Evaluation of a new Etest for detecting metallo-beta-lactamases in routine clinical testing

Hidden VIM-1 metallo-beta-lactamase phenotypes among Acinetobacter baumannii clinical isolates

A total of 87 Acinetobacter baumannii nonrepetitive consecutive clinical isolates were tested for the presence of metallo-beta-lactamases (MBLs). Results of phenotypic assays (MBL Etest, imipenem/imipenem-EDTA combined-disk test, and imipenem/EDTA double-disk synergy test) were negative in all cases, but molecular testing revealed the presence of two bla(VIM-1)-carrying isolates. One isolate had bla(VIM-1) preceded by a weak P1 promoter, and both had inactivated P2 promoters and reduced bla(VIM-1) expression, partially justifying the results revealing hidden MBL phenotypes.

Specific detection of blaVIM and blaIMP metallo-beta-lactamase genes in a single real-time PCR

This study describes the development of a real-time PCR protocol for rapid detection of the most common bla(VIM) (bla(VIM-1), bla(VIM-2), bla(VIM-3), bla(VIM-4), bla(VIM-5), bla(VIM-6), bla(VIM-10), bla(VIM-11), bla(VIM-12)) and bla(IMP) (bla(IMP-1), bla(IMP-2), bla(IMP-6), bla(IMP-8), bla(IMP-10), bla(IMP-15), bla(IMP-19), bla(IMP-20)) genes in a single reaction. The genes were specifically detected and clearly differentiated into four groups, i.e., (i) bla(VIM-1)-like (bla(VIM-1), bla(VIM-4), bla(VIM-5), bla(VIM-12)); (ii) bla(VIM-2)-like (bla(VIM-2), bla(VIM-3), bla(VIM-6), bla(VIM-10), bla(VIM-11)); (iii) bla(IMP-1)-like (bla(IMP-1), bla(IMP-6), bla(IMP-10)); and (iv) bla(IMP-2)-like (bla(IMP-2), bla(IMP-8), bla(IMP-15), bla(IMP-19), bla(IMP-20)), by melting curve analysis of the real-time PCR products. The protocol was used to screen positive bla(VIM-1), bla(VIM-2) and bla(IMP-1) control strains, 70 Gram-negative isolates resistant to carbapenems, and 30 Gram-negative isolates susceptible to carbapenems (negative controls).

Nosocomial spread of Pseudomonas aeruginosa producing the metallo-β-lactamase VIM-2 in a Spanish hospital: clinical and epidemiological implications

Thirty-four isolates of pan-resistant Pseudomonas aeruginosa producing VIM-2 metallo-beta-lactamase (MBL) were detected at a university hospital in Spain (July 2004-September 2006). Eleven (32%) patients had clinically significant infections, and three (27%) of these patients died. A single clone of MBL-producing P. aeruginosa was identified by pulsed-field gel electrophoresis. A cluster of isolates associated with the vascular surgery ward involved ten patients and appeared as a series of low-grade, sustained and misdiagnosed endemic infections in the hospital. The identification of MBL-positive P. aeruginosa should be considered mandatory in the surveillance of pan-resistant P. aeruginosa and requires a high index of suspicion in the context of endemic infections with a low attack rate.

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.

Outbreak of Acinetobacter baumannii producing the carbapenem-hydrolyzing oxacillinase OXA-58 in Rome, Italy

In this study 45 epidemic and sporadic isolates of Acinetobacter baumannii were investigated by antimicrobial resistance, integron identifications and genotyping. Isolates were genotyped by random amplified polymorphism (RAPD) DNA and pulsed-field gel electrophoresis (PFGE). Four different RAPD patterns were observed among the isolates of our collection, further discerned in six PFGE types. Two prevalent genotypes were identified, one corresponding to a carbapenem resistant epidemic clone, causing an outbreak at the intensive care unit of a hospital of Rome. Two class 1 integrons, carrying different gene cassette arrays, were identified among the two prevalent genotypes. Nucleotide analysis of the integron-variable regions revealed the presence of the aacA4, orfO, bla(OXA-20), and aacC1, orfX, orfX', aadA1 gene cassette arrays, respectively. All the carbapenem resistant strains analyzed in this study carried the bla (OXA-58) gene located on plasmids.

Convenient test using a combination of chelating agents for detection of metallo-beta-lactamases in the clinical laboratory

Although transmissible metallo-beta-lactamases (MBLs) are a serious threat to beta-lactam antibiotic therapy, the CLSI currently does not recommend testing methods for the detection of MBLs. The aim of this study was to evaluate the capability of double-disk tests (DDTs) by using disks containing a combination of the chelators 2-mercaptopropionic acid (MPA) and Tris-EDTA (TE) to detect MBLs. Sixteen isolates (4 Acinetobacter baumannii isolates, 6 Pseudomonas aeruginosa isolates, 1 Serratia marcescens isolate, 1 Aeromonas hydrophila isolate, 1 Aeromonas veronii isolate, 2 Chryseobacterium meningosepticum isolates, and 1 Stenotrophomonas maltophilia isolate) producing IMP-1, IMP-1-like, IMP-18, GIM-1, SPM-1, VIM-2, VIM-2-like, and chromosomal MBLs and 20 isolates (7 Klebsiella pneumoniae isolates, 3 Escherichia coli isolates, 5 Enterobacter cloacae isolates, 2 S. marcescens isolates, 1 Proteus mirabilis isolate, and 2 A. baumannii isolates) producing non-MBL carbapenemases, AmpC beta-lactamases, and extended-spectrum beta-lactamases were tested. The DDT method was evaluated by using four types of chelator disks (TE, high-strength TE, MPA, and TE plus 20 microl of MPA [at various concentrations]) and the beta-lactams imipenem (IPM), meropenem (MEM), ertapenem (ERT), and ceftazidime (CAZ). DDTs with IPM and a TE disk supplemented with 1:320 MPA detected all MBLs and yielded no false-positive results. Some, but not all, MBL producers were detected in IPM-based tests involving the single chelator TE or MPA alone or by ERT- or CAZ-based tests. IPM-based tests with MPA concentrations other than 1:320 and all MEM-based tests had suboptimal sensitivities or specificities. DDT with IPM and a TE disk supplemented with 20 microl of 1:320 MPA appears to be convenient for the detection of MBLs in the clinical laboratory.

