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Antimicrobial Agents and Chemotherapy, April 2007, p. 1530-1533, Vol. 51, No. 4
0066-4804/07/$08.00+0 doi:10.1128/AAC.01132-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
Genetics and Expression of the Carbapenem-Hydrolyzing Oxacillinase Gene blaOXA-23 in Acinetobacter baumannii
Stéphane Corvec,1,2
Laurent Poirel,1
Thierry Naas,1
Henri Drugeon,2 and
Patrice Nordmann1*
Service de Bactériologie-Virologie, Hôpital de Bicêtre, Assistance Publique/Hôpitaux de Paris, Faculté de Médecine Paris-Sud, 94275 K Bicêtre,1
Laboratoire de Bactériologie-Virologie, Hygiène Hospitalière, CHU, Nantes, France2
Received 8 September 2006/
Returned for modification 4 November 2006/
Accepted 27 December 2006

ABSTRACT
The genetic structures surrounding the plasmid-carried
blaOXA-23 oxacillinase gene, encoding resistance to carbapenems, were
studied in
Acinetobacter baumannii. IS
Aba1 and the novel element
IS
Aba4 were detected upstream of the
blaOXA-23 gene, providing
promoter sequences for its expression. These insertion elements
were likely involved in transposition processes at the origin
of acquisition of this ß-lactamase gene.

