MexXY-OprM efflux pump is necessary for a adaptive resistance of Pseudomonas aeruginosa to aminoglycosides

Résistance de bas niveau aux fluoroquinolones par surexpression de l'efflux chez Pseudomonas aeruginosa : conséquences thérapeutiques et détection au laboratoire

The aim of this study was (i) to assess the impact of stable overproduction of efflux systems MexAB-OprM and MexXY-OprM on the bacteriostatic activities of fluoroquinolones in clinical Pseudomonas aeruginosa strains and (ii) to find a convenient test for screening isolates with a low level resistance to fluoroquinolones. The minimal inhibitory concentrations (MICs) of ciprofloxacin and levofloxacin were determined for clinical isolates of P. aeruginosa overexpressing MexAB-OprM or MexXY-OprM. Efflux pumps derepression was associated with a modest two- to fourfold increase in resistance to the tested fluoroquinolones. Clinical significance of low level resistance conferred by the efflux mechanism was evaluated with a Monte Carlo simulation with various fluoroquinolone regimens. With this model, low levels of resistance to ciprofloxacin (MIC > or =0.25 mg/L) or levofloxacin (MIC > or =1 mg/L) such as those due to overproduced MexAB-OprM or MexXY-OprM were predicted to result in poor clinical outcomes. Altogether these data strongly suggest that when derepressed MexAB-OprM or MexXY-OprM provides P. aeruginosa with a resistance that may be sufficient to impair the efficacy of single therapy with highly potent fluoroquinolones such as ciprofloxacin and levofloxacin. Routine detection of clinical strains that displayed low-level resistance to fluoroquinolones with a Mueller Hinton agar containing 0.20 mg/L of ciprofloxacin will help clinician in his therapeutical choice.

Breakpoints for predicting Pseudomonas aeruginosa susceptibility to inhaled tobramycin in cystic fibrosis patients: use of high-range Etest strips

Inhaled administration of tobramycin assures high concentrations in cystic fibrotic lungs, improving the therapeutic ratio over that of parenteral tobramycin levels, particularly against Pseudomonas aeruginosa. Conventional Clinical and Laboratory Standards Institute (CLSI; formerly National Committee for Clinical Laboratory Standards) breakpoints only consider parenteral levels and do not take into account these high antimicrobial concentrations. The Spanish Antibiogram Committee (The MENSURA Group) has tentatively defined specific breakpoint values for inhaled tobramycin when testing P. aeruginosa isolates from cystic fibrosis (CF) patients (susceptible, < or =64 microg/ml; resistant, > or =128 microg/ml). The antimicrobial susceptibilities of 206 prospectively collected CF P. aeruginosa isolates were determined by the reference agar dilution method. For tobramycin, the performance of high range tobramycin Etest strips (AB Biodisk, Solna, Sweden) and conventional tobramycin disks were assessed with the same collection. Applying MENSURA proposed breakpoints, 95.1% of the strains were categorized as susceptible to tobramycin, either using agar dilution or Etest high-range strips (99% categorical agreement between both methods). With CLSI breakpoints, susceptibility rates decreased to 79.1 and 81.1% for agar dilution and Etest strips, respectively (83.5% categorical agreement). Minor, major, and very major errors for Etest strips (CLSI criteria) were 13.6, 1.2, and 14.8%, respectively. Upon applying the new proposed criteria for inhaled tobramycin, only one major and one very major error were observed with Etest strips. Whenever inhaled tobramycin is considered for therapy, we suggest that P. aeruginosa strains from CF patients categorized as intermediate or resistant to tobramycin according to the CLSI criteria should be retested with high-range Etest strips and recategorized using MENSURA interpretive criteria. CLSI breakpoints should still be followed when intravenous tobramycin is used in CF patients, particularly during the course of exacerbations.

Efflux-mediated antimicrobial resistance

Antibiotic resistance continues to plague antimicrobial chemotherapy of infectious disease. And while true biocide resistance is as yet unrealized, in vitro and in vivo episodes of reduced biocide susceptibility are common and the history of antibiotic resistance should not be ignored in the development and use of biocidal agents. Efflux mechanisms of resistance, both drug specific and multidrug, are important determinants of intrinsic and/or acquired resistance to these antimicrobials, with some accommodating both antibiotics and biocides. This latter raises the spectre (as yet generally unrealized) of biocide selection of multiple antibiotic-resistant organisms. Multidrug efflux mechanisms are broadly conserved in bacteria, are almost invariably chromosome-encoded and their expression in many instances results from mutations in regulatory genes. In contrast, drug-specific efflux mechanisms are generally encoded by plasmids and/or other mobile genetic elements (transposons, integrons) that carry additional resistance genes, and so their ready acquisition is compounded by their association with multidrug resistance. While there is some support for the latter efflux systems arising from efflux determinants of self-protection in antibiotic-producing Streptomyces spp. and, thus, intended as drug exporters, increasingly, chromosomal multidrug efflux determinants, at least in Gram-negative bacteria, appear not to be intended as drug exporters but as exporters with, perhaps, a variety of other roles in bacterial cells. Still, given the clinical significance of multidrug (and drug-specific) exporters, efflux must be considered in formulating strategies/approaches to treating drug-resistant infections, both in the development of new agents, for example, less impacted by efflux and in targeting efflux directly with efflux inhibitors.