Collateral damage of flomoxef therapy: in vivo development of porin deficiency and acquisition of blaDHA-1 leading to ertapenem resistance in a clinical isolate of Klebsiella pneumoniae producing CTX-M-3 and SHV-5 beta-lactamases

OBJECTIVES

The study aimed to characterize the genetic basis of flomoxef and collateral ertapenem resistance in a clinical isolate of extended-spectrum beta-lactamase-producing Klebsiella pneumoniae (ESBL-KP) after flomoxef exposure.

METHODS

Four ESBL-KP isolates (Lkp11-14) were recovered sequentially from four episodes of bacteraemia in an elderly patient. Laboratory investigations included genotyping by PFGE, resistance gene analysis by PCR and sequencing, and outer membrane protein analysis by SDS-PAGE. Plasmid analysis by DNA-DNA hybridization, electroporation and conjugation was also performed.

RESULTS

Lkp14 was recovered after 21 days of flomoxef therapy. It demonstrated an indistinguishable PFGE pattern when compared with those produced by Lkp11-13. However, resistance to both flomoxef and ertapenem emerged in Lkp14. Molecular characterization revealed that, in addition to the pre-existing ESBL production (CTX-M-3 and SHV-5) and OmpK35 deficiency found in Lkp11-13, Lkp14 had acquired an extra plasmid-mediated AmpC beta-lactamase gene (blaDHA-1) and failed to express OmpK36, because of insertional inactivation by an insertion sequence IS5. Other resistance mechanisms, such as production of carbapenem-hydrolysing enzymes or expression of chromosomal efflux, were apparently not involved. Conjugational transfer of the plasmid-mediated blaDHA-1 gene into Lkp11 resulted in a significant increase in the MICs of cephamycins and beta-lactamase inhibitors, but not in those of carbapenems.

CONCLUSIONS

Lkp14 was apparently derived from the previously flomoxef-susceptible isolates, Lkp11-13. After flomoxef exposure, the in vivo acquisition of the plasmid-mediated blaDHA-1 gene has led to flomoxef resistance in Lkp14, and the concomitant depletion of OmpK36 expression has resulted in a collateral effect of ertapenem resistance and diminished susceptibilities to imipenem and meropenem.

Detection of plasmid-mediated class C β-lactamases

Plasmid-mediated class C beta-lactamases are reported from Enterobacteriaceae with increasing frequency. They likely originate from chromosomal AmpC of certain Gram-negative bacterial species and subsequently are mobilized onto transmissible plasmids. There are reports of unfavorable clinical outcomes in patients infected with these organisms and treated with broad-spectrum cephalosporins. However, unlike class A extended-spectrum beta-lactamases (ESBLs), no screening and confirmatory tests have been uniformly established for strains that produce class C beta-lactamases. Reduced susceptibility to cefoxitin is a sensitive but not specific indicator of class C beta-lactamase production. Simple confirmatory tests including tests using boronic acid compounds as specific class C beta-lactamase inhibitors have recently been developed. Their utilization will enable clinical microbiology laboratories to report those strains producing plasmid-mediated class C beta-lactamases as being resistant to all broad-spectrum cephalosporins, thus allowing physicians to prescribe appropriate antimicrobial therapy.

Multicopy blaOXA-58 gene as a source of high-level resistance to carbapenems in Acinetobacter baumannii

The mechanisms at the origin of heterogeneous carbapenem resistance levels observed among Acinetobacter baumannii isolates collected in 2005 in a large University Hospital of Rome, Italy, were investigated. These isolates were related and possessed similar plasmids carrying the carbapenem-hydrolyzing oxacillinase gene bla(OXA-58) but showed variable levels of resistance to carbapenems. Analysis of sequences surrounding the bla(OXA-58) gene showed genetic variability, with the presence in several isolates of multiple copies of the bla(OXA-58) gene; this extra copy number was likely related to an IS26-mediated transposition or recombination process.

Influence of disk preparation on detection of metallo-beta-lactamase-producing isolates by the combined disk assay

The combined disk assay has been used for detection of metallo-beta-lactamase-producing isolates. We have observed that the size of inhibition zones produced by many beta-lactam/metallo-beta-lactamase inhibitor (IMBL) combinations may differ depending on the way that the combined disks were prepared. Among the 10 beta-lactam/IMBL combinations tested, only the imipenem/EDTA combination produced similar results.

Beta-lactam resistance in the gram negatives: increasing complexity of conditional, composite and multiply resistant phenotypes

The greatest impact of microbiology data on clinical care is in the critically ill. Unfortunately, this is also the area in which microbiology laboratories are most often non-contributive. Attempts to move to rapid, culture-independent diagnostics are driven by the need to expedite urgent results. This is difficult in Gram-negative infection because of the complexity of the antibiotic resistance phenotype. Here, we discuss resistance to modern beta-lactams as a case in point. Recent outbreaks of transmissible carbapenem resistance among Gram-negative enteric pathogens in Sydney and Melbourne serve to illustrate the pitfalls of traditional phenotypical approaches. A better understanding of the epidemiology and mosaic nature of antibiotic resistance elements in the microflora is needed for us to move forward.

Microbiological and epidemiological characterization of imipenem-resistant Pseudomonas aeruginosa strains from a Brazilian tertiary hospital: report from the SENTRY Antimicrobial Surveillance Program

OBJECTIVES

To evaluate the antimicrobial susceptibility profile, the genetic similarity, and the mechanisms of carbapenem resistance among imipenem-resistant Pseudomonas aeruginosa isolates collected from a Brazilian tertiary teaching hospital.