TEXT
Acinetobacter baumannii is a typical opportunistic pathogen,
often involved in nosocomial outbreaks, for which resistance
to carbapenems is increasingly reported (
23) and may be linked
to the production of Ambler class B metallo-ß-lactamases
(
30) but also to the production of carbapenem-hydrolyzing class
D ß-lactamases (CHDLs) (
18,
31). Although integrons
are associated with many metallo-ß-lactamase or oxacillinase
genes, they are likely not the genetic vehicles for acquisition
of CHDL genes since these genes are not in the form of gene
cassettes (
10,
22,
23). Three main acquired CHDL gene clusters
have been described for
A. baumannii, namely, the
blaOXA-23-,
blaOXA-24-, and
blaOXA-58-like genes, whereas the
blaOXA-51 gene cluster is naturally occurring and chromosomally located
in
A. baumannii (
5,
6,
11,
16,
23,
29). The
blaOXA-23 gene has
been identified worldwide in
A. baumannii (
4,
8,
9,
13,
15,
27) and in
Proteus mirabilis in France (
3). Whereas the
blaOXA-24-like
genes have been identified as chromosomally encoded, the
blaOXA-23 and
blaOXA-58 genes are mostly found on plasmids (
23). The acquisition
of
blaOXA-58 in an
A. baumannii isolate from France has been
associated with a homologous recombination process (
21,
22).
ISAba1 has been found upstream of blaOXA-58, blaOXA-51-like, and blaampC genes in A. baumannii and is involved in their expression (7, 12, 23, 26, 28). ISAba1 belongs to the IS4 family of insertion sequences, possesses two 16-bp imperfect inverted repeats (IRs), and generates a 9-bp target site duplication upon transposition. Its transposase is made of two open reading frames, encoding 189 and 178 amino acids, leading to a functional protein when a frameshift occurs during the translation process (12).
The aim of this study was to analyze the genetics of acquisition and expression of the blaOXA-23 gene in unrelated A. baumannii isolates recovered from different countries (Table 1). Genes coding for CHDLs were searched for by PCR, using primers specific for the blaOXA-23-like, blaOXA-24-like, and blaOXA-58 genes (15). Two blaOXA-23-positive A. baumannii isolates (Ab13 and Ab14) resistant to all ß-lactams, including carbapenems, were studied in detail (Table 1). Genetic structures surrounding the blaOXA-23 gene in A. baumannii Ab13 and Ab14 were cloned by restricting total DNA by BamHI or SacI, ligating it into BamHI- or SacI-restricted plasmid pBK-CMV, and transforming the recombinant plasmids into Escherichia coli DH10B, as described previously (11). Recombinant plasmids were selected on Trypticase soy agar plates containing amoxicillin (50 µg/ml) and kanamycin (30 µg/ml). The cloned DNA fragments of several recombinant plasmids (pAB13B, pAB13S, and pAB14B) were sequenced and analyzed as described previously (21).
Recombinant plasmids pAB13B and pAB13S, obtained from
A. baumannii Ab13, revealed that
blaOXA-23 was bracketed by two copies of
an identical IS
Aba1 element that were in opposite orientations
(Fig.
1). Detailed DNA sequence analysis revealed a 9-bp target
site duplication at the inverted repeat right (IRR) extremities
of the two IS
Aba1 elements, suggesting that both copies formed
a putative composite transposon, Tn
2006, likely at the origin
of
blaOXA-23 acquisition (Fig.
1). The Tn
2006 insertion occurred
inside a gene encoding a putative sulfonamide resistance protein
sharing 89% amino acid identity with a protein identified in
A. baumannii AYE (accession no.
CAJ31116). Sequencing of the
internal sequence of that transposon revealed a 2,445-bp sequence
containing, in addition to the
blaOXA-23 gene, two other genes,
encoding part of a putative AAA ATPase (83% amino acid identity
with that of
Acinetobacter baylyi ADP-1 [
2] [accession no.
YP_046025]
but lacking the first 70 N-terminal amino acids) and part of
a putative DEAD helicase sharing 69% identity with that of
Ralstonia solanacearum but lacking its 493-amino-acid C-terminal extremity
(accession no.
ZP_00943964) (Fig.
1). These two proteins were
truncated at the exact same position, suggesting that a recombination
event had occurred. Even though IS
Aba1 has been shown to be
very prevalent in
A. baumannii and might be "customized" for
that species (
25), this is the first description of an IS
Aba1-based
putative composite transposon.
Sequencing of the recombinant plasmid pAB14B obtained from isolate
Ab14 identified a novel IS
Aba4 element upstream of the
blaOXA-23 gene. IS
Aba4 belongs to the IS
982 family, is 975 bp long, possesses
two 18-bp IRs, and encodes a 292-amino-acid putative transposase.
No target site duplication was observed on either end of IS
Aba4.
PCR mapping with different sets of primers did not detect any
extra copy of IS
Aba4 downstream of the
blaOXA-23 gene. Detailed
analysis of sequences located downstream of the
blaOXA-23 gene
identified a 1,497-bp sequence that was identical to that identified
in Tn
2006, with the same gene encoding a putative AAA ATPase.
This gene was truncated at its 5' extremity, leading to a protein
lacking the first 108 N-terminal amino acids. Detailed analysis
of the site of truncation identified a 7-bp sequence with an
A+T-rich content (TAATATA) that was also identified at the extremity
of the IRR of IS
Aba4 (Fig.
1). This feature was very likely
the signature of a transposition process mediated by IS
