Genetic context and biochemical characterization of the IMP-18 metallo-beta-lactamase identified in a Pseudomonas aeruginosa isolate from the United States

Functional and structural analyses of IMP-27 metallo-β-lactamase: evolution of IMP-type enzymes to overcome Zn(II) deprivation

IMP-type metallo-β-lactamases are di-Zn(II) enzymes that can inactivate a wide range of bicyclic β-lactam agents used in clinical practice. IMP-27 shares 82% amino acid sequence identity with IMP-1, the first IMP-type enzyme identified. Herein, we conducted structural determination, kinetic, and chelating agent resistance analyses of IMP-27. Once determined, IMP-27 was then compared to its mutant, namely, G262S, and IMP-1. Crystallographic structural analysis of IMP-27 showed an overall structure comparable to that of IMP-1 and other IMP-type enzymes; the positions of the zinc (Zn) ions varied across enzymes. Kinetic analysis showed that IMP-27 had lower catalytic efficiency against penicillins, ceftazidime, cephalexin, and imipenem than IMP-1; however, it had higher affinity and catalytic efficiency against meropenem, especially in the presence of Zn(II). This suggests that the catalytic site of IMP-27 is optimized to hydrolyze meropenem during molecular evolution at the expense of catalytic efficiency against penicillins. However, Zn(II) content analysis after EDTA treatment revealed no significant difference between enzymes. Moreover, analysis of IMP-27 mutants indicated that the differences in kinetic properties and chelator resistance between IMP-1 and IMP-27 were mainly due to an amino acid substitution at position 262.IMPORTANCEThe residue at position 262 has been reported as a key determinant of substrate specificity in IMP-type enzymes. Among more than 80 IMP-type metallo-β-lactamase (MBL) variants, IMP-27 was the first reported IMP-type MBL isolated from Proteus mirabilis. This enzyme has a glycine residue at position 262, which is occupied by serine in IMP-1. Compared with IMP-1, IMP-27 had a significantly higher affinity and catalytic efficiency against meropenem and improved metal-binding capacity, maintaining its activity under Zn(II)-limited conditions better than IMP-1. The analysis of the IMP-27 mutants indicated that differences between IMP-27 and IMP-1 were mainly due to an amino acid substitution at position 262. In the case of IMP-27, the G262S mutation optimized the catalytic site of IMP-27 for meropenem hydrolysis, at the expense of catalytic efficiency against penicillins.

Emergence of carbapenem resistant gram-negative pathogens with high rate of colistin resistance in Egypt: A cross sectional study to assess resistance trends during the COVID-19 pandemic

The current study investigated the temporal phenotypic and genotypic antimicrobial resistance (AMR) trends among multi-drug resistant and carbapenem-resistant Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa recovered from Egyptian clinical settings between 2020 and 2021. Bacterial identification and antimicrobial sensitivity of 111 clinical isolates against a panel of antibiotics were performed. Molecular screening for antibiotic resistance determinants along with integrons and associated gene cassettes was implemented. An alarming rate (98.2%) of these isolates were found to be phenotypically resistant to carbapenem. Although 23.9 % K. pneumoniae isolates were phenotypically resistant to colistin, no mobile colistin resistance (mcr) genes were detected. Among carbapenem-resistant isolates, blaNDM and blaOXA-48-like were the most prevalent genetic determinants and were significantly overrepresented among K. pneumoniae. Furthermore, 84.78% of K. pneumoniae isolates co-produced these two carbapenemase genes. The plasmid-mediated quinolone resistance genes (qnrS and qnrB) were detected among the bacterial species and were significantly more prevalent among K. pneumoniae. Moreover, Class 1 integron was detected in 82% of the bacterial isolates. This study alarmingly reveals elevated resistance to last-resort antibiotics such as carbapenems as well as colistin which impose a considerable burden in the health care settings in Egypt. Our future work will implement high throughput sequencing-based antimicrobial resistance surveillance analysis for characterization of novel AMR determinants. This information could be applied as a step forward to establish a robust antibiotic stewardship program in Egyptian clinical settings, thereby addressing the rising challenges of AMR.

Endemicity of Pseudomonas aeruginosa producing IMP-18 and/or VIM-2 MBLs from the high-risk clone ST111 in Central America

Background

Pseudomonas aeruginosa is an important cause of serious nosocomial infections. Despite the overall genetic diversity of this species, highly conserved clonal complexes (CCs) have been observed among MDR isolates. Many of these CCs are associated with MBL-producing isolates.