Measurement of Pseudomonas aeruginosa multidrug efflux pumps by quantitative real-time polymerase chain reaction

Multidrug efflux pumps contribute to multiple antibiotic resistance in Pseudomonas aeruginosa. Pump expression usually has been quantified by Western blotting. Quantitative real-time polymerase chain reaction has been developed to measure mRNA expression for genes of interest. Whether this method correlates with pump protein quantities is unclear. We devised a real-time PCR for mRNA expression of MexAB-OprM and MexXY-OprM multidrug efflux pumps. In laboratory strains differing in MexB and MexY expression and in several clinical isolates, protein and mRNA expression correlated well. Quantitative real-time PCR should be a useful alternative in quantitating expression of multidrug efflux pumps by P. aeruginosa isolates in clinical laboratories.

Deux systèmes d'efflux exprimés simultanément chez des souches cliniques de Pseudomonas aeruginosa

Active efflux systems MexAB-OprM and MexXY were found to be overexpressed simultaneously in 12 multiresistant clinical isolates of Pseudomonas aeruginosa. Nine of these strains (agrZ mutants) harbored mutations in gene mexZ, the product of which down-regulates expression of operon mexXY. Eight of the 12 strains exhibited mutations in genes known to control transcription of operon mexAB-oprM, such as mexR (four nalB mutants) or PA3721 (three nalC mutants). One strain was a nalB/nalC double mutant. For MexAB-OprM as well as for MexXY, no clear correlation could be established between (i) the types of mutations, (ii) the over-expression levels of genes mexA or mexX, and (iii) the resistance levels to effluxed antibiotics. Finally, three and four isolates overproduced MexXY (agrW mutants) or MexAB-OprM (nalD mutants), respectively, without any mutation in the known regulator genes. These data show that clinical isolates are able to broaden their drug resistance profiles by coexpressing two Mex efflux pumps and suggest the existence of additional regulators for MexAB-OprM and MexXY.

Clinical strains of Pseudomonas aeruginosa overproducing MexAB-OprM and MexXY efflux pumps simultaneously

Simultaneous overexpression of the MexAB-OprM and MexXY efflux systems was demonstrated by real-time reverse transcription-PCR and immunoblotting experiments for 12 multiresistant clinical isolates of Pseudomonas aeruginosa. DNA sequencing analysis showed that nine of these strains (named agrZ mutants) harbored mutations in mexZ, the product of which downregulates the expression of the mexXY operon. In addition, 8 of the 12 strains exhibited mutations in genes known to control transcription of the mexAB-oprM operon. Four of them were nalB mutants with alterations in the repressor gene mexR, three of them appeared to be nalC mutants deficient in gene PA3721 and overexpressing gene PA3720, and one strain was a nalB nalC double mutant. For MexAB-OprM as well as for MexXY, no clear correlation could be established between (i) the types of mutations, (ii) the expression level of mexA or mexX, and (iii) resistance to effluxed antibiotics. Finally, three isolates, named agrW mutants, overproduced MexXY and had an intact mexZ gene, and four strains overproduced MexAB-OprM and had intact mexR and PA3721 genes (nalD mutants). These data show that clinical isolates are able to broaden their drug resistance profiles by coexpressing two Mex efflux pumps and suggest the existence of additional regulators for MexAB-OprM and MexXY.

Role of the multidrug efflux system MexXY in the emergence of moderate resistance to aminoglycosides among Pseudomonas aeruginosa isolates from patients with cystic fibrosis

This study investigates the role of active efflux system MexXY in the emergence of aminoglycoside (AG) resistance among cystic fibrosis (CF) isolates of Pseudomonas aeruginosa. Three genotypically related susceptible and resistant (S/R) bacterial pairs and three other AG-resistant CF strains were compared to four non-CF strains moderately resistant to AGs. As demonstrated by immunoblot experiments, pump MexY was strongly overproduced in all of the resistant bacteria. This MexXY upregulation was associated with a 2- to 16-fold increase in the MICs of AGs in the S/R pairs and lower intracellular accumulation of dihydrostreptomycin. Alterations in mexZ, the repressor gene of operon mexXY, were found in all of the AG-resistant CF isolates and in one non-CF strain. Complementation of these bacteria with a plasmid-borne mexZ gene dramatically reduced the MICs of AGs, thus highlighting the role played by MexXY in the development of moderate resistance in CF patients. In contrast, complementation of the three non-CF strains showing wild-type mexZ genes left residual levels of resistance to AGs. These data indicate that a locus different from mexZ may be involved in overproduction of MexXY and that other nonenzymatic mechanisms contribute to AG resistance in P. aeruginosa.

