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JAC Advance Access originally published online on May 13, 2008
Journal of Antimicrobial Chemotherapy 2008 62(3):484-489; doi:10.1093/jac/dkn205
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© The Author 2008. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Original research

Emergence of carbapenem resistance in Acinetobacter baumannii in the Czech Republic is associated with the spread of multidrug-resistant strains of European clone II

Alexandr Nemec1,2,*, Lenka Krízová1, Martina Maixnerová1, Laure Diancourt3, Tanny J. K. van der Reijden4, Sylvain Brisse3, Peterhans van den Broek4 and Lenie Dijkshoorn4

1 Centre of Epidemiology and Microbiology, National Institute of Public Health, Prague, Czech Republic 2 3rd Faculty of Medicine, Charles University in Prague, Prague, Czech Republic 3 Genotyping of Pathogens and Public Health, Institut Pasteur, Paris, France 4 Department of Infectious Diseases, Leiden University Medical Center, Leiden, The Netherlands

* Correspondence address. Centre of Epidemiology and Microbiology, National Institute of Public Health, Srobárova 48, 100 42 Prague 10, Czech Republic. Tel: +420-267082266; Fax: +420-267082538; E-mail: anemec{at}szu.cz

Received 11 February 2008; returned 25 March 2008; revised 11 April 2008; accepted 16 April 2008


   Abstract
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 Materials and methods
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Objectives: The aim of this study was to analyse the emergence of carbapenem resistance among hospital strains of Acinetobacter in the Czech Republic.

Methods: Acinetobacter isolates were collected prospectively in 2005–06 from 19 diagnostic laboratories. They were identified to species level by AFLP, typed using AFLP, pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing, and tested for susceptibility to 14 antimicrobials and for the presence of 20 genes associated with antimicrobial resistance.

Results: A total of 150 Acinetobacter isolates were obtained from 56 intensive care units of 20 hospitals in 15 cities. They were identified as Acinetobacter baumannii (n = 108) or other species. A. baumannii isolates were allocated to EU clone I (n = 5), EU clone II (n = 66) or other, mostly unique genotypes. Two-thirds of the clone II isolates had nearly identical AFLP and PFGE fingerprints. As many as 85% and 88% isolates were susceptible to meropenem and imipenem (≤4 mg/L), respectively. Carbapenem MICs of ≥8 mg/L were found in 23 A. baumannii isolates, of which 20 belonged to clone II. Isolates with blaOXA-58-like (n = 3), blaOXA-24-like (n = 1) or ISAba1 adjacent to blaOXA-51-like (n = 34) had carbapenem MICs of 2 to >16 mg/L, while those without these elements showed MICs of ≤0.5–4 mg/L. Clone II isolates varied in susceptibility to some antibiotics including carbapenems and carried 6–12 resistance genes in 17 combinations.

Conclusions: The emergence of Acinetobacter carbapenem resistance in the Czech Republic is associated with the spread of A. baumannii strains of EU clone II. The variation in susceptibility in these strains is likely to result from both the horizontal spread of resistance genes and differential expression of intrinsic genes.

Keywords: European clonal lineages , AFLP , PCR gene detection , OXA-type carbapenemases


   Introduction
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Bacteria of the genus Acinetobacter, with Acinetobacter baumannii in particular, are notorious for their involvement in nosocomial infections and spread among severely ill patients.1 These organisms are frequently resistant to multiple antimicrobial agents and there are recent reports on strains resistant to virtually all clinically relevant drugs. Extensive genotypic characterization has shown that, within A. baumannii, clusters of highly similar strains occur, which are assumed to represent distinct clonal lineages. Of these, the so-called European (EU) clones I, II and III are widely spread across Europe and include strains that are usually multidrug-resistant (MDR) and associated with outbreaks of hospital infections.1–3