Objectives

To evaluate five P. aeruginosa isolates from Central America that carried IMP-18- and/or VIM-2-encoding genes from the SENTRY Antimicrobial Surveillance Program (2017-2018).

Methods

Susceptibility testing was performed by broth microdilution (CLSI). WGS was performed using MiSeq (Illumina) and MinION (Oxford Nanopore). Assembled contigs from short and long reads were combined for in silico screening of resistance genes, MLST, core genome (cg)MLST and SNP analysis.

Results

The P. aeruginosa isolates were collected in Panama and Mexico from patients with urinary tract infections or pneumonia. Isolates were categorized as XDR (CLSI/EUCAST). All isolates belonged to ST111 but carried different combinations of resistance-encoding genes. Transposon-associated MBL genes, blaIMP-18 and/or blaVIM-2, were chromosomally located. blaIMP-18 was detected in an In1666 integron whereas blaVIM-2 was embedded in an In59-like integron. Isolates were closely related based on cgMLST (average allele distance 2-34) and SNP analysis (5-423 different SNPs).

Conclusions

MBL-producing ST111 P. aeruginosa have become endemic in Panama and may have spread to Mexico via clonal dissemination. Recombination events are apparent in the evolution of this CC. Surveillance is warranted to track the expansion and movement of this clone.

Metallo-β-lactamases in the Age of Multidrug Resistance: From Structure and Mechanism to Evolution, Dissemination, and Inhibitor Design

Antimicrobial resistance is one of the major problems in current practical medicine. The spread of genes coding for resistance determinants among bacteria challenges the use of approved antibiotics, narrowing the options for treatment. Resistance to carbapenems, last resort antibiotics, is a major concern. Metallo-β-lactamases (MBLs) hydrolyze carbapenems, penicillins, and cephalosporins, becoming central to this problem. These enzymes diverge with respect to serine-β-lactamases by exhibiting a different fold, active site, and catalytic features. Elucidating their catalytic mechanism has been a big challenge in the field that has limited the development of useful inhibitors. This review covers exhaustively the details of the active-site chemistries, the diversity of MBL alleles, the catalytic mechanism against different substrates, and how this information has helped developing inhibitors. We also discuss here different aspects critical to understand the success of MBLs in conferring resistance: the molecular determinants of their dissemination, their cell physiology, from the biogenesis to the processing involved in the transit to the periplasm, and the uptake of the Zn(II) ions upon metal starvation conditions, such as those encountered during an infection. In this regard, the chemical, biochemical and microbiological aspects provide an integrative view of the current knowledge of MBLs.

Genomic context of the two integrons of ST-111 Pseudomonas aeruginosa AG1: A VIM-2-carrying old-acquaintance and a novel IMP-18-carrying integron

Pseudomonas aeruginosa is an opportunist and versatile organism responsible for infections mainly in immunocompromised hosts. This pathogen has high intrinsic resistance to most antimicrobials. P. aeruginosa AG1 (PaeAG1) is a Costa Rican high-risk ST-111 strain with resistance to multiple antibiotics, including carbapenems, due to the activity of VIM-2 and IMP-18 metallo-β-lactamases (MBLs). These genes are harbored in two class 1 integrons located inone out of the 57 PaeAG1 genomic islands. However, the genomic context associated to these determinants in PaeAG1 and other P. aeruginosa strains is unclear. Thus, we first assessed the transcriptional activity of VIM-2 and IMP-18 genes when exposed to imipenem (a carbapenem) by RT-qPCR. To select related genomes to PaeAG1, we implemented a pan-genome analysis to define and up-date the phylogenetic relationship among complete P. aeruginosa genomes. We also studied the PaeAG1 genomic islands content in the related strains and finally we described the architecture and possible evolutionary steps of the genomic regions around the VIM-2- and IMP-18-carrying integrons. Expression of VIM-2 and IMP-18 genes was demonstrated to be induced after imipenem exposure. In a subsequent comparative genomics analysis with 211 strains, the P. aeruginosa pan-genome revealed that complete genome sequences are able to separate clones by MLST profile, including a clear ST-111 cluster with PaeAG1. The PaeAG1 genomic islands were found to define a diverse presence/absence pattern among related genomes. Finally, landscape reconstruction of genomic regions showed that VIM-2-carrying integron (In59-like) is an old-acquaintance element harbored in the same known region found in other two ST-111 strains. Also, PaeAG1 has an exclusive genomic region containing a novel IMP-18-carrying integron (registered as In1666), with an arrangement never reported before. Altogether, we provide new insights about the genomic determinants associated with the resistance to carbapenems in this high-risk P. aeruginosa using comparative genomics.

