Pseudomonas aeruginosa - a phenomenon of bacterial resistance

The impact of membrane lipid composition on macrophage activation in the immune defense against Rhodococcus equi and Pseudomonas aeruginosa

Nutritional fatty acids are known to have an impact on membrane lipid composition of body cells, including cells of the immune system, thus providing a link between dietary fatty acid uptake, inflammation and immunity. In this study we reveal the significance of macrophage membrane lipid composition on gene expression and cytokine synthesis thereby highlighting signal transduction processes, macrophage activation as well as macrophage defense mechanisms. Using RAW264.7 macrophages as a model system, we identified polyunsaturated fatty acids (PUFA) of both the n-3 and the n-6 family to down-regulate the synthesis of: (i) the pro-inflammatory cytokines IL-1β, IL-6 and TNF-α; (ii) the co-stimulatory molecule CD86; as well as (iii) the antimicrobial polypeptide lysozyme. The action of the fatty acids partially depended on the activation status of the macrophages. It is particularly important to note that the anti-inflammatory action of the PUFA could also be seen in case of infection of RAW264.7 with viable microorganisms of the genera R. equi and P. aeruginosa. In summary, our data provide strong evidence that PUFA from both the n-3 and the n-6 family down-regulate inflammation processes in context of chronic infections caused by persistent pathogens.

Inhaled antibiotics for Gram-negative respiratory infections

Several disease states create conditions that lead to opportunistic Gram-negative respiratory infections. Inhalation is the most direct and, until recently, underutilized means of antimicrobial drug targeting for respiratory tract infections. All approved antimicrobial agents for administration by inhalation are indicated for Pseudomonas aeruginosa infections in patients with cystic fibrosis. These inhaled therapies have directly contributed to a significant reduction in exacerbations and hospitalizations in this patient population over the last few decades. The relentless adaptation of pathogenic organisms to current treatment options demands that the pharmaceutical industry continue designing next-generation antimicrobial agents over 70 years after they were first introduced. Recent technological advances in inhalation devices and drug formulation techniques have broadened the scope of antimicrobial structural classes that can be investigated by inhalation; however, there is an urgent need to discover novel compounds with improved resistance profiles relative to those drugs that are already marketed.

Characterization of the χψ subcomplex of Pseudomonas aeruginosa DNA polymerase III

BACKGROUND

DNA polymerase III, the main enzyme responsible for bacterial DNA replication, is composed of three sub-assemblies: the polymerase core, the β-sliding clamp, and the clamp loader. During replication, single-stranded DNA-binding protein (SSB) coats and protects single-stranded DNA (ssDNA) and also interacts with the χψ heterodimer, a sub-complex of the clamp loader. Whereas the χ subunits of Escherichia coli and Pseudomonas aeruginosa are about 40% homologous, P. aeruginosa ψ is twice as large as its E. coli counterpart, and contains additional sequences. It was shown that P. aeruginosa χψ together with SSB increases the activity of its cognate clamp loader 25-fold at low salt. The E. coli clamp loader, however, is insensitive to the addition of its cognate χψ under similar conditions. In order to find out distinguishing properties within P. aeruginosa χψ which account for this higher stimulatory effect, we characterized P. aeruginosa χψ by a detailed structural and functional comparison with its E. coli counterpart.

RESULTS

Using small-angle X-ray scattering, analytical ultracentrifugation, and homology-based modeling, we found the N-terminus of P. aeruginosa ψ to be unstructured. Under high salt conditions, the affinity of the χψ complexes from both organisms to their cognate SSB was similar. Under low salt conditions, P. aeruginosa χψ, contrary to E. coli χψ, binds to ssDNA via the N-terminus of ψ. Whereas it is also able to bind to double-stranded DNA, the affinity is somewhat reduced.

CONCLUSIONS

The binding to DNA, otherwise never reported for any other ψ protein, enhances the affinity of P. aeruginosa χψ towards the SSB/ssDNA complex and very likely contributes to the higher stimulatory effect of P. aeruginosa χψ on the clamp loader. We also observed DNA-binding activity for P. putida χψ, making this activity most probably a characteristic of the ψ proteins from the Pseudomonadaceae.

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.

Influence of Pseudomonas aeruginosa pvdQ gene on altering antibiotic susceptibility under swarming conditions