Carbapenem resistance in Acinetobacter spp. has emerged as a significant health problem over the last decade, leaving limited options for antimicrobial therapy.1 This resistance has been attributed to the production of carbapenem-hydrolysing β-lactamases (carbapenemases), although other mechanisms can also be involved, including those that reduce membrane permeability, alter penicillin-binding proteins or expel drugs from the cell.1 Carbapenemases found in Acinetobacter belong to molecular class D (OXA enzymes) or class B (metalloenzymes of IMP- and VIM-type or SIM-1). The OXA carbapenemases of Acinetobacter are divided into four phylogenetic subgroups: acquired enzymes OXA-23-like, OXA-24-like and OXA-58-like, and OXA-51-like enzymes that are intrinsic to A. baumannii. OXA-51-like enzymes are normally expressed at low levels but can be overexpressed as a consequence of the insertion of an ISAba1 sequence upstream of their genes.4,5

Our previous studies showed that resistance of Acinetobacter isolates to carbapenems was rare in the Czech Republic till the early 2000s.3,6 However, in 2003 and 2004, A. baumannii isolates resistant to these antibiotics were received by the National Institute of Public Health (NIPH) in Prague from several hospitals. This observation gave rise to the current study to analyse the emergence of carbapenem resistance among clinical Acinetobacter isolates at the country level. We investigated prospectively collected Acinetobacter isolates for their species and strain diversity, and for susceptibility to carbapenems and the presence of genes linked to carbapenem resistance. Furthermore, to obtain a comprehensive view of the situation, the susceptibility and presence of genes conferring resistance to other clinically important agents were determined.


   Materials and methods
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Results
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Collection of Acinetobacter isolates

Acinetobacter strains were collected prospectively from 19 diagnostic laboratories in the Czech Republic between January 2005 and April 2006. The laboratories were asked to send clinically relevant isolates of Acinetobacter spp. obtained from patients hospitalized at intensive care units (ICUs) with no more than one isolate per patient and 10 isolates per ICU. Isolates sent to the NIPH were confirmed for the genus identity and presumptively identified to species using a set of biochemical tests7 and assessed for susceptibility to 12 antimicrobial agents using disc diffusion (see below). Isolates from the same ICU that were indistinguishable from each other according to phenotype were further typed using ApaI macrorestriction analysis by pulsed-field gel electrophoresis (PFGE). From each group of isolates with a common PFGE profile, sharing phenotypic properties and originating from the same ICU, one isolate was selected for further investigation. Thus, a final set of 150 Acinetobacter isolates remained from a total of 265 isolates received by the NIPH. The 150 isolates were from 56 ICUs of 20 hospitals in 15 cities and were recovered from sputum (n = 69), wounds or pus (n=19), urine (n = 17), blood or intravenous catheters (n = 15) or from other clinical specimens (n = 30).

Genomic fingerprinting and multilocus sequence typing (MLST)

AFLP genomic fingerprinting performed as described8 was used both to identify strains to species and to classify them at the subspecies (clone, strain) level. DNA macrorestriction analysis by PFGE included digestion of agarose plugs containing genomic DNA with ApaI (New England Biolabs; 30 U per plug) for 2 h at 25°C, followed by separation of restriction fragments with a CHEF-DRII device (Bio-Rad) through a 1.2% SeaKem LE agarose gel (Cambrex) in TBE buffer at 14°C for 19 h (pulse times 5–20 s at 6 V/cm). The resulting PFGE fingerprints were compared visually: patterns that differed in the position of more than six bands were designated by different capitals, while those differing in the positions of one to six bands were marked with the same letter followed by different numerals (Figure 1). MLST was based on a sequence analysis of the internal portions of seven housekeeping genes (cpn60, fusA, gltA, pyrG, recA, rplB and rpoB). Details of the MLST scheme including amplification and sequencing primers, allele sequences and STs are available at Institut Pasteur's MLST Web site (www.pasteur.fr/mlst).


Figure 1
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  		Figure 1. Dendrogram of cluster analysis of AFLP fingerprints of 108 A. baumannii isolates in the present study. Clusters of more than two isolates defined at 80% are marked by vertical lines. Numbers following the city name indicate different hospitals in the city; capitals denote different ICUs in the same hospital. Indicated are the numbers of antimicrobial agents to which an isolate was resistant using disc diffusion with 12 antimicrobial agents. Positive PCR results are presented by black boxes. All strains were positive both for ampC-like and blaOXA-51-like, and negative for blaIMP-like, blaVIM-like, blaSIM-1, aacC2 and aacA4. All strains positive for aacC1 were also positive for aadA1. VR 2.5, 3.0 and 3.5 denote three respective variable class 1 integron regions. Positive PCR results for primer combinations ISAba1-blaOXA-51-like and ISAba1-ampC-like indicate the location of ISAba1 in the promotor regions of blaOXA-51-like and ampC-like, respectively. The designations of isolates studied by MLST are underlined. RUH 134 and RUH 875 are the reference strains of EU clones II and I, respectively.2 NT, not tested; blank, negative (except for RUH 134 and RUH 875 for which no data are shown).