Treatment of severe infections due to metallo-β-lactamases-producing Gram-negative bacteria

In the last decades, there was an important paucity of agents for adequately treating infections due to metallo-β-lactamases-producing Gram-negative bacteria (MBL-GNB). Cefiderocol, a novel siderophore cephalosporin showing in vitro activity against MBL-GNB, has been recently marketed, and a combination of aztreonam and ceftazidime/avibactam has shown a possible favorable effect on survival of patients with severe MBL-GNB infections in observational studies. Other agents showing in vitro activity against MBL-GNB are currently in clinical development (e.g., cefepime/taniborbactam, LYS228, cefepime/zidebactam) that could be an important addition to our future armamentarium for severe MBL-GNB infections. Nonetheless, we should not discontinue our efforts to optimize the use of non-β-lactams agents, since they could remain an essential last-resort or alternative option in selected cases.

High quality 3C de novo assembly and annotation of a multidrug resistant ST-111 Pseudomonas aeruginosa genome: Benchmark of hybrid and non-hybrid assemblers

Genotyping methods and genome sequencing are indispensable to reveal genomic structure of bacterial species displaying high level of genome plasticity. However, reconstruction of genome or assembly is not straightforward due to data complexity, including repeats, mobile and accessory genetic elements of bacterial genomes. Moreover, since the solution to this problem is strongly influenced by sequencing technology, bioinformatics pipelines, and selection criteria to assess assemblers, there is no systematic way to select a priori the optimal assembler and parameter settings. To assembly the genome of Pseudomonas aeruginosa strain AG1 (PaeAG1), short reads (Illumina) and long reads (Oxford Nanopore) sequencing data were used in 13 different non-hybrid and hybrid approaches. PaeAG1 is a multiresistant high-risk sequence type 111 (ST-111) clone that was isolated from a Costa Rican hospital and it was the first report of an isolate of P. aeruginosa carrying both blaVIM-2 and blaIMP-18 genes encoding for metallo-β-lactamases (MBL) enzymes. To assess the assemblies, multiple metrics regard to contiguity, correctness and completeness (3C criterion, as we define here) were used for benchmarking the 13 approaches and select a definitive assembly. In addition, annotation was done to identify genes (coding and RNA regions) and to describe the genomic content of PaeAG1. Whereas long reads and hybrid approaches showed better performances in terms of contiguity, higher correctness and completeness metrics were obtained for short read only and hybrid approaches. A manually curated and polished hybrid assembly gave rise to a single circular sequence with 100% of core genes and known regions identified, >98% of reads mapped back, no gaps, and uniform coverage. The strategy followed to obtain this high-quality 3C assembly is detailed in the manuscript and we provide readers with an all-in-one script to replicate our results or to apply it to other troublesome cases. The final 3C assembly revealed that the PaeAG1 genome has 7,190,208 bp, a 65.7% GC content and 6,709 genes (6,620 coding sequences), many of which are included in multiple mobile genomic elements, such as 57 genomic islands, six prophages, and two complete integrons with blaVIM-2 and blaIMP-18 MBL genes. Up to 250 and 60 of the predicted genes are anticipated to play a role in virulence (adherence, quorum sensing and secretion) or antibiotic resistance (β-lactamases, efflux pumps, etc). Altogether, the assembly and annotation of the PaeAG1 genome provide new perspectives to continue studying the genomic diversity and gene content of this important human pathogen.

B1-Metallo-β-Lactamases: Where Do We Stand?

Metallo-β-Lactamases (MBLs) are class Bβ-lactamases that hydrolyze almost all clinically-availableβ-lactam antibiotics. MBLs feature the distinctive αβ/βα sandwich fold of the metallo-hydrolase/oxidoreductase superfamily and possess a shallow active-site groove containing one or two divalent zinc ions, flanked by flexible loops. According to sequence identity and zinc ion dependence, MBLs are classified into three subclasses (B1, B2 and B3), of which the B1 subclass enzymes have emerged as the most clinically significant. Differences among the active site architectures, the nature of zinc ligands, and the catalytic mechanisms have limited the development of a common inhibitor. In this review, we will describe the molecular epidemiology and structural studies of the most prominent representatives of class B1 MBLs (NDM-1, IMP-1 and VIM-2) and describe the implications for inhibitor design to counter this growing clinical threat.

