Influence of OprM expression on multiple antibiotic resistance in Pseudomonas aeruginosa

Implications of indoor microbial ecology and evolution on antibiotic resistance

The indoor environment is an important source of microbial exposures for its human occupants. While we naturally want to favor positive health outcomes, built environment design and operation may counter-intuitively favor negative health outcomes, particularly with regard to antibiotic resistance. Indoor environments contain microbes from both human and non-human origins, providing a unique venue for microbial interactions, including horizontal gene transfer. Furthermore, stressors present in the built environment could favor the exchange of genetic material in general and the retention of antibiotic resistance genes in particular. Intrinsic and acquired antibiotic resistance both pose a potential threat to human health; these phenomena need to be considered and controlled separately. The presence of both environmental and human-associated microbes, along with their associated antibiotic resistance genes, in the face of stressors, including antimicrobial chemicals, creates a unique opportunity for the undesirable spread of antibiotic resistance. In this review, we summarize studies and findings related to various interactions between human-associated bacteria, environmental bacteria, and built environment conditions, and particularly their relation to antibiotic resistance, aiming to guide "healthy" building design.

Mechanism of fibroblast inflammatory responses to Pseudomonas aeruginosa elastase

Receptor tyrosine kinases, including the epidermal growth factor receptors (EGFR), are able to activate the mitogen-activated protein kinases (MAPK) via several adaptor proteins and protein kinases such as Raf. EGFR can be activated by a variety of extracellular stimuli including neutrophil elastase, but we are aware of no report as to whether Pseudomonas aeruginosa produced elastase (PE) could elicit such signalling through EGFR activation. We sought to test the inference that PE modulates inflammatory responses in human lung fibroblasts and that the process occurs by activation of the EGFR/MAPK pathways. We utilized IL-8 cytokine expression as a pathway-specific end point measure of the fibroblast inflammatory response to PE. Western blot analysis was performed to detect phosphorylation of EGFR and signal transduction intermediates. Northern blot, real-time PCR, and ELISA methods were utilized to determine cytokine gene expression levels. We found that PE induces phosphorylation of the EGFR and the extracellular signal-regulated proteins (ERK1/2) of the MAPK pathway, and nuclear translocation of NF-κB. Furthermore, enzymically active PE enhances IL-8 mRNA and protein secretion. Pretreatment of the cells with specific inhibitors of EGFR, MAPK kinase and NF-κB markedly attenuated the PE-induced signal proteins phosphorylation and IL-8 gene expression and protein secretion. Collectively, the data show that PE produced by Pseudomonas aeruginosa can modulate lung inflammation by exploiting the EGFR/ERK signalling cascades and enhancing IL-8 production in the lungs via NF-κB activation.

Contribution of cell elongation to the biofilm formation of Pseudomonas aeruginosa during anaerobic respiration

Pseudomonas aeruginosa, a gram-negative bacterium of clinical importance, forms more robust biofilm during anaerobic respiration, a mode of growth presumed to occur in abnormally thickened mucus layer lining the cystic fibrosis (CF) patient airway. However, molecular basis behind this anaerobiosis-triggered robust biofilm formation is not clearly defined yet. Here, we identified a morphological change naturally accompanied by anaerobic respiration in P. aeruginosa and investigated its effect on the biofilm formation in vitro. A standard laboratory strain, PAO1 was highly elongated during anaerobic respiration compared with bacteria grown aerobically. Microscopic analysis demonstrated that cell elongation likely occurred as a consequence of defective cell division. Cell elongation was dependent on the presence of nitrite reductase (NIR) that reduces nitrite (NO₂⁻) to nitric oxide (NO) and was repressed in PAO1 in the presence of carboxy-PTIO, a NO antagonist, demonstrating that cell elongation involves a process to respond to NO, a spontaneous byproduct of the anaerobic respiration. Importantly, the non-elongated NIR-deficient mutant failed to form biofilm, while a mutant of nitrate reductase (NAR) and wild type PAO1, both of which were highly elongated, formed robust biofilm. Taken together, our data reveal a role of previously undescribed cell biological event in P. aeruginosa biofilm formation and suggest NIR as a key player involved in such process.