 
Susceptibility testing

Resistance to 12 antimicrobial agents that are primarily effective against susceptible A. baumannii strains was determined by disc diffusion following the CLSI guidelines.9 The cut-off values for resistance were adjusted according to the distribution of inhibition zone diameters among A. baumannii strains.3 The agents (content in µg/disc; resistance breakpoint in mm) included gentamicin (10; ≤14), netilmicin (30; ≤14), tobramycin (10; ≤14), amikacin (30; ≤16), ampicillin + sulbactam (10 + 10; ≤14), piperacillin (100; ≤17), ceftazidime (30; ≤17), meropenem (10; ≤15), imipenem (10; ≤15), ofloxacin (5; ≤15), sulfamethoxazole + trimethoprim (23.75 + 1.25; ≤15) and doxycycline (30; ≤15) (Oxoid). MICs were determined by the agar dilution method according to the CLSI guidelines using the CLSI susceptibility and resistance breakpoints.9 Etest MBL strips (AB Biodisk, Solna, Sweden) as well as a synergy test using imipenem- and EDTA-containing discs10 were used to screen for metallo-β-lactamase production.

Gene detection

The presence of the following genes was determined by PCR amplification of: the genes encoding the class D carbapenemases OXA-23-like, OXA-24-like, OXA-51-like and OXA-58-like;11 the genes encoding the metallo-β-lactamases IMP, VIM and SIM-1;12 the genes encoding aminoglycoside-modifying phosphotransferases APH(3')-Ia (aphA1) and APH(3')-VIa (aphA6), acetyltransferases AAC(3)-Ia (aacC1), AAC(3)-IIa (aacC2) and AAC(6')-Ib (aacA4), and nucleotidyltransferases ANT(2'')-Ia (aadB) and ANT(3'')-Ia (aadA1);13 the blaTEM-1-like gene encoding TEM-1-like β-lactamases;14 the ampC-like gene encoding class C β-lactamases intrinsic to A. baumannii;15 the tet(A) and tet(B) genes encoding the respective tetracycline-specific efflux pumps;16 the class 1 integrase gene intI1;13 the adeB and adeR genes encoding the structural and regulatory proteins of the AdeABC efflux system, respectively,8 and the ISAba1 insertion sequence gene.4 To determine the structure of class 1 integron variable regions, PCR mapping and restriction analysis of amplicons obtained by PCR with primers targeting 5' and 3' conserved integron segments were carried out as previously.13 The location of ISAba1 in the upstream region of the chromosomal genes encoding OXA-51-like and AmpC-like β-lactamases was determined according to Turton et al.4 and Ruiz et al.,17 respectively.


   Results
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Species diversity and antimicrobial susceptibility of non-A. baumannii isolates

Using AFLP analysis, the 150 Acinetobacter isolates were identified as A. baumannii (n = 108), genomic sp. 3 (n = 30), genomic sp. 13 TU (n = 8), Acinetobacter calcoaceticus (n = 1), Acinetobacter schindleri (n = 1) or Acinetobacter junii (n = 1). One isolate could not be allocated to any of the known Acinetobacter species. The MICs of imipenem and meropenem for the non-A. baumannii isolates ranged from ≤0.125 to 0.5 mg/L. All non-A. baumannii isolates were fully susceptible to the 12 antimicrobials tested by disc diffusion, except for four isolates of gen. sp. 3, which were resistant to gentamicin and tobramycin and/or ofloxacin, and for one gen. sp. 13 TU isolate that was resistant to gentamicin and netilmicin. The non-A. baumannii isolates were negative for all resistance genes except for three aminoglycoside resistant isolates of gen. sp. 3, which were PCR-positive for aadB, and for all gen. sp. 13 TU isolates, which were positive for at least one of the genes associated with the AdeABC efflux system.