In Pseudomonas aeruginosa PAO1, the pvdQ gene has been shown to have at least two functions. It encodes the acylase enzyme and hydrolyzes 3-oxo-C12-HSL, the key signaling molecule of quorum sensing system. In addition, pvdQ is involved in swarming motility. It is required for up-regulated during swarming motility, which is triggered by high cell densities. As high-density bacterial populations also display elevated antibiotic resistance, studies have demonstrated that swarm-cell differentiation in P. aeruginosa promotes increased resistance to various antibiotics. PvdQ acts as a signal during swarm-cell differentiation, and thus may play a role in P. aeruginosa antibiotic resistance. The aim of this study is to examine whether pvdQ was involved in modifying antibiotic susceptibility during swarming conditions, and to investigate the mechanism by which this occurred. We constructed the PAO1pMEpvdQ strain, which overproduced PvdQ. PAO1pMEpvdQ promotes swarming motility, while PAO1ΔpvdQ abolishes swarming motility. In addition, both PAO1 and PAO1pMEpvdQ acquired resistance to ceftazidime, ciprofloxacin, meropenem, polymyxin B, and gentamicin, though PAO1pMEpvdQ exhibited a two to eightfold increase in antibiotic resistance compared to PAO1. These results indicate that pvdQ plays an important role in elevating antibiotic resistance via swarm-cell differentiation and possibly other mechanisms as well. We analyzed outer membrane permeability. Our data also suggest that pvdQ decreases P. aeruginosa outer membrane permeability, thereby elevating antibiotic resistance under swarming conditions. Our results suggest new approaches for reducing P. aeruginosa resistance.

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?

Effect of ciprofloxacin concentration on the frequency and nature of resistant mutants selected from Pseudomonas aeruginosa mutS and mutT hypermutators

The rapid emergence of drug resistance upon treatment of Pseudomonas aeruginosa infections with fluoroquinolones is a serious concern. In this study, we report the effect of hypermutability on the mutant selection window for ciprofloxacin (CIP) by comparing the hypermutator MPAO1 mutS and mutT strains with the wild-type strain. The mutant selection window was shifted to higher CIP concentrations for both hypermutators, presenting the mutS strain with a broader selection window in comparison to the wild-type strain. The mutation prevention concentrations (MPC) determined for mutT and mutS strains were increased 2- and 4-fold over the wild-type level, respectively. In addition, we analyzed the molecular bases for resistance in the bacterial subpopulations selected at different points in the window. At the top of the window, the resistant clones isolated were mainly mutated in GyrA and ParC topoisomerase subunits, while at the bottom of the window, resistance was associated with the overexpression of MexCD-OprJ and MexAB-OprM efflux pumps. Accordingly, a greater proportion of multidrug-resistant clones were found among the subpopulations isolated at the lower CIP concentrations. Furthermore, we found that the exposure to CIP subinhibitory concentrations favors the accumulation of cells overexpressing MexCD-OprJ (due to mutations in the transcriptional repressor NfxB) and MexAB-OprM efflux pumps. We discuss these results in the context of the possible participation of this antibiotic in a mutagenic process.

Sequencing and characterization of Pseudomonas aeruginosa phage JG004

Background

Phages could be an important alternative to antibiotics, especially for treatment of multiresistant bacteria as e.g. Pseudomonas aeruginosa. For an effective use of bacteriophages as antimicrobial agents, it is important to understand phage biology but also genes of the bacterial host essential for phage infection.

Results

We isolated and characterized a lytic Pseudomonas aeruginosa phage, named JG004, and sequenced its genome. Phage JG004 is a lipopolysaccharide specific broad-host-range phage of the Myoviridae phage family. The genome of phage JG004 encodes twelve tRNAs and is highly related to the PAK-P1 phage genome. To investigate phage biology and phage-host interactions, we used transposon mutagenesis of the P. aeruginosa host and identified P. aeruginosa genes, which are essential for phage infection. Analysis of the respective P. aeruginosa mutants revealed several characteristics, such as host receptor and possible spermidine-dependance of phage JG004.

Conclusions

Whole genome sequencing of phage JG004 in combination with identification of P. aeruginosa host genes essential for infection, allowed insights into JG004 biology, revealed possible resistance mechanisms of the host bacterium such as mutations in LPS and spermidine biosynthesis and can also be used to characterize unknown gene products in P. aeruginosa.

Efflux pump inhibitors (EPIs) as new antimicrobial agents against Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic human pathogen and one of the leading causes of nosocomial infections worldwide. The difficulty in treatment of pseudomonas infections arises from being multidrug resistant (MDR) and exhibits resistance to most antimicrobial agents due to the expression of different mechanisms overcoming their effects. Of these resistance mechanisms, the active efflux pumps in Pseudomonas aeruginosa that belong to the resistance nodulation division (RND) plays a very important role in extruding the antibiotics outside the bacterial cells providing a protective means against their antibacterial activity. Beside its role against the antimicrobial agents, these pumps can extrude biocides, detergents, and other metabolic inhibitors. It is clear that efflux pumps can be targets for new antimicrobial agents. Peptidomimetic compounds such as phenylalanine arginyl β-naphthylamide (PAβN) have been introduced as efflux pump inhibitors (EPIs); their mechanism of action is through competitive inhibition with antibiotics on the efflux pump resulting in increased intracellular concentration of antibiotic, hence, restoring its antibacterial activity. The advantage of EPIs is the difficulty to develop bacterial resistance against them, but the disadvantage is their toxic property hindering their clinical application. The structure activity relationship of these compounds showed other derivatives from PAβN that are higher in their activity with higher solubility in biological fluids and decreased toxicity level. This raises further questions on how can we compact Pseudomonas infections. Of particular importance, the recent resurgence in the use of older antibiotics such as polymyxins and probably applying stricter control measures in order to prevent their spread in clinical sittings.