METHODS

Seventy-eight consecutive samples of P. aeruginosa were evaluated during 2000 and 2001. The antimicrobial susceptibility was evaluated by reference broth microdilution methods and the imipenem-resistant isolates were screened for metallo-beta-lactamase (MbetaL) production throughout disc approximation test and MbetaL Etest strips and isolates with positive screen test result were submitted to PCR assays using primers blaIMP-1, bla VIM-1, blaVIM-2 e blaSPM-1. The genetic similarity of MbetaL-producing strains was evaluated by automated ribotyping for epidemiological typing purpose.

RESULTS

Resistance rates were high to the majority of antimicrobial agents tested except polymyxin B, which inhibited all samples at the Clinical and Laboratory Standards Institute breakpoint (< or = 2 microg/ml). Twenty-nine (37.2%) isolates were resistant to imipenem and these isolates showed great genomic variability. MbetaL production was demonstrated in two imipenem-resistant isolates, which were detected using blaSPM-1 and blaIMP-2-specific primers. Sequence analysis revealed the presence of blaSPM-1 and a novel blaIMP-type gene, blaIMP-16.

CONCLUSION

The results of this study showed high resistance rates to the majority of antimicrobial agents among P. aeruginosa samples. High imipenem resistance rates were probably due to continuous selection of resistant mutants. The production of MbetaL did not represent a frequent mechanism of carbapenem resistance in this medical center; but a novel MbetaL was identified. Continued antimicrobial surveillance and infection control measures should be emphasized to minimize the emergence and dissemination of antimicrobial resistance.

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.

Phenotypic detection of carbapenem-susceptible metallo-beta-lactamase-producing gram-negative bacilli in the clinical laboratory

Rapid detection of metallo-beta-lactamase (MBL)-producing gram-negative pathogens is critical to prevent their widespread dissemination. Thus far, no standardized phenotypic method is available, and previously reported techniques have poor sensitivity for detecting carbapenem-susceptible MBL-carrying isolates, an increasingly described phenomenon. We developed a phenotypic detection method using both a double-disk synergy test and a combined-disk test with imipenem and 292 microg EDTA on one agar plate. Genotypic confirmation was used for validation. Of the 134 clinical isolates, 84 were confirmed to carry an MBL. Of these, 51 (61%) were susceptible to at least one carbapenem, and 22 (26%) were isolated from blood. The phenotypic method correctly differentiated all MBL-producing isolates (sensitivity, 100%). Fifty-one of the 52 MBL-negative isolates were correctly differentiated (specificity, 98%). This study reports the validation of a simple and accurate MBL detection method that can be easily incorporated into the daily routine of a clinical laboratory. Early detection of MBL-carrying organisms, including those with susceptibility to carbapenems, is of paramount clinical importance, as it allows rapid initiation of strict infection control practices as well as therapeutic guidance for confirmed infection.

Class II transposon-borne structure harboring metallo-beta-lactamase gene blaVIM-2 in Pseudomonas putida

A plasmid-encoded class II transposon element was identified in a carbapenem-resistant Pseudomonas putida isolate. Tn1332, closely related to Tn1331, harbored the metallo-beta-lactamase gene bla(VIM-2) in addition to four other antibiotic resistance genes, aacA4, aadA1, bla(OXA-9), and bla(TEM-1), and two novel insertion sequences, ISPpu17 and ISPpu18.

Evaluation of imipenem/imipenem+EDTA disk method for detection of metallo-beta-lactamase-producing Klebsiella pneumoniae isolated from blood cultures

The objective of this study was to evaluate the imipenem (IMP) and IMP+EDTA (IMP/IMP+EDTA) disk method for the detection of metallo-beta-lactamases (MBLs) in clinical isolates of Klebsiella pneumoniae with various MIC levels to IMP. Forty-one blood isolates of K. pneumoniae with MIC to IMP ranging from < or =0.5 to > or =16 microg/ml were examined. The MICs were determined by VITEK-2 (bioMerieux Vitek two, France). Disks of 10 microg IMP with and without the addition of 0.5 M EDTA were used for the IMP/IMP+EDTA disk method. The E-test (AB Biodisk, Solna, Sweden) for MBL detection was also used. All isolates were examined for the bla (VIM-1) gene by PCR and for clonality of VIM-1-producing isolates by pulsed-field gel electrophoresis (PFGE). All isolates with MIC values of IMP < or =0.5 microg/ml exhibited no differences in inhibition zone diameters (IZD) produced by IMP and IMP+EDTA disks, whereas the isolates with MICs > or =1 microg/ml showed an increase in IZD, ranging from 8 to 26 mm. All isolates with MIC values of > or =1 microg/ml were found positive for the bla (VIM-1) gene by PCR and for MBL production by the E-test, whereas none of isolates with MICs <0.5 microg/ml was found positive by any of the tests. DNA restriction fragments generated by PFGE of VIM-1-producing isolates were classified in four main types. The IMP/IMP+EDTA disk method is simple to perform, sensitive, and specific for detection of MBL-producing K. pneumoniae clinical isolates. K. pneumoniae isolates with MICs of IMP > or =1 microg/ml and/or IZD produced by IMP disk <19 mm should be tested for MBL production.

Dissemination of IMP-1 metallo- beta -lactamase-producing Acinetobacter species in a Brazilian teaching hospital

OBJECTIVE

To evaluate the emergence and dissemination of metallo- beta -lactamase (MBL)-producing Acinetobacter species.

DESIGN

All carbapenem-resistant Acinetobacter strains (1 strain per patient) collected during the period 1993-2001 were evaluated.

SETTING

A Brazilian tertiary care teaching hospital (Hospital Sao Paulo, Sao Paulo).

METHODS

Seventy-three strains of carbapenem-resistant Acinetobacter species were recovered from the organism bank of the hospital. All isolates were tested for antimicrobial susceptibility by broth microdilution methods, and the production of MBL was initially assessed by phenotypic tests (MBL Etest strip and a disk approximation test). The MBL enzymes were identified by polymerase chain reaction using primers for bla(IMP), bla(VIM), and bla(SPM), followed by gene sequencing. Genetic similarity among the carbapenem-resistant strains was evaluated by automated ribotyping.