Population structure of A. baumannii isolates

The results of cluster analysis of the AFLP fingerprinting of 108 A. baumannii isolates are shown in Figure 1. Using a cut-off of 80% (which corresponds to the approximate grouping level of strains of the same clone3,18), the isolates were classified into one major cluster with 66 isolates, two clusters with 5 isolates each, 5 pairs and 22 single isolates. The major cluster and one of the small clusters corresponded to EU clones II and I, respectively, while none of the strains was found to group with strains of EU clone III (data not shown). Most clone II isolates yielded identical or highly similar PFGE patterns (Figure 2) and 45 (68%) of the clone II isolates clustered together according to their AFLP patterns at ≥90% (Figure 1), indicating that they were genetically related at the subclonal level.18 MLST was performed for seven clone II isolates that differed from each other in PFGE/AFLP patterns or/and in resistance phenotype (Figure 1). Six of them had ST2 (2-2-2-2-2-2-2), which seems to be the typical ST of EU clone II (L. Diancourt, V. Passet, A. Nemec, L. Dijkshoorn and S. Brisse, unpublished results), while NIPH 2578 yielded ST47 (2-13-2-2-2-2-2), a single locus variant of ST2.


Figure 2
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  		Figure 2. Examples of the ApaI macrorestriction patterns of A. baumannii isolates. Strains are indicated by the numbers above the lanes: 1, NIPH 2571; 2, NIPH 2895; 3, NIPH 2700; 4, NIPH 2519; 5, NIPH 2981; 6, NIPH 2867; 7, NIPH 2982; 8, NIPH 2990; 9, NIPH 2874; 10, NIPH 2610; 11, NIPH 2578; 12, NIPH 2713; 13, NIPH 2605; 14, NIPH 2666; 15, NIPH 2988; 16, NIPH 2706. Lane M, molecular size markers (48.5 kb ladder).

 
Resistance of A. baumannii isolates to carbapenems

According to MICs, 85 (79%) A. baumannii isolates were susceptible (MIC ≤ 4 mg/L) to both imipenem and meropenem, while 23 (21%) isolates were either intermediate (8 mg/L) or resistant (≥16 mg/L) to at least one carbapenem (Figure 1). Out of the 85 susceptible isolates, 40 had MICs ≤ 0.5 mg/L for both carbapenems, but 45 showed reduced susceptibility (MICs 1.0–4.0 mg/L) to at least one carbapenem. All 68 isolates with carbapenem MICs ≥ 1 mg/L were also resistant to at least one other antimicrobial agent and belonged to clone II, clone I or three unique AFLP genotypes. None of the isolates was PCR-positive for the genes encoding metallo-β-lactamase IMP-like, VIM-like or SIM-1. In addition, no metallo-β-lactamase activity was detected in any of 16 isolates with imipenem MICs ≥ 16 mg/L, using Etest MBL and a double-disc synergy test.

The results of PCR detection of OXA-type carbapenemases are shown in Table 1. All isolates were positive for blaOXA-51-like, three were positive for blaOXA-58-like and one for blaOXA-24-like. Using the ISAba1 forward primer and the OXA-51-like gene reverse primer, 34 isolates yielded a PCR amplicon of ~1.2 kb, which indicates the location of ISAba1 in the upstream region of the OXA-51-like gene of these isolates.4 The remaining 74 isolates showed no PCR product, although 37 of them were positive for the ISAba1 sequence (Figure 1). The isolates with blaOXA-58-like, blaOXA-24-like or/and ISAba1 adjacent to blaOXA-51-like had carbapenem MICs of 2 to >16 mg/L (MIC50 8 mg/L and MIC90≥16 mg/L), while those without evidence of these genetic structures showed MICs of ≤0.5–4 mg/L (MIC50 0.5 mg/L and MIC90 2 mg/L) (Table 1).