Investigation of the acylation mechanism of class C beta-lactamase: pKa calculation, molecular dynamics simulation and quantum mechanical calculation

β-Lactamases are bacterial enzymes that act as a bacterial defense system against β-lactam antibiotics. β-Lactamase cleaves the β-lactam ring of the antibiotic by a two step mechanism involving acylation and deacylation steps. Although class C β-lactamases have been investigated extensively, the details of their mechanism of action are not well understood at the molecular level. In this study, we investigated the mechanism of the acylation step of class C β-lactamase using pKa calculations, molecular dynamics (MD) simulations and quantum mechanical (QM) calculations. Serine64 (Ser64) is an active site residue that attacks the β-lactam ring. In this study, we considered three possible scenarios for activation of the nucleophile Ser64, where the activation base is (1) Tyrosine150 (Tyr150), (2) Lysine67 (Lys67), or (3) substrate. From the pKa calculation, we found that Tyr150 and Lys67 are likely to remain in their protonated states in the pre-covalent complex between the enzyme and substrate, although their role as activator would require them to be in the deprotonated state. It was found that the carboxylate group of the substrate remained close to Ser64 for most of the simulation. The energy barrier for hydrogen abstraction from Ser64 by the substrate was calculated quantum mechanically using a large truncated model of the enzyme active site and found to be close to the experimental energy barrier, which suggests that the substrate can initiate the acylation mechanism in class C β-lactamase.

Suppression of ciprofloxacin-induced resistant Pseudomonas aeruginosa in a dynamic kill curve system

Current dosing approaches for treating microbial infections ignore resistant subpopulations. A clinical isolate of Pseudomonas aeruginosa was cultured in a dynamic in vitro kill curve system designed to simulate the half-lives of drugs in order to evaluate the drug-microbial response relationship. The first dose of ciprofloxacin (CIP) uses a concentration equivalent to the unbound fraction of a 200mg clinical dose. A second dose of 200mg or 600 mg CIP, or ceftriaxone (CFX) or gentamicin (GEN) was administered at 12h. Dynamics of the minimum inhibitory concentration (MIC) were assessed using Etest strips before and throughout the CIP treatment period. In addition, the microbroth dilution method was used to evaluate drug susceptibility across a wide range of antibiotics using samples from before and after CIP exposure. A significant loss of CIP effects was observed at the second dose. Cross-resistance to many antibiotics (cefoxitin, cefuroxime, cefotetan, ampicillin and ertapenem) was observed. GEN, but not CFX or high-dose CIP, was sufficient to suppress the developed resistant subpopulation following the initial CIP exposure. The CIP MIC increased substantially from 0.13 μg/mL pre dose to 4 μg/mL at 12h after a CIP dose. In addition, aztreonam induced a similar resistance pattern as CIP, indicating that induction of resistance was not limited to fluoroquinolones. In conclusion, the in vitro dynamic kill curve system revealed that aminoglycosides, more than other classes of antibiotics, were effective against the CIP-induced resistant subpopulations.

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?

Role of persister cells in chronic infections: clinical relevance and perspectives on anti-persister therapies

Certain infectious diseases caused by pathogenic bacteria are typically chronic in nature. Potentially deadly examples include tuberculosis, caused by Mycobacterium tuberculosis, cystic fibrosis-associated lung infections, primarily caused by Pseudomonas aeruginosa, and candidiasis, caused by the fungal pathogen Candida albicans. A hallmark of this type of illness is the recalcitrance to treatment with antibiotics, even in the face of laboratory tests showing the causative agents to be sensitive to drugs. Recent studies have attributed this treatment failure to the presence of a small, transiently multidrug-tolerant subpopulation of cells, so-called persister cells. Here, we review our current understanding of the role that persisters play in the treatment and outcome of chronic infections. In a second part, we offer a perspective on the development of anti-persister therapies based on genes and mechanisms that have been implicated in persistence over the last decade.

