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Antimicrobial Agents and Chemotherapy, September 2005, p. 3940-3943, Vol. 49, No. 9
0066-4804/05/$08.00+0 doi:10.1128/AAC.49.9.3940-3943.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Department of Oral Biology, Dental Faculty, University of Oslo, Oslo, Norway,1 Laboratoire Mixte Médecine-Odontologie, Ecologie Microbienne et Résistance aux Antibiotiques, Hopital L'Archet 2, Centre Hospitalier Universitaire de Nice, Nice, France,2 Department of Airborne Infections, Norwegian Institute of Public Health, Oslo, Norway3
Received 10 January 2005/ Returned for modification 7 April 2005/ Accepted 5 July 2005
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Twenty-five ß-lactamase-producing strains of Capnocytophaga spp. were included (Table 1). DNA was isolated by the cetyltrimethylammonium bromide procedure (20). Random amplified polymorphic DNA (RAPD) reactions were performed using a Ready-To-Go RAPD analysis kit (Amersham Biosciences, Cleveland, OH). Pulsed-field gel electrophoresis (PFGE) (6), antimicrobial susceptibility testing, PCRs, DNA sequencing (8, 9, 22), isoelectric focusing (18), and Southern hybridization (21) were performed as described previously. Plasmid DNA was isolated by alkaline lysis with a QIAprep Spin miniprep kit (QIAGEN, GmbH, Germany) and the alkaline lysis method of Ish-Horowicz and Burke (11). For comparative purposes, plasmid DNA was digested with PvuII endonuclease (Amersham Biosciences). A Gene Clean spin kit (Qbiogene, Montreal, Quebec, Canada) was used for gel extraction of plasmid DNA. Dot blotting was performed with Gene Screen Plus membranes (Dupont, NEN Research Products, Boston, MA). Probe DNA (CfxA and TEM PCR products from control strains) was purified by using a QIAquick PCR purification kit (QIAGEN). The plasmids of four strains (321, 595, 616, and 800) were cut at the unique ClaI site, subcloned in the pBC SK+ chloramphenicol resistance vector (Stratagene, La Jolla, CA), and introduced by electroporation into Escherichia coli XL1-Blue MRF. Upstream and downstream sequences of the ß-lactamase genes were determined by direct sequencing of purified DNA. The plasmid sequence from strain 321 was determined by primer walking (GENOME Express, Meylan, France).
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TABLE 1. Antibiotic susceptibilities and ß-lactamases characterized for oral Capnocytophaga isolatesa
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TABLE 2. PCR primers used in this study
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FIG. 1. RAPD profiles of Capnocytophaga spp. by use of primer 1. Lanes 1 to 11, isolates 507, 161, 149, 136, 10-02B, 10-02C, 10-02A, 7-02, 2-02, 11-01, and 8-01; lanes M, molecular size markers (100-bp ladder).
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Isoelectric focusing analysis revealed that the ß-lactamases of different Capnocytophaga spp. migrated with a main band with a pI of around 5.6. DNA sequencing of PCR products showed that both cfxA2 and cfxA3 genes were prominent in Capnocytophaga spp. (Table 1). The CfxA2/CfxA3 division was not related to species differentiation or MICs. Five strains, all identified as C. sputigena, were CfxA negative, but two of them (507 and 581) were positive with the CfxA probe, suggesting the presence of a gene coding for an unidentified chromosomal ß-lactamase related to the CfxA family.
Six of the 25 strains harbored a 9-kb plasmid (C. ochracea 321, 595, 616, and 800 and C. gingivalis 140 and 11-01) with the cfxA3 gene, and for one strain (C. gingivalis 10-02A), the ß-lactamase gene (cfxA2) was located on a 4-kb plasmid. PvuII restriction profiles of plasmids from C. ochracea strains were identical but differed from those of the C. gingivalis strains. In the C. gingivalis strains harboring either the 9-kb or the 4-kb plasmid, the cfxA gene (cfxA3 or cfxA2, respectively) was present on the 1.2-kb PvuII fragment. Thus, it is likely that the plasmid-mediated CfxA resistance in C. gingivalis was acquired independently by these strains. Interestingly, in a different strain of C. gingivalis from the same patient (strain 10-02B), the cfxA2 gene was chromosome encoded, as demonstrated by Southern hybridization studies. PCR experiments with purified plasmid DNA were positive with specific CfxA primers and negative with TEM primers for all strains, including the C. ochracea reference strain (strain 321) previously described by Rosenau et al. (18) to harbor a TEM-17 plasmid-encoded ß-lactamase. This strain possessed, however, the 9-kb plasmid with the cfxA gene, as did other C. ochracea strains.
Hybridization of plasmid DNA and total DNA with the CfxA probe indicated that the cfxA gene was located on the plasmid and not on the chromosome. CfxA-positive, plasmid-negative strains were all positive by dot blotting, but no hybridization was detected between 50- and 300-kb bands after PFGE migration, suggesting that the cfxA gene is located at a chromosomal band of larger size.
Nucleotide sequence analysis of the plasmid from strain 321 (Fig. 2) (GenBank accession no. AY860640) revealed identity with the mobA-cfxA region of Tn4555 (CfxA of Bacteroides vulgatus) (19), but without the origin of transfer (oriT) of the transposon. A new insertion sequence (ISCoc1) was found upstream of the cfxA gene (Fig. 3). ISCoc1 had a size of 1,038 bp with two 25-bp terminal inverted repeats. The deduced protein sequence of the transposase showed homology, although low, with a transposase-like protein of the rumen bacterium Mannheimia succiniciproducens (10). Upstream, the plasmid sequence showed a characteristic repA region with an origin of replication, containing three iterons. The repA region was similar to the B. vulgatus pIP417 plasmid gene (7), but with relatively low amino acid identities (47%) for replication proteins. The cfxA flanking regions from the three other C. ochracea strains (595, 616, and 800) were identical. The mobA-cfxA region could represent the minimum DNA sequence responsible for the mobility of the cfxA gene in Bacteroidaceae. To verify this hypothesis, further sequence studies on the cfxA-surrounding region are needed, particularly for C. sputigena strains where the cfxA gene had a chromosomal location.
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FIG. 2. Schematic comparison of the genetic environment of blacfxA of (A) C. ochracea strain 321 (CfxA3) (GenBank accession no. AY860640) with that of (B) Bacteroides fragilis (CfxA) and (C) Prevotella intermedia (CfxA2). The arrangements of the common region mobA-cfxA are based upon updated sequences (GenBank accession no. U75371 and AF118110, respectively). IR, inverted repeat; DR2, direct repeat 2.
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FIG. 3. Comparison of the upstream nucleotide sequences of the cfxA3 gene of strains 321, 595, 616, and 800, along with the partial sequence upstream of cfxA1 and cfxA2 (GenBank accession no. U75371 and AF118110, respectively). Sequence identity among the cfxA genes is indicated with asterisks. Sequence identity between blacfxA2 and blacfxA3 is indicated with dots. The 35 and 10 regions of putative promoter sequences of the cfxA genes are shown in boldface. The horizontal arrow indicates an inverted repeat (IR) of 25 bp found upstream of blacfxA3.
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This project was partially supported by a grant from Ministère de la Santé, Projet Hospitalier de Recherche Clinique 2001 PHRC 01-01, CHU de Nice, to T.F., a grant of the Scandinavian Society for Antimicrobial Chemotherapy to D.A.C., and a grant from the Dental Faculty, University of Oslo.
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