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Applied and Environmental Microbiology, October 2008, p. 6369-6377, Vol. 74, No. 20
0099-2240/08/$08.00+0     doi:10.1128/AEM.01218-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Genetic Tools for Studying Capnocytophaga canimorsus

Manuela Mally and Guy R. Cornelis^*

Infection Biology, Biozentrum, University of Basel, Klingelbergstrasse 50/70, CH-4056 Basel, Switzerland

Received 2 June 2008/ Accepted 13 August 2008

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Capnocytophaga canimorsus, a commensal bacterium from canine oral flora, has been isolated throughout the world from severe human infections caused by dog bites. Due to the low level of evolutionary relationship to Proteobacteria, genetic methods suitable for the genus Capnocytophaga needed to be established. Here, we show that Tn4351, derived from Bacteroides fragilis, could be introduced by conjugation into C. canimorsus and conferred resistance to erythromycin. By mapping and sequencing a naturally occurring plasmid isolated from a clinical isolate of C. canimorsus, we identified a repA gene that allowed us to construct Escherichia coli-Capnocytophaga shuttle vectors. Most commonly used antibiotic markers were not functional in C. canimorsus, but cefoxitin (cfxA), tetracycline (tetQ), and erythromycin (ermF) resistances could be used as markers for plasmid maintenance in C. canimorsus and even in some other Capnocytophaga spp. Shuttle vectors were introduced into C. canimorsus either by conjugation using the origin of transfer (oriT) of RP4 or by electrotransformation. Taking advantage of the promoter of ermF, an expression vector was constructed. Finally, a method that allows site-directed mutagenesis is described. All these genetic tools pave the way, not only for molecular studies of the pathogenesis of C. canimorsus, but also for studies of other oral Capnocytophaga species.

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Capnocytophaga canimorsus is a commensal bacterium found in the oral cavities of dogs and cats. Since its discovery in a patient who had developed septicemia and meningitis after a dog bite in 1976 (4), more than 160 cases of severe human infections by C. canimorsus have been reported (33). Human infections can result in septicemia or meningitis, with mortality rates of 30% and 5%, respectively (17). Bacteria from the genus Capnocytophaga form part of the resident oral flora of humans and domestic animals (6). Seven species, including Capnocytophaga ochracea and Capnocytophaga gingivalis, are found in the normal human oral flora, whereas the dog's oral flora contains C. canimorsus and Capnocytophaga cynodegmi. In spite of this diversity, C. canimorsus is the only Capnocytophaga species that has been associated with severe human infections. Recently, we started to unravel the molecular mechanisms underlying C. canimorsus infections (22, 28), but we had to establish genetic methods adapted to this group of bacteria. The genus Capnocytophaga belongs to the family Flavobacteriaceae in the phylum Bacteroidetes. Many genetic methods that function in Proteobacteria have been shown to fail in Bacteroidetes (26), and commonly used broad-host-range plasmids did not result in ampicillin-resistant (Ap^r), tetracycline-resistant (Tc^r), or kanamycin-resistant (Km^r) colonies of Flavobacterium johnsoniae (20). However, transposons and selectable markers identified and used in Bacteroides spp. (30, 32) have been successfully adapted for the family Flavobacteriaceae (2, 19, 20).

In the present work, we describe the tools necessary to genetically manipulate Capnocytophaga spp. Taking advantage of genetic methods originating from Bacteroides spp., we established ways to introduce DNA using functional selection markers and to perform transposon mutagenesis. Finally, we identified an endogenous plasmid in a clinical isolate of C. canimorsus, and we generated the first shuttle vectors that allow plasmid replication in Capnocytophaga spp. Taken together, these tools will facilitate studies of bacteria in the genus Capnocytophaga.

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Bacterial strains, growth conditions, and selective agents.
The strains and plasmids used in this study are shown in Table 1. Escherichia coli strains were routinely grown in LB broth at 37°C. Capnocytophaga spp. were grown on plates of heart infusion agar (Difco) supplemented with 5% sheep blood (Oxoid) (SB plates) for 2 days at 37°C in the presence of 5% CO₂. Bacteria were harvested by gently scraping colonies off the agar surface, washed, and resuspended in phosphate-buffered saline (PBS). Alternatively, C. canimorsus was grown in 50 ml heart infusion broth (HIB) (Difco) supplemented with 10% (vol/vol) fetal bovine serum (FBS) (Invitrogen) for approximately 24 h without shaking in a 37°C incubator with 5% CO₂ using Erlenmeyer flasks. To select for plasmids or transposons, antibiotics were added at the following concentrations: 10 µg/ml erythromycin (Em), 10 µg/ml cefoxitin (Cf), 20 µg/ml gentamicin (Gm), 100 µg/ml Ap, 5 µg/ml Tc, 50 µg/ml Km, and 10 µg/ml chloramphenicol (Cm).