Methylthioinosine phosphorylase from Pseudomonas aeruginosa. Structure and annotation of a novel enzyme in quorum sensing

The PA3004 gene of Pseudomonas aeruginosa PAO1 was originally annotated as a 5'-methylthioadenosine phosphorylase (MTAP). However, the PA3004 encoded protein uses 5'-methylthioinosine (MTI) as a preferred substrate and represents the only known example of a specific MTI phosphorylase (MTIP). MTIP does not utilize 5'-methylthioadenosine (MTA). Inosine is a weak substrate with a k(cat)/K(m) value 290-fold less than MTI and is the second best substrate identified. The crystal structure of P. aeruginosa MTIP (PaMTIP) in complex with hypoxanthine was determined to 2.8 Å resolution and revealed a 3-fold symmetric homotrimer. The methylthioribose and phosphate binding regions of PaMTIP are similar to MTAPs, and the purine binding region is similar to that of purine nucleoside phosphorylases (PNPs). The catabolism of MTA in P. aeruginosa involves deamination to MTI and phosphorolysis to hypoxanthine (MTA → MTI → hypoxanthine). This pathway also exists in Plasmodium falciparum, where the purine nucleoside phosphorylase (PfPNP) acts on both inosine and MTI. Three tight-binding transition state analogue inhibitors of PaMTIP are identified with dissociation constants in the picomolar range. Inhibitor specificity suggests an early dissociative transition state for PaMTIP. Quorum sensing molecules are associated with MTA metabolism in bacterial pathogens suggesting PaMTIP as a potential therapeutic target.

Prevalence and antimicrobial-resistance of Pseudomonas aeruginosa in swimming pools and hot tubs

Pseudomonas aeruginosa is an important opportunistic pathogen in recreational waters and the primary cause of hot tub folliculitis and otitis externa. The aim of this surveillance study was to determine the background prevalence and antimicrobial resistance profile of P. aeruginosa in swimming pools and hot tubs. A convenience sample of 108 samples was obtained from three hot tubs and eight indoor swimming pools. Water and swab samples were processed using membrane filtration, followed by confirmation with polymerase chain reaction. Twenty-three samples (21%) were positive for P. aeruginosa, and 23 isolates underwent susceptibility testing using the microdilution method. Resistance was noted to several antibiotic agents, including amikacin (intermediate), aztreonam, ceftriaxone, gentamicin, imipenem, meropenem (intermediate), ticarcillin/clavulanic acid, tobramycin (intermediate), and trimethoprim/sulfamethoxazole. The results of this surveillance study indicate that 96% of P. aeruginosa isolates tested from swimming pools and hot tubs were multidrug resistant. These results may have important implications for cystic fibrosis patients and other immune-suppressed individuals, for whom infection with multidrug-resistant P. aeruginosa would have greater impact. Our results underlie the importance of rigorous facility maintenance, and provide prevalence data on the occurrence of antimicrobial resistant strains of this important recreational water-associated and nosocomial pathogen.

Beyond Susceptible and Resistant, Part I: Treatment of Infections Due to Gram-Negative Organisms With Inducible β-Lactamases

Inactivation of β-lactams by the action of β-lactamase enzymes is the most common mode of resistance to these drugs among Gram-negative organisms. The genomes of some key clinical pathogens such as Enterobacter and Pseudomonas encode AmpC, an inducible chromosomal β-lactamase. The potent activity of AmpC against broad-spectrum β-lactams complicates treatment of organisms with this gene. Antibiotic exposure can select for mutants expressing high levels of this enzyme, leading to the emergence of resistant isolates and failure of therapy, even when the initial isolate is fully susceptible. The risk of selecting for resistant organisms varies according to the particular β-lactam used for treatment. This article reviews the microbiology of these enzymes, summarizes clinical data on the frequency emergence of resistance, and discusses considerations for antimicrobial treatment of these organisms.

Bactericidal effects of non-thermal argon plasma in vitro, in biofilms and in the animal model of infected wounds

Non-thermal (low-temperature) physical plasma is under intensive study as an alternative approach to control superficial wound and skin infections when the effectiveness of chemical agents is weak due to natural pathogen or biofilm resistance. The purpose of this study was to test the individual susceptibility of pathogenic bacteria to non-thermal argon plasma and to measure the effectiveness of plasma treatments against bacteria in biofilms and on wound surfaces. Overall, Gram-negative bacteria were more susceptible to plasma treatment than Gram-positive bacteria. For the Gram-negative bacteria Pseudomonas aeruginosa, Burkholderia cenocepacia and Escherichia coli, there were no survivors among the initial 105 c.f.u. after a 5 min plasma treatment. The susceptibility of Gram-positive bacteria was species- and strain-specific. Streptococcus pyogenes was the most resistant with 17 % survival of the initial 105 c.f.u. after a 5 min plasma treatment. Staphylococcus aureus had a strain-dependent resistance with 0 and 10 % survival from 105 c.f.u. of the Sa 78 and ATCC 6538 strains, respectively. Staphylococcus epidermidis and Enterococcus faecium had medium resistance. Non-ionized argon gas was not bactericidal. Biofilms partly protected bacteria, with the efficiency of protection dependent on biofilm thickness. Bacteria in deeper biofilm layers survived better after the plasma treatment. A rat model of a superficial slash wound infected with P. aeruginosa and the plasma-sensitive Staphylococcus aureus strain Sa 78 was used to assess the efficiency of argon plasma treatment. A 10 min treatment significantly reduced bacterial loads on the wound surface. A 5-day course of daily plasma treatments eliminated P. aeruginosa from the plasma-treated animals 2 days earlier than from the control ones. A statistically significant increase in the rate of wound closure was observed in plasma-treated animals after the third day of the course. Wound healing in plasma-treated animals slowed down after the course had been completed. Overall, the results show considerable potential for non-thermal argon plasma in eliminating pathogenic bacteria from biofilms and wound surfaces.

