nfxC-type quinolone resistance in a clinical isolate of Pseudomonas aeruginosa

Combination Therapy with Ciprofloxacin and Pentamidine against Multidrug-Resistant Pseudomonas aeruginosa: Assessment of In Vitro and In Vivo Efficacy and the Role of Resistance-Nodulation-Division (RND) Efflux Pumps

The aim of this work was to (i) evaluate the efficacy of a combination treatment of pentamidine with ciprofloxacin against Galleria mellonella larvae infected with an MDR strain of P. aeruginosa and (ii) determine if pentamidine acts as an efflux-pump inhibitor. Resistant clinical isolates, mutant strains overexpressing one of three RND efflux pumps (MexAB-OprM, MexCD-OprJ, and MexEF-OprN), and a strain with the same three pumps deleted were used. MIC assays confirmed that the clinical isolates and the mutants overexpressing efflux pumps were resistant to ciprofloxacin and pentamidine. The deletion of the three efflux pumps induced sensitivity to both compounds. Exposure to pentamidine and ciprofloxacin in combination resulted in the synergistic inhibition of all resistant strains in vitro, but no synergy was observed versus the efflux-pump deletion strain. The treatment of infected G. mellonella larvae with the combination of pentamidine and ciprofloxacin resulted in enhanced efficacy compared with the monotherapies and significantly reduced the number of proliferating bacteria. Our measurement of efflux activity from cells revealed that pentamidine had a specific inhibitory effect on the MexCD-OprJ and MexEF-OprN efflux pumps. However, the efflux activity and membrane permeability assays revealed that pentamidine also disrupted the membrane of all cells. In conclusion, pentamidine does possess some efflux-pump inhibitory activity, in addition to a more general disruptive effect on membrane integrity that accounts for its ability to potentiate ciprofloxacin activity. Notably, the enhanced efficacy of combination therapy with pentamidine and ciprofloxacin versus MDR P. aeruginosa strains in vivo merits further investigation into its potential to treat infections via this pathogen in patients.

Ramaswamy V, Vargiu AV, Malloci G, Bosin A, Ruggerone P. Common recognition topology of mex transporters of Pseudomonas aeruginosa revealed by molecular modelling

The secondary transporters of the resistance-nodulation-cell division (RND) superfamily mediate multidrug resistance in Gram-negative bacteria like Pseudomonas aeruginosa. Among these RND transporters, MexB, MexF, and MexY, with partly overlapping specificities, have been implicated in pathogenicity. Only the structure of the former has been resolved experimentally, which together with the lack of data about the functional dynamics of the full set of transporters, limited a systematic investigation of the molecular determinants defining their peculiar and shared features. In a previous work (Ramaswamy et al., Front. Microbiol., 2018, 9, 1144), we compared at an atomistic level the two main putative recognition sites (named access and deep binding pockets) of MexB and MexY. In this work, we expand the comparison by performing extended molecular dynamics (MD) simulations of these transporters and the pathologically relevant transporter MexF. We employed a more realistic model of the inner phospholipid membrane of P. aeruginosa and more accurate force-fields. To elucidate structure/dynamics-activity relationships we performed physico-chemical analyses and mapped the binding propensities of several organic probes on all transporters. Our data revealed the presence, also in MexF, of a few multifunctional sites at locations equivalent to the access and deep binding pockets detected in MexB. Furthermore, we report for the first time about the multidrug binding abilities of two out of five gates of the channels deputed to peripheral (early) recognition of substrates. Overall, our findings help to define a common “recognition topology” characterizing Mex transporters, which can be exploited to optimize transport and inhibition propensities of antimicrobial compounds.

The role of chemotaxis and efflux pumps on nitrate reduction in the toxic regions of a ciprofloxacin concentration gradient

Spatial concentration gradients of antibiotics are prevalent in the natural environment. Yet, the microbial response in these heterogeneous systems remains poorly understood. We used a microfluidic reactor to create an artificial microscopic ecosystem that generates diffusive gradients of solutes across interconnected microenvironments. With this reactor, we showed that chemotaxis toward a soluble electron acceptor (nitrate) allowed Shewanella oneidensis MR-1 to inhabit and sustain metabolic activity in highly toxic regions of the antibiotic ciprofloxacin (>80× minimum inhibitory concentration, MIC). Acquired antibiotic resistance was not observed for cells extracted from the reactor, so we explored the role of transient adaptive resistance by probing multidrug resistance (MDR) efflux pumps, ancient elements that are important for bacterial physiology and virulence. Accordingly, we constructed an efflux pump deficient mutant (∆mexF) and used resistance-nodulation-division (RND) efflux pump inhibitors (EPIs). While batch results showed the importance of RND efflux pumps for microbial survival, microfluidic studies indicated that these pumps were not necessary for survival in antibiotic gradients. Our work contributes to an emerging body of knowledge deciphering the effects of antibiotic spatial heterogeneity on microorganisms and highlights differences of microbial response in these systems versus well-mixed batch conditions.

Pseudomonas aeruginosa: targeting cell-wall metabolism for new antibacterial discovery and development

Pseudomonas aeruginosa is a leading cause of hospital-acquired infections and is resistant to most antibiotics. With therapeutic options against P. aeruginosa dwindling, and the lack of new antibiotics in advanced developmental stages, strategies for preserving the effectiveness of current antibiotics are urgently required. β-Lactam antibiotics are important agents for treating P. aeruginosa infections, thus, adjuvants that potentiate the activity of these compounds are desirable for extending their lifespan while new antibiotics - or antibiotic classes - are discovered and developed. In this review, we discuss recent research that has identified exploitable targets of cell-wall metabolism for the design and development of compounds that hinder resistance and potentiate the activity of antipseudomonal β-lactams.

Interaction of antibacterial compounds with RND efflux pumps in Pseudomonas aeruginosa

Pseudomonas aeruginosa infections are becoming increasingly difficult to treat due to intrinsic antibiotic resistance and the propensity of this pathogen to accumulate diverse resistance mechanisms. Hyperexpression of efflux pumps of the Resistance-Nodulation-Cell Division (RND)-type multidrug efflux pumps (e.g., MexAB-OprM), chromosomally encoded by mexAB-oprM, mexCD-oprJ, mexEF-oprN, and mexXY (-oprA) is often detected in clinical isolates and contributes to worrying multi-drug resistance phenotypes. Not all antibiotics are affected to the same extent by the aforementioned RND efflux pumps. The impact of efflux on antibiotic activity varies not only between different classes of antibiotics but also between members of the same family of antibiotics. Subtle differences in physicochemical features of compound-pump and compound-solvent interactions largely determine how compounds are affected by efflux activity. The combination of different high-resolution techniques helps to gain insight into the functioning of these molecular machineries. This review discusses substrate recognition patterns based on experimental evidence and computer simulations with a focus on MexB, the pump subunit of the main RND transporter in P. aeruginosa.