Heterogeneous virulence potential and high antibiotic resistance of Pseudomonas aeruginosa strains isolated from Korean pneumonia patients

Pseudomonas aeruginosa is an opportunistic human pathogen of clinical importance that causes airway infections in immunocompromised patients. Here, we report the virulence-associated characteristics of strains of P. aeruginosa, isolated from the sputa of 25 Korean pneumonia patients. A high degree of genomic plasticity was observed by random amplified polymorphic DNA genotype analysis, suggesting that the infections were caused by strains with diverse genomic backgrounds. Biofilm formation of each isolate was heterogeneous in terms of their relative motilities. In addition, 48% of isolates were defective in the production of 3-oxo-C(12)-HSL (PAI-1), a quorum sensing signal molecule. In these strains, PAI-1-dependent elastase production was correspondingly decreased, suggesting that a large number of strains were presumed to be quorum sensing deficient. Multidrug resistance (MDR) was seen in 56% of the isolates tested, and 44% of the MDR strains were resistant to five or more antibiotics. Taken together, our results provide additional insights into the virulence traits of P. aeruginosa clinical isolates, which will aid in treating P. aeruginosa infections in pneumonia patients.

Investigation on response of the metabolites in tricarboxylic acid cycle of Escherichi coli and Pseudomonas aeruginosa to antibiotic perturbation by capillary electrophoresis

Metabolomics is a new branch of systems biology exerting its influence in many aspects. In order to appraise the effects of antibiotics on central carbon metabolism, a CE based method was set up. With this platform, we estimated the organic acid metabolite pools' fluctuation of Escherichia coli and Pseudomonas aeruginosa cultured under 11 different antibiotics' stimuli. Multivariate data analysis showed that different antibiotics had clustered distributions for each strain and could be easily distinguished. Genetic, metabolic and antibiotic mechanism differences could also be deduced by the aid of further correlation analysis. For P. aeruginosa, even synergy action amid antibiotics could be ascertained.

Evaluation of a structural model of Pseudomonas aeruginosa outer membrane protein OprM, an efflux component involved in intrinsic antibiotic resistance

The outer membrane protein OprM of Pseudomonas aeruginosa is involved in intrinsic and mutational multiple-antibiotic resistance as part of two resistance-nodulation-division efflux systems. The crystal structure of TolC, a homologous protein in Escherichia coli, was recently published (V. Koronakis, A. Sharff, E. Koronakis, B. Luisl, and C. Hughes, Nature 405:914-919, 2000), demonstrating a distinctive architecture comprising outer membrane beta-barrel and periplasmic helical-barrel structures, which assemble differently from the common beta-barrel-only conformation of porins. Based on their sequence similarity, a similar content of alpha-helical and beta-sheet structure determined by circular dichroism spectroscopy, and our observation that OprM, like TolC, reconstitutes channels in planar bilayer membranes, OprM and TolC were considered to be structurally homologous, and a model of OprM was constructed by threading its sequence to the TolC crystal structure. Residues thought to be important for the TolC structure were conserved in space in this OprM model. Analyses of deletion mutants and previously isolated insertion mutants of OprM in the context of this model allowed us to propose roles for different protein domains. Our data indicate that the helical barrel of the protein is critical for both the function and the integrity of the protein, while a C-terminal domain localized around the equatorial plane of this helical barrel is dispensable. Extracellular loops appear to play a lesser role in substrate specificity for this efflux protein compared to classical porins, and there appears to be a correlation between the change in antimicrobial activity for OprM mutants and the pore size. Our model and channel formation studies support the "iris" mechanism of action for TolC and permit us now to form more focused hypotheses about the functional domains of OprM and its related family of efflux proteins.

Mutational analysis of the OprM outer membrane component of the MexA-MexB-OprM multidrug efflux system of Pseudomonas aeruginosa