Contribution of the MexXY multidrug transporter to aminoglycoside resistance in Pseudomonas aeruginosa clinical isolates

MexXY is an aminoglycoside-inducible multidrug transporter shown to contribute to intrinsic and acquired aminoglycoside resistance in laboratory isolates of Pseudomonas aeruginosa. To assess its contribution to aminoglycoside resistance in 14 clinical isolates demonstrating a panaminoglycoside resistance phenotype unlikely to be explained solely by aminoglycoside modification, expression of mexXY by these isolates was examined by reverse transcription-PCR. Elevated levels of mexXY expression were evident for most strains compared with those detected for an aminoglycoside-susceptible control strain, although there was no correlation between mexXY levels and the aminoglycoside MICs for the resistant strains, indicating that if MexXY was playing a role, other factors were also contributing. Deletion of mexXY from 9 of the 14 isolates resulted in enhanced susceptibilities to multiple aminoglycosides, confirming the contribution of this efflux system to the aminoglycoside resistance of these clinical isolates. Still, the impact of MexXY loss varied, with some strains clearly more or less dependent on MexXY for aminoglycoside resistance. Expression of mexXY also varied in these strains, with some showing high-level expression of the efflux genes independent of aminoglycoside exposure (aminoglycoside-independent hyperexpression) and others showing hyperexpression of the efflux genes that was to a greater or lesser degree aminoglycoside dependent. None of these strains carried mutations in mexZ, which encodes a negative regulator of mexXY expression, or in the mexZ-mexXY intergenic region. Thus, mexXY hyperexpression in aminoglycoside-resistant clinical isolates occurs via mutation in one or more as yet unidentified genes.

Mécanisme de la résistance adaptative de Pseudomonas aeruginosa aux aminosides

Exposure of Pseudomonas aeruginosa to aminoglycosides frequently selects for recalcitrant subpopulations exhibiting an unstable, << adaptive >> resistance to these antibiotics. In this study, we investigated the implication in the phenomenon of MexXY-OprM, an active efflux system known to export aminoglycosides in P. aeruginosa. Immunoblotting experiments demonstrated that the transporter MexY, but not the outer membrane pore OprM, was overproduced during the post-drug exposure adaptation period in wild-type strain PAO1. Furthermore, MexY production was dependent upon the degree of bacterial exposure to gentamicin (drug concentration). In contrast to parental strain PAO1, mutants defective in MexXY or in OprM were unable to develop adaptive resistance. Altogether, these results indicate that the resistance process requires the rapid production of MexXY and the interaction of these proteins with the constitutively produced component OprM.

Genetic and phenotypic variations of a resistant Pseudomonas aeruginosa epidemic clone

From May 1997 to December 2001, a serotype O:6 multidrug-resistant strain of Pseudomonas aeruginosa colonized or infected 201 patients in the University Hospital of Besançon (France). The susceptibility profile of this epidemic clone to fluoroquinolones and aminoglycosides was relatively stable during the outbreak but showed important isolate-to-isolate variations (up to 64-fold) in the MICs of beta-lactams. Analysis of 18 genotypically related isolates selected on a quarterly basis demonstrated alterations in the two DNA topoisomerases II and IV (Thr83-->Ile in GyrA and Ser87-->Leu in ParC) and production of an ANT(2")-I enzyme. Although constitutively overproduced in these bacteria, the MexXY efflux system did not appear to contribute significantly to aminoglycoside resistance. beta-Lactam resistance was associated with derepression of intrinsic AmpC beta-lactamase (with isolate-to-isolate variations of up to 58-fold) and sporadic deficiency in a 46-kDa protein identified as the carbapenem-selective porin OprD. Of the 18 isolates, 14 were also found to overproduce the efflux system MexAB-OprM as a result of alteration of the repressor protein MexR (His107-->Pro). However, complementation experiments with the cloned mexR gene demonstrated that MexAB-OprM contributed only marginally to beta-lactam and fluoroquinolone resistance. Of the four isolates exhibiting wild-type MexAB-OprM expression despite the MexR alteration, two appeared to harbor secondary mutations in the mexA-mexR intergenic region and one harbored secondary mutations in the putative ribosome binding site located upstream of the mexAB oprM operon. In conclusion, this study shows that many mechanisms were involved in the multiresistance phenotype of this highly epidemic strain of P. aeruginosa. Our results also demonstrate that the clone sporadically underwent substantial genetic and phenotypic variations during the course of the outbreak, perhaps in relation to local or individual selective drug pressures.