The challenge of efflux-mediated antibiotic resistance in Gram-negative bacteria

The global emergence of multidrug-resistant Gram-negative bacteria is a growing threat to antibiotic therapy. The chromosomally encoded drug efflux mechanisms that are ubiquitous in these bacteria greatly contribute to antibiotic resistance and present a major challenge for antibiotic development. Multidrug pumps, particularly those represented by the clinically relevant AcrAB-TolC and Mex pumps of the resistance-nodulation-division (RND) superfamily, not only mediate intrinsic and acquired multidrug resistance (MDR) but also are involved in other functions, including the bacterial stress response and pathogenicity. Additionally, efflux pumps interact synergistically with other resistance mechanisms (e.g., with the outer membrane permeability barrier) to increase resistance levels. Since the discovery of RND pumps in the early 1990s, remarkable scientific and technological advances have allowed for an in-depth understanding of the structural and biochemical basis, substrate profiles, molecular regulation, and inhibition of MDR pumps. However, the development of clinically useful efflux pump inhibitors and/or new antibiotics that can bypass pump effects continues to be a challenge. Plasmid-borne efflux pump genes (including those for RND pumps) have increasingly been identified. This article highlights the recent progress obtained for organisms of clinical significance, together with methodological considerations for the characterization of MDR pumps.

Investigating the link between imipenem resistance and biofilm formation by Pseudomonas aeruginosa

Pseudomonas aeruginosa, a ubiquitous environmental organism, is a difficult-to-treat opportunistic pathogen due to its broad-spectrum antibiotic resistance and its ability to form biofilms. In this study, we investigate the link between resistance to a clinically important antibiotic, imipenem, and biofilm formation. First, we observed that the laboratory strain P. aeruginosa PAO1 carrying a mutation in the oprD gene, which confers resistance to imipenem, showed a modest reduction in biofilm formation. We also observed an inverse relationship between imipenem resistance and biofilm formation for imipenem-resistant strains selected in vitro, as well as for clinical isolates. We identified two clinical isolates of P. aeruginosa from the sputum of cystic fibrosis patients that formed robust biofilms, but were sensitive to imipenem (MIC ≤ 2 μg/ml). To test the hypothesis that there is a general link between imipenem resistance and biofilm formation, we performed transposon mutagenesis of these two clinical strains to identify mutants defective in biofilm formation, and then tested these mutants for imipenem resistance. Analysis of the transposon mutants revealed a role for previously described biofilm factors in these clinical isolates of P. aeruginosa, including mutations in the pilY1, pilX, pilW, algC, and pslI genes, but none of the biofilm-deficient mutants became imipenem resistant (MIC ≥ 8 μg/ml), arguing against a general link between biofilm formation and resistance to imipenem. Thus, assessing biofilm formation capabilities of environmental isolates is unlikely to serve as a good predictor of imipenem resistance. We also discuss our findings in light of the limited literature addressing planktonic antibiotic resistance factors that impact biofilm formation.

Carbapenems in the USA: focus on doripenem

The threat of antibacterial resistance continues to increase globally, and therapeutic options for the treatment of some serious infectious diseases are diminishing. The carbapenems are a potent class of broad-spectrum drugs, and their stability against hydrolysis by many important beta-lactamases make them an important weapon in the treatment of beta-lactamase-producing bacterial pathogens. This review focuses on four carbapenems of clinical importance in the USA: imipenem, meropenem, ertapenem and doripenem. After a historical review of carbapenem development, these four carbapenems are evaluated based on their mechanism of action, spectrum of activity, potency, pharmacodynamics, clinical pharmacokinetics, clinical profiles and toxicity issues.

Adaptive and mutational resistance: role of porins and efflux pumps in drug resistance

The substantial use of antibiotics in the clinic, combined with a dearth of new antibiotic classes, has led to a gradual increase in the resistance of bacterial pathogens to these compounds. Among the various mechanisms by which bacteria endure the action of antibiotics, those affecting influx and efflux are of particular importance, as they limit the interaction of the drug with its intracellular targets and, consequently, its deleterious effects on the cell. This review evaluates the impact of porins and efflux pumps on two major types of resistance, namely, mutational and adaptive types of resistance, both of which are regarded as key phenomena in the global rise of antibiotic resistance among pathogenic microorganisms. In particular, we explain how adaptive and mutational events can dramatically influence the outcome of antibiotic therapy by altering the mechanisms of influx and efflux of antibiotics. The identification of porins and pumps as major resistance markers has opened new possibilities for the development of novel therapeutic strategies directed specifically against these mechanisms.

MexEF-OprN efflux pump exports the Pseudomonas quinolone signal (PQS) precursor HHQ (4-hydroxy-2-heptylquinoline)

Bacterial cells have evolved the capacity to communicate between each other via small diffusible chemical signals termed autoinducers. Pseudomonas aeruginosa is an opportunistic pathogen involved, among others, in cystic fibrosis complications. Virulence of P. aeruginosa relies on its ability to produce a number of autoinducers, including 4-hydroxy-2-alkylquinolines (HAQ). In a cell density-dependent manner, accumulated signals induce the expression of multiple targets, especially virulence factors. This phenomenon, called quorum sensing, promotes bacterial capacity to cause disease. Furthermore, P. aeruginosa possesses many multidrug efflux pumps conferring adaptive resistance to antibiotics. Activity of some of these efflux pumps also influences quorum sensing. The present study demonstrates that the MexEF-OprN efflux pump modulates quorum sensing through secretion of a signalling molecule belonging to the HAQ family. Moreover, activation of MexEF-OprN reduces virulence factor expression and swarming motility. Since MexEF-OprN can be activated in infected hosts even in the absence of antibiotic selective pressure, it could promote establishment of chronic infections in the lungs of people suffering from cystic fibrosis, thus diminishing the immune response to virulence factors. Therapeutic drugs that affect multidrug efflux pumps and HAQ-mediated quorum sensing would be valuable tools to shut down bacterial virulence.

Pseudomonas aeruginosa: resistance to the max

Pseudomonas aeruginosa is intrinsically resistant to a variety of antimicrobials and can develop resistance during anti-pseudomonal chemotherapy both of which compromise treatment of infections caused by this organism. Resistance to multiple classes of antimicrobials (multidrug resistance) in particular is increasingly common in P. aeruginosa, with a number of reports of pan-resistant isolates treatable with a single agent, colistin. Acquired resistance in this organism is multifactorial and attributable to chromosomal mutations and the acquisition of resistance genes via horizontal gene transfer. Mutational changes impacting resistance include upregulation of multidrug efflux systems to promote antimicrobial expulsion, derepression of ampC, AmpC alterations that expand the enzyme's substrate specificity (i.e., extended-spectrum AmpC), alterations to outer membrane permeability to limit antimicrobial entry and alterations to antimicrobial targets. Acquired mechanisms contributing to resistance in P. aeruginosa include β-lactamases, notably the extended-spectrum β-lactamases and the carbapenemases that hydrolyze most β-lactams, aminoglycoside-modifying enzymes, and 16S rRNA methylases that provide high-level pan-aminoglycoside resistance. The organism's propensity to grow in vivo as antimicrobial-tolerant biofilms and the occurrence of hypermutator strains that yield antimicrobial resistant mutants at higher frequency also compromise anti-pseudomonal chemotherapy. With limited therapeutic options and increasing resistance will the untreatable P. aeruginosa infection soon be upon us?