RESULTS

Only colistin and ampicillin-sulbactam showed reasonable in vitro activity against carbapenem-resistant isolates (97% and 74% of isolates susceptible, respectively). More than half of the isolates (55%) had a positive MBL phenotypic test result and a positive polymerase chain reaction result for bla(IMP-1). The proportion of IMP-1-producing Acinetobacter isolates among carbapenem-resistant strains increased from 0% in the 1993-1997 period to 29% in 1998 and 100% in the 1999-2001 period. No carbapenem-resistant Acinetobacter isolates that harbored bla(VIM) or bla(SPM) were detected. Molecular typing results revealed 20 ribogroups among carbapenem-resistant isolates. During the study period of 1994-2001, we identified 2 major ribogroups, 52-1 (MBL-negative and MBL-positive strains) and 60-7 (MBL-positive strains), that had a coefficient of similarity of 0.85 or higher.

CONCLUSIONS

Our results indicate that IMP-1-producing strains of Acinetobacter emerged in our institution in 1998. Since then, production of this MBL was detected not only in the major ribogroups of carbapenem-resistant Acinetobacter species but also among isolates that belonged to 17 distinct ribogroups, indicating that this important mechanism of antimicrobial resistance was disseminated among distinct clones.

First detection of metallo-beta-lactamase VIM-2 in Pseudomonas aeruginosa isolates from Colombia

Carbapenem resistance rates in Pseudomonas aeruginosa isolates in Colombia, as in many South American countries, are high for reasons that remain unclear. From our nationwide network, we describe the first detection of the metallo-beta-lactamase VIM-2 in clinical isolates of P. aeruginosa from multiple cities within Colombia. Metallo-beta-lactamases were not detected in the two centers with the highest imipenem resistance rates. Clonality was noted in five of the eight centers with strains meeting the criteria for molecular typing. The high carbapenem resistance in P. aeruginosa in Colombia may be attributable to a combination of factors, including the presence of metallo-beta-lactamases and nosocomial transmission.

Sensitive EDTA-based microbiological assays for detection of metallo-{beta}-lactamases in nonfermentative gram-negative bacteria

The worldwide spread of metallo-beta-lactamase (MBL)-producing gram-negative bacilli represents a great concern nowadays. Sensitive assays for their specific detection are increasingly demanded to aid infection control and to prevent their dissemination. We have developed a novel microbiological assay employing crude bacterial extracts, designated EDTA-imipenem microbiological assay (EIM), to identify MBLs in nonfermentative gram-negative clinical strains. We also evaluated the ability of EIM to detect MBLs in comparison to those of other currently employed screening methods, such as the EDTA disk synergy test (EDS) with imipenem as a substrate and the Etest method. The sensitivities of EIM and Etest were similar (1 versus 0.92, respectively) and much higher than that of EDS (0.67). Moreover, both EIM and Etest displayed the maximum specificity. Modifications were introduced to EDS, including the simultaneous testing of three different beta-lactams (imipenem, meropenem, and ceftazidime) and two different EDTA concentrations. This resulted in a sensitivity improvement (0.92), albeit at a cost to its specificity. A simple strategy to accurately detect MBL producers is proposed; this strategy combines (i) an initial screening of the isolates by the extended EDS assay to select the potential candidates and (ii) confirmation of the true presence of MBL activity by EIM.

Characterization of a carbapenemase-producing clinical isolate of Bacteroides fragilis in Scandinavia: genetic analysis of a unique insertion sequence

In 2003 a Bacteroides fragilis blood culture isolate (K2-28) was recovered from a 61-y-old male with severe general atherosclerosis during treatment with meropenem. K2-28 was shown to possess a functional metallo-beta-lactamase with a reduction in imipenem MIC from 256 to 3 mg/l in the presence of EDTA using the MBL E-test strip. PCR results were for positive for the cfiA gene. Analysis of the cfiA from K2-28 revealed it was 100% identical to previously described cfiA-1 genes. Analysis of the upstream region of cfiA revealed a novel insertion sequence (IS) element, being most similar (94% identity) to IS612 recently described from Japan designating the element within the IS4 family. The element possessed a perfect terminal inverted repeat sequence at the distal ends of the IS element and provided a putative promoter for transcription of the cfiA gene. The distance between the hybrid promoter and the cfiA start codon was 158 base pairs and inserted into a different DNA sequence upstream of cfiA to that previously reported. The -10 promoter region was most similar to that of IS613 (100%) and the -35 promoter region to IS612 (100%), demonstrating the plasticity of these genetic regions.

First nosocomial outbreak of Pseudomonas aeruginosa producing an integron-borne metallo-beta-lactamase (VIM-2) in the United States

Carbapenemases are rare in the United States. This is the first report of a United States nosocomial outbreak of pan-resistant Pseudomonas aeruginosa infections due to an integron-borne metallo-beta-lactamase, VIM-2. This emergence of carbapenemases on mobile genetic elements in the United States warrants surveillance.

Nosocomial outbreak caused by multidrug-resistant Pseudomonas aeruginosa producing IMP-13 metallo-beta-lactamase

An outbreak of Pseudomonas aeruginosa showing a multidrug-resistant (MDR) phenotype (including carbapenems, ceftazidime, cefepime, gentamicin, tobramycin, and fluoroquinolones) was observed, during a 5-month period, in a general intensive care unit of a large tertiary care and clinical research hospital in southern Italy. The outbreak involved 15 patients, with a total of 87 isolates, mostly from lower respiratory tract specimens. Analysis of isolates involved in the outbreak revealed production of metallo-beta-lactamase (MBL) activity, and genotyping by pulsed-field gel electrophoresis of genomic DNA digested by SpeI revealed clonal relatedness among isolates. Molecular analysis of the MBL determinant showed the presence of a bla(IMP-13) gene carried on a gene cassette inserted in a class 1 integron which also contained an aacA4 aminoglycoside resistance cassette encoding an AAC(6')-Ib enzyme. The bla(IMP-13)-containing integron and its genetic environment appeared to be similar to those found in P. aeruginosa isolates producing IMP-13 from a hospital in Rome. The bla(IMP-13) gene was not transferable by conjugation and was apparently carried on the chromosome. The outbreak was coincidental with a shortage of nursing personnel, and resolution was apparently associated with reinstatement of nursing personnel and reinforcement of general infection control practices within the intensive care unit. To our best knowledge this is the first description of a nosocomial outbreak of relatively large size caused by an IMP-producing gram-negative pathogen in Europe.

