EDTA susceptibility leading to false detection of metallo-beta-lactamase in Pseudomonas aeruginosa by Etest and an imipenem-EDTA disk method

Phenotypic and molecular characterization of extended spectrum- and metallo- beta lactamase producing Pseudomonas aeruginosa clinical isolates from Egypt

BACKGROUND

Antimicrobial resistance among Pseudomonas aeruginosa (P. aeruginosa), a leading cause of nosocomial infections worldwide, is escalating. This study investigated the prevalence of extended-spectrum β-lactamases (ESBLs) and metallo-β-lactamases (MBLs) among 104 P. aeruginosa clinical isolates from Alexandria Main University Hospital, Alexandria, Egypt.

METHODS

Antimicrobial susceptibility testing was performed using agar dilution technique, or broth microdilution method in case of colistin. ESBL and MBL prevalence was assessed phenotypically and genotypically using polymerase chain reaction (PCR). The role of plasmids in mediating resistance to extended-spectrum β-lactams was studied via transformation technique using plasmids isolated from ceftazidime-resistant isolates.

RESULTS

Antimicrobial susceptibility testing revealed alarming resistance rates to carbapenems, cephalosporins, and fluoroquinolones. Using PCR as the gold standard, phenotypic methods underestimated ESBL production while overestimating MBL production. Eighty-five isolates (81.7%) possessed only ESBL encoding genes, among which 69 isolates harbored a single ESBL gene [blaOXA-10 (n = 67) and blaPER (n = 2)]. Four ESBL-genotype combinations were detected: blaPER + blaOXA-10 (n = 8), blaVEB-1 + blaOXA-10 (n = 6), blaPSE + blaOXA-10 (n = 1), and blaPER + blaVEB-1 + blaOXA-10 (n = 1). Three isolates (2.9%) possessed only the MBL encoding gene blaVIM. Three ESBL + MBL- genotype combinations: blaOXA-10 + blaAIM, blaOXA-10 + blaVIM, and blaPER + blaOXA-10 + blaAIM were detected in 2, 1 and 1 isolate(s), respectively. Five plasmid preparations harboring blaVEB-1 and blaOXA-10 were successfully transformed into chemically competent Escherichia coli DH5α with transformation efficiencies ranging between 6.8 × 10 3 and 3.7 × 10 4    CFU/μg DNA plasmid. Selected tested transformants were ceftazidime-resistant and harbored plasmids carrying blaOXA-10.

CONCLUSIONS

The study highlights the importance of the expeditious characterization of ESBLs and MBLs using genotypic methods among P. aeruginosa clinical isolates to hinder the development and dissemination of multidrug resistant strains.

Dissemination of carbapenem-resistant Acinetobacter baumannii strains carrying the bla GES, bla NDM and bla OXA23 in Morocco

Background

Acinetobacter baumannii is a microorganism which has been classified by the World Health Organization in the list of the bacterial strains that pose the biggest danger to human health. This study was performed to determine the susceptibility profile to carbapenems and to detect carbapenemases production in 111 A. baumannii isolates. Among these 30 are environmental isolates and 81 are from the three major hospitals in Morocco.

Methods

All strains of A. baumannii were tested against diverse antimicrobial agents (13 antibiotic drugs) by the agar diffusion test. Minimum inhibitory concentration (MIC) of imipenem on carbapenem-resistant strains (CRAB) was determined by the E-test technique. Simple phenotypic tests were used to detect carbapenemases and metallo-β-lactamases (MBLs) production including the modified Hodge test, EDTA test, and the cloxacillin test. The presence of carbapenemases-encoding resistance genes of CRAB strains was examined using polymerase chain reaction (PCR).

Results

Carbapenem resistance was observed in 23 clinical Acinetobacter isolates showing dissemination of the multiresistance profile. Molecular biology techniques indicated that all these strains encoded the naturally occurring bla _OXA-51-like gene and were proved as A. baumannii. The bla _OXA-23 gene was detected in 16 strains (69.6%). The metallo-β-lactamase bla _NDM gene was detected in five isolates (21.7%). GES-type carbapenemases were found in 15 strains, the existence of three classes of carbapenemases (bla _GES, bla _NDM, and bla _OXA-23) was detected in three strains, while none of the CRAB isolates contained the bla _OXA-58, bla _OXA-24, bla _VIM, bla _OXA-48 or bla _KPC encoding genes.