Pseudomonas aeruginosa: resistance to the max

Pseudomonas aeruginosa is intrinsically resistant to a variety of antimicrobials and can develop resistance during anti-pseudomonal chemotherapy both of which compromise treatment of infections caused by this organism. Resistance to multiple classes of antimicrobials (multidrug resistance) in particular is increasingly common in P. aeruginosa, with a number of reports of pan-resistant isolates treatable with a single agent, colistin. Acquired resistance in this organism is multifactorial and attributable to chromosomal mutations and the acquisition of resistance genes via horizontal gene transfer. Mutational changes impacting resistance include upregulation of multidrug efflux systems to promote antimicrobial expulsion, derepression of ampC, AmpC alterations that expand the enzyme's substrate specificity (i.e., extended-spectrum AmpC), alterations to outer membrane permeability to limit antimicrobial entry and alterations to antimicrobial targets. Acquired mechanisms contributing to resistance in P. aeruginosa include β-lactamases, notably the extended-spectrum β-lactamases and the carbapenemases that hydrolyze most β-lactams, aminoglycoside-modifying enzymes, and 16S rRNA methylases that provide high-level pan-aminoglycoside resistance. The organism's propensity to grow in vivo as antimicrobial-tolerant biofilms and the occurrence of hypermutator strains that yield antimicrobial resistant mutants at higher frequency also compromise anti-pseudomonal chemotherapy. With limited therapeutic options and increasing resistance will the untreatable P. aeruginosa infection soon be upon us?

mexEF-oprN multidrug efflux operon of Pseudomonas aeruginosa: regulation by the MexT activator in response to nitrosative stress and chloramphenicol

A null mutation in the mexS gene of Pseudomonas aeruginosa yielded an increased level of expression of a 3-gene operon containing a gene, xenB, whose product is highly homologous to a xenobiotic reductase in Pseudomonas fluorescens shown previously to remove nitro groups from trinitrotoluene and nitroglycerin (D. S. Blehert, B. G. Fox, and G. H. Chambliss, J. Bacteriol. 181:6254, 1999). This expression, which paralleled an increase in mexEF-oprN expression in the same mutant, was, like mexEF-oprN, dependent on the MexT LysR family positive regulator previously implicated in mexEF-oprN expression. As nitration is a well-known result of nitrosative stress, a role for xenB (and the coregulated mexEF-oprN) in a nitrosative stress response was hypothesized and tested. Using s-nitrosoglutathione (GSNO) as a source of nitrosative stress, the expression of xenB and mexEF-oprN was shown to be GSNO inducible, although in the case of xenB, this was seen only for a mutant lacking MexEF-OprN. In both instances, this GSNO-inducible expression was dependent upon MexT. Chloramphenicol, a nitroaromatic antimicrobial that is a substrate for MexEF-OprN, was shown to induce mexEF-oprN but not xenB, again dependent upon the MexT regulator, possibly because it resembles a nitrosated nitrosative stress product accommodated by MexEF-OprN.

Transcriptional regulation of the mexEF-oprN multidrug efflux pump operon by MexT and an unidentified repressor in nfxC-type mutant of Pseudomonas aeruginosa

Expression of the MexEF-OprN efflux pump in Pseudomonas aeruginosa seems to be upregulated by MexT. The region upstream of the mexEF-oprN operon contains mexT arranged in tandem with mexEF-oprN and separated by 230 bp of mexT-mexE intergenic DNA. Therefore, it is likely that this intergenic DNA contains the promoter-operator element of mexEF-oprN. To characterize how the expression of the mexEF-oprN operon was controlled, we analyzed the mexT-mexE intergenic DNA by constructing a series of intergenic DNA deletions connected with the mexE∷lacZ reporter and made two important discoveries. The first was that the central region of the DNA contained two nod boxes. The mexT-proximal nod box was identified as the MexT-binding site by gel-shift assays using purified MexT. The mexT-distal nod box was required for the transcription of the mexEF-oprN operon, but not for the binding of MexT, suggesting that this region accommodates the binding of the RNA polymerase. The second observation is that there is a 13 bp inverted repeat sequence separated by 10 bp immediately upstream of the mexE gene. Deletion of this region caused a sudden rise in MexEF-OprN production, suggesting that this region accommodates the binding of a putative repressor protein.

Antibacterial-resistant Pseudomonas aeruginosa: clinical impact and complex regulation of chromosomally encoded resistance mechanisms

Treatment of infectious diseases becomes more challenging with each passing year. This is especially true for infections caused by the opportunistic pathogen Pseudomonas aeruginosa, with its ability to rapidly develop resistance to multiple classes of antibiotics. Although the import of resistance mechanisms on mobile genetic elements is always a concern, the most difficult challenge we face with P. aeruginosa is its ability to rapidly develop resistance during the course of treating an infection. The chromosomally encoded AmpC cephalosporinase, the outer membrane porin OprD, and the multidrug efflux pumps are particularly relevant to this therapeutic challenge. The discussion presented in this review highlights the clinical significance of these chromosomally encoded resistance mechanisms, as well as the complex mechanisms/pathways by which P. aeruginosa regulates their expression. Although a great deal of knowledge has been gained toward understanding the regulation of AmpC, OprD, and efflux pumps in P. aeruginosa, it is clear that we have much to learn about how this resourceful pathogen coregulates different resistance mechanisms to overcome the antibacterial challenges it faces.

Antibacterial-resistant Pseudomonas aeruginosa: clinical impact and complex regulation of chromosomally encoded resistance mechanisms

Treatment of infectious diseases becomes more challenging with each passing year. This is especially true for infections caused by the opportunistic pathogen Pseudomonas aeruginosa, with its ability to rapidly develop resistance to multiple classes of antibiotics. Although the import of resistance mechanisms on mobile genetic elements is always a concern, the most difficult challenge we face with P. aeruginosa is its ability to rapidly develop resistance during the course of treating an infection. The chromosomally encoded AmpC cephalosporinase, the outer membrane porin OprD, and the multidrug efflux pumps are particularly relevant to this therapeutic challenge. The discussion presented in this review highlights the clinical significance of these chromosomally encoded resistance mechanisms, as well as the complex mechanisms/pathways by which P. aeruginosa regulates their expression. Although a great deal of knowledge has been gained toward understanding the regulation of AmpC, OprD, and efflux pumps in P. aeruginosa, it is clear that we have much to learn about how this resourceful pathogen coregulates different resistance mechanisms to overcome the antibacterial challenges it faces.

mvaT mutation modifies the expression of the Pseudomonas aeruginosa multidrug efflux operon mexEF-oprN

Pseudomonas aeruginosa carries several multidrug efflux operons, including mexEF-oprN, that contribute to its resistance to multiple antibiotics. mvaT affects the expression of several P. aeruginosa genes. In this study, we show that the mvaT mutant PAODeltamvaT is more resistant than its parent PAO1 strain to chloramphenicol and norfloxacin but more sensitive to imipenem; yet both were less resistant to chloramphenicol, norfloxacin, and imipenem than 'typical'nfxC-type mutants. Neither strain carries the deletion described for nfxC-type mutants in mexT, the mexEF-oprN regulatory gene. Expression of mexEF-oprN is increased by five- to sixfold in PAODeltamvaT, while the expression of oprD is reduced by approximately twofold. mvaT mutation had no effect on the expression of other multidrug resistance operons, although it increased the expression of several ATP-binding cassette transporter genes. We show that mvaT mutation does not affect mexEF-oprN expression through mexT or mexS. We also explored several other potential mechanisms.