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  		Table 1. Relationship between carbapenem MICs and the presence of genes associated with the decreased susceptibility or resistance to carbapenems in 108 A. baumannii isolates

 
Resistance of A. baumannii isolates to non-carbapenem agents

According to disc diffusion, 33 isolates were fully susceptible to all 12 antimicrobials, while 75 isolates showed resistance to ≥3 agents (Figure 1). The MICs of imipenem and meropenem for the 33 fully susceptible isolates ranged from ≤0.125 to 0.5 mg/L, while those of the 75 isolates resistant to ≥3 agents were between 0.25 and >16 mg/L. Out of these 75 isolates, 5 and 66 belonged to clones I and II, respectively.

There was a good correlation between the presence of the genes associated with resistance to non-carbapenem agents and MIC of these agents. All isolates positive for aphA6 (n = 24), aacC1 (n = 60) and aadB (n = 1) were resistant to or, in a few cases, intermediate to amikacin, gentamicin and tobramycin + gentamicin, respectively. All 61 tet(B)-positive isolates showed doxycycline MICs ≥ 32 mg/L, while the tet(B)-negative isolates had MICs of ≤8 mg/L. The MICs of ampicillin/sulbactam of ≥16/8 mg/L were found only in isolates harbouring the blaTEM-1-like gene (n = 53), while all but one isolate with MICs of ≤8/4 mg/L were blaTEM-1-like-negative. Using the ISAba1 forward primer and the ampC-like gene reverse primer, 68 isolates yielded a PCR product of ~750 bp, which indicates the presence of ISAba1 in the promoter region of the ampC-like gene.17 Ceftazidime MICs against these 68 isolates were ≥32 mg/L, whereas those against 40 isolates without ISAba1 in the promoter region were ≤8 mg/L (Figure 1).

Both the adeB and adeS genes that are associated with the AdeABC efflux system were detected in 96 isolates, while seven isolates were negative for both genes and five isolates were positive for only one of the genes. Only the isolates positive for both genes showed increased netilmicin MICs (≥4 mg/L) (Figure 1), which may indicate up-regulation of the efflux.8 The class 1 integrase gene was found in 60 isolates (belonging to either clone I or II) and was unequivocally associated with the aacC1 and aadA1 genes and with PCR products obtained with the primers aimed to amplify variable integron regions. Three different variable regions with the respective sizes of 2.5, 3.0 and 3.5 kb were detected (Figure 1), and restriction analysis and PCR mapping of these structures revealed that they contained the same genes in the same order [aacC1-(orfX)1-3-orfX'-aadA1], differing only in the number of orfX copies.13

Heterogeneity of resistance phenotypes and genotypes within EU clone II isolates

The susceptibility rates of clone II isolates (n = 66) according to the MIC and the CLSI breakpoints9 were as follows (% susceptible isolates): imipenem (76), meropenem (70), ceftazidime (5), piperacillin (0), ampicillin + sulbactam (23), gentamicin (14), tobramycin (80), amikacin (68), netilmicin (12), sulfamethoxazole + trimethoprim (12), doxycycline (6), ciprofloxacin (0) and colistin (98). As many as 21 different resistance profiles were identified among these isolates and a similar heterogeneity was revealed at the gene level. The isolates were positive for the tested genes as follows (% PCR-positive isolates): blaTEM-1-like (80), tet(B) (92), tet(A) (5), aacC1 (83), aphA1 (80), aphA6 (30), blaOXA-58-like (3), intI1 (83) and ISAba1 (95). The integron variable regions of 2.5, 3 and 3.5 kb were found in 8, 46 and 1 isolate, respectively. Individual strains carried from 6 to 12 resistance-associated genes in 17 different combinations. Some isolates with the same PFGE patterns and obtained from the same ICU (e.g. NIPH 2893 and NIPH 2873) differed in the combination of resistance genes, whereas other isolates indistinguishable from each other by genotype and phenotype originated from different cities (e.g. NIPH 2601 and NIPH 2991) (Figure 1).