Nosocomial Spread ofPseudomonas aeruginosaIsolates Expressing the Metallo-β-Lactamase VIM-2 in a Hematology Unit of a French Hospital

Dissemination of metallo-beta-lactamases (carbapenemases) was investigated retrospectively among ceftazidimeand imipenem-resistant Pseudomonas aeruginosa isolates in a hematology unit in Marseilles, France, from September, 1995, to March, 1999. Sixteen clinical isolates and 23 environmental strains were identified, with a same bla (VIM-2) gene that encoded a carbapenemase identified in Southern Europe and South Korea. Five different genotypes were identified among clinical and environmental P. aeruginosa isolates all harboring an approximately 45-kb plasmid with bla (VIM-2)-positive class 1 integrons varying in structures. This study identified a hidden reservoir of carbapenemase producers.

Detection of Pseudomonas aeruginosa producing metallo-beta-lactamases in a large centralized laboratory

Metallo-beta-lactamases (MBLs) have been increasingly recognized from clinical isolates worldwide, but the laboratory detection of these strains is not well defined. We report a study that developed an EDTA disk screen test and a molecular diagnostic assay for the detection of MBL-producing Pseudomonas aeruginosa. Using NCCLS disk methodology, inhibition zone diameters were determined in tests with imipenem (IPM) and meropenem (MEM) disks alone and in combination with 930 microg of EDTA. This test was compared with the MBL Etest. The duplex PCR assay showed 100% sensitivity and specificity for detecting MBL-producing control strains. Of the 241 clinical strains of IPM-nonsusceptible P. aeruginosa from the Calgary Health Region isolated from 2002 to 2004, 110/241 (46%) were MBL positive using phenotypic methods while 107/241 (45%) were PCR positive for MBL genes: 103/241 (43%) for bla(VIM) and 4/241 (2%) for bla(IMP). The EDTA disk screen test using MEM showed 100% sensitivity and 97% specificity for detecting MBLs in control and clinical strains. The EDTA disk screen test is simple to perform and to interpret and can easily be introduced into the workflow of a clinical laboratory. We recommend that all IPM-nonsusceptible P. aeruginosa isolates be routinely screened for MBL production using the EDTA disk screen test and that PCR confirmation be performed at a regional laboratory.

Emerging Metallo‐β‐Lactamase–Mediated Resistances: A Summary Report from the Worldwide SENTRY Antimicrobial Surveillance Program

The rates of occurrence of metallo-beta-lactamase-mediated resistances in Pseudomonas aeruginosa, Acinetobacter species, and Serratia marcescens, among other gram-negative bacilli, have escalated since 2000, severely limiting treatment options in Asia, Europe, and Latin America to non-beta-lactam antimicrobial classes. Clinical isolates harboring metallo-beta-lactamases have also recently been reported in western Canada and in Texas, signaling the need for development of accurate diagnostic tests by clinical laboratories to detect their presence and for new, and more potent, antimicrobial agents.

In vitro synergy test of meropenem and sulbactam against clinical isolates of Acinetobacter baumannii

Meropenem and imipenem are often the drugs of choice for the treatment of infections due to multidrug-resistant Acinetobacter baumannii. The present study aimed at evaluating the interaction between meropenem and sulbactam through microdilution and checkerboard methods against 48 clinical isolates of A. baumannii collected from Brazilian hospitals. All the isolates presented elevated minimum inhibitory concentration (>or=2 microg/mL) to either meropenem or sulbactam. The checkerboard method with the combination of meropenem and sulbactam demonstrated 29.2% (14/48) synergism, 47.9% (23/48) partial synergism, 10.5% (5/48) additive, 6.2% (3/48) indifference, and 6.2% (3/48) antagonism (SigmaFIC(min)=0.09 and SigmaFIC(max)=8). Thus, combinations of meropenem and sulbactam may show synergism or partial synergism for most A. baumannii isolates. Further studies may help identify treatment options for patients with infections caused by these organisms, particularly with this combination, where both drugs have time-dependent activities and might be suitable for therapy optimization studies.

Evaluation of Etest MBL for detection of blaIMP-1 and blaVIM-2 allele-positive clinical isolates of Pseudomonas spp. and Acinetobacter spp

The Etest MBL (AB BIODISK, Solna, Sweden) correctly differentiated all 57 isolates of Acinetobacter spp. and Pseudomonas aeruginosa with the bla(IMP-1) allele and 135 of 137 (98.5%) Acinetobacter spp. and Pseudomonas spp. isolates with the bla(VIM-2) allele. The Etest MBL was reliable for detecting the IMP-1- and VIM-2-producing Pseudomonas and Acinetobacter isolates.

Carbapenem resistance mechanisms in Pseudomonas aeruginosa: alterations of porin OprD and efflux proteins do not fully explain resistance patterns observed in clinical isolates

Imipenem resistance in Pseudomonas aeruginosa is considered to be associated with loss of the porin OprD combined with activity of chromosomal beta-lactamase (AmpC), while overexpression of multidrug efflux pumps is considered to confer meropenem resistance. Carbapenem resistance can also result from production of metallo-beta-lactamases. Transcription of oprD and efflux pump genes mexB, mexY and mexF was analysed in 23 clinical isolates of P. aeruginosa by quantitative RT-PCR. oprD was sequenced in all, and mexR, regulator of efflux pump MexAB-OprM, in selected isolates. Four isolates that were imipenem susceptible had significant reduction of oprD mRNA and presence of oprD mutations causing frameshift or translational stop. In strains only resistant to imipenem no significant difference in transcription of oprD was observed between low-level and high-level resistant isolates. The differences could not be explained by either pattern of oprD mutations. Increased transcription of mexB generally correlated well with meropenem resistance. One high-level meropenem-resistant isolate showed no significant change in mexB mRNA, but sequencing confirmed presence of a nalB mutation. Furthermore, one meropenem-susceptible isolate showed significant increase in mexB transcription, but no mexR mutations. In summary, our findings indicate that the resistance patterns observed cannot be fully explained by the currently described carbapenem resistance mechanisms.