Conclusions

This study established baseline proof of three classes of carbapenemases producing A. baumannii in Morocco, showing the important role of surveillance in controlling their spread.

In vitro activities of imipenem, vancomycin, and rifampicin against clinical Elizabethkingia species producing BlaB and GOB metallo-beta-lactamases

Elizabethkingia genus is emerging in hospitals and resistant to multiple antibiotics. The intrinsic imipenem resistance of Elizabethkingia genus is related to two chromosome-encoded metallo-beta-lactamases (MBLs), BlaB and GOB. This study was aimed to investigate the in vitro activity of imipenem, vancomycin, and rifampicin in clinical Elizabethkingia species. The distribution and heterogeneity of MBLs responsible for imipenem resistance were also evaluated. A total of 167 Elizabethkingia isolates from different patients were collected, including E. anophelis (142), E. meningoseptica (11), and E. miricola (14). All isolates were evaluated by the broth microdilution assay, ethylenediaminetetraacetic acid (EDTA) combination disk test, and EDTA-based microdilution test. The characteristics of BlaB and GOB were evaluated in phylogenetic analysis and heterologous expression experiments. Most of the isolates were susceptible to rifampin (94%), whereas none of the isolates were susceptible to imipenem. Vancomycin showed intermediate effectiveness. EDTA could reduce 4 folds or more minimum inhibitory concentrations (MICs) of imipenem in 105 isolates (62.9%). Of the isolates, the amino acid sequences of BlaB and GOB were divided into 22 and 25 different types, respectively. Phylogenetic analysis showed BlaB and GOB are species-specific proteins. Furthermore, GOB and BlaB from E. anophelis showed higher imipenem hydrolysis efficiency than those from the other two species. Rifampicin remained the most active agent in the current study. The mechanism of Elizabethkingia resistance to imipenem primarily stemmed from MBLs but other mechanisms could also exist, which requires further investigation.

Emergence of blaVEB and blaGES among VIM-producing Pseudomonas aeruginosa clinical isolates in Alexandria, Egypt

Thirty-three Pseudomonas aeruginosa isolates, resistant to one or more β-lactams, were included in this study. Identification of tested strains was confirmed using MALDI-TOF/MS. Phenotypic and genotypic β-lactamase patterns were investigated. Most of the isolates were resistant to carbapenems (32 out of 33) and to the extended-spectrum cephalosporins (ESC) (30 out of 33). Phenotypically, the production of extended-spectrum beta-lactamase (ESBL), metallo-β-lactamases (MBL), and carbapenemases was detected in 10, 23, and 9 isolates, respectively. However, AmpC hyperproduction was not phenotypically detected among all isolates. Genotypically, ESBL and MBL encoding genes were detected in 23 and 27 isolates, respectively. Altogether 27 strains were detected as bla_VIM positive and 16 strains carried bla_OXA-10 gene. To the best of our knowledge, this is the first report of P. aeruginosa clinical isolates harboring bla_VEB together with bla_GES in Egypt, where 5 of our 30 ESC-resistant isolates showed this genotype. Our results confirmed that resistance of P. aeruginosa isolates to β-lactam antibiotics is mediated via multiple β-lactamases belonging to different molecular classes. To the best of our knowledge, this is the first report of bla_VEB among P. aeruginosa clinical isolates from Egypt. Ten isolates harbored bla_VEB and five of them co-harbored bla_VEB together with bla_GES, bla_VIM, and bla_OXA-10.

Detection and analysis of different interactions between resistance mechanisms and carbapenems in clinical isolates of Klebsiella pneumoniae

Carbapenems are considered last-line agents for the treatment of serious infections caused by Klebsiella pneumoniae, and this microorganism may exhibit resistance to β-lactam antibiotics due to different mechanisms of resistance. We evaluated 27 isolates of K. pneumoniae resistant to carbapenems recovered from inpatients at the University Hospital of Santa Maria-RS from July 2013 to August 2014. We carried out antimicrobial susceptibility, carbapenemase detection, testing for the presence of efflux pump by broth microdilution and loss of porin by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Genetic similarity was evaluated by ERIC-PCR. High levels of resistance were verified by the minimum inhibitory concentration for the antimicrobials tested. The bla_KPC gene was present in 89% of the clinical isolates. Blue-Carba and combined disk with AFB tests showed 100% concordance, while the combined disk test with EDTA showed a high number of false-positives (48%) compared with the gold-standard genotypic test. Four isolates showed a phenotypic resistance profile consistent with the overexpression of the efflux pump, and all clinical isolates had lost one or both porins. The ERIC-PCR dendrogram demonstrated the presence of nine clusters. The main mechanism of resistance to carbapenems found in the assessed isolates was the presence of the bla_KPC gene.