Pharmacodynamics of levofloxacin against Pseudomonas aeruginosa with reduced susceptibility due to different efflux pumps: do elevated MICs always predict reduced in vivo efficacy?

The Pseudomonas aeruginosa efflux pumps MexAB-OprM, MexCD-OprJ, and MexEF-OprN play an important role in susceptibility to fluoroquinolones in vitro. To determine if levofloxacin MICs arising from different levels of expression of efflux pumps result in a proportional reduction in the response to levofloxacin in vivo, isogenic strains of P. aeruginosa were tested with levofloxacin in two mouse models of infection (sepsis and neutropenic mouse thigh models). The levofloxacin 50% effective doses (ED(50)s) increased proportionally with the MICs for most strains. Similarly, the 24-h area under the concentration-time curve (AUC)/MIC ratio that resulted in 90% of the maximum bactericidal activity (90% E(max)) exceeded 75 for all strains except those with elevated MICs due to MexEF-OprN overexpression. In these strains, levofloxacin ED(50)s were 2- to 10-fold lower than the ED(50)/MIC ratios in the other strains and 90% E(max) AUC/MIC ratios were 2- to 4-fold lower than those predicted from pharmacodynamic modeling of efficacy against other strains. These data show that while the MexEF-OprN efflux pump can provide P. aeruginosa resistance to levofloxacin in vitro, it appears to be less efficient in providing resistance to levofloxacin in animal models of infection.

Levofloxacin/imipenem prevents the emergence of high-level resistance among Pseudomonas aeruginosa strains already lacking susceptibility to one or both drugs

OBJECTIVES

Previous studies have demonstrated that a combination of levofloxacin with imipenem could prevent the emergence of resistance during the treatment of susceptible Pseudomonas aeruginosa isolates in a two-compartment pharmacodynamic model of infection. In this study, the efficacy of levofloxacin/imipenem was further evaluated against a panel of characterized P. aeruginosa strains that lacked susceptibility to one or both drugs in the combination.

METHODS

Five P. aeruginosa strains with characterized resistance mechanisms were evaluated. Log-phase cultures were inoculated into the peripheral compartment of the in vitro pharmacokinetic model and treated using simulated doses of 750 mg levofloxacin (dosed every 24 h) and 250 mg or 1 g doses of imipenem (dosed every 12 h). Peak levels were adjusted for protein binding. Pharmacodynamic interactions were evaluated by measuring the changes in viable counts over 30 h. To evaluate the emergence of resistance, samples removed at 30 h were plated onto agar containing the drug at 4x MIC, and potential mutants were evaluated for changes in susceptibility.

RESULTS

Against strains overexpressing MexAB-OprM, MexCD-OprJ and MexEF-OprN efflux pumps, levofloxacin/imipenem prevented the emergence of resistance and achieved a 5 log total kill of one strain and eradication of two strains. Levofloxacin/imipenem also eradicated an imipenem-resistant strain lacking OprD. Although the combination initially killed 6-7 logs of a dual-resistant strain lacking OprD and overexpressing MexXY, it could not prevent the emergence of resistance when the 250 mg dose of imipenem was simulated in the combination. However, when the 1 g dose of imipenem was simulated with the combination, resistance was suppressed.

CONCLUSIONS

These data suggest that levofloxacin/imipenem may be an effective combination for preventing the emergence of resistance among P. aeruginosa, even with strains already lacking susceptibility to one or both drugs in the combination. Clinical evaluation of this combination is warranted.

Levofloxacin-imipenem combination prevents the emergence of resistance among clinical isolates of Pseudomonas aeruginosa

A 2-compartment in vitro pharmacokinetic model (IVPM) was used to assess the potential of a levofloxacin-imipenem combination to prevent the emergence of resistance during treatment of Pseudomonas aeruginosa infection. Log-phase cultures (10(8) cfu/mL) of 3 clinical isolates were inoculated into the peripheral compartment of the IVPMs and were treated with simulated human doses of levofloxacin (750 mg) and imipenem (250 mg). Pharmacodynamics and the emergence of resistance were evaluated over the course of 24 h. Resistant mutants were evaluated for transcriptional expression of specific efflux pumps. Initially, rapid killing was observed in association with each regimen. However, with levofloxacin and imipenem alone, rapid regrowth was observed as a result of the selection of resistant subpopulations. Analysis of mutants selected by levofloxacin demonstrated that mexEF-oprN-overexpressing subpopulations resistant to both levofloxacin and imipenem were selected from cultures of all 3 strains. Nevertheless, the levofloxacin-imipenem combination rapidly eradicated all 3 P. aeruginosa strains. These data suggest that levofloxacin-imipenem may be an effective combination for preventing the emergence of resistance among P. aeruginosa strains, even when subpopulations resistant to both drugs are present. Further studies are warranted to evaluate the use of this combination against strains with established resistance to either or both drugs.

Alterations in the GyrA and GyrB subunits of topoisomerase II and the ParC and ParE subunits of topoisomerase IV in ciprofloxacin-resistant clinical isolates of Pseudomonas aeruginosa

The presence of fluoroquinolone resistance-associated alterations in topoisomerase II and IV were investigated for 103 nfxC-like type Pseudomonas aeruginosa isolates. The most nfxC-like type isolates (98.1%) possessed the substitution of Ile for Thr-83 in GyrA. A single alteration in GyrA (Thr-83-->Ile) was the most frequently detected and the next common alteration was two alterations with Thr-83-->Ile in GyrA and Ser-87-->Leu in ParC. A novel alteration at position Glin-106 of GyrA, which was suggested to be responsible for fluoroquinolone resistance, was identified. Our study revealed that the alterations in GyrB (Glu-468-->Asp) and in ParE (Asp-419-->Asn or Glu-459-->Asp) play a complementary role in the acquisition of resistance to fluoroquinolone. There was a correlation between the ciprofloxacin MIC and the number of resistance-associated alterations in GyrA, GyrB, ParC and ParE of P. aeruginosa isolates.