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TABLE 1. Strains and plasmids

Conjugation.
E. coli strains BW19581 and S17-1, used for conjugative transfer of mobilizable plasmids, were grown without antibiotics to early exponential phase in LB broth. C. canimorsus, which is naturally resistant to Gm, was grown for 2 days on SB plates at 37°C and harvested by scraping. Bacteria were washed and resuspended in PBS. Donor and recipient were mixed at a 1:10 ratio, centrifuged for 2 min at 8,000 x g, and resuspended in 50 µl of PBS, and 2.2 x 10⁸ CFU were spotted on 22-µm mesh nitrocellulose filters (Millipore) laid on the surface of an SB plate. The plates were incubated overnight in 5% CO₂ at 37°C. Each filter was washed with 2 ml of HIB and 10% FBS containing Gm and kept for 1 h at room temperature, and the bacteria were diluted and plated on selective SB plates containing Gm and the appropriate antibiotic to select for plasmid or transposon transfer. The plates were incubated for 2 to 3 days.

Electroporation.
C. canimorsus was grown in HIB and 10% FBS overnight to early or mid-exponential phase without shaking, cooled to 4°C, harvested by centrifugation at 5,500 x g for 15 min at 4°C, washed three times in ice-cold double-distilled H₂O and twice in double-distilled H₂O plus 10% glycerol, and resuspended to a cell density of approximately 1 x 10¹⁰/ml in 10% glycerol. After being shock frozen in liquid nitrogen, the bacteria were either thawed and used for transformation or stored at –80°C. Plasmid DNA was added to 100 µl of bacterial suspension in Bio-Rad Genepulser cuvettes with 0.2-cm electrodes and pulsed with 2.5 kV. After electroporation, the bacteria were transferred to 900 µl prewarmed HIB and 10% FBS and incubated at 37°C for 2 to 3 h to allow expression of antibiotic resistance. The bacteria were plated on SB plates with the appropriate antibiotic and incubated for 2 to 3 days.

Analysis of Tn4351 insertions.
Tn4351 was introduced into C. canimorsus by conjugation as described above. Genomic DNA from Em^r colonies was isolated with the GenElute bacterial genomic DNA kit (Sigma) following the manufacturer's instructions, digested with HindIII, and analyzed by Southern hybridization using standard procedures (27). IS4351 probes were prepared by PCR amplification using primers 3505 and 3506, plasmid pEP4351 DNA as a template, and digoxigenin (DIG)-11-dUTP (Roche) according to the manufacturer's recommendations. To test for vector cointegration, the Cm acetyltransferase gene (cat), which is present on the Tn4351 delivery vector pEP4351, was amplified as a 633-bp PCR product from genomic DNA using primers 3576 and 3577. All primers used are listed in Table 2.

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TABLE 2. Oligonucleotides used in this study

Isolation and identification of naturally occurring plasmids in C. canimorsus.
Plasmids were isolated from Capnocytophaga spp. by hot alkaline lysis (13) or alkaline lysis in combination with Qiagen columns (Qiaprep spin miniprep kit; Qiagen). For analysis of pCC7, a 1.95-kb HindIII-EcoRI fragment was inserted into the corresponding restriction sites of the cloning vector pBSIIKS(+), resulting in pMM7, which was subsequently sequenced. Based on the sequence information obtained, the native pCC7 plasmid was sequenced by primer walking, using the BigDye Terminator ready reaction kit (PE Biosystems) and primers (3574, 3575, 3601, 3623, 3625, 3626, 3639, 3641, 3675, 3676, 3677, and 3678) described in Table 2. The results were analyzed using Vector NTI 10.0 software (Invitrogen) (Fig. 1A).

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FIG. 1. Engineering of an expression shuttle vector from a natural C. canimorsus plasmid. (A) Genetic and restriction map of the endogenous plasmid pCC7 showing the primer binding sites used for amplification of the replicon (3601 and 4274). (B) Map of the shuttle expression vector pMM47.A containing the cfxA gene (Cfr) for selection in C. canimorsus, the repA replicon of pCC7, and the promoter of ermF (–33 and –7 boxes) upstream from the NcoI, XbaI, and XhoI restriction sites that allow the insertion of a coding sequence in frame or out of frame with six histidine codons. Unique restriction sites are shown in red. (C) Partial nucleotide sequence of pMM47.A showing the promoter with its –33 and –7 boxes (boldface and underlined), the transcription initiation site (TIS) (boldface italics), and the Shine-Dalgarno sequence (boldface) (24). Restriction sites (italics) that are unique are shown in red. The ATG codon within the NcoI site can be used as the start codon, giving the translation shown below the nucleotide sequence.