Phenotypic and genotypic characteristics of carbapenem-resistant Enterobacteriaceae in a tertiary-level reference hospital in Turkey

Background

Enterobacteriaceae are among the most common pathogens that are responsible for serious community-acquired, hospital-acquired, and health-care associated infections. The emergence and spread of carbapenem-resistant Enterobacteriaceae (CRE) have become an increasing concern for healthcare services worldwide. Infections caused by these bacteria have been associated with significant morbidity and mortality and treatment options have been limited. The rapid and accurate detection of carbapenem resistance in these bacteria is important for infection control. The aim of this study was to investigate the phenotypic and genotypic features of CRE strains isolated in a tertiary-level reference hospital in Turkey.

Methods

A total of 181 CRE strains were included in the study. Antimicrobial susceptibility rates were tested using Vitek 2 system. Modified Hodge test (MHT) was performed using meropenem and ertapenem discs. Metallo-β-lactamase antimicrobial gradient test (E-test MBL strips) were used for evaluation of metallo-β-lactamase production. A multiplex PCR was used for detection of carbapenems resistance genes (IMP, VIM, KPC, NDM-1 and OXA-48).

Results

The OXA-48 gene was detected in 86 strains, NDM-1 gene in six strains, VIM gene in one strain. IMP and KPC genes were not identified. Three strains produced both OXA-48 and NDM-1 and one strain produced both OXA-48 and VIM. In two patients more than one genus of OXA-48 positive CREs was isolated. Ninety-two of the isolates were multidrug-resistant. One hundred and ten isolates were MHT with meropenem (MEM-MHT) positive and 109 isolates were MHT with ertapenem (ERT-MHT) positive. Nine of the isolates were positive with E-test MBL strips. The sensitivity of MEM-MHT and ERT-MHT for detection of OXA-48 was 70.9 and 70.6 %, respectively. MEM-MHT was found highly discriminative for OXA-48 Escherichia coli (p < 0.001). The sensitivity of E-test MBL for NDM-1 was 66.7 %. A statistically significant correlation was observed between OXA-48 gene and MHT positivity and between NDM-1, VIM gene and E-test MBL positivity (p < 0.001).

Conclusions

OXA-48 gene is spreading rapidly to many different species of Enterobacteriaceae in the hospital environment. While OXA-48 is still the most common source of carbapenem resistance in Enterobacteriaceae in our country, NDM-1 is increasingly being isolated from patients without a history of foreign contact.

Characterization of Oxacillinase and Metallo-β-Lactamas Genes and Molecular Typing of Clinical Isolates of Acinetobacter baumannii in Ahvaz, South-West of Iran

Background:

Carbapenem resistant Acinetobacter baumannii is an important nosocomial pathogen associated with a variety of infections.

Objectives:

The current study aimed to characterize the antimicrobial susceptibility, analyze the prevalence of oxacillinase and metallo-β-lactamase (MBL) genes and molecular typing of clinical isolates of A. baumannii.

Materials and Methods:

A total of 124 non-repetitive isolates of A. baumannii were collected from various clinical specimens in two teaching hospitals in Ahvaz, south-west of Iran. Antimicrobial susceptibility test was carried out by disk diffusion method. The minimum inhibitory concentrations (MICs) of imipenem, meropenem, colistin and tigecycline were determined using E-test. To screen for MBL production, double disk synergy (DDs) test and MBL E-test were performed. The presence of bla_OXA-23-like, bla_OXA-24-like, bla_OXA-51-like, bla_OXA-58-like, bla_VIM, bla_IMP and bla_SPM genes was assessed by polymerase chain reaction (PCR). To identify clonal relatedness, all isolates were subjected to repetitive sequence-based PCR (rep-PCR)

Results:

Based on disk diffusion results, the highest rate of resistance was observed in rifampin (96.8%). Colistin and polymyxin-B were the most effective agents in vitro. According to the MIC results, the rate of resistance to imipenem, meropenem, colistin and tigecycline were 78.2%, 73.4%, 0.8% and 0, respectively. Metallo-β-lactamase production was positive in 42.3% and 79.4% of the isolates by DDs test and E-test, respectively. All isolates (100%) carried bla_OXA-51-like gene. According to the results of multiplex PCR, bla_OXA-23-like and bla_OXA-24-like genes were detected in 85.6% and 6.2% of carbapenem resistant isolates, respectively. No bla_{OXA-58- like}, bla_VIM, bla_IMP and bla_SPM were detected. By rep-PCR, carbapenem resistant isolates were separated into six genotypes (A to F). Genotype A (30.9%) was the most prevalent (P value < 0.001). Genotypes B and C were found in 28.9% and 26.8% of the isolates, respectively.

Conclusions:

The rate of carbapenem resistant A. baumannii isolates were high in this study. Since, bla_OXA-58-like or MBL genes were not detected, it seems that resistance to carbapenems is related to bla_OXA-23-like and bla_OXA-24-like. Moreover, bla_OXA-23-like was the most prevalent oxacillinase (OXA) gene. Most of the isolates belonged to one of the four dominant genotypes indicating clonal dissemination in the hospitals under study. In order to control the spread of carbapenem-resistant A. baumannii, infection- control strategies are needed.

Investigation of Metallo Beta Lactamases and Oxacilinases in Carbapenem Resistant Acinetobacter baumannii Strains Isolated from Inpatients

BACKGROUND

Resistance to beta-lactam antibiotics is widespread among Acinetobacter strains. Plasmid-mediated metallo beta lactamases (MBL) are responsible for carbapenem resistance, as are oxacillinases (OXA). In recent years, MBL producing carbapenem-resistant strains have been reported in the world and in Turkey in increasing rates. In our country, besides the OXA 51-like enzyme which is inherent in A. baumannii strains, OXA 58-like and OXA 23-like carbapenemases producing strains have also been widely detected. In addition, Verona Imipenemase (VIM) and (IMP)-type MBL have been reported in some centers.

AIMS

The aim of our study was to investigate the presence of carbapenemases in Acinetobacter strains isolated from hospitalized patients in Edirne.

STUDY DESIGN

Cross-sectional study.

METHODS

A total of 52 imipenem-resistant A. baumannii strains isolated between January and March 2013 were investigated. The presence of MBL was described phenotypically by the combined disk diffusion test (CDDT), double disk synergy test (DDST), MBL E-test (only performed in 28 strains) and modified Hodge test. blaIMP, blaVIM, blaGIM, blaSIM, blaSPM genes and blaOXA-23, blaOXA-51, blaOXA-40, blaOXA-58 genes were investigated by multiplex polymerase chain reaction (PCR). The blaNDM-1 gene was determined by PCR.

RESULTS

By modified Hodge test, 50 strains (96%) were found to be MBL positive. Positivity of MBL was 21% by both CDDT (0.1 M EDTA) and DDST. Twenty-four of 28 strains (85.7%) were positive by MBL E-test. OXA 23-like and OXA 51-like carbapenemases were detected in all strains, but OXA 58-like and OXA 40-like carbapenemases-producing A. baumannii were not detected. Also, MBL genes were not detected by genotypic methods.

CONCLUSION

Only OXA 23-like carbapenemase was responsible for carbapenem resistance in carbapenem-resistant Acinetobacter strains in Edirne. The MBL-producing Acinetobacter strain is not yet a problem in our hospital. MBL resistance was found by phenotyping tests, which must be confirmed by genotypic methods; multiplex PCR tests can be easily used for screening MBL.

Evaluation of tests to predict metallo-β-lactamase in cystic fibrosis (CF) and non-(CF) Pseudomonas

Double disks synergy test (DDST) and combined disks test (CD) were evaluated to predict the presence of metallo-β-lactamase in 70 Pseudomonas aeruginosa isolates recovered from cystic fibrosis and non-cystic fibrosis patients. DDST_{CAZ-EDTA 1 cm} and CD_IMP-EDTA tests showed the best accuracy (94.3%). Furthermore, for other combinations, accuracy unsatisfactory was obtained.