OprM is the outer membrane component of the MexA-MexB-OprM efflux system of Pseudomonas aeruginosa. Multiple-sequence alignment of this protein and its homologues identified several regions of high sequence conservation that were targeted for site-directed mutagenesis. Of several deletions which were stably expressed, two, spanning residues G199 to A209 and A278 to N286 of the mature protein, were unable to restore antibiotic resistance in OprM-deficient strains of P. aeruginosa. Still, mutation of several conserved residues within these regions did not adversely affect OprM function. Mutation of the highly conserved N-terminal cysteine residue, site of acylation of this presumed lipoprotein, also did not affect expression or activity of OprM. Similarly, substitution of the OprM lipoprotein signal, including consensus lipoprotein box, with the signal peptide of OprF, the major porin of this organism, failed to impact on expression or activity. Apparently, acylation is not essential for OprM function. A large deletion at the N terminus, from A12 to R98, compromised OprM expression to some extent, although the deletion derivative did retain some activity. Several deletions failed to yield an OprM protein, including one lacking an absolutely conserved LGGGW sequence near the C terminus of the protein. The pattern of permissive and nonpermissive deletions was used to test a topology model for OprM based on the recently published crystal structure of the OprM homologue, TolC (V. Koronakis, A. Sharff, E. Koronakis, B. Luisi, and C. Hughes, Nature 405:914-919, 2000). The data are consistent with OprM monomer existing as a substantially periplasmic protein with four outer membrane-spanning regions.

Insertion mutagenesis and membrane topology model of the Pseudomonas aeruginosa outer membrane protein OprM

Pseudomonas aeruginosa OprM is a protein involved in multiple-antibiotic resistance as the outer membrane component for the MexA-MexB-OprM efflux system. Planar lipid bilayer experiments showed that OprM had channel-forming activity with an average single-channel conductance of only about 80 pS in 1 M KCl. The gene encoding OprM was subjected to insertion mutagenesis by cloning of a foreign epitope from the circumsporozoite form of the malarial parasite Plasmodium falciparum into 11 sites. In Escherichia coli, 8 of the 11 insertion mutant genes expressed proteins at levels comparable to those obtained with the wild-type gene and the inserted malarial epitopes were surface accessible as assessed by indirect immunofluorescence. When moved to a P. aeruginosa OprM-deficient strain, seven of the insertion mutant genes expressed proteins at variable levels comparable to that of wild-type OprM and three of these reconstituted MIC profiles resembling those of the wild-type protein, while the other mutant forms showed variable MIC results. Utilizing the data from these experiments, in conjunction with multiple sequence alignments and structure predictions, an OprM topology model with 16 beta strands was proposed.

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.

Characterization of a Pseudomonas aeruginosa efflux pump contributing to aminoglycoside impermeability

Pseudomonas aeruginosa can employ many distinct mechanisms of resistance to aminoglycoside antibiotics; however, in cystic fibrosis patients, more than 90% of aminoglycoside-resistant P. aeruginosa isolates are of the impermeability phenotype. The precise molecular mechanisms that produce aminoglycoside impermeability-type resistance are yet to be elucidated. A subtractive hybridization technique was used to reveal gene expression differences between PAO1 and isogenic, spontaneous aminoglycoside-resistant mutants of the impermeability phenotype. Among the many genes found to be up-regulated in these laboratory mutants were the amrAB genes encoding a recently discovered efflux system. The amrAB genes appear to be the same as the recently described mexXY genes; however, the resistance profile that we see in P. aeruginosa is very different from that described for Escherichia coli with mexXY. Direct evidence for AmrAB involvement in aminoglycoside resistance was provided by the deletion of amrB in the PAO1-derived laboratory mutant, which resulted in the restoration of aminoglycoside sensitivity to a level nearly identical to that of the parent strain. Furthermore, transcription of the amrAB genes was shown to be up-regulated in P. aeruginosa clinical isolates displaying the impermeability phenotype compared to a genotypically matched sensitive clinical isolate from the same patient. This suggests the possibility that AmrAB-mediated efflux is a clinically relevant mechanism of aminoglycoside resistance. Although it is unlikely that hyperexpression of AmrAB is the sole mechanism conferring the impermeability phenotype, we believe that the Amr efflux system can contribute to a complex interaction of molecular events resulting in the aminoglycoside impermeability-type resistance phenotype.

Insertion mutagenesis of the ferric pyoverdine receptor FpvA of Pseudomonas aeruginosa: identification of permissive sites and a region important for ligand binding

Insertion of an 18-amino-acid-encoding sequence within the fpvA gene identified permissive sites at residues Y350, A402, R451, R521, and R558, consistent with these residues occurring in extramembranous loop regions of the protein. Insertions at R451, R521, and R558 did not adversely affect receptor function, although insertions at Y350 and A402 compromised ferric pyoverdine binding and uptake. The latter region likely contributes to or interacts with the ligand-binding site.