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Of the 150 Acinetobacter isolates in the present study, 146 (97%) were identified as A. baumannii (72%) or other genomic species of the Acinetobacter calcoaceticus–A. baumannii complex (25%). Nearly all strains resistant to multiple antimicrobial agents belonged to A. baumannii, and the vast majority of these MDR isolates were allocated to EU clone I or II. These results are consistent with those of our retrospective study on the A. calcoaceticus–A. baumannii complex isolates collected in Czech hospitals in 1991–97.3,6 However, whereas in the present study, 5 and 66 isolates were allocated to clones I and II, respectively, 39 and 9 isolates from the 1990s were classified into the respective clones. Even though the results of the two studies are not directly comparable as the strain inclusion criteria differed, the data suggest a shift in the recent A. baumannii population towards clone II.

In the present study, 90% of the isolates with decreased susceptibility or resistance to carbapenems (≥1 mg/L) and 83% of those resistant to one or more non-carbapenem agents belonged to EU clone II. The wide spread of clone II may have resulted from its selective advantage in the antibiotic-rich hospital environment and could further be facilitated by the absence of effective measures to prevent the transmission of MDR microorganisms, a problem commonly encountered in Czech hospitals. Other European studies have also recently reported on the spread of strains of clone II and on the association of carbapenem resistance with these strains.18,19 EU clone II thus seems to be particularly successful in its spread in European countries and it is conceivable that the ability of clone II strains to develop carbapenem resistance has substantially contributed to this spread.

Neither metallo-β-lactamase activity nor the genes encoding these enzymes were detected in any of the studied isolates. The genes encoding OXA-23-type carbapenemases were not found either, and those for OXA-24-type or OXA-58-like enzymes were identified only in four carbapenem-resistant isolates. These data indicate that carbapenem resistance in Czech Acinetobacter strains does not result from the presence of acquired carbapenemases. It has recently been shown that the insertion of an ISAba1 sequence upstream of the chromosomal genes encoding OXA-51-type β-lactamases can increase the expression of these genes, which are normally expressed at a low level, and result in carbapenem resistance.4,5 In the present study, ISAba1 was located in the promoter region of the blaOXA-51-like gene in half of the clone II isolates and most of these isolates had higher carbapenem MICs when compared with those devoid of ISAba1 in the promoter region. However, similar MICs (2–4 mg/L) were obtained for some isolates regardless of the presence or absence of ISAba1 adjacent to the blaOXA-51-like gene. Carbapenem resistance thus seems to result from the overexpression of the blaOXA-51-like gene, but other mechanisms are probably also involved.

In conclusion, the present study shows that the emergence of Acinetobacter resistance to carbapenems in the Czech Republic was associated with the spread of MDR A. baumannii strains belonging to EU clone II. Carbapenem resistance of these strains is likely to result from up-regulation of the chromosomal OXA-51-like β-lactamase rather than from acquisition of other OXA- or metallo-β-lactamases, but the precise molecular basis of this resistance remains to be resolved. Although the high genomic similarity of most clone II isolates suggests that they represent a recent lineage within the clone, these isolates show a striking variation in the phenotype and genotype of resistance to several clinically important antibiotics. This variation is likely to result from a relatively frequent horizontal acquisition and/or loss of resistance genes as well as from differential expression of intrinsic genes. This striking genetic versatility may endow EU clone II with the ability to develop resistance to nearly all clinically relevant agents.


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The study was supported by grant NR 8554-3 of the Internal Grant Agency of the Ministry of Health of the Czech Republic awarded to A. N.


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None to declare.


   Acknowledgements
 
We are grateful to all colleagues who generously provided strains included in this study. We thank B. van Strijen, M. van den Barselaar and J. Smísek for their excellent technical assistance. J. Hrabák is acknowledged for the determination of metallo-β-lactamase activity.


   References
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1 Dijkshoorn L, Nemec A, Seifert H. An increasing threat in the hospital: multidrug resistant Acinetobacter baumannii. Nat Rev Microbiol (2007) 5:939–51.[CrossRef][Web of Science][Medline]

2 Dijkshoorn L, Aucken HM, Gerner-Smidt P, et al. Comparison of outbreak and nonoutbreak Acinetobacter baumannii strains by genotypic and phenotypic methods. J Clin Microbiol (1996) 34:1519–25.[Abstract]

3 Nemec A, Dijkshoorn L, van der Reijden TJK. Long-term predominance of two pan-European clones among multi-resistant Acinetobacter baumannii strains in the Czech Republic. J Med Microbiol (2004) 53:147–53.[Abstract/Free Full Text]