The prevalence of aminoglycoside-modifying enzyme genes (aac (6')-I, aac (6')-II, ant (2")-I, aph (3')-VI) in Pseudomonas aeruginosa

INTRODUCTION

Pseudomonas aeruginosa (P. aeruginosa) is one of the primary opportunistic pathogens responsible for nosocomial infections. Aminoglycosides are an import ant component of antipseudomonal chemotherapy. The inactivation of drugs by modifying enzymes is the most common mechanism of aminoglycoside resistance.

OBJECTIVES

The inactivation of aminoglycosides by modifying enzymes is the primary resistance mechanism employed by P. aeruginosa. The aim of the present study was to investigate the occurrence of aminoglycoside resistance and the prevalence of four import ant modifying enzyme genes (aac (6')-I, aac (6')-II, ant (2")-I, aph (3')-VI) in P. aeruginosa in Iran.

METHODS

A total of 250 clinical isolates of P. aeruginosa were collected from several hospitals in seven cities in Iran. Antimicrobial susceptibility tests (using the disk diffusion method and E-tests) were performed for all 250 isolates. In addition, all isolates were screened for the presence of modifying enzyme genes by polymerase chain reaction.

RESULTS

The resistance rates, as determined by the disk diffusion method, were as follows: gentamicin 43%, tobramycin 38%, and amikacin 24%. Of the genes examined, aac (6')-II (36%) was the most frequently identified gene in phenotypic resist ant isolates, followed by ant (2")-I, aph (3')-VI, and aac (6')-I.

CONCLUSIONS

Aminoglycoside resistance in P. aeruginosa remains a significant problem in Iran. Therefore, there is considerable local surveillance of aminoglycoside resistance.

Current challenges in antimicrobial chemotherapy: focus on ß-lactamase inhibition

The use of the three classical beta-lactamase inhibitors (clavulanic acid, tazobactam and sulbactam) in combination with beta-lactam antibacterials is currently the most successful strategy to combat beta-lactamase-mediated resistance. However, these inhibitors are efficient in inactivating only class A beta-lactamases and the efficiency of the inhibitor/antibacterial combination can be compromised by several mechanisms, such as the production of naturally resistant class B or class D enzymes, the hyperproduction of AmpC or even the production of evolved inhibitor-resistant class A enzymes. Thus, there is an urgent need for the development of novel inhibitors. For serine active enzymes (classes A, C and D), derivatives of the beta-lactam ring such as 6-beta-halogenopenicillanates, beta-lactam sulfones, penems and oxapenems, monobactams or trinems seem to be potential starting points to design efficient molecules (such as AM-112 and LK-157). Moreover, a promising non-beta-lactam molecule, NXL-104, is now under clinical development. In contrast, an ideal inhibitor of metallo-beta-lactamases (class B) remains to be found, despite the huge number of potential molecules already described (biphenyl tetrazoles, cysteinyl peptides, mercaptocarboxylates, succinic acid derivatives, etc.). The search for such an inhibitor is complicated by the absence of a covalent intermediate in their catalytic mechanisms and the fact that beta-lactam derivatives often behave as substrates rather than as inhibitors. Currently, the most promising broad-spectrum inhibitors of class B enzymes are molecules presenting chelating groups (thiols, carboxylates, etc.) combined with an aromatic group. This review describes all the types of molecules already tested as potential beta-lactamase inhibitors and thus constitutes an update of the current status in beta-lactamase inhibitor discovery.

Coastal bacterioplankton community diversity along a latitudinal gradient in Latin America by means of V6 tag pyrosequencing

The bacterioplankton diversity of coastal waters along a latitudinal gradient between Puerto Rico and Argentina was analyzed using a total of 134,197 high-quality sequences from the V6 hypervariable region of the small-subunit ribosomal RNA gene (16S rRNA) (mean length of 60 nt). Most of the OTUs were identified into Proteobacteria, Bacteriodetes, Cyanobacteria, and Actinobacteria, corresponding to approx. 80% of the total number of sequences. The number of OTUs corresponding to species varied between 937 and 1946 in the seven locations. Proteobacteria appeared at high frequency in the seven locations. An enrichment of Cyanobacteria was observed in Puerto Rico, whereas an enrichment of Bacteroidetes was detected in the Argentinian shelf and Uruguayan coastal lagoons. The highest number of sequences of Actinobacteria and Acidobacteria were obtained in the Amazon estuary mouth. The rarefaction curves and Good coverage estimator for species diversity suggested a significant coverage, with values ranging between 92 and 97% for Good coverage. Conserved taxa corresponded to aprox. 52% of all sequences. This study suggests that human-contaminated environments may influence bacterioplankton diversity.