Structural and Mutagenic Analysis of Metallo-β-Lactamase IMP-18

IMP-type metallo-β-lactamases (MBLs) are exogenous zinc metalloenzymes that hydrolyze a broad range of β-lactams, including carbapenems. Here we report the crystal structure of IMP-18, an MBL cloned from Pseudomonas aeruginosa, at 2.0-Å resolution. The overall structure of IMP-18 resembles that of IMP-1, with an αβ/βα "folded sandwich" configuration, but the loop that covers the active site has a distinct conformation. The relationship between IMP-18's loop conformation and its kinetic properties was investigated by replacing the amino acid residues that can affect the loop conformation (Lys44, Thr50, and Ile69) in IMP-18 with those occupying the corresponding positions in the well-described enzyme IMP-1. The replacement of Thr50 with Pro considerably modified IMP-18's kinetic properties, specifically those pertaining to meropenem, with the kcat/Km value increased by an order of magnitude. The results indicate that this is a key residue that defines the kinetic properties of IMP-type β-lactamases.

The resistance and transmission mechanism of Acinetobacter baumannii isolates in a tertiary care hospital, China

The aim of this study was to analyse the resistance and epidemiological data of 117 Acinetobacter baumannii isolates from Southwest Hospital, Chongqing, China. Except for polymyxin B, tigecycline, minocycline, cefoperazone/sulbactam, amikacin and levofloxacin, the resistance rates of other antimicrobial agents were above 90%. All the clinical isolates had the bla_OXA-51 gene and 114 isolates had the bla_OXA-23 gene. Forty-nine isolates were found to contain the bla_IMP-4 gene. PFGE data showed that 117 isolates were divided into 25 groups. Sixty-three (53.85%) were found to carry the class 1 integron, and the sequence analysis of the class 1 integron internal variable regions - five types, one of which had the bla_IMP-4 gene. For the bla_IMP-4 positive strain without class 1 integron, we found the flanking sequence had the TnpA gene. The result suggested that the resistance gene was widely distributed in our hospital; moreover, the modes of presence and transmission are different and complicated. The results of our study can improve the infection empirical treatment method and infection control programme.

Epidemiology and Characteristics of Metallo-β-Lactamase-Producing Pseudomonas aeruginosa

Metallo-β-lactamase-producing Pseudomonas aeruginosa (MPPA) is an important nosocomial pathogen that shows resistance to all β-lactam antibiotics except monobactams. There are various types of metallo-β-lactamases (MBLs) in carbapenem-resistant P. aeruginosa including Imipenemase (IMP), Verona integron-encoded metallo-β-lactamase (VIM), Sao Paulo metallo-β-lactamase (SPM), Germany imipenemase (GIM), New Delhi metallo-β-lactamase (NDM), Florence imipenemase (FIM). Each MBL gene is located on specific genetic elements including integrons, transposons, plasmids, or on the chromosome, in which they carry genes encoding determinants of resistance to carbapenems and other antibiotics, conferring multidrug resistance to P. aeruginosa. In addition, these genetic elements are transferable to other Gram-negative species, increasing the antimicrobial resistance rate and complicating the treatment of infected patients. Therefore, it is essential to understand the epidemiology, resistance mechanism, and molecular characteristics of MPPA for infection control and prevention of a possible global health crisis. Here, we highlight the characteristics of MPPA.

Description of IMP-31, a novel metallo-β-lactamase found in an ST235 Pseudomonas aeruginosa strain in Western Germany

OBJECTIVES

The objective of this study was to characterize a novel IMP-type metallo-β-lactamase (MBL) found in an MDR clinical isolate of Pseudomonas aeruginosa.

METHODS

The P. aeruginosa isolate NRZ-00156 was recovered from an inguinal swab from a patient hospitalized in Western Germany and showed high MICs of carbapenems. MBL production was analysed by Etest for MBLs, an EDTA combined disc test and an EDTA bioassay. Typing of the isolate was performed by MLST. Genetic characterization of the new blaIMP gene was performed by sequencing the PCR products. A phylogenetic tree was constructed. The novel blaIMP gene was expressed in Escherichia coli TOP10 and the enzyme was subjected to biochemical characterization.