Mutations in PA2491 (mexS) promote MexT-dependent mexEF-oprN expression and multidrug resistance in a clinical strain of Pseudomonas aeruginosa

Disruption of the PA2491 gene in a mini-Tn5-tet insertion mutant of a clinical isolate of Pseudomonas aeruginosa increased expression of the mexEF-oprN multidrug efflux genes and decreased production of outer membrane protein OprD, concomitant with enhanced resistance to chloramphenicol, quinolones, and imipenem, which was reminiscent of previously described nfxC mutants. PA2491 encodes a probable oxidoreductase previously shown to be positively regulated by the MexT positive regulator of mexEF-oprN expression (T. Kohler, S. F. Epp, L. K. Curty, and J. C. Pechere, J. Bacteriol. 181:6300-6305, 1999). Spontaneous multidrug-resistant mutants of the P. aeruginosa clinical isolate hyperexpressing mexEF-oprN and showing reduced production of OprD were readily selected in vitro, and all of them were shown to carry mutations in PA2491, highlighting the probable significance of such mutations as determinants of MexEF-OprN-mediated multidrug resistance in vivo.

Antimicrobial activity of chlorhexidine diacetate and benzalkonium chloride against Pseudomonas aeruginosa and its response to biocide residues

AIMS

The aims of this study were to evaluate the antimicrobial activity of chlorhexidine diacetate (CHX) and benzalkonium chloride (BZK) for strains of Pseudomonas aeruginosa exhibiting increased minimum inhibitory concentrations (MIC) for CHX, and to determine whether residues of chlorhexidine digluconate (CHG) and Hibiscrub (Hib, a formulation containing CHG) affect the susceptibility of P. aeruginosa to these biocides and a number of antibiotics.

METHODS AND RESULTS

The bactericidal activity of CHX and BZK was evaluated for strains of P. aeruginosa exhibiting increased MIC for CHX with established suspension and surface disinfection tests. None of the strains of P. aeruginosa exhibiting raised MIC for CHX was less sensitive than the parent strain to CHX or BZK in either method. A test was designed to investigate the effects of dried CHG and Hib residues on P. aeruginosa cells. Exposure of P. aeruginosa to dried residues of CHG or Hib did not result in the organism becoming less sensitive to either biocide or a number of antibiotics.

CONCLUSIONS

Pseudomonas aeruginosa strains with raised MIC to CHX were no less sensitive than the parent strain to CHX and BZK in bactericidal investigations. Exposure to dried residues of CHG and Hib did not render P. aeruginosa less sensitive to either of these agents or a number of antibiotics.

SIGNIFICANCE AND IMPACT OF THE STUDY

An increase in the MIC for a biocide in a micro-organism does not necessarily result in a failure of the biocide to effectively kill the organism. The residue that remains after the use of an antimicrobial agent can be at a far lower concentration than that initially applied and this study highlights the necessity for further investigations into the effect of residues, at low concentration, on bacterial populations and their role, if any, in the continued problem of antibiotic resistance.

The role of multidrug efflux pumps in the antibiotic resistance of Pseudomonas aeruginosa and other gram-negative bacteria. Insights from the Society of Infectious Diseases Pharmacists

Gram-negative bacteria remain clinically important pathogens in both hospital and community settings. Recent research indicates that efflux pumps play a prominent role in the multidrug resistance of Pseudomonas aeruginosa and many other gram-negative bacteria. Four multidrug efflux pump systems have been well characterized in P. aeruginosa: MexA-MexB-OprM, MexC-MexD-OprJ, MexE-MexF-OprN, and MexX-MexY-OprM. These efflux pumps have different substrate specificities, and their production and activity can be increased by many factors commonly present in infections (e.g., high inocula of bacteria, low pH, and stationary-phase growth). Moreover, fluoroquinolone antibiotics can commonly select mutants that constitutively overproduce Mex-Opr efflux pump systems. Based on most recent studies, the prevalence of efflux pump overproduction in clinical strains of P. aeruginosa may range from 14-75%. The best treatment for infections caused by bacteria that overproduce efflux pumps is unknown, but pharmacodynamic optimization of antibiotics and the use of antibiotic combinations that are substrates for different pump systems may represent reasonable strategies until more data are available.

Efflux-mediated resistance to tigecycline (GAR-936) in Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa strains are less susceptible to tigecycline (previously GAR-936; MIC, 8 micro g/ml) than many other bacteria (P. J. Petersen, N. V. Jacobus, W. J. Weiss, P. E. Sum, and R. T. Testa, Antimicrob. Agents Chemother. 43:738-744, 1999). To elucidate the mechanism of resistance to tigecycline, P. aeruginosa PAO1 strains defective in the MexAB-OprM and/or MexXY (OprM) efflux pumps were tested for susceptibility to tigecycline. Increased susceptibility to tigecycline (MIC, 0.5 to 1 micro g/ml) was specifically associated with loss of MexXY. Transcription of mexX and mexY was also responsive to exposure of cells to tigecycline. To test for the emergence of compensatory efflux pumps in the absence of MexXY-OprM, mutants lacking MexXY-OprM were plated on medium containing tigecycline at 4 or 6 micro g/ml. Resistant mutants were readily recovered, and these also had decreased susceptibility to several other antibiotics, suggesting efflux pump recruitment. One representative carbenicillin-resistant strain overexpressed OprM, the outer membrane channel component of the MexAB-OprM efflux pump. The mexAB-oprM repressor gene, mexR, from this strain contained a 15-bp in-frame deletion. Two representative chloramphenicol-resistant strains showed expression of an outer membrane protein slightly larger than OprM. The mexCD-OprJ repressor gene, nfxB, from these mutants contained a 327-bp in-frame deletion and an IS element insertion, respectively. Together, these data indicated drug efflux mediated by MexCD-OprJ. The MICs of the narrower-spectrum semisynthetic tetracyclines doxycycline and minocycline increased more substantially than did those of tigecycline and other glycylcyclines against the MexAB-OprM- and MexCD-OprJ-overexpressing mutant strains. This suggests that glycylcyclines, although they are subject to efflux from P. aeruginosa, are generally inferior substrates for P. aeruginosa efflux pumps than are narrower-spectrum tetracyclines.

Analysis of the Pseudomonas aeruginosa oprD gene from clinical and environmental isolates

Genomes are constantly evolving. Our report highlights the wide mutational diversity of clinical as well as environmental isolates, compared with the laboratory strain(s), through the systematic genetic analysis of a chromosomal porin gene (oprD) in relation to a specific antibiotic resistance. Mutational inactivation of the oprD gene is associated with carbapenem resistance in Pseudomonas aeruginosa. The sequence of the oprD gene of 55 Pseudomonas aeruginosa natural isolates obtained from across the world--from sources as diverse as patients and rhizospheres--was analysed. A microscale mosaic structure for this gene--resulting from multiple intra- and possibly interspecies recombinational events--is reported. An array of independent and seemingly fast-occurring defective oprD mutations were found, none of which had been described before. A burn wound isolate demonstrated unusually high overall sequence variability typical of mutator strains. We also present evidence for the existence of OprD homologues in other fluorescent pseudomonads.