Efflux pump contribution to multidrug resistance in clinical isolates of Pseudomonas aeruginosa

STUDY OBJECTIVE

To determine if increased expression of efflux pumps, mutations in the genes encoding regulatory proteins for efflux pumps, or the combination is associated with multidrug-resistant (MDR) Pseudomonas aeruginosa isolates.

DESIGN

Microbiologic evaluation of prospectively collected Pseudomonas aeruginosa isolates.

SETTING

University teaching hospital. ISOLATES: One hundred eight unique P. aeruginosa isolates-50 non-MDR and 58 MDR isolates-obtained from pulmonary or blood sources from patients admitted to the intensive care unit between January 1, 1999, and December 31, 2004.

MEASUREMENTS AND MAIN RESULTS

Isolates were considered MDR if they were resistant to at least three of the following four drugs: ciprofloxacin, tobramycin, ceftazidime, or imipenem. Possible mutations in efflux regulatory genes mexR, nfxB, and mexZ were analyzed by using polymerase chain reaction amplification and DNA sequencing. Determination of the expression of outer membrane proteins OprM and OprJ was performed by using sodium dodecyl sulfate- polyacrylamide gel electrophoresis immunoblotting. Differences in regulatory gene mutations and outer membrane protein expression were compared between non-MDR and MDR isolates. Among the 108 P. aeruginosa isolates, the MDR isolates were more likely to overexpress OprM compared with non-MDR isolates (64% vs 2%, p<0.001). Mutations in mexR and mexZ were present in 64% and 26% of MDR strains, respectively, but were not associated with OprM overexpression or multidrug resistance. Expression of OprJ was not associated with MDR isolates (odds ratio [OR] 3.7, 95% confidence interval [CI] 0.7-18.5, p=0.11). Mutations in nfxB (12% of MDR strains) were also not associated with multidrug resistance (OR 3.5, 95% CI 0.7-17.8, p=0.13). Eight (100%) of 8 isolates with OprJ expression plus OprM overexpression, 12 (92%) of 13 isolates with combined mexR and mexZ mutations, 5 (100%) of 5 isolates with nfxB plus mexZ mutations, and 16 (100%) of 16 isolates with OprM overexpression plus mexZ mutations were MDR isolates.

CONCLUSION

The presence of one regulatory gene mutation or simple expression of a single outer membrane protein was not linked to multidrug resistance. However, OprM overexpression and multiple efflux regulatory gene mutations or efflux protein expression were associated with MDR P. aeruginosa isolates.

Multilevel modelling of the prevalence of hospitalized patients infected with Pseudomonas aeruginosa

Pseudomonas aeruginosa is one of the leading nosocomial pathogens. The question of the respective contribution of endogenous and exogenous sources remains controversial. In this study, we shed new light on this issue by means of a multilevel logistic regression analysis which allowed a simultaneous investigation of factors associated with prevalence of patients infected with P. aeruginosa at two levels: patient and healthcare facility (HCF) in the eastern regions of France. A total of 25 533 in-patients from 51 HCFs were included in the analysis. The overall prevalence was 0·37% (range 0-1·65%). Multilevel modelling estimated that <14% of total variability of the outcome variable was explained by differences between HCFs and that after adjusting for patient-level variables, which explained 52% of HCF-level variance, the latter became non-significantly different from zero. A compositional effect (patient factors), rather than a contextual effect (ecological factors), explains heterogeneity of the prevalence of patients infected with P. aeruginosa in the eastern HCFs of France.

Application of an inducible system to engineer unmarked conditional mutants of essential genes of Pseudomonas aeruginosa

The Phi CTX-based integration vector pYM101 harboring a tightly controlled modified phage T7 early gene promoter/LacI(q) repressor (T7/LacI) system was constructed for the generation of unmarked conditional mutants in Pseudomonas aeruginosa. Promoter activity of the T7/LacI system was demonstrated to be dependent on the presence of the inducer isopropyl -beta-D-1-thiogalactopyranoside (IPTG), as evaluated by measuring beta-galactosidase activity. In the absence of the inducer, the promoter was silent as its activity was lower than those of a promoter-less lacZ control. Unmarked conditional mutants of four predicted essential genes (lolCDE (PA2988-86), lpxC (PA4406), rho (PA5239), and def (PA0019)) were successfully constructed using this recombination system. In the absence of IPTG, the growth of all mutants was repressed; however, the addition of either 0.1 or 1mM IPTG restored growth rates to levels nearly identical to wild-type cells. It was therefore demonstrated that the inducible integration vector pYM101 is suitable for the creation of unmarked conditional mutants of P. aeruginosa, and is particularly useful for examining the function of essential genes.