Multidrug efflux pump AcrAB of Salmonella typhimurium excretes only those beta-lactam antibiotics containing lipophilic side chains

We found that the previously reported SS-B drug-supersusceptible mutant of Salmonella typhimurium (S. Sukupolvi, M. Vaara, I. M. Helander, P. Viljanen, and P. H. Mäkelä, J. Bacteriol. 159:704-712, 1984) had a mutation in the acrAB operon. Comparison of this mutant with its parent strain and with an AcrAB-overproducing strain showed that the activity of the AcrAB efflux pump often produced significant resistance to beta-lactam antibiotics in the complete absence of beta-lactamase. The effect of AcrAB activity on resistance was more pronounced with agents containing more lipophilic side chains, suggesting that such compounds were better substrates for this pump. This correlation is consistent with the hypothesis that only those molecules that become at least partially partitioned into the lipid bilayer of the cytoplasmic membrane are captured by the AcrAB pump. According to this mechanism, the pump successfully excretes even those beta-lactams that fail to traverse the cytoplasmic membrane, because these compounds are likely to become partitioned into the outer leaflet of the bilayer. Even the compounds with lipophilic side chains were shown to penetrate across the outer membrane relatively rapidly, if the pump was inactivated genetically or physiologically. The exclusion of such compounds, exemplified by nafcillin, from cells of the wild-type S. typhimurium was previously interpreted as the result of poor diffusion across the outer membrane (H. Nikaido, Biochim. Biophys. Acta 433:118-132, 1976), but it is now recognized as the consequence of efficient pumping out of entering antibiotics by the active efflux process.

Contribution of outer membrane efflux protein OprM to antibiotic resistance in Pseudomonas aeruginosa independent of MexAB

A Pseudomonas aeruginosa strain carrying an insertion of an omega Hg interposon in the mexB gene (mexB::omega Hg; strain K879) produced markedly reduced but still detectable levels of OprM, the product of the third gene of the mexAB-oprM multidrug efflux operon. By using a lacZ transcriptional fusion vector, promoter activity likely responsible for OprM expression in the mexB::omega Hg mutant was identified upstream of oprM. Introduction of the oprM gene, but not the mexAB genes, into a P. aeruginosa multidrug-susceptible delta mexAB-oprM mutant increased resistance to quinolones, cephalosporins, erythromycin, and tetracycline. A delta mexAB-oprM strain carrying the oprM gene accumulated markedly less antibiotic than the deletion strain without oprM. Antibiotic accumulation by the MexAB- OprM+ strain was markedly enhanced upon treatment of cells with the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP), indicating that MexAB-independent OprM function likely involves an efflux process. Moreover, pretreatment of cells with CCCP prior to the accumulation assay abrogated any differences in accumulation levels between the MexAB- OprM+ and MexAB- OprM- strains, indicating that reduced drug accumulation by the OprM+ strain (in the absence of CCCP) cannot be due to OprM-mediated reduction in outer membrane permeability. It appears, therefore, the OprM can be expressed and function in a drug efflux capacity independent of MexAB.

Inner membrane efflux components are responsible for beta-lactam specificity of multidrug efflux pumps in Pseudomonas aeruginosa

A major feature of the MexAB-OprM multidrug efflux pump which distinguishes it from the MexCD-OprJ and MexEF-OprN multidrug efflux systems in Pseudomonas aeruginosa is its ability to export a wide variety of beta-lactam antibiotics. Given the periplasmic location of their targets it is feasible that beta-lactams exit the cell via the outer membrane OprM without interaction with MexA and MexB, though the latter appear to be necessary for OprM function. To test this, chimeric MexAB-OprJ and MexCD-OprM efflux pumps were reconstituted in delta mexCD delta oprM and delta mexAB delta oprJ strains, respectively, and the influence of the exchange of outer membrane components on substrate (i.e., beta-lactam) specificity was assessed. Both chimeric pumps were active in antibiotic efflux, as evidenced by their contributions to resistance to a variety of antimicrobial agents, although there was no change in resistance profiles relative to the native pumps, indicating that OprM is not the determining factor for the beta-lactam specificity of MexAB-OprM. Thus, one or both of inner membrane-associated proteins MexA and MexB are responsible for drug recognition, including recognition of beta-lactams.