Aba4 that occurred at the origin of acquisition of the
blaOXA-23 gene. This potential transposon, termed Tn
2007, was 2,471 bp
long and included IS
Aba4 and
blaOXA-23. The IS
Aba4-mediated
mobilization process likely corresponded to a one-ended transposition
mechanism, in contrast to what has been observed with IS
Ecp1,
which during its mobilization process uses a wide range of DNA
sequences as IRRs, which are, however, not absolutely random
(
19,
20). In this case, the sequence identified at the right
end of Tn
2007 did not exhibit homology with the IRs of IS
Aba4.
Alternatively, it could be hypothesized that a second copy of
IS
Aba4 in a likely IS
Aba4-made composite transposon might have
been lost by excision. At the left extremity of Tn
2007, an open
reading frame (ORF) (Fig.
1B,
orf1) encoding 62 N-terminal amino
acids sharing 68% amino acid identity with a plasmid maintenance
killer protein of
Photorabdus luminescens (accession no.
NP_928205)
was identified. At the right extremity of Tn
2007, an ORF was
identified that encoded 309 amino acids that shared 51% amino
acid identity with a MobA/MobL mobilization protein found on
the pP plasmid of
Salmonella enterica serovar Enteritidis (accession
no.
NP_604396).
Using 5' rapid amplification of cDNA ends-PCR (22), the sites of initiation of transcription of the blaOXA-23 gene were mapped in both A. baumannii isolates Ab13 and Ab14, possessing ISAba1 and ISAba4, respectively, upstream of blaOXA-23. In Ab13, the ISAba1 element was located 25 bp upstream of blaOXA-23, and the +1 transcription start was identified 60 bp upstream of the start codon of the blaOXA-23 gene, located inside the ISAba1 element. The corresponding promoter, made of a 35 sequence (TTAGAA) and separated by 16 bp from the 10 sequence (TTATTT), was identical to that identified previously in ISAba1, located at the origin of overexpression of the naturally occurring blaampC and blaOXA-51-like genes in A. baumannii (7, 12) (Fig. 2). The ISAba4 element was located 25 bp upstream of blaOXA-23 in Ab14, and the +1 transcription start was located 31 bp upstream of the start codon of the blaOXA-23 gene, just inside the left IRs of ISAba4. The 35 sequence (TAACTA) and a 10 sequence (TTTCTT) separated by 17 bp acted as promoter sequences (Fig. 2).
Subsequent PCR mapping was performed to evaluate the presence
of these transposons in 11
blaOXA-23-positive
A. baumannii isolates,
using the primers detailed in Table
2. Pulsed-field gel electrophoresis
analysis revealed that the 12 isolates studied were unrelated
(data not shown). In these isolates, the
blaOXA-23 gene was
plasmid borne, according to the results of the Kieser technique
(
14) and of hybridization experiments (data not shown). Tn
2006-like
elements were identified in 10 of 12 isolates, whereas the 2
others harbored the Tn
2007 structure.
IS
Aba1, which enhances the expression of several unrelated resistance
genes, seems to be quite an important factor of genetic plasticity
in
A. baumannii (
25). The large distribution and high copy number
of IS
Aba1 may facilitate the creation of composite transposons
(
24). On the other hand, a single copy of IS
Aba4 might mobilize
sequences located at its right-end extremity in what is considered
a one-ended transposition process (
1). Further in vitro experiments
are now required to establish under which conditions and at
what frequency these transposition mechanisms occur. Interestingly,
we have identified sequences located just downstream of the
blaOXA-23 gene possessing significant homology with those of
A. baylyi. Together with the fact that IS
Aba1 could be considered
widespread in the
Acinetobacter genus, this could suggest that
blaOXA-23 originated from an
Acinetobacter-like species. It
could also be hypothesized that the Tn
2006 putative composite
transposon might have been formed in
A. baumannii after a previous
acquisition process not related to IS
Aba1.
Nucleotide sequence accession numbers.
The nucleotide sequences of the insertion elements reported in this paper have been submitted to the IS Finder website (http://www-is.biotoul.fr). The entire sequences identified and described in this study have been assigned accession no. EF127491, for Tn2006, and EF059914, for Tn2007.

ACKNOWLEDGMENTS
This work was funded by a grant from the Ministère de
l'Education Nationale et de la Recherche (UPRES-EA3539), Université
Paris XI, Paris, France, and mostly by a grant from the European
community (LSHM-CT-2005-018705). L.P. is a researcher from INSERM,
France.
We thank A. Carattoli for precious advice on plasmid feature analysis. We also thank I. Podglajen and D. Colak for their gifts of isolates DOS and TN850, respectively.

FOOTNOTES
* Corresponding author. Mailing address: Service de Bactériologie-Virologie, Hôpital de Bicêtre, 78 rue du Général Leclerc, 94275 Le Kremlin-Bicêtre, France. Phone: 33-1-45-21-36-32. Fax: 33-1-45-21-63-40. E-mail:
nordmann.patrice{at}bct.aphp.fr 
Published ahead of print on 12 January 2007. 

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Antimicrobial Agents and Chemotherapy, April 2007, p. 1530-1533, Vol. 51, No. 4
0066-4804/07/$08.00+0 doi:10.1128/AAC.01132-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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