Metallo-beta-lactamases: the quiet before the storm?

The ascendancy of metallo-beta-lactamases within the clinical sector, while not ubiquitous, has nonetheless been dramatic; some reports indicate that nearly 30% of imipenem-resistant Pseudomonas aeruginosa strains possess a metallo-beta-lactamase. Acquisition of a metallo-beta-lactamase gene will invariably mediate broad-spectrum beta-lactam resistance in P. aeruginosa, but the level of in vitro resistance in Acinetobacter spp. and Enterobacteriaceae is less dependable. Their clinical significance is further embellished by their ability to hydrolyze all beta-lactams and by the fact that there is currently no clinical inhibitor, nor is there likely to be for the foreseeable future. The genes encoding metallo-beta-lactamases are often procured by class 1 (sometimes class 3) integrons, which, in turn, are embedded in transposons, resulting in a highly transmissible genetic apparatus. Moreover, other gene cassettes within the integrons often confer resistance to aminoglycosides, precluding their use as an alternative treatment. Thus far, the metallo-beta-lactamases encoded on transferable genes include IMP, VIM, SPM, and GIM and have been reported from 28 countries. Their rapid dissemination is worrisome and necessitates the implementation of not just surveillance studies but also metallo-beta-lactamase inhibitor studies securing the longevity of important anti-infectives.

IMPs, VIMs and SPMs: the diversity of metallo-beta-lactamases produced by carbapenem-resistant Pseudomonas aeruginosa in a Brazilian hospital

Pseudomonas aeruginosa isolates (n=183), collected from bacteraemic patients hospitalised in Sao Paulo Hospital (Brazil) during 2000-2001, were screened for susceptibility to antimicrobial agents. The polymyxins were the most active compounds (100% susceptibility), followed by amikacin and cefepime (59.0%), meropenem (57.4%), and imipenem and gentamicin (55.2%). Imipenem-resistant isolates were ribotyped and screened for production of metallo-beta-lactamases (MBLs) by PCR with primers for bla(IMP), bla(VIM) and bla(SPM). MBL production was detected in 36 isolates (19.7% of the entire collection; 43.9% of the imipenem-resistant isolates) and the MBLs included SPM-1-like (55.6%), VIM-2-like (30.6%) and IMP-1-like (8.3%) enzymes.

Molecular characterization of an epidemic clone of panantibiotic-resistant Pseudomonas aeruginosa

We describe the molecular characterization of a multiresistant Pseudomonas aeruginosa clone causing an outbreak in the intensive care unit (ICU) of a tertiary-care university hospital. Analysis included antimicrobial susceptibility profile, O-serotyping, pulsed-field gel electrophoresis, and amplified fragment length polymorphism. Resistance mechanisms were characterized, including production of naturally occurring and acquired beta-lactamases, porin expression, and efflux pump systems. Eighteen patients were colonized or infected with multiresistant P. aeruginosa. Multiresistant P. aeruginosa was panresistant to penicillins, cephalosporins, carbapenems, aminoglycosides, and fluoroquinolones and remained susceptible only to colistin. Sixteen isolates (89%) belonged to serotype O:11, pulsed-field gel electrophoresis type A1, and amplified fragment length polymorphism type A. Resistance characterization of this epidemic clone showed an overexpression of the chromosomal cephalosporinase AmpC combined with decreased expression of porin OprD and the absence of metallo-beta-lactamase or extended-spectrum beta-lactamase. An upregulation of the MexXY efflux system due to an agrZ mutation in the mexZ repressor was detected. This epidemic clone was restricted to the ICU and was not found elsewhere in hospital. Contamination of the ICU environment and the hands of an ICU nurse with this clone suggests possible hand-borne transmission. Implementation of contact precautions effectively controlled transmission of the epidemic clone. This study illustrates the ability of multiresistant P. aeruginosa to cause an outbreak with significant morbidity and mortality and underscores the need to identify clonal outbreaks, which require targeted infection control measures.

Genetic methods for detection of antimicrobial resistance

Accurate and rapid diagnostic methods are needed to guide antimicrobial therapy and infection control interventions. Advances in real-time PCR have provided a user-friendly, rapid and reproducible testing platform catalysing an increased use of genetic assays as part of a wider strategy to minimize the development and spread of antimicrobial-resistant bacteria. In this review we outline the principal features of genetic assays in the detection of antimicrobial resistance, their advantages and limitations, and discuss specific applications in the detection of methicillin-resistant Staphylococcus aureus, glycopeptide-resistant enterococci, aminoglycoside resistance in staphylococci and enterococci, broad-spectrum resistance to beta-lactam antibiotics in gram-negative bacteria, as well as genetic elements involved in the assembly and spread of antimicrobial resistance.