RESULTS

The P. aeruginosa isolate NRZ-00156 expressed the ST235 allelic profile and was resistant to all the β-lactams tested except aztreonam. The isolate was positive for MBL production and harboured a new IMP allele, blaIMP-31, located on a disrupted class I integron [also carrying the blaOXA-35, aac(6')-Ib, aac(3)-Ic and aphA15 genes]. Its closest relative was IMP-35, with 96.7% amino acid identity. Expression of blaIMP-31 demonstrated that E. coli TOP10 producing IMP-31 had elevated resistance to all the β-lactams tested except aztreonam. Kinetic data were obtained for both IMP-31 and IMP-1. In comparison with IMP-1, IMP-31 showed weaker hydrolytic activity against all the β-lactams tested, which resulted from lower kcat values.

CONCLUSIONS

The characterization of the new IMP-type gene blaIMP-31 from an ST235 P. aeruginosa isolate indicates an ongoing spread of highly divergent IMP-type carbapenemases in clinical P. aeruginosa strains and highlights the continuous need for the prevention of nosocomial infections caused by MDR Gram-negative bacteria.

Predominance of carbapenem-resistant Pseudomonas aeruginosa isolates carrying blaIMP and blaVIM metallo-β-lactamases in a major hospital in Costa Rica

This study aimed to assess the molecular basis of the resistance to carbapenems in clinical isolates of Pseudomonas aeruginosa recovered from a tertiary-level health facility in San José, Costa Rica. A total of 198 non-duplicated isolates were evaluated for their susceptibility to β-lactams, aminoglycosides and fluoroquinolones. The production of metallo-β-lactamases (MBLs), the presence of MBL encoding genes (blaIMP, blaVIM and blaGIM-1) and the occurrence of these genes within class 1 integrons were investigated. In addition, an ERIC2 PCR fingerprinting method was used to elucidate the distribution of the detected MBL genes within the strain collection. Of the 198 isolates tested, 125 (63.1 %) were categorized as carbapenem-resistant. The majority (88.8 %) of the carbapemen-resistant isolates also showed resistance to ceftazidime, cefepime, aztreonam, ticarcillin/clavulanic acid, amikacin, gentamicin, tobramycin, ciprofloxacin and gatifloxacin. Among the carbapenem-resistant isolates, 102 (81.6 %) showed MBL activity. Strikingly, both blaIMP and blaVIM genes were simultaneously detected in most (94.1 %) of the 102 MBL producers. Five carbapenem-resistant MBL producers were positive only for blaIMP genes. Almost 70 % of the isolates examined harboured the intI1 gene, accompanied by the sul1 and qacEΔ1 genes in 136 (99 %) and 122 (89 %) isolates, respectively. The majority (94.4 %) of the carbapenem-resistant isolates carried the intI1 gene, in contrast to 26 % of the carbapenem-susceptible isolates. Ninety-three out of 96 (96.9 %) isolates carrying both blaIMP and blaVIM genes also harboured the intI1, sul1 and qacEΔ1 genes. Gene cassettes from carbapenem-susceptible and MBL-negative carbapenem-resistant isolates encoded aminoglycoside-resistance enzymes (aadA2, aadA4 and aadA6) as well as orfD and qacF genes. RAPD analysis distributed 126 of the isolates in 29 clusters. Eighty of the 90 blaIMP (+) blaVIM (+) isolates were sorted into 16 different clusters, suggesting that the blaIMP and blaVIM genes detected were located within a genetic element capable of lateral transfer. Carbapenem-resistant MBL-positive isolates were recovered from almost all hospital wards and were over-represented in samples obtained from the surgical emergency and intensive care therapy units. Remarkably, three carbapenem-resistant isolates, exhibiting MBL activity and carrying both blaIMP and blaVIM genes, were recovered from outpatients. Sequence analysis of both bla genes in various isolates revealed that they correspond to the alleles blaIMP-18 and blaVIM-2. To our knowledge, this is the first report of the combination of two metallo-β-lactamases encoded by the blaIMP-18 and blaVIM-2 genes in P. aeruginosa.