4 Turton JF, Ward ME, Woodford N, et al. The role of ISAba1 in expression of OXA carbapenemase genes in Acinetobacter baumannii. FEMS Microbiol Lett (2006) 258:72–7.[CrossRef][Web of Science][Medline]

5 Hu WS, Yao SM, Fung CP, et al. An OXA-66/OXA-51-like carbapenemase and possibly an efflux pump are associated with resistance to imipenem in Acinetobacter baumannii. Antimicrob Agents Chemother (2007) 51:3844–52.[Abstract/Free Full Text]

6 Nemec A, Janda L, Melter O, et al. Genotypic and phenotypic similarity of multiresistant Acinetobacter baumannii isolates in the Czech Republic. J Med Microbiol (1999) 48:287–96.[Abstract/Free Full Text]

7 Nemec A, Dijkshoorn L, Jezek P. Recognition of two novel phenons of the genus Acinetobacter among non-glucose-acidifying isolates from human specimens. J Clin Microbiol (2000) 38:3937–41.[Abstract/Free Full Text]

8 Nemec A, Maixnerová M, van der Reijden TJK, et al. Relationship between the AdeABC efflux system gene content, netilmicin susceptibility and multidrug resistance in a genotypically diverse collection of Acinetobacter baumannii isolates. J Antimicrob Chemother (2007) 60:483–9.[Abstract/Free Full Text]

9 Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing: Fifteenth Informational Supplement M100-S15 (2005) Wayne, PA, USA: CLSI.

10 Arakawa Y, Shibata N, Shibayama K, et al. Convenient test for screening metallo-β-lactamase-producing Gram-negative bacteria by using thiol compounds. J Clin Microbiol (2000) 38:40–3.[Abstract/Free Full Text]

11 Woodford N, Ellington MJ, Coelho JM, et al. Multiplex PCR for genes encoding prevalent OXA carbapenemases in Acinetobacter spp. Int J Antimicrob Agents (2006) 27:351–3.[CrossRef][Web of Science][Medline]

12 Ellington MJ, Kistler J, Livermore DM, et al. Multiplex PCR for rapid detection of genes encoding acquired metallo-β-lactamases. J Antimicrob Chemother (2007) 59:321–2.[Free Full Text]

13 Nemec A, Dolzani L, Brisse S, et al. Diversity of aminoglycoside resistance genes and their association with class 1 integrons among strains of pan-European Acinetobacter baumannii clones. J Med Microbiol (2004) 53:1233–40.[Abstract/Free Full Text]

14 Bou G, Oliver A, Martínez-Beltrán J. OXA-24 a novel class D β-lactamase with carbapenemase activity in an Acinetobacter baumannii clinical strain. Antimicrob Agents Chemother (2000) 44:1556–61.[Abstract/Free Full Text]

15 Bou G, Martínez-Beltrán J. Cloning, nucleotide sequencing, and analysis of the gene encoding an AmpC β-lactamase in Acinetobacter baumannii. Antimicrob Agents Chemother (2000) 44:428–32.[Abstract/Free Full Text]

16 Huys G, Cnockaert M, Vaneechoutte M, et al. Distribution of tetracycline resistance genes in genotypically related and unrelated multiresistant Acinetobacter baumannii strains from different European hospitals. Res Microbiol (2005) 156:348–55.[Medline]

17 Ruiz M, Marti S, Fernandez-Cuenca F, et al. Prevalence of ISAba1 in epidemiologically unrelated Acinetobacter baumannii clinical isolates. FEMS Microbiol Lett (2007) 274:63–6.[CrossRef][Web of Science][Medline]

18 Da Silva G, Dijkshoorn L, van der Reijden T, et al. Identification of widespread, closely related Acinetobacter baumannii isolates in Portugal as a subgroup of European clone II. Clin Microbiol Infect (2007) 13:190–5.[Web of Science][Medline]

19 Turton JF, Gabriel SN, Valderrey C, et al. Use of sequence-based typing and multiplex PCR to identify clonal lineages of outbreak strains of Acinetobacter baumannii. Clin Microbiol Infect (2007) 13:807–15.[CrossRef][Web of Science][Medline]


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