Detection of carbapenemases in Enterobacteriaceae: a challenge for diagnostic microbiological laboratories

Carbapenemase-producing bacteria have now spread all over the world. Infections caused by those bacteria are difficult to treat. Therefore, there is an urgent need for accurate and fast detection of carbapenemases in diagnostic laboratories. In this review, we summarize screening methods for suspected isolates, direct assays for confirmation of carbapenemase activity (e.g. the Carba NP test and matrix-assisted laser desorption ionization time-of-flight mass spectrometry carbapenem hydrolysis assay), inhibitor-based methods for carbapenemase classification, and molecular-genetic techniques for precise identification of carbapenemase genes. We also propose a workflow for carbapenemase identification in diagnostic laboratories.

Comparison of phenotypic tests for the detection of metallo-beta-lactamases in clinical isolates of Pseudomonas aeruginosa

Metallo-β-lactamase (MBL)-producing gram-negative bacteria are an increasing public health concern worldwide. Screening tests for the rapid and specific identification of these pathogens are essential, and should be included among routine diagnostics in laboratories. This study aimed to determine the MBL frequency among carbapenem-resistant Pseudomonas aeruginosa isolates, and to evaluate the accuracy of different tests in screening for MBL production. From January 2001 to December 2008, a total of 142 imipenem-non-susceptible P. aeruginosa strains were isolated from distinct clinical samples from hospitalized patients. These isolates were examined by PCR, MBL E-test, double-disk synergy test (DDST), and combined disk (CD) test. The minimal inhibitory concentration (MIC; μg/mL) was determined by agar dilution, and pulsed field gel electrophoresis (PFGE) was performed on all samples. Sequencing was performed to confirm and define the MBL variant and subtype. Using PCR and DNA sequence analysis, 93 strains were confirmed positive for MBLs, 91 strains for the blaSPM-1 gene, 1 strain for the blaIMP-1 gene, and 1 strain for the blaIMP-16 gene. PFGE displayed a clonal pattern. The sensitivities, specificities, positive and negative predictive values were evaluated for all tests. The DDST assay (CAZ-MPA) was the optimal method for screening MBL production in P. aeruginosa strains. However, the results of the CD assay (IMP/EDTA) showed close agreement with those of the DDST. In addition, the CD assay allowed a more objective interpretation and did not require the use of a toxic substance.

Phenotypic detection of metallo-β-lactamase in imipenem-resistant Pseudomonas aeruginosa

Carbapenems are the primary choice of treatment for severe Pseudomonas aeruginosa infection. However, the emergence of carbapenem resistance due to the production of metallo-β-lactamases (MBLs) is of global concern. In this study, 90 imipenem- (IPM- or IP-) resistant P. aeruginosa (IRPA) isolates, including 32 previously tested positive and genotyped for MBL genes by PCR, were subjected to double-disk synergy test (DDST), combined disk test (CDT), and imipenem/imipenem-inhibitor (IP/IPI) E-test to evaluate their MBLs detection capability. All three methods were shown to have a sensitivity of 100%. However, DDST was the most specific of the three (96.6%), followed by IP/IPI E-test interpreted based on the single criteria of IP/IPI ≥8 as positive (62.1%), and CDT was the least specific (43.1%). Based on the data from this evaluation, we propose that only IRPA with IP MIC >16 μg/mL and IP/IPI ≥8 by IP/IPI E-test should be taken as positive for MBL activity. With the new dual interpretation criteria, the MBL IP/IPI E-test was shown to achieve 100% sensitivity as well as specificity for the IRPA in this study. Therefore, the IP/IPI E-test is a viable alternative phenotypic assay to detect MBL production in IRPA in our population in circumstances where PCR detection is not a feasible option.

The emerging NDM carbapenemases

Carbapenems were the last β-lactams retaining near-universal anti-Gram-negative activity, but carbapenemases are spreading, conferring resistance. New Delhi metallo-β-lactamase (NDM) enzymes are the latest carbapenemases to be recognized and since 2008 have been reported worldwide, mostly in bacteria from patients epidemiologically linked to the Indian subcontinent, where they occur widely in hospital and community infections, and also in contaminated urban water. The main type is NDM-1, but minor variants occur. NDM enzymes are present largely in Enterobacteriaceae, but also in non-fermenters and Vibrionaceae. Dissemination predominantly involves transfer of the blaNDM-1 gene among promiscuous plasmids and clonal outbreaks. Bacteria with NDM-1 are typically resistant to nearly all antibiotics, and reliable detection and surveillance are crucial.

