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Antimicrobial Agents and Chemotherapy, August 2008, p. 2955-2958, Vol. 52, No. 8
0066-4804/08/$08.00+0 doi:10.1128/AAC.00072-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Characterization of the Carbapenem-Hydrolyzing Oxacillinase Oxa-58 in an Acinetobacter Genospecies 3 Clinical Isolate
Sara Marti,1
Javier Sánchez-Céspedes,1
M. Dolores Blasco,2
Marc Ruiz,1
Paula Espinal,1
Verónica Alba,1
Felipe Fernández-Cuenca,3
Alvaro Pascual,3 and
Jordi Vila1*
Servei de Microbiologia, Centre de Diagnóstic Biomèdic, Hospital Clínic, Facultat de Medicina, Universitat de Barcelona, Barcelona, Spain,1
Departament de Microbiologia i Ecologia, Universitat de Valencia, Valencia, Spain,2
Departamento de Microbiología, Facultad de Medicina, Universidad de Sevilla, Sevilla, Spain3
Received 17 January 2008/
Returned for modification 20 March 2008/
Accepted 16 May 2008

ABSTRACT
Based on imipenem resistance in an
Acinetobacter genospecies
3 clinical isolate, we were able to identify, for the first
time in this genomic species, a plasmid-encoded
blaOXA-58 gene
that was 100% homologous to the same gene in
Acinetobacter baumannii.