Construction of a series of mutants lacking all of the four major mex operons for multidrug efflux pumps or possessing each one of the operons from Pseudomonas aeruginosa PAO1: MexCD-OprJ is an inducible pump

We constructed a series of deletion mutants lacking all of the four major mex operons for Mex multidrug efflux pumps or possessing each one of the operons from Pseudomonas aeruginosa PAO1. The drug specificity of MexAB-OprM, MexXY-OprM and MexCD-OprJ was investigated. Surprisingly, we found that the MexCD-OprJ was an inducible pump, inducers of which were tetraphenylphosphonium chloride, ethidium bromide, rhodamine 6G and acriflavine. Fluoroquinolones, chloramphenicol, erythromycin and tetracycline were not inducers although they were substrates of MexCD-OprJ.

Type II topoisomerase mutations in fluoroquinolone-resistant clinical strains of Pseudomonas aeruginosa isolated in 1998 and 1999: role of target enzyme in mechanism of fluoroquinolone resistance

The major mechanism of resistance to fluoroquinolones for Pseudomonas aeruginosa is the modification of type II topoisomerases (DNA gyrase and topoisomerase IV). We examined the mutations in quinolone-resistance-determining regions (QRDR) of gyrA, gyrB, parC, and parE genes of recent clinical isolates. There were 150 isolates with reduced susceptibilities to levofloxacin and 127 with reduced susceptibilities to ciprofloxacin among 513 isolates collected during 1998 and 1999 in Japan. Sequencing results predicted replacement of an amino acid in the QRDR of DNA gyrase (GyrA or GyrB) for 124 of the 150 strains (82.7%); among these, 89 isolates possessed mutations in parC or parE which lead to amino acid changes. Substitutions of both Ile for Thr-83 in GyrA and Leu for Ser-87 in ParC were the principal changes, being detected in 48 strains. These replacements were obviously associated with reduced susceptibilities to levofloxacin, ciprofloxacin, and sparfloxacin; however, sitafloxacin showed high activity against isolates with these replacements. We purified GyrA (The-83 to Ile) and ParC (Ser-87 to Leu) by site-directed mutagenesis and compared the inhibitory activities of the fluoroquinolones. Sitafloxacin showed the most potent inhibitory activities against both altered topoisomerases among the fluoroquinolones tested. These results indicated that, compared with other available quinolones, sitafloxacin maintained higher activity against recent clinical isolates with multiple mutations in gyrA and parC, which can be explained by the high inhibitory activities of sitafloxacin against both mutated enzymes.

Identification and characterization of inhibitors of multidrug resistance efflux pumps in Pseudomonas aeruginosa: novel agents for combination therapy

Whole-cell assays were implemented to search for efflux pump inhibitors (EPIs) of the three multidrug resistance efflux pumps (MexAB-OprM, MexCD-OprJ, MexEF-OprN) that contribute to fluoroquinolone resistance in clinical isolates of Pseudomonas aeruginosa. Secondary assays were developed to identify lead compounds with exquisite activities as inhibitors. A broad-spectrum EPI which is active against all three known Mex efflux pumps from P. aeruginosa and their close Escherichia coli efflux pump homolog (AcrAB-TolC) was discovered. When this compound, MC-207,110, was used, the intrinsic resistance of P. aeruginosa to fluoroquinolones was decreased significantly (eightfold for levofloxacin). Acquired resistance due to the overexpression of efflux pumps was also decreased (32- to 64-fold reduction in the MIC of levofloxacin). Similarly, 32- to 64-fold reductions in MICs in the presence of MC-207,110 were observed for strains with overexpressed efflux pumps and various target mutations that confer resistance to levofloxacin (e.g., gyrA and parC). We also compared the frequencies of emergence of levofloxacin-resistant variants in the wild-type strain at four times the MIC of levofloxacin (1 microg/ml) when it was used either alone or in combination with EPI. In the case of levofloxacin alone, the frequency was approximately 10(-7) CFU/ml. In contrast, with an EPI, the frequency was below the level of detection (<10(-11)). In summary, we have demonstrated that inhibition of efflux pumps (i) decreased the level of intrinsic resistance significantly, (ii) reversed acquired resistance, and (iii) resulted in a decreased frequency of emergence of P. aeruginosa strains that are highly resistant to fluoroquinolones.

Variation of the mexT gene, a regulator of the MexEF-oprN efflux pump expression in wild-type strains of Pseudomonas aeruginosa

We found three variations of wild-type strains in terms of mexT-mediated regulation of the MexEF-OprN efflux pump, in which overexpression of the pump results in nfxC-type antibiotic resistance in Pseudomonas aeruginosa. Type-I: the mexT gene of the wild-type strain encoded inactive MexT and the nfxC-type mutants derived from this parent had an additional mutation in mexT converting MexT from the inactive to the active form. Type-II: The mexT gene in the wild-type strain had an 8-bp insert producing inactive MexT and the nfxC-type mutants from this parent had a deletion of the 8-bp insert converting inactive MexT to the active form. Type-III: Both the wild-type strain and its nfxC-type derivative produced identical and active MexT. The nfxC mutant from this parent must have an additional mutation. The original nfxC mutant isolated in 1990 might be derived from the Type-I parent strain.

Large outbreak in a surgical intensive care unit of colonization or infection with Pseudomonas aeruginosa that overexpressed an active efflux pump

During a 30-month survey, 55 patients were colonized or infected by a single clone of Pseudomonas aeruginosa in a surgical intensive care unit (ICU). This clone overexpressed an efflux pump system, and its antibiotic resistance pattern was extremely stable as it spread from patient to patient. Pulsed-field gel electrophoresis showed that isolates from different patients were genetically identical or very similar. We were unable to identify an environmental reservoir, but cultures of hand specimens from 2 health care workers were positive. It was not clear whether this carriage was the source of the epidemic or a consequence of it. However, the propagation of the epidemic clone was probably linked to its transmission by the staff from patient to patient. The outbreak was controlled, with difficulty, by strengthening isolation procedures, replacing the antiseptic soap being used by the staff, and changing the antibiotic prescription policy. This observation emphasizes the importance of compliance with hand washing and universal precautions.

Role of putative loops 2 and 3 in imipenem passage through the specific porin OprD of Pseudomonas aeruginosa

Mutant proteins with eight amino acid deletions in putative surface loops 2 and 3 of the imipenem-specific porin OprD of Pseudomonas aeruginosa failed to reconstitute imipenem susceptibility in an oprD-deficient background. The loop 3 deletion prevented the ability of imipenem to inhibit KCl conductance through the OprD channel, as previously shown for a loop 2 deletion. This suggests that both loops 2 and 3 have a role in imipenem binding to the OprD channel.