Sensitive and specific phenotypic assay for metallo-beta-lactamase detection in Enterobacteria by use of a moxalactam disk supplemented with EDTA

Moxalactam is highly hydrolyzed by plasmid-mediated metallo-β-lactamases (MBLs), whereas it is poorly inactivated by serine-active carbapenemases. This study demonstrated that moxalactam resistance constituted an effective screen for MBL expression in enterobacteria, which could be confirmed, even in low-MBL-producing isolates, by a disk potentiation test using moxalactam and EDTA.

Acquired carbapenemases in Gram-negative bacterial pathogens: detection and surveillance issues

Acquired carbapenemases are emerging resistance determinants in Gram-negative pathogens, including Enterobacteriaceae, Pseudomonas aeruginosa and other Gram-negative non-fermenters. A consistent number of acquired carbapenemases have been identified during the past few years, belonging to either molecular class B (metallo-beta-lactamases) or molecular classes A and D (serine carbapenemases), and genes encoding these enzymes are associated with mobile genetic elements that allow their rapid dissemination in the clinical setting. Therefore, detection and surveillance of carbapenemase-producing organisms have become matters of major importance for the selection of appropriate therapeutic schemes and the implementation of infection control measures. As carbapenemase production cannot be simply inferred from the resistance profile, criteria must be established for which isolates should be suspected and screened for carbapenemase production, and for which tests (phenotypic and/or genotypic) should be adopted for confirmation of the resistance mechanism. Moreover, strategies should be devised for surveillance of carbapenemase producers in order to enable the implementation of effective surveillance programmes. The above issues are addressed in this article, as a follow-up to an expert meeting on acquired carbapenemases that was recently organized by the ESCMID Study Group for Antibiotic Resistance Surveillance.

Metallo-beta-lactamases among imipenem-resistant Pseudomonas aeruginosa in a Brazilian university hospital

INTRODUCTION

Imipenem-resistant Pseudomonas aeruginosa resulting from metallo-β-lactamases has been reported to be an important cause of nosocomial infection and is a critical therapeutic problem worldwide, especially in the case of bacteremia.

OBJECTIVES

To determine the frequency of metallo-β-lactamases among imipenem-resistant Pseudomonas aeruginosa isolates and to compare methods of phenotypic and molecular detection.

METHODS

During 2006, 69 imipenem-resistant Pseudomonas aeruginosa samples were isolated from blood and tested for metallo-β-lactamase production using both phenotypic methods. Minimal Inhibitory Concentratrions (MIC) (μg/mL) was determined with commercial microdilution panels. Pulsed Field Gel Electrophoresis (PFGE) was performed among metallo-β-lactamase producers.

RESULTS

Of all the blood isolates, 34.5% were found to be imipenem-resistant Pseudomonas aeruginosa. Positive phenotypic tests for metallo-β-lactamases ranged from 28%-77%, and Polymerase Chain Reaction (PCR) were positive in 30% (of note, 81% of those samples were bla(SPM)₋₁ and 19% were bla(VIM)₋₂). Ethylenediamine tetracetic acid (EDTA) combinations for the detected enzymes had low kappa values; thus, care should be taken when use it as a phenotypic indicator of MBL. Despite a very resistant antibiogram, four isolates demonstrated the worrisome finding of a colistin MIC in the resistant range. PFGE showed a clonal pattern.

CONCLUSION

Metallo-β-lactamases among imipenem-resistant Pseudomonas aeruginosa were detected in 30.4% of imipenem-resistant Pseudomonas aeruginosa isolates. This number might have been higher if other genes were included. SPM-1 was the predominant enzyme found. Phenotypic tests with low kappa values could be misleading when testing for metallo-β-lactamases. Polymerase Chain Reaction detection remains the gold standard.

Evaluation of dipicolinic acid-based Mueller Hinton agar biplate for detection of IMP-1 and VIM-2 type metallo-beta-lactamase in imipenem non-susceptible gram negative bacilli

BACKGROUND

Since metallo-beta-lactamase (MBL)-producing isolates can hydrolyze carbapenem and also easily transfer the resistance genes to other bacteria, a rapid and accurate detection of MBL has become very important. We evaluated the utility of Mueller Hinton agar (MHA) biplate containing dipicolinic acid (DPA) as a screening method to detect IMP-1 and VIM-2 type MBL-producing isolates.

