different affinities to three sites in the gapA promoter. lacZ reporter assays with mutated RamA binding sites in the gapA promoter showed that the middle binding site is the most important for RamA to activate gapA expression and that binding of RamA to the gapA promoter activates the gene expression not only in glucose-grown cells but also in acetate-grown cells. Furthermore, RamA also directly activates sugR expression, indicating that two global regulators, RamA and SugR, are coordinately involved in the complex regulation of gapA expression in C. glutamicum.
The transcriptional regulation of Corynebacterium glutamicum gnd, encoding 6-phosphogluconate dehydrogenase, was investigated. Two transcriptional regulators, GntR1 and RamA, were isolated by affinity purification using gnd promoter DNA. GntR1 was previously identified as a repressor of gluconate utilization genes, including gnd. Involvement of RamA in gnd expression had not been investigated to date. The level of gnd mRNA was barely affected by the single deletion of ramA. However, gnd expression was downregulated in the ramA gntR1 double mutant compared to that of the gntR1 single mutant, suggesting that RamA activates gnd expression. Two RamA binding sites are found in the 5′ upstream region of gnd. Mutation proximal to the transcriptional start site diminished the gluconate-dependent induction of gnd-lacZ. DNase I footprinting assay revealed two GntR1 binding sites, with one corresponding to a previously proposed site that overlaps with the −10 region. The other site overlaps the RamA binding site. GntR1 binding to this newly identified site inhibits DNA binding of RamA. Therefore, it is likely that GntR1 represses gnd expression by preventing both RNA polymerase and RamA binding to the promoter. In addition, DNA binding activity of RamA was reduced by high concentrations of NAD(P)H but not by NAD(P), implying that RamA senses the redox perturbation of the cell.
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.
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.
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.
Five Klebsiella pneumoniae isolates with reduced susceptibility to tigecycline (MIC, 2 μg/ml) were analyzed. A gene homologous to ramR of Salmonella enterica was identified in Klebsiella pneumoniae. Sequencing of ramR in the nonsusceptible Klebsiella strains revealed deletions, insertions, and point mutations. Transformation of mutants with wild-type ramR genes, but not with mutant ramR genes, restored susceptibility to tigecycline and repressed overexpression of ramA and acrB. Thus, this study reveals a molecular mechanism for tigecycline resistance in Klebsiella pneumoniae.
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.
Escherichia coli and Salmonella enterica serovar Typhimurium have evolved genetic systems, such as the soxR/S and marA regulons, to detoxify reactive oxygen species, like superoxide, which are formed as by-products of metabolism. Superoxide also serves as a microbicidal effector mechanism of the host's phagocytes. Here, we investigate whether regulatory genes other than soxR/S and marA are active in response to oxidative stress in Salmonella and may function as virulence determinants. We identified a bacterial gene, which was designated ramA (342 bp) and mapped at 13.1 min on the Salmonella chromosome, that, when overexpressed on a plasmid in E. coli or Salmonella, confers a pleiotropic phenotype characterized by increased resistance to the redox-cycling agent menadione and to multiple unrelated antibiotics. The ramA gene is present in Salmonella serovars but is absent in E. coli. The gene product displays 37 to 52% homology to the transcriptional activators soxR/S and marA and 80 to 100% identity to a multidrug resistance gene in Klebsiella pneumoniae and Salmonella enterica serovar Paratyphi A. Although a ramA soxR/S double null mutant is highly susceptible to intracellular superoxide generated by menadione and displays decreased Mn-superoxide dismutase activity, intracellular survival of this mutant within macrophage-like RAW 264.7 cells and in vivo replication in the spleens in Ityr mice are not affected. We concluded that despite its role in the protective response of the bacteria to oxidative stress in vitro, the newly identified ramA gene, together with soxR/S, does not play a role in initial replication of Salmonella in the organs of mice.
Mechanisms of antibiotic resistance were examined in nalidixic acid-resistant Salmonella enterica serovar Enteritidis field isolates displaying decreased susceptibility to ciprofloxacin and in in vitro-derived ciprofloxacin-resistant mutants (104-cip and 5408-cip). All field isolates harbored a single gyrA mutation (D87Y). Deletion of acrB and complementation with wild-type gyrA increased quinolone susceptibility. Selection for ciprofloxacin resistance was associated with the development of an additional gyrA (S83F) mutation in 104-cip, novel gyrB (E466D) and parE (V461G) mutations in 5408-cip, overexpression of acrB and decreased susceptibility to nonquinolone antibiotics in both mutants, and decreased OmpF production and altered lipopolysaccharide in 104-cip. Complementation of mutated gyrA and gyrB with wild-type alleles restored susceptibility to quinolones in 104-cip and significantly decreased the ciprofloxacin MIC in 5408-cip. Complementation of parE had no effect on quinolone MICs. Deletion of acrB restored susceptibility to ciprofloxacin and other antibiotics tested. Both soxS and marA were overexpressed in 104-cip, and ramA was overexpressed in 5408-cip. Inactivation of each of these global regulators lowered ciprofloxacin MICs, decreased expression of acrB, and restored susceptibility to other antibiotics. Mutations were found in soxR (R20H) and in soxS (E52K) in 104-cip and in ramR (G25A) in 5408-cip. In conclusion, both efflux activity and a single gyrA mutation contribute to nalidixic acid resistance and reduced ciprofloxacin sensitivity. Ciprofloxacin resistance and decreased susceptibility to multiple antibiotics can result from different genetic events leading to development of target gene mutations, increased efflux activity resulting from differential expression of global regulators associated with mutations in their regulatory genes, and possible altered membrane permeability.