Mechanisms of quinolone resistance in clinical strains of Pseudomonas aeruginosa

Principal mechanisms of bacterial resistance to quinolones are modification of target enzymes, DNA gyrase (gyrA) and topoisomerase IV (parC), or reduction of intracellular concentration due to mutations in the regulatory genes for efflux systems, such as mexR and nfxB. We have examined gyrA, parC, mexR, and nfxB genes from 16 quinolone-resistant clinical isolates of Pseudomonas aeruginosa to determine the relation between mutations in DNA replicating enzymes or regulatory genes for efflux systems and to correlate the mutations with minimal inhibitory concentrations (MICs). The quinolone resistance-determining regions (QRDR) of these genes were amplified by PCR and sequenced by capillary electrophoresis. Fourteen of 16 isolates had mutations in gyrA, and 13/14 strains with MIC to norfloxacin > or = 8 mg/L had threonine at position 83 changed to isoleucine. Seven of 8 strains with MIC > or = 32 mg/L had mutations in parC. One of these strains showed a parC mutation at position 74 without any mutation in gyrA. Four strains had mexR and two strains nfxB mutations. The data indicate that gyrA mutation is the most important component of quinolone resistance, and simultaneous presence of parC mutations is associated with high-level resistance. parC mutation alone may contribute to resistance, and gyrA mutation may not be a prerequisite for parC mutation to express resistance. mexR and nfxB mutations were found mostly in strains with high-level resistance.

A dose-response study of antibiotic resistance in Pseudomonas aeruginosa biofilms

Bacterial biofilms show enormous levels of antibiotic resistance, but little is known about the underlying molecular mechanisms. Multidrug resistance pumps (MDRs) are responsible for the extrusion of chemically unrelated antimicrobials from the bacterial cell. Contribution of the MDR-mediated efflux to antibiotic resistance of Pseudomonas aeruginosa biofilms was examined by using strains overexpressing and lacking the MexAB-OprM pump. Resistance of P. aeruginosa biofilms to ofloxacin was dependent on the expression of MexAB-OprM but only in the low concentration range. Unexpectedly, biofilm resistance to ciprofloxacin, another substrate of MexAB-OprM, did not depend on the presence of this pump. Dose-dependent killing indicated the presence of a small "superresistant" cell fraction. This fraction was primarily responsible for very high resistance of P. aeruginosa biofilms to quinolones. Bacterial cells recovered from a biofilm and tested under nongrowing conditions with tobramycin exhibited higher resistance levels than planktonic cells but lower levels than cells of an intact biofilm.

Rapid identification of mutations in a multidrug efflux pump in Pseudomonas aeruginosa

The gene mexR regulates negatively the expression of the MexA-MexB-OprM efflux pump in Pseudomonas aeruginosa, and mutations in mexR cause a multiple antibiotic resistance phenotype. Five hundred and forty resistant clones of P. aeruginosa PAO503 were isolated after selection for resistance to chloramphenicol or tetracycline. All isolates showed similar phenotypes and were resistant to tetracycline, chloramphenicol and norfloxacin. Nineteen randomly selected isolates were analyzed. Since mutational analysis by direct sequencing of all regions of interest in several strains is time-consuming and expensive, a screening method, Non-Isotopic RNase Cleavage Assay (NIRCA), was applied to identify mutant genes so that they could be targeted for DNA sequencing. NIRCA is a simple but rapid method for mutational analysis and can be performed in 3-4 h. Results of NIRCA analysis were compared with DNA sequencing. Both NIRCA and DNA sequencing analysis showed mexR gene mutations in 11 of 19 isolates but no alterations in 8 strains. An immunoblot assay showed overexpression of OprN, a component of another multidrug efflux pump, MexE-MexF-OprN, in those eight isolates. Nucleotide sequencing of quinolone resistance-determining regions of DNA gyrase (gyrA) or topoisomerase IV (parC) showed no alterations in any of the 19 mutants. The data indicate that two efflux pump systems, MexA-MexB-OprM and MexE-MexF-OprN, were involved in multidrug resistance including quinolones and that NIRCA is a sensitive method for screening mutations.

Involvement of an active efflux system in the natural resistance of Pseudomonas aeruginosa to aminoglycosides

A mutant, named 11B, hypersusceptible to aminoglycosides, tetracycline, and erythromycin was isolated after Tn501 insertion mutagenesis of Pseudomonas aeruginosa PAO1. Cloning and sequencing experiments showed that 11B was deficient in an, at that time, unknown active efflux system that contains homologs of MexAB. This locus also contained a putative regulatory gene, mexZ, transcribed divergently from the efflux operon. Introduction of a recombinant plasmid that carries the genes of the efflux system restored the resistance of 11B to parental levels, whereas overexpression of these genes strongly increased the MICs of substrate antibiotics for the PAO1 host. Antibiotic accumulation studies confirmed that this new system is an energy-dependent active efflux system that pumps out aminoglycosides. Furthermore, this system appeared to function with an outer membrane protein, OprM. While the present paper was being written and reviewed, genes with a sequence identical to our pump genes, mexXY of P. aeruginosa, have been reported to increase resistance to erythromycin, fluoroquinolones, and organic cations in Escherichia coli hosts, although efflux of aminoglycosides was not examined (Mine et al., Antimicrob. Agents Chemother. 43:415-417, 1999). Our study thus shows that the MexXY system plays an important role in the intrinsic resistance of P. aeruginosa to aminoglycosides. Although overexpression of MexXY increased the level of resistance to fluoroquinolones, disruption of the mexXY operon in P. aeruginosa had no detectable effect on susceptibility to these agents.

Characterization of MexT, the regulator of the MexE-MexF-OprN multidrug efflux system of Pseudomonas aeruginosa

We investigated the regulation of the MexEF-OprN multidrug efflux system of Pseudomonas aeruginosa, which is overexpressed in nfxC-type mutants and confers resistance to quinolones, chloramphenicol and trimethoprim. Sequencing of the DNA region upstream of the mexEF-oprN operon revealed the presence of an open reading frame (ORF) of 304 amino acids encoding a LysR-type transcriptional activator, termed MexT. By using T7-polymerase, a 34-kDa protein was expressed in Escherichia coli from a plasmid carrying the mexT gene. Expression of a mexE::lacZ fusion was 10-fold higher in nfxC-type mutants than in the wild-type strain; however, transcription of mexT as well as the mexT DNA region was unchanged. Located adjacent to mexT but transcribed in opposite direction, the beginning of an ORF termed qrh (quinone oxidoreductase homologue) was identified. Expression of a qrh::lacZ fusion was also found to be activated by MexT. Further, we present evidence for coregulation at the transcriptional and the posttranscriptional level between the MexEF-OprN efflux system and the OprD porin responsible for cross-resistance of nfxC-type mutants to carbapenem antibiotics.

Use of a genetic approach to evaluate the consequences of inhibition of efflux pumps in Pseudomonas aeruginosa

Drug efflux pumps in Pseudomonas aeruginosa were evaluated as potential targets for antibacterial therapy. The potential effects of pump inhibition on susceptibility to fluoroquinolone antibiotics were studied with isogenic strains that overexpress or lack individual efflux pumps and that have various combinations of efflux- and target-mediated mutations. Deletions in three efflux pump operons were constructed. As expected, deletion of the MexAB-OprM efflux pump decreased resistance to fluoroquinolones in the wild-type P. aeruginosa (16-fold reduction for levofloxacin [LVX]) or in the strain that overexpressed mexAB-oprM operon (64-fold reduction for LVX). In addition to that, resistance to LVX was significantly reduced even for the strains carrying target mutations (64-fold for strains for which LVX MICs were >4 microg/ml). We also studied the frequencies of emergence of LVX-resistant variants from different deletion mutants and the wild-type strain. Deletion of individual pumps or pairs of the pumps did not significantly affect the frequency of emergence of resistant variants (at 4x the MIC for the wild-type strain) compared to that for the wild type (10(-6) to 10(-7)). In the case of the strain with a triple deletion, the frequency of spontaneous mutants was undetectable (<10(-11)). In summary, inhibition of drug efflux pumps would (i) significantly decrease the level of intrinsic resistance, (ii) reverse acquired resistance, and (iii) result in a decreased frequency of emergence of P. aeruginosa strains highly resistant to fluoroquinolones in clinical settings.