Dissemination and diversity of metallo-β-lactamases in Latin America: report from the SENTRY Antimicrobial Surveillance Program

Carbapenem resistance among Pseudomonas aeruginosa and Acinetobacter spp. is becoming a critical therapeutic problem worldwide. The SENTRY Antimicrobial Surveillance Program monitors pathogen frequency and antimicrobial resistance patterns of nosocomial and community-acquired infections through sentinel hospitals on five continents. Pseudomonas spp. and Acinetobacter spp. strains resistant to imipenem (MIC, >/=16 mg/l), meropenem (MIC, >/=16 mg/l), and ceftazidime (MIC, >/=32 mg/l) collected from January 2001 to December 2003 were routinely screened for antimicrobial resistance genes. Resistant isolates were initially tested for metallo-beta-lactamase (MbetaL) production by phenotypic tests (disk approximation or MbetaL Etest strip) and then characterization of the MbetaL (hydrolysis assays, PCR for bla(IMP), bla(VIM), bla(SPM), gene sequencing). Eighty-nine isolates (33 Acinetobacter spp., 54 Pseudomonas aeruginosa, and 2 P. fluorescens) had positive phenotypic screening tests. Among those, 34 isolates producing MbetaL were identified, including 7 Acinetobacter spp., 25 P. aeruginosa and 2 P. fluorescens. The MbetaLs identified were IMP-1, VIM-2 and two newly described enzymes: SPM-1 and IMP-16. The greatest concentration of MbetaL strains was in Brazil, where imipenem-resistant P. aeruginosa increased significantly in the time period evaluated by the SENTRY Program. MbetaL-producing P. aeruginosa was detected in São Paulo (SPM-1) and Brasilia (SPM-1 and IMP-16), Brazil and Caracas, Venezuela (VIM-2); while MbetaL-producing Acinetobacter spp. isolates were detected in São Paulo, Brazil (IMP-1). P. fluorescens isolates producing IMP-1 and VIM-2 were detected in São Paulo, Brazil and Santiago, Chile, respectively. The emergence and dissemination of mobile MbetaL-producing isolates represent an alarming factor for increasing resistance to carbapenems in several medical centres evaluated by the SENTRY Program in Latin America.

Antibacterial susceptibility testing in the clinical laboratory

This article familiarizes the clinician with the principles of bacterial susceptibility testing and reporting to facilitate communication with the clinical microbiology laboratory. The emergence of resistance in nearly all commonly isolated bacterial organisms has highlighted the need for ongoing dialogue between the laboratory and those who use its services.

Metallo-β-lactamase expressing multi-resistant Acinetobacter baumannii transmitted in the operation area

Outbreaks of Acinetobacter baumannii demonstrating multiple antibiotic resistance, including meropenem resistance, have been described as severe therapeutic problems. Here we describe a monoclonal outbreak of infection and colonization with multidrug-resistant A. baumannii over a two-month period. Resistance to meropenem was mediated by expression of a metallo-beta-lactamase enzyme. Four of 14 patients showed clinical signs of infection and two died. Contamination of the environment, water, or instruments were excluded as causes of the outbreak. All patients, except one, underwent surgery in a specific operation theatre where surgery of contamination class IV (infected, dirty) was performed. Although individual surgeon error was eliminated, analyses of the patients' histories suggested that bacterial transmission had occurred during surgery. Five patients showed signs of A. baumannii infection and two of these patients suffered from large abdominal wounds infected with a high density of A. baumannii requiring repeated revisions. Presumably, these revisions favoured the transmission of A. baumannii, which is remarkably resistant to various environmental stresses including soaps, disinfectants and dry conditions. No case of meropenem-resistant A. baumannii had been observed in the hospital before the outbreak. Interestingly, the resistant bacteria appear to have been imported by a patient returning from West Africa. This indicates that, similar to MRSA, multiresistant A. baumannii may be introduced by patients from foreign hospitals. The outbreak was stopped in the following months by reinforcing standard procedures and by taking all necessary precautions such as patient isolation, and finally only one new case was detected.

Evaluation of the contemporary occurrence rates of metallo-β-lactamases in multidrug-resistant Gram-negative bacilli in Japan: Report from the SENTRY Antimicrobial Surveillance Program (1998–2002)

Metallo-beta-lactamases (M beta L) were initially characterized in Japan, usually of the IMP-type, and found in Pseudomonas aeruginosa (PSA), Acinetobacter spp. (ACB), or Serratia marcescens (SM). The number of M beta L types has increased worldwide, but geographic dissemination within Japan has appeared limited. This study compares baseline levels of M beta L resistance from two 22-center studies (1996-1997) to the longitudinal sample (3 sites) of Japanese isolates from the SENTRY Antimicrobial Surveillance Program (1998-2002). All minimal inhibitory concentration results were determined by reference methods. A total of 26.8% PSA, 3.4% ACB, and 3.1% Enterobacteriaceae (enterobacters and SM) with resistance to monitored carbapenems (CARB) (minimal inhibitory concentration, > or =8 microg/mL) were screened for M beta L production by disk approximation tests (EDTA and 2-MPA inhibitors), CARB hydrolysis by enzyme extracts, and selected PCR primers for known M beta L types. All M beta L-positive strains (10) were sequenced to determine enzyme identification. Clonality in each center was determined by automated ribotyping and PFGE. The CARB susceptibility rates in PSA decreased (80.7% to 62.0%) over the monitored interval (1998-2002), but varied by medical center location. Among CARB-resistant isolates, 10.8% were attributed to M beta L strains (1.1% of all PSA tested). M beta L identification showed the following: five PSA (three IMP-1, two IMP-2), four SM (one IMP-1, two IMP-1 + OXA-1, and one IMP-11). Also a single ACB had an IMP-1. Eight of 10 M beta L isolations occurred between 2000 and 2002; four occurred in 2002. BRL42715, an AMP-C inhibitor, confirmed AMP-C-mediated resistance in 87.3% of PSA, and outer membrane protein changes were also discovered by membrane studies. Prior results (22 sites, 1997-1998) showed CARB resistance at 22.4-25.6% and 0.5-0.9% M beta Ls (IMP-1) overall; it was slightly elevated in this SENTRY Program sample. In conclusion, M beta L-producing strains from several species persist in Japan, but represent a distinct minority of all CARB-resistant strains (1998-2002). Although M beta L rates appear generally stable in Japan, continued surveillance for these mechanisms seems to be a prudent practice, because of the mobility of the genetic determinants and the emergence of novel enzyme types, especially among the Enterobacteriaceae.