TEXT
Since 1986 members of the genus
Acinetobacter are determined
by DNA-DNA hybridization. Genospecies 1 (
Acinetobacter calcoaceticus),
2 (
A. baumannii), 3, and 13TU are genetically closely related
and are commonly known as the
A. calcoaceticus-A. baumannii complex. With the exception of genospecies 1, the other members
of this complex have been involved in nosocomial infections
and have the ability to spread in hospitals (
3,
9,
19,
23,
25,
26). Treatment of these nosocomial infections is becoming a
problem because increasing resistance to antibiotics, especially
in the case of
A. baumannii. In the last decade, carbapenem
resistance in
Acinetobacter spp. has been reported worldwide
(
3,
16,
23), mostly associated with the synthesis of carbapenem-hydrolyzing
β-lactamases, reduced outer membrane permeability and,
occasionally, modification of penicillin-binding proteins (
7,
16,
22,
27). The most prevalent carbapenemases in
Acinetobacter spp. are the carbapenem-hydrolyzing class D β-lactamases,
which are divided into four phylogenetic subgroups: OXA-23,
OXA-24, and OXA-58 with all their variants and the OXA-51 family,
which is intrinsic to
A. baumannii (
16,
27).
OXA-58 confers reduced susceptibility to carbapenems, but it produces high-level resistance to carbapenems when additional efflux mechanisms are expressed (12, 15). It was first identified in France in 2003 and, at present, is found worldwide in A. baumannii isolates (13, 16, 20, 21), as well as in A. junii isolates from Romania and Australia (12, 14).
The clinical isolates Ac057 (Acinetobacter sp. strain G3) and Ac058 (A. baumannii) were obtained from the same hospital in November 2000 and were identified by amplified ribosomal DNA restriction analysis (ARDRA) (6, 24). The epidemiological difference was corroborated by pulsed-field gel electrophoresis (PFGE) with ApaI (Promega, Madrid, Spain) under conditions described elsewhere (11).
Antimicrobial susceptibility analysis was performed by Etest according to the manufacturer's instructions (AB Biodisk, Sölna, Sweden) and determined that both strains had an imipenem MIC of >32 µg/ml (Table 1) . The breakpoints for imipenem were those proposed by the Clinical and Laboratory Standards Institute (5).
PCR analysis with specific primers for all class D β-lactamases
(Table
2) determined the presence of the
blaOXA-58 gene in both
strains;
A. baumannii strain Ac058 was also positive for the
blaOXA-51 gene. Additional primers were designed at the beginning
and end of the
blaOXA-58 gene (Table
2) to amplify the whole
fragment. This gene presented 100% homology with the
blaOXA-58 gene from
A. baumannii listed in GenBank.
Plasmid DNA identification was attempted by using genomic mapping
with I-CeuI (
10) and by digestion with the S1 nuclease (
1).
I-CeuI cuts a 26-bp site in the
rrl gene (23S rRNA), shearing
the bacterial genome into an analyzable number of fragments
(
10). The S1 nuclease transforms supercoiled plasmids into linear
molecules (
1). Digested genomic DNA and plasmids were sepa-rated
by PFGE (Fig.
1). Probes were marked with the PCR DIG probe
synthesis kit (Roche, Barcelona, Spain), and detection was performed
with anti-digoxigenin antibody conjugated to alkaline phosphatase
and the color substrates NBT/BCIP (Roche) according to the manufacturer's
instructions. In Fig.
1a, the most intense bands would represent
fragments of genomic DNA, and the faded bands represent plasmid
DNA. Hybridization with probes for the
blaOXA-58 gene (Fig.
1c) and the 23S rRNA gene (Fig.
1b) suggest that in both isolates
the
blaOXA-58 gene is present in a plasmid. With the S1 nuclease
(Fig.
2a), the highest band would be the genomic DNA and the
remaining bands would be linear plasmids. Hybridization with
the probe for the OXA-58 gene (Fig.
2c) gives the same pattern
as obtained with I-CeuI. The hybridization signal with the probe
for the 23S rRNA gene was only observed in the undigested genomic
DNA (Fig.
2b). Although conjugation experiments did not show
any plasmid transfer between strains, Southern blot analysis
suggests that the
blaOXA-58 gene could be present in a plasmid
in both strains, and the plasmid from
A. baumannii is possibly
different from the plasmid in the
Acinetobacter genospecies
3 isolate.
In order to determine the genetic structure surrounding of the
blaOXA-58 gene, DNA from both isolates was digested with MspI
"C*CGG" (Promega). The fragments obtained were autoligated overnight
at 16°C using a T4 DNA ligase (Promega). The fragment of
DNA containing the
blaOXA-58 gene was used as a template for
a PCR with inverse primers designed from the
blaOXA-58 gene
sequence (Table
2). All PCR fragments were sequenced using a
BigDye Terminator v3.1 cycle sequencing kit (Applied Biosystems,
Warrington, United Kingdom) and analyzed in an automatic DNA
sequencer (3100 Genetic Analyzer; Applied Biosystems).
Analysis of the genetic surrounding confirms that both plasmids are different (Fig. 3). In Ac057, the blaOXA-58 gene is surrounded by two copies of the Insertion Sequence ISAba3; the copy downstream has the same direction as the blaOXA-58 gene, and the upstream copy has the opposite direction (Fig. 3). This structure has already been described in A. baumannii by Poirel et al. (17, 18).
The presence of the OXA-58 alone does not account for the level
of resistance to imipenem of these isolates (MIC of >32 µg/ml).
Further work is needed to determine whether additional efflux
pumps or porin modifications are involved.
A. baumannii is certainly the most frequently isolated species in hospitals and also the microorganism of greatest clinical interest in this genus. However, Acinetobacter genospecies 3 and 13 are also nosocomial pathogens, and they should be considered in hospital settings. Previous studies in Acinetobacter genospecies 3 have revealed the presence of AmpC (2), IMP-4 (4), and blaVIM-2 (28). In addition to these previously described enzymes, we report here, for the first time, the presence of the blaOXA-58 in this microorganism. The main reason for the lack of interest on non-baumannii Acinetobacter isolates is probably their susceptibility to antimicrobial agents (9). However, as suggested by Horrevorts et al. (8), the clinical significance of genospecies 3 can be underestimated because the resistant strains can be erroneously classified as A. baumannii.
Nucleotide sequence accession number.
The GenBank accession number for the blaOXA-58 in Acinetobacter genospecies 3 is EU642594.

ACKNOWLEDGMENTS
This study has been supported by grant SGR050444 from the Departament
d'Universitats, Recerca I Societat de la Informació de
la Generalitat de Catalunya, Spain, and by the Spanish Ministry
of Health (FIS 04/0068 to J.V.). This study was supported by
the Ministerio de Sanidad y Consumo, Instituto de Salud Carlos
III, Spanish Network for the Research in Infectious Diseases
(REIPI RD06/0008), as well.

FOOTNOTES
* Corresponding author. Mailing address: Servei de Microbiologia, Centre de Diagnóstic Biomèdic, Hospital Clínic, Facultat de Medicina, Universitat de Barcelona, Barcelona, Spain. Phone: 34 93 227 55 22. Fax: 34 93 227 93 72. E-mail:
jvila{at}ub.edu 
Published ahead of print on 27 May 2008. 

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Antimicrobial Agents and Chemotherapy, August 2008, p. 2955-2958, Vol. 52, No. 8
0066-4804/08/$08.00+0 doi:10.1128/AAC.00072-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
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