Mutation of nfxB causes global changes in the physiology and metabolism of Pseudomonas aeruginosa

Loss-of-function mutations in nfxB lead to up-regulation of mexCD-oprJ expression and, consequently, increased resistance to fluoroquinolone antibiotics. Such nfxB mutants have also been reported to exhibit altered virulence profiles, diminished type III secretion system-dependent cytotoxicity, and impaired fitness. However, it is not clear whether these phenotypes are directly linked to NfxB activity or whether inappropriate expression of the MexCD-OprJ pump has pleiotropic effects, thereby impacting indirectly on the phenotype of the cells. The aim of the current work is to investigate which of these possibilities is correct. We isolated a novel type of nfxB mutant generated by a spontaneous polygenic deletion and show that this mutant is rapidly out-competed when grown in a mixed culture with the wild-type progenitor. This competitive fitness defect only manifested itself during the stationary phase of growth. The endoproteome of the nfxB mutant, assessed using 2D-DiGE (difference gel electrophoresis), showed major alterations compared with the wild-type. Consistent with this, we found that the nfxB mutant was impaired in all forms of motility (swimming, swarming, and twitching) as well as in the production of siderophores, rhamnolipid, secreted protease, and pyocyanin. Further investigation showed that the exoproteome, endometabolome, and exometabolome of the nfxB mutant were all globally different compared with the wild-type. The exometabolome of the nfxB mutant was enriched in a selection of long chain fatty acids raising the possibility that these might be substrates for the MexCD-OprJ pump. The nfxB mutant metabotype could be complemented by expression of nfxB in trans and was abolished in an nfxB mexD double mutant, suggesting that inappropriate overexpression of a functional MexCD-OprJ efflux pump causes pleiotropic changes. Taken together, our data suggest that many of the nfxB mutant phenotypes are not caused by the direct effects of the NfxB regulator, but instead by inappropriate mexCD-oprJ expression. Furthermore, the pleiotropic nature of the phenotypes indicate that these may simply reflect the globally dysregulated physiology of the strain.

Development of an immunochromatographic assay for the rapid detection of AAC(6')-Iae-producing multidrug-resistant Pseudomonas aeruginosa

OBJECTIVES

To develop an easy-to-use method for the rapid detection of antibiotic-resistant bacteria. Here, a new immunochromatographic assay specific for aminoglycoside 6'-N-acetyltransferase AAC(6')-Iae was designed. AAC(6')-Iae is a significant marker molecule for multidrug-resistant (MDR) Pseudomonas aeruginosa isolates in Japan.

METHODS

Monoclonal antibodies specific for AAC(6')-Iae were used to construct the assay. The assessment of the assay was performed using 116 P. aeruginosa clinical isolates obtained from hospitals in the Kanto area of Japan where little was known about AAC(6')-Iae producers. PCR analyses of the aac(6')-Iae and class 1 integron, antimicrobial susceptibility testing and PFGE analysis were performed to characterize positive strains.

RESULTS

The detection limit of the assay was 1.0 x 10(5) cfu. Of 116 clinical isolates, 60 were positive for AAC(6')-Iae using the assay. The results of assessment with clinical isolates were fully consistent with those of aac(6')-Iae PCR analyses, showing no false positives or negatives. All positive strains detected by the assay showed MDR phenotypes that were resistant to several classes of antibiotic. PFGE analysis showed that 59 of 60 positive strains tightly clustered, and these included clonal expansions.

CONCLUSIONS

The developed assay is an easy-to-use and reliable detection method for AAC(6')-Iae-producing MDR P. aeruginosa. This approach may be applicable for screening and investigation of antibiotic-resistant bacteria as an alternative to PCR analysis.

The outer membrane protein OprQ and adherence of Pseudomonas aeruginosa to human fibronectin

Outer membrane proteins of the Gram-negative organism Pseudomonas aeruginosa play a significant role in membrane permeability, antibiotic resistance, nutrient uptake, and virulence in the infection site. In this study, we show that the P. aeruginosa outer membrane protein OprQ, a member of the OprD superfamily, is involved in the binding of human fibronectin (Fn). Some members of the OprD subfamily have been reported to be important in the uptake of nutrients from the environment. Comparison of wild-type and mutant strains of P. aeruginosa revealed that inactivation of the oprQ gene does not reduce the growth rate. Although it does not appear to be involved in nutrient uptake, an increased doubling time was reproducibly observed with the loss of OprQ in P. aeruginosa. Utilizing an oprQ–xylE transcriptional fusion, we determined that the PA2760 gene, encoding OprQ, was upregulated under conditions of decreased iron and magnesium. This upregulation appears to occur in early exponential phase. Insertional inactivation of PA2760 in the P. aeruginosa wild-type background did not produce a significant increase in resistance to any antibiotic tested, a phenotype that is typical of OprD family members. Interestingly, the in trans expression of OprQ in the ΔoprQ PAO1 mutant resulted in increased sensitivity to certain antibiotics. These findings suggest that OprQ is under dual regulation with other P. aeruginosa genes. Intact P. aeruginosa cells are capable of binding human Fn. We found that loss of OprQ resulted in a reduction of binding to plasmatic Fn in vitro. Finally, we present a discussion of the possible role of the P. aeruginosa outer membrane protein OprQ in adhesion to epithelial cells, thereby increasing colonization and subsequently enhancing lung destruction by P. aeruginosa.

