MpeR Regulates the mtr Efflux Locus in Neisseria gonorrhoeae and Modulates Antimicrobial Resistance by an Iron-Responsive Mechanism

Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, Georgia, USA.
Antimicrobial Agents and Chemotherapy (Impact Factor: 4.48). 01/2012; 56(3):1491-501. DOI: 10.1128/AAC.06112-11
Source: PubMed


Previous studies have shown that the MpeR transcriptional regulator produced by Neisseria gonorrhoeae represses the expression of mtrF, which encodes a putative inner membrane protein (MtrF). MtrF works as an accessory protein with the Mtr efflux pump, helping gonococci to resist high levels of diverse hydrophobic antimicrobials. Regulation of mpeR has been reported to occur by an iron-dependent mechanism involving Fur (ferric uptake regulator). Collectively, these observations suggest the presence of an interconnected regulatory system in gonococci that modulates the expression of efflux pump protein-encoding genes in an iron-responsive manner. Herein, we describe this connection and report that levels of gonococcal resistance to a substrate of the mtrCDE-encoded efflux pump can be modulated by MpeR and the availability of free iron. Using microarray analysis, we found that the mtrR gene, which encodes a direct repressor (MtrR) of mtrCDE, is an MpeR-repressed determinant in the late logarithmic phase of growth when free iron levels would be reduced due to bacterial consumption. This repression was enhanced under conditions of iron limitation and resulted in increased expression of the mtrCDE efflux pump operon. Furthermore, as judged by DNA-binding analysis, MpeR-mediated repression of mtrR was direct. Collectively, our results indicate that both genetic and physiologic parameters (e.g., iron availability) can influence the expression of the mtr efflux system and modulate levels of gonococcal susceptibility to efflux pump substrates.

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Available from: William M Shafer, Mar 31, 2014
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    • "In order to determine the transcriptional response of gonococci to the reported iron-limiting conditions inside of phagocytic cells [7], [12], we investigated the expression of gonococcal iron-responsive genes that encode the transferrin-binding protein complex (tbpA and tbpB), an enterobactin-like siderophore receptor (fetA) and a transcriptional activator (mpeR) of fetA that is negatively regulated by the gonococcal ferric uptake regulator (Fur) in the presence of iron; two non-iron-responsive genes that encode the gonococcal reduction modification protein (rmpM) and phosphoserine aminotransferase (serC) were also tested as controls [34], [35], [36], [37]. We found that these iron-responsive genes were significantly upregulated in monocyte-associated gonococci at 5 hr compared with 1 hr of phagocytosis (Figure 1C). "
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    ABSTRACT: Neisseria gonorrhoeae is a strict human pathogen that causes the sexually transmitted infection termed gonorrhea. The gonococcus can survive extracellularly and intracellularly, but in both environments the bacteria must acquire iron from host proteins for survival. However, upon infection the host uses a defensive response by limiting the bioavailability of iron by a number of mechanisms including the enhanced expression of hepcidin, the master iron-regulating hormone, which reduces iron uptake from the gut and retains iron in macrophages. The host also secretes the antibacterial protein NGAL, which sequesters bacterial siderophores and therefore inhibits bacterial growth. To learn whether intracellular gonococci can subvert this defensive response, we examined expression of host genes that encode proteins involved in modulating levels of intracellular iron. We found that N. gonorrhoeae can survive in association (tightly adherent and intracellular) with monocytes and macrophages and upregulates a panel of its iron-responsive genes in this environment. We also found that gonococcal infection of human monocytes or murine macrophages resulted in the upregulation of hepcidin, NGAL, and NRAMP1 as well as downregulation of the expression of the gene encoding the short chain 3-hydroxybutyrate dehydrogenase (BDH2); BDH2 catalyzes the production of the mammalian siderophore 2,5-DHBA involved in chelating and detoxifying iron. Based on these findings, we propose that N. gonorrhoeae can subvert the iron-limiting innate immune defenses to facilitate iron acquisition and intracellular survival.
    PLoS ONE 01/2014; 9(1):e87688. DOI:10.1371/journal.pone.0087688 · 3.23 Impact Factor
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    • "As stated above, the MtrCDE system is involved in resistance to host-derived antibacterial peptides (Shafer et al., 1998). It was recently reported that mtrCDE expression is indirectly regulated by free levels of iron through the regulation of its major transcriptional repressor, MtrR, by the MpeR transcriptional regulator (Mercante et al., 2012). Under the proposed model, expression of the efflux system would increase under iron-limited conditions, a situation that bacteria can encounter over the course of the infection process (Martinez et al., 1990). "
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    ABSTRACT: Multidrug efflux pumps are chromosomally encoded genetic elements capable of mediating resistance to toxic compounds in several life forms. In bacteria, these elements are involved in intrinsic and acquired resistance to antibiotics. Unlike other well-known horizontally acquired antibiotic resistance determinants, genes encoding for multidrug efflux pumps belong to the core of bacterial genomes and thus have evolved over millions of years. The selective pressure stemming from the use of antibiotics to treat bacterial infections is relatively recent in evolutionary terms. Therefore, it is unlikely that these elements have evolved in response to antibiotics. In the last years, several studies have identified numerous functions for efflux pumps that go beyond antibiotic extrusion. In this review we present some examples of these functions that range from bacterial interactions with plant or animal hosts, to the detoxification of metabolic intermediates or the maintenance of cellular homeostasis.
    Frontiers in Microbiology 02/2013; 4:7. DOI:10.3389/fmicb.2013.00007 · 3.99 Impact Factor
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    • "Additionally, transcription of mtrCDE may be induced in the presence of sub-lethal concentrations of nonionic, membrane-acting detergents through the action of an AraC/XylS family transcriptional activator, MtrA (Rouquette et al., 1999). Expression of mtrF is negatively regulated by both MtrR and the AraC family regulator MpeR (Folster and Shafer, 2005), as well as the availability of free iron (Mercante et al., 2012). "
    Antibiotic Resistant Bacteria - A Continuous Challenge in the New Millennium, 04/2012; , ISBN: 978-953-51-0472-8
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