Negative regulation of the Pseudomonas aeruginosa outer membrane porin OprD selective for imipenem and basic amino acids

Pseudomonas aeruginosa OprD is a specific porin which facilitates the uptake of basic amino acids and imipenem, a carbapenem antibiotic. Resistance to imipenem due to the loss of OprD is an important mechanism for the loss of clinical effectiveness. To investigate the negative regulatory mechanisms influencing oprD expression, a gene upstream of the coregulated mexEF-oprN efflux operon, designated mexT, was cloned. The predicted 304-amino-acid mature MexT protein showed strong homology to LysR-type regulators. When overexpressed it induced the expression of the mexEF-oprN efflux operon while decreasing the level of expression of OprD. The use of an oprD::xylE transcriptional fusion indicated that it acted by repressing the transcription of oprD. Salicylate, a weak aromatic acid known to reduce porin expression and induce low levels of multiple antibiotic resistance in Escherichia coli, was able to induce imipenem resistance and reduce the expression of OprD but not multiple antibiotic resistance or OprN expression in P. aeruginosa. This was also demonstrated to occur at the level of transcription. Acetyl salicylate and benzoate, but not catechol, were also able to reduce the levels of OprD in the P. aeruginosa outer membranes. These OprD-suppressing compounds increased imipenem resistance even in a mexT-overexpressing and nfxC mutant backgrounds, suggesting that such resistance is independent of the MexT repressor and that oprD is influenced by more than a single mechanism of repression.

In vivo emergence of multidrug-resistant mutants of Pseudomonas aeruginosa overexpressing the active efflux system MexA-MexB-OprM

During a 6-month period, 21 pairs of Pseudomonas aeruginosa isolates susceptible (pretherapy) and resistant (posttherapy) to antipseudomonal beta-lactam antibiotics were isolated from hospitalized patients. In vivo emergence of beta-lactam resistance was associated with the overexpression of AmpC beta-lactamase in 10 patients. In the other 11 patients, the posttherapy isolates produced only low, basal levels of beta-lactamase and had increased levels of resistance to a variety of non-beta-lactam antibiotics (e.g., quinolones, tetracyclines, and trimethoprim) compared with the levels of beta-lactamase production and resistance of their pretherapy counterparts. These data suggested the involvement of the MexA-MexB-OprM active efflux system in the multidrug resistance phenotype of the posttherapy strains. Immunoblotting of the outer membrane proteins of these 11 bacterial pairs with a specific polyclonal antibody raised against OprM demonstrated the overexpression of OprM in all the posttherapy isolates. To determine whether mutations in mexR, the regulator gene of the mexA-mexB-oprM efflux operon, could account for the overproduction of the efflux system, sequencing experiments were carried out with the 11 bacterial pairs. Eight posttherapy isolates were found to contain insertions or deletions that led to frameshifts in the coding sequences of mexR. Two resistant strains had point mutations in mexR that yielded single amino acid changes in the protein MexR, while another strain did not show any mutation in mexR or in the promoter region upstream of mexR. Introduction of a plasmid-encoded wild-type mexR gene into five posttherapy isolates partially restored the susceptibility of the bacteria to selected antibiotics. These results indicate that in the course of antimicrobial therapy multidrug-resistant active efflux mutants overexpressing the MexA-MexB-OprM system may emerge as a result of mutations in the mexR gene.

Mechanisms of fluoroquinolone resistance

Mechanisms of bacterial resistance to fluoroquinolones fall into two principal categories, alterations in drug target enzymes and alterations that limit permeation of drug to the target, both resulting from chromosomal mutations. No specific resistance mechanisms of quinolone degradation or modification have been found. The target enzymes, DNA gyrase and topoisomerase IV are most commonly altered in domains near the enzyme active sites and in some cases reduced drug binding affinity has been demonstrated. Drug permeation is altered by mutations that increase expression of endogenous multidrug efflux pumps, alter outer membrane diffusion channels, or both. Recently a new plasmid-mediated resistance of an as yet undefined mechanism was found in clinical isolates of Klebsiella pneumoniae. Copyright 1999 Harcourt Publishers Ltd.

Type II topoisomerase mutations in ciprofloxacin-resistant strains of Pseudomonas aeruginosa

We determined the sequences of the quinolone resistance-determining regions of gyrA, gyrB, and parC genes for 30 clinical strains of Pseudomonas aeruginosa resistant to ciprofloxacin that were previously complemented by wild-type gyrA and gyrB plasmid-borne alleles and studied for their coresistance to imipenem (E. Cambau, E. Perani, C. Dib, C. Petinon, J. Trias, and V. Jarlier, Antimicrob. Agents Chemother. 39:2248-2252, 1995). In the present study, we found mutations in type II topoisomerase genes for all strains. Twenty-eight strains had a missense mutation in gyrA (codon 83 or 87). Ten of them had an additional mutation in parC (codon 80 or 84), including a novel mutation of Ser-80 to Trp, but all were fully complemented by a plasmid-borne wild-type gyrA allele. The remaining two strains harbored the first gyrB mutation described in P. aeruginosa, leading to the substitution of phenylalanine for serine 464. The strains which had two mutations in type II topoisomerase genes (i.e., gyrA and parC) were significantly more resistant to fluoroquinolones than those with a single mutation in gyrA or gyrB (geometric mean MICs of ciprofloxacin, 39.4 versus 10.9 microg/ml, P < 0.01; geometric mean MICs of sparfloxacin, 64.0 versus 22.6, P < 0. 01). No mutant with a parC mutation alone was observed, which favors DNA gyrase being the primary target for fluoroquinolones. These results demonstrate that gyrA mutations are the major mechanism of resistance to fluoroquinolones for clinical strains of P. aeruginosa and that additional mutations in parC lead to a higher level of quinolone resistance.

Differential selection of multidrug efflux systems by quinolones in Pseudomonas aeruginosa

Resistance mechanisms selected after in vitro exposure to 12 quinolones were analyzed for Pseudomonas aeruginosa. Efflux-type mutants were predominant. Quinolones differed in their ability to select a particular efflux system. While the newer fluoroquinolones favored the MexCD-OprJ system, the older quinolones selected exclusively the MexEF-OprN or MexAB-OprM systems. A protonable C-7 substituent in combination with a C-6 fluorine atom is a structural determinant of quinolones involved in efflux pump substrate specificity.