METHODS

Based on our preliminary tests using various concentrations of DPA, 200 and 300 microg/mL concentration of DPA were chosen for further study. Bacterial lawns were grown on MHA biplate, one half of which contained DPA while the other did not. The inhibition zone around the imipenem (IPM) disk on both sides of this plate was compared. The stability of DPA in the stored DPA-MHA biplate was also evaluated during three months using two MBL- and one non-MBL-producing isolates.

RESULTS

When the criterion of a > or =7 mm increase of inhibition zone around the IPM disk on the MHA containing DPA compared to MHA without DPA was used, the sensitivities and specificities were 94.7% and 97.6% for 200 microg/mL DPA-MHA biplate, and 98.2% and 97.6% for 300 microg/mL DPA-MHA biplate, respectively. The activity of the DPA in this biplate was stable for three months.

CONCLUSIONS

Assays using DPA 300-MHA biplate were highly sensitive and specific for the detection of IMP-1 and VIM-2 type MBL-producing bacteria. In addition, it is easy to perform; so, it may be useful to apply this method for detection of IMP-1 and VIM-2 type MBL in clinical laboratories.

Molecular characterisation of the metallo-beta-lactamase genes in imipenem-resistant Gram-negative bacteria from a university hospital in southern Taiwan

In this study, 260 non-replicate imipenem-resistant Gram-negative bacteria isolated between January 2002 and December 2006 were subjected to a screening test for detection of metallo-beta-lactamase (MBL) using the Etest containing imipenem and ethylene diamine tetra-acetic acid (EDTA). Among the 260 strains, 123 (47.3%) appeared to produce MBL. Of these 123 strains, 113 (91.9%) were found by polymerase chain reaction (PCR) to carry MBL genes of types blaVIM-2, blaVIM-3, blaVIM-11 (blaVIM-11a), blaIMP-8 and novel blaIMP-24. One strain of Serratia marcescens harboured two MBL genes (blaVIM-11 and blaIMP-8) simultaneously. Of the 123 strains, 116 strains (94.3%) carrying the intI1 gene and 21 strains carrying integron-associated blaVIM-3, blaVIM-11 and blaIMP-8 genes were identified among Acinetobacter baumannii, Pseudomonas aeruginosa, Acinetobacter haemolyticus and S. marcescens. Using pulsed-field gel electrophoresis (PFGE) and Southern hybridisation with the blaVIM gene probe for I-CeuI-digested genomic DNA, P. aeruginosa 9527 strain harboured two class 1 integron-associated MBL genes in the chromosome, including blaVIM-3-orf2-aacA4 and novel bla(VIM-3)-orf2-aacA4-aadB-aacA4. This is the first description of the blaVIM-11 gene spreading among P. aeruginosa and A. baumannii strains in southern Taiwan. This finding suggests that clinical spread of this blaVIM-11 gene is a matter of great concern for carbapenem resistance in southern Taiwan.

Dipicolinic acid-based disk methods for detection of metallo-beta-lactamase-producing Pseudomonas spp. and Acinetobacter spp

We evaluated novel and simple dipicolinic acid (DPA)-based phenotypic screening methods to detect metallo-beta-lactamase (MBL)-producing isolates: DPA disk synergy tests and DPA-based disk tests (DPA-imipenem, DPA-meropenem, or DPA-ceftazidime disk tests). Upon testing these methods on 79 MBL-producing and 95 MBL-nonproducing isolates of Pseudomonas spp. and Acinetobacter spp., their specificity and sensitivity were superior or comparable with those of the EDTA-based phenotypic screening methods.

New and sensitive assay for determining Pseudomonas aeruginosa metallo-beta-lactamase resistance to imipenem

We present an imipenem lysate metallo-beta-lactamase (MBL) functional assay. This assay eliminates false-positive results due to the bactericidal effects of EDTA, can be performed with inexpensive reagents available in most laboratories, and is as accurate as the MBL Etest. It is appropriate for both high-accuracy screens and laboratories in developing countries with limited resources.