Comparison of the double-disk, combined disk, and Etest methods for detecting metallo-beta-lactamases in gram-negative bacilli

Three phenotypic methods for detecting metallo-beta-lactamases (MBLs) in gram-negative bacilli were tested on 60 MBL producers and 155 MBL nonproducers. The 2-mercaptopropionic acid double-disk potentiation method using ceftazidime and cefepime with and without clavulanate was found to be most sensitive (100%). The combined disk method depends on comparing the zones given by disks containing a beta-lactam with and without ethylenediaminetetraacetic acid. Various drug combinations were tested, and the best results were obtained with imipenem for Pseudomonas species and Acinetobacter baumannii, ceftazidime-clavulanate for Klebsiella pneumoniae, and cefepime-clavulanate for Enterobacter cloacae and Citrobacter freundii. The overall sensitivity of the combined disk method was 86.7%. The Etest (AB BIODISK, Solna, Sweden) method using imipenem- ethylenediaminetetraacetic acid strips detected imipenem-resistant MBL-producing isolates only, resulting in a sensitivity of 36.7%. Our data suggest that the double-disk and combined disk tests are acceptable methods for MBL detection in both imipenem-resistant and imipenem-susceptible isolates.

Determination of epidemic clonality among multidrug-resistant strains of Acinetobacter spp. and Pseudomonas aeruginosa in the MYSTIC Programme (USA, 1999–2003)

The Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Programme was initiated in 1997 (1999 for the United States). This program monitors resistance in participant medical centers where carbapenems are prescribed and drug use data can be obtained. An earlier report found antimicrobial use was not a clear cause of local or aggregate changes in resistance rates. This study addresses the role of dissemination of resistant clones on susceptibility rates for nonfermentors, Acinetobacter spp. (ACB) and Pseudomonas aeruginosa (PSA). Carbapenem (CARB)-multidrug-resistant strains (MDR) from among 236 ACB and 1,111 PSA were tested by reference broth microdilution methods, automated ribotyping, and pulsed field gel electrophoresis to determine possible clonal dissemination. Each strain was also tested for metallo-beta-lactamases (MbetaL) (phenotypic and polymerase chain reaction); and then analyzed by CARB-R rate and defined daily dose (DDD)/100 days use groupings (high, moderate, and low). For the aggregate 15 sites in the MYSTIC Programme each year, the CARB-resistant rate decreased over 5 years; but other drug-resistance rates generally escalated. Changes were not related to antimicrobial use calculations. The discovered clonally spread MDR-PSA strains were more frequent in high- (1.8 clones/site) and moderate-resistance (0.6 clones/site) rate centers (21.7% to 29.5% were clonal), compared with unique strains in low-resistance hospitals. ACB clonality was extreme in one geographic area, with dissemination of 5 different clones (931.7/B, C, or D; 1090.2/A; 167.5/A) in 4 centers (02, 04, 06, and 18). Resistance rates in ACB and PSA were clearly related to clonal occurrence and spread, and one MbetaL (VIM-7) was detected. Decreased CARB resistance rates from 1999 through 2002 were directly attributed to the disappearance of resistance clones in some locations. In conclusion, ACB and PSA CARB and MDR resistance rates in MYSTIC Programme institutions have been greatly influenced by clonal dissemination and less by antimicrobial use patterns. The most serious examples of resistance were the clonality observed among ACB in New York City and the documented endemic nature of VIM-7-producing PSA (0.09% of all PSA isolates). Meropenem remained the most active antimicrobial agent tested in the program, and surveillance networks must implement epidemiologic typing to accurately assess the role of clonal spread on the study results.

Prevalence and characterization of metallo-beta-lactamases in clinical isolates of pseudomonas aeruginosa

The prevalence and the type(s) of metallo-beta-lactamases (MBLs) produced by isolates of Pseudomonas aeruginosa were investigated. During 2001, 506 nonduplicate isolates were obtained from hospitalized patients. Eighty-two strains were selected because of resistance to carbapenems and/or ceftazidime. Screening for MBL production was performed in the latter isolates by the Etest MBL strips (AB Biodisk, Solna, Sweden) and by a broth microdilution method measuring minimum inhibitory concentrations (MICs) of imipenem alone and in the presence of metal-chelating agents (EDTA and o-phenanthroline). Specific DNA probes were used to investigate the presence of genes coding for IMP- or VIM-type enzymes. Overall, four isolates of P. aeruginosa (obtained from independent patients) were found to carry a blaVIM gene. Polymerase chain reaction (PCR) experiments and DNA sequencing revealed that the VIM-2 determinant was present in three cases, whereas VIM-1 was detected in one isolate. Surveillance programs should be adopted to avoid the spread of these worrisome resistance genes.

In vitro double and triple synergistic activities of Polymyxin B, imipenem, and rifampin against multidrug-resistant Acinetobacter baumannii

Eight unrelated clinical Acinetobacter baumannii isolates resistant to all commonly used antibiotics were subjected to three-dimensional checkerboard microtiter plate dilution and time-kill studies at one-fourth of their MICs of polymyxin B, imipenem, and rifampin. Synergy was demonstrated with combinations of polymyxin B and imipenem, polymyxin B and rifampin, and polymyxin B, imipenem, and rifampin. Double combinations of polymyxin B and imipenem and of polymyxin B and rifampin were bactericidal for seven of eight isolates, and triple combinations were bactericidal for all isolates within 24 h. Future clinical studies using double and triple therapy with these antibacterials may provide an effective option against potentially lethal infection due to multiresistant Acinetobacter baumannii.

blaVIM-7, an evolutionarily distinct metallo-beta-lactamase gene in a Pseudomonas aeruginosa isolate from the United States

As part of the CANCER Antimicrobial Surveillance Program in North America, a Pseudomonas aeruginosa isolate, strain 07-406, was shown to possess a metallo-beta-lactamase, designated VIM-7. bla(VIM-7) is located on a 24-kb plasmid which can be readily transferred into Enterobacteriaceae and other pseudomonads. This is the first report of a mobile metallo-beta-lactamase gene, bla(VIM-7), being detected within the United States.