Antimicrobial susceptibility differences among mucoid and non-mucoid Pseudomonas aeruginosa isolates

Pseudomonas aeruginosa is one of the most important opportunistic bacteria, causing a wide variety of infections particularly in immunocompromised patients. The extracellular glycocalyx is produced in copious amounts by mucoid strains of P. aeruginosa. Mucoid and non-mucoid P. aeruginosa strains show some differences in their antimicrobial susceptibility pattern. The aim of this study was to investigate the frequency of mucoid and non-mucoid types and their antimicrobial susceptibility patterns isolated from Milad and Mostafa Khomeini Hospital in Tehran, Iran.

One hundred P. aeruginosa isolates were collected which all were confirmed by conventional biochemical tests and PCR assay using specific primers for oprI and oprL lipoproteins. Mucoid and non-mucoid types of isolates were determined by culturing isolates on BHI agar containing Congo red and Muir mordant staining method. The susceptibility pattern of isolates against 23 different antibiotics was assessed using MIC sensititre susceptibility plates.

Fifty of 100 of isolates were mucoid type, of which 14 isolates were from Mostafa Khomeini Hospital. Frequency of mucoid type of P. aeruginosa in Mostafa Khomeini hospital (70%) was higher than that seen in Milad hospital (45%). The statistical analysis of MICs results showed significant differences in antimicrobial resistance among mucoid and non-mucoid types (non mucoid strains showed more resistance against tested antibiotics). This may be due to the tendency of some antibiotics to attach to extracellular glycocalyx of mucoid strains.

Current status of carbapenemases in Latin America

Enterobacteriaceae and non fermenting Gram-negative bacilli have become a threat to public health, in part due to their resistance to multiple antibiotic classes, which ultimately have led to an increase in morbidity and mortality. β-lactams are currently the mainstay for combating infections caused by these microorganisms, and β-lactamases are the major mechanism of resistance to this class of antibiotics. Within the β-lactamases, carbapenemases pose one of the gravest threats, as they compromise one of our most potent lines of defense, the carbapenems. Carbapenemases are being continuously identified worldwide; and in Latin America, numerous members of these enzymes have been reported. In this region, the high incidence of reports implies that carbapenemases have become a menace and that they are an issue that must be carefully studied and analyzed.

Purification, crystallization and preliminary X-ray analysis of IMP-18, a class B carbapenemase from Pseudomonas aeruginosa

Class B β-lactamases are known as metallo-β-lactamases (MBLs) and they hydrolyze most β-lactams, including carbapenems. IMP-18, an MBL cloned from Pseudomonas aeruginosa, was overexpressed, purified and crystallized by vapour diffusion for X-ray crystallographic analysis. Preliminary X-ray analysis showed that the crystal diffracted to 2.4 Å resolution and belonged to the tetragonal space group P4(1)2(1)2, with unit-cell parameters a = b = 120.77, c = 96.54 Å, α = β = γ = 90°, suggesting the presence of two molecules in the asymmetric unit.

New integron gene arrays from multiresistant clinical isolates of members of the Enterobacteriaceae and Pseudomonas aeruginosa from hospitals in Malaysia

This study investigated 147 multidrug-resistant Enterobacteriaceae and Pseudomonas aeruginosa isolates from hospitalized patients in Malaysia. Class 1 integrons were the most dominant class identified (45.6%). Three isolates were shown to contain class 2 integrons (2.0%), whilst one isolate harboured both class 1 and 2 integrons. No class 3 integrons were detected in this study. In addition, the sul1 gene was amplified in 35% of isolates and was significantly associated with the presence of integrase genes in an integron structure. RFLP and DNA sequencing analyses revealed the presence of 19 different cassette arrays among the detected integrons. The most common gene cassettes were those encoding resistance towards aminoglycosides (aad) and trimethoprim (dfr). As far as is known, this study is the first to identify integron-carrying cassette arrays such as aadA2-linF, aacC3-cmlA5 and aacA4-catB8-aadA1 in the Malaysian population. Patients' age was demonstrated as a significant risk factor for the acquisition of integrons (P=0.028). Epidemiological typing using PFGE also demonstrated a clonal relationship among isolates carrying identical gene cassettes in Klebsiella pneumoniae and P. aeruginosa but not in Escherichia coli isolates.