A Salmonella enterica serovar Hadar strain resistant to tigecycline (MIC, 16 μg/ml) was isolated. Molecular characterization revealed the presence of a plasmid-borne tet(A) variant associated with Tn1721 mediating a rise of the MIC for tigecycline when transferred to Escherichia coli. Additionally, a truncating mutation in ramR was detected. Transformation with wild-type ramR but not with the mutated ramR lowered the MIC for tigecycline. Characterization of this Salmonella isolate implicates ramR in resistance to tigecycline.
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 is an expanded broad-spectrum antibacterial agent that is active against many clinically relevant species of bacterial pathogens, including Klebsiella pneumoniae. The majority of K. pneumoniae isolates are fully susceptible to tigecycline; however, a few strains that have decreased susceptibility have been isolated. One isolate, G340 (for which the tigecycline MIC is 4 μg/ml and which displays a multidrug resistance [MDR] phenotype), was selected for analysis of the mechanism for this decreased susceptibility by use of transposon mutagenesis with IS903φkan. A tigecycline-susceptible mutant of G340, GC7535, was obtained (tigecycline MIC, 0.25 μg/ml). Analysis of the transposon insertion mapped it to ramA, a gene that was previously identified to be involved in MDR in K. pneumoniae. For GC7535, the disruption of ramA led to a 16-fold decrease in the MIC of tigecycline and also a suppression of MDR. Trans-complementation with plasmid-borne ramA restored the original parental phenotype of decreased susceptibility to tigecycline. Northern blot analysis revealed a constitutive overexpression of ramA that correlated with an increased expression of the AcrAB transporter in G340 compared to that in tigecycline-susceptible strains. Laboratory mutants of K. pneumoniae with decreased susceptibility to tigecycline could be selected at a frequency of approximately 4 × 10−8. These results suggest that ramA is associated with decreased tigecycline susceptibility in K. pneumoniae due to its role in the expression of the AcrAB multidrug efflux pump.
A screening for non-target mutations affecting fluoroquinolone susceptibility was conducted in epidemic multidrug-resistant Salmonella enterica serovar Kentucky ST198. Among a panel of representative isolates (n = 27), covering the epidemic, only three showed distinct mutations in ramR resulting in enhanced expression of genes encoding the AcrAB-TolC efflux system and low increase in ciprofloxacin MIC. No mutations were detected in other regulatory regions of this efflux system. Ciprofloxacin resistance in serovar Kentucky ST198 is thus currently mainly due to multiple target gene mutations.
Salmonella; ciprofloxacin resistance; efflux pump; regulation; ram
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.
Streptomycetes are filamentous soil bacteria that produce spores through a complex process of morphological differentiation. The ram cluster plays an important part during the development. The ram genes encode a membrane-bound kinase (RamC), a small protein (RamS), components of an ABC transporter (RamAB), and a response regulator (RamR). While the introduction of an extra copy of the ram cluster accelerates development in Streptomyces lividans, ramABR disruption mutants are unable to produce aerial hyphae and spores. The developmental regulation of ram gene transcription was analyzed. Transcription of the ram genes occurred only on solid rich media and not on minimal media. The ramR gene is transcribed from a single promoter during all growth stages, with the highest levels during aerial growth. The ramCSAB genes comprise one operon and are transcribed from one principal promoter, P1, directly upstream of ramC. Transcription of ramCSAB was already observed during vegetative growth, but was strongly upregulated upon initiation of formation of aerial hyphae and was decreased during late stages of development. A large inverted repeat located downstream of ramS terminated the majority of transcripts. The introduction of ramR on a multicopy vector in S. lividans strongly induced P1 activity, while disruption of this regulator eliminated all P1 promoter activity. This shows that ramR is a crucial activator of ramCSAB transcription. Importantly, in bldA, bldB, bldD, or bldH mutants, ramR and ramCSAB are not transcribed, while ram gene transcription was observed in the earliest whi mutant, whiG. This indicates that the transcription of the ram genes marks the transition from vegetative to aerial growth.
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
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.