Insights into antifolate activity of phytochemicals against Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic drug resistant pathogen. Drug interaction studies for phytochemicals (protocatechuic acid (PA), gallic acid (GA), quercetin (QUER), and myricetin (MYR)) in combination with antifolates (sulfamethoxazole (SMX) and trimethoprim (TMP)) are presented. Our results show that the combinations of SMX and phytochemicals are synergistic, whereas TMP in combination with phytochemicals results in additive mode of interaction. Molecular docking of phytochemicals in the active site of modeled P. aeruginosa dihydrofolate reductase (DHFR), an important enzyme in the folic acid biosynthesis pathway, shows that the phytochemicals QUER and MYR dock in the active site of P. aeruginosa DHFR with promoted binding at the NADP site, PA, and GA dock in the active site of P. aeruginosa DHFR with promoted binding at the folate binding site. Possible mode of action of these phytochemicals as anti-DHFR compounds in this bacterium is suggested. Taken together, the above findings provide novel insights to mode of interactions of these phytochemicals with antibiotics and may have significance as prospective leads in the development of antipseudomonal drug developments.

Assessing the emergence of resistance: the absence of biological cost in vivo may compromise fosfomycin treatments for P. aeruginosa infections

BACKGROUND

Fosfomycin is a cell wall inhibitor used efficiently to treat uncomplicated urinary tract and gastrointestinal infections. A very convenient feature of fosfomycin, among others, is that although the expected frequency of resistant mutants is high, the biological cost associated with mutation impedes an effective growth rate, and bacteria cannot offset the obstacles posed by host defenses or compete with sensitive bacteria. Due to the current scarcity of new antibiotics, fosfomycin has been proposed as an alternative treatment for other infections caused by a wide variety of bacteria, particularly Pseudomonas aeruginosa. However, whether fosfomycin resistance in P. aeruginosa provides a fitness cost still remains unknown.

PRINCIPAL FINDINGS

We herein present experimental evidence to show that fosfomycin resistance cannot only emerge easily during treatment, but that it is also cost-free for P. aeruginosa. We also tested if, as has been reported for other species such as Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis, fosfomycin resistant strains are somewhat compromised in their virulence. As concerns colonization, persistence, lung damage, and lethality, we found no differences between the fosfomycin resistant mutant and its sensitive parental strain. The probability of acquisition in vitro of resistance to the combination of fosfomycin with other antibiotics (tobramycin and imipenem) has also been studied. While the combination of fosfomycin with tobramycin makes improbable the emergence of resistance to both antibiotics when administered together, the combination of fosfomycin plus imipenem does not avoid the appearance of mutants resistant to both antibiotics.

CONCLUSIONS

We have reached the conclusion that the use of fosfomycin for P. aeruginosa infections, even in combined therapy, might not be as promising as expected. This study should encourage the scientific community to assess the in vivo cost of resistance for specific antibiotic-bacterial species combinations, and therefore avoid reaching universal conclusions from single model organisms.

Multidrug-resistant epidemic clones among bloodstream isolates of Pseudomonas aeruginosa in the Czech Republic

To determine whether the high proportion of antimicrobial resistance among hospital isolates of Pseudomonas aeruginosa in the Czech Republic is associated with the spread of multidrug-resistant clones, we investigated 108 bloodstream isolates collected prospectively in 2007. The isolates originated from 48 hospitals in 36 cities and were serotyped, tested for susceptibility to 10 anti-Pseudomonas agents and studied by multilocus sequence typing, macrorestriction analysis and class 1 integron typing. Forty-five isolates were fully susceptible, while 14 and 49 isolates were resistant to 1-2 and 3-9 agents, respectively. A total of 42 multilocus sequence types (ST) were identified, of which ST235 (serotype O11), ST175 (O4) and ST132 (O6) included 19, 16 and 5 isolates, respectively. These three STs encompassed 40 (82%) of 49 isolates resistant to more than two agents and originated from 29 hospitals in 22 cities. Isolates of the same ST had highly similar macrorestriction patterns. Twelve ST235 isolates harbored an integron variable region with the gene cassette array of aacA7-aadA6-orfD, while 15 ST175 isolates shared a region with the aadB-aadA13 array and all ST132 isolates carried a region with aacA4. A carbapenemase-encoding gene (bla(IMP-7)) was detected in a single strain (ST357). In conclusion, the multidrug resistance of Czech P. aeruginosa bloodstream isolates in 2007 was predominantly associated with three epidemic clones, one of which belongs to international clonal complex CC235.