Phenotypic screening of extended-spectrum ß-lactamase and metallo-ß-lactamase in multidrug-resistant Pseudomonas aeruginosa from infected burns

Pseudomonas aeruginosa is an important opportunistic pathogen causing nosocomial infections, especially in immunocompromised patients such as burn patients. Pseudomonas aeruginosa is potentially resistant to different broad-spectrum antibiotics due to its ability to produce extended-spectrum β-lactamase (ESBL) and metallo-β-lactamase (MBL). In the present 6 month study, 220 strains of multidrug-resistant (MDR) Pseudomonas aeruginosa were isolated from male and female burn patients who had been hospitalized for at least one week in Motahari Hospital in Tehran. These strains were screened by the disc diffusion and double disc methods to determine the capacity of producing ESBL and MBL. Of all strains, 18% were ESBL-positive, resulting in a significant inhibition zone (≥5 mm) with cefotaxime and ceftazidime plus clavulanic acid discs when compared to the plain cefotaxime or ceftazidime discs. 38% of the strains were MBL-positive, showing at least 7 mm difference between the inhibition zone around the imipenem discs alone in comparison with imipenem plus EDTA discs, and at least 5 mm difference between the inhibition zone around imipenem plus EDTA discs and EDTA discs alone. In the light of our results, the rapidly spreading resistance among bacterial populations due to the extensive use of antibiotics is a matter of concern for the optimal treatment of patients, particularly in burn wards, and the determination of ESBL and MBL production of MDR Pseudomonas aeruginosa strains is essential.

Characterization of a novel IMP-28 metallo-β-lactamase from a Spanish Klebsiella oxytoca clinical isolate

An isolate of Klebsiella oxytoca carrying a novel IMP metallo-β-lactamase was discovered in Madrid, Spain. The bla(IMP-28) gene is part of a chromosomally located class I integron. The IMP-28 k(cat)/K(m) values for ampicillin, ceftazidime, and cefepime and, to a lesser extent, imipenem and meropenem, are clearly lower than those of IMP-1. The His306Gln mutation may induce important modifications of the L3 loop and thus of substrate accessibility and hydrolysis and be the main reason for this behavior.

IMP-29, a novel IMP-type metallo-β-lactamase in Pseudomonas aeruginosa

Analysis of two clonally related multiresistant Pseudomonas aeruginosa isolates led to the identification of a novel IMP-type metallo-β-lactamase. IMP-29 was significantly different from the other IMP variants (the closest variant being IMP-5 with 93% amino acid identity). The bla(IMP-29) gene cassette was carried by a class 1 integron in strain 10.298, while in strain 10.266 it was located in a rearranged DNA region on a 30-kb conjugative plasmid. Biochemical analysis confirmed that IMP-29 efficiently hydrolyzed carbapenems.

Two novel class I integron arrays containing IMP-18 metallo-β-lactamase gene in Pseudomonas aeruginosa clinical isolates from Puerto Rico

During a β-lactam resistance surveillance study, 12 IMP-18-positive Pseudomonas aeruginosa isolates belonging to 9 different pulsed-field gel electrophoresis groups were identified. In nine isolates, a class I integron with a novel gene array was identified that contained bla(IMP-18) and bla(OXA-224), while in two isolates the class I integron contained bla(IMP-18) and bla(OXA-2) but in a new arrangement. Our findings show the dissemination of two novel class I integrons in P. aeruginosa from different regions of Puerto Rico.

Structure and function of OprD protein in Pseudomonas aeruginosa: from antibiotic resistance to novel therapies

Pseudomonas aeruginosa (P. aeruginosa) is a common pathogen isolated from patients with nosocomial infections. Due to its intrinsic and acquired antimicrobial resistance, limited classes of antibiotics can be used for the treatment of infection with P. aeruginosa. Of these, the carbapenems are very important; however, the occurrence of carbapenem-resistant strains is gradually increasing over time. Deficiency of the outer membrane protein OprD confers P. aeruginosa a basal level of resistance to carbapenems, especially to imipenem. Functional studies have revealed that loops 2 and 3 in the OprD protein contain the entrance and/or binding sites for imipenem. Therefore, any mutation in loop 2 and/or loop 3 that causes conformational changes could result in carbapenem resistance. OprD is also a common channel for some amino acids and peptides, and competition with carbapenems through the channel may also occur. Furthermore, OprD is a highly regulated protein at transcriptional and post-transcriptional levels by some metals, small bioactive molecules, amino acids, and efflux pump regulators. Because of its hypermutability and highly regulated properties, OprD is thought to be the most prevalent mechanism for carbapenem resistance in P. aeruginosa. Developing new strategies to combat infection with carbapenem-resistant P. aeruginosa lacking OprD is an ongoing challenge.