Metallo-beta-lactamase detection: comparative evaluation of double-disk synergy versus combined disk tests for IMP-, GIM-, SIM-, SPM-, or VIM-producing isolates

The emergence of metallo-beta-lactamase (MBL)-producing isolates is a challenge to routine microbiology laboratories, since there are no standardized methods for detecting such isolates. The aim of this study was to evaluate the accuracy of different phenotypic methods to detect MBL production among Pseudomonas spp., Acinetobacter spp., and enterobacterial isolates, including GIM, IMP, SIM, SPM, and VIM variants. A total of 46 genetically unrelated Pseudomonas aeruginosa, Pseudomonas putida, Acinetobacter sp., and enterobacterial strains producing distinct MBLs were tested. Nineteen strains were included as negative controls. The inhibition of bacterial growth and beta-lactam hydrolysis caused by MBL inhibitors (IMBL) also were evaluated. The isolates were tested for MBL production by both a double-disk synergy test (DDST) and a combined disk assay (CD) using imipenem and ceftazidime as substrates in combination with distinct IMBL. One hundred percent sensitivity and specificity were achieved by DDST using 2-mercaptopropionic acid in combination with ceftazidime and imipenem for the detection of MBL production among P. aeruginosa and Acinetobacter species isolates, respectively. The CD test showed the same results for detecting MBL-producing enterobacteria by combining imipenem and EDTA, with a 5.0-mm-breakpoint increase in the size of the inhibition zone. Our results indicate that both phenotypic methods to detect MBL-producing isolates should be based on the genera to be tested, regardless of the enzyme produced by such isolates, as well as on the local prevalence of MBL producers.

Metallo beta lactamases in Pseudomonas aeruginosa and Acinetobacter species

The multi drug resistant gram negative bacteria especially Pseudomonas aeruginosa and Acinetobacter species are on the rise. The major defense in these bacteria against beta-lactam antibiotics is production of metallo beta lactamases (MBLs) which degrade this group of antibiotics including carbapenems. Till now five main types of MBLs have been described throughout the World--IMP, VIM, SPM, GIM and SIM. A new MBL has been recently reported in P. aeruginosa from Australia--bla AIM-1. There are no standard guidelines by CLSI for detection of these enzymes in various bacteria. A number of phenotypic tests based on different beta lactam-inhibitor combinations are being evaluated and used for routine testing. Regarding the treatment options--colistin, various antibiotic combinations and a few novel antibiotics are being tried and evaluated. Prevention is based on age old practices of strict infection control and judicious use of antibiotics.

Carbapenemases: the versatile beta-lactamases

Carbapenemases are beta-lactamases with versatile hydrolytic capacities. They have the ability to hydrolyze penicillins, cephalosporins, monobactams, and carbapenems. Bacteria producing these beta-lactamases may cause serious infections in which the carbapenemase activity renders many beta-lactams ineffective. Carbapenemases are members of the molecular class A, B, and D beta-lactamases. Class A and D enzymes have a serine-based hydrolytic mechanism, while class B enzymes are metallo-beta-lactamases that contain zinc in the active site. The class A carbapenemase group includes members of the SME, IMI, NMC, GES, and KPC families. Of these, the KPC carbapenemases are the most prevalent, found mostly on plasmids in Klebsiella pneumoniae. The class D carbapenemases consist of OXA-type beta-lactamases frequently detected in Acinetobacter baumannii. The metallo-beta-lactamases belong to the IMP, VIM, SPM, GIM, and SIM families and have been detected primarily in Pseudomonas aeruginosa; however, there are increasing numbers of reports worldwide of this group of beta-lactamases in the Enterobacteriaceae. This review updates the characteristics, epidemiology, and detection of the carbapenemases found in pathogenic bacteria.

Metallo-beta-lactamases as emerging resistance determinants in Gram-negative pathogens: open issues

The rapid spread of acquired metallo-beta-lactamases (MBLs) among major Gram-negative pathogens is a matter of particular concern worldwide and primarily in Europe, one of first continents where the emergence of acquired MBLs has been reported and possibly the geographical area where the increasing diversity of these enzymes and the number of bacterial species affected are most impressive. This spread has not been paralleled by accuracy/standardisation of detection methods, completeness of epidemiological knowledge or a clear understanding of what MBL production entails in terms of clinical impact, hospital infection control and antimicrobial chemotherapy. A number of European experts in the field met to review the current knowledge on this phenomenon, to point out open issues and to reinforce and relate to one another the existing activities set forth by research institutes, scientific societies and European Union-driven networks.