Directed gene replacement by allelic exchange.
The replacement cassette with flanking regions spanning approximately 500 bp homologous to the siaC gene (accession number EU329392) was constructed with a three-fragment overlapping-PCR strategy (see Fig. 4A). First, two PCRs were performed on 100 ng of genomic DNA of C. canimorsus 5 with primers 4783 and 4784 for the upstream flanking region of siaC and with primers 4787 and 4788 for the downstream regions homologous to siaC. Primer 4784 for the upstream siaC region and primer 4787 for amplification of the downstream siaC region contained 20 bp of sequence homology to the ermF insertion cassette as a 5' extension. The ermF resistance cassette was amplified from pEP4351 with primers 4785 and 4786, which contained as 5' extensions 30 bp of the siaC gene. All three PCR products were cleaned and then mixed in equal amounts for PCR using Phusion polymerase (Finnzymes). The initial denaturation was at 98°C for 2 min, followed by 12 cycles without primers to allow annealing and elongation of the overlapping fragments (98°C for 30 s, 50°C for 40 s, and 72°C for 2 min). After the addition of external primers (4783 and 4788), the program was continued with 20 cycles (98°C for 30 s, 50°C for 40 s, and 72°C for 2 min 30 s) and finally 10 min at 72°C. The final PCR product linking the three initial fragments led to the siaC::ermF insertion cassette and was then digested with PstI and SpeI for cloning into the appropriate sites of the C. canimorsus suicide vector pMM25. The resulting plasmid, pMM106, was transferred by RP4-mediated conjugative DNA transfer from E. coli S17-1 to C. canimorsus 5 as previously described to allow integration of the insertion cassette by its homologous regions to siaC.

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FIG. 4. Generation of a C. canimorsus 5 sialidase knockout and complementation in trans by siaCHis cloned in the expression shuttle vector pMM47.A. (A) Schematic representation of the deletion strategy. The upstream flanking region was amplified from genomic C. canimorsus 5 DNA with primers 4783 (PstI; black) and 4784 containing an additional 5' 20-nucleotide extension homologous to the resistance cassette ermF (magenta) (PCR 1). The same was done for the downstream flanking region with primers 4787, including 20 nucleotides complementary to the resistance cassette in 5' (magenta), and 4788, including an SpeI restriction site (black) (PCR 2). The ermF resistance cassette (magenta) was amplified with primer 4785, which included the 30-bp homology to the end of the upstream flanking region at the 5' end (green), and primer 4786, with 30-bp homology to the downstream flanking region (green) (PCR 3). All three PCR products were subjected in equal amounts to another PCR after the addition of external primers (4783 and 4788). The final PCR product linking the three initial fragments was digested with PstI and SpeI and cloned into the suicide vector pMM25, giving pMM106. (B) Immunoblot analysis of crude cell extracts of the wild type (C. canimorsus 5 [Cc5]), an siaC-deficient Tn4351 mutant (siaC), the site-directed mutant of siaC (siaC::ermF), and both mutants (siaC and siaC::ermF) complemented in trans with pMM52 (psiaCHis), using a polyclonal serum against SiaC (top) and a monoclonal antibody against the C-terminal His tag encoded by pMM52 (psiaCHis) (bottom). (C) Sialidase activity was measured by monitoring the fluorescence at 445 nm generated by the cleavage of 2'-(4-methylumbelliferyl)--D-N-acetylneuraminic acid (means plus standard deviations of the results for a representative experiment).

Immunoblotting.
Total cell extracts were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotted according to standard procedures. Monoclonal antibody against C-terminal His was purchased from Invitrogen, and polyclonal anti-SiaC serum is described elsewhere (18).

Nucleotide sequence accession number.
The sequence of pCC7, which is reported here, has been deposited in the GenBank database under accession number EU741249.

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Conjugative DNA transfer.
We first tried to introduce IncP and pBBR1 broad-host-range vectors into C. canimorsus 5 (Table 1) by conjugative DNA transfer. Conjugation-proficient E. coli strains (Table 1) were used to mobilize pMR20 (IncP; Tc^r), pBBR1MSC3 (Tc^r), or pBBR1MSC4 (Ap^r), but no C. canimorsus transconjugant could be isolated. Next we constructed pBBR1 derivatives (pMM2 and pMM3) with randomly cloned genomic DNA (500 to 650 bp) of C. canimorsus 5 in order to allow plasmid integration by crossover, but no Ap^r colonies were found, hinting either that conjugation did not occur or that the selection marker was not expressed in C. canimorsus. Since C. canimorsus belongs to the family Flavobacteriaceae, we tested plasmid pCP29 derived from a natural plasmid of Flavobacterium psychrophilum and containing the Em^r gene ermF. This plasmid, which has been shown to replicate in F. johnsoniae, was transferred by an RP4-mediated system from E. coli S17-1 to E. coli recipient strains and to F. johnsoniae, but no Em^r C. canimorsus transconjugant colony could be obtained. To ensure that this failure was due to the replication origin and not to the selection marker or the DNA transfer itself, we turned to transposon Tn4351. Tn4351, carrying the ermF gene, was isolated in 1985 from pBF4, a self-transmissible plasmid from Bacteroides fragilis (30). As a delivery vector for Tn4351, we used plasmid pEP4351, which can be mobilized from E. coli BW19851 by the chromosome-encoded RP4 conjugation machinery but which was not replicating in C. canimorsus. Em^r transconjugants of C. canimorsus 5 could be isolated in this way, showing first that conjugation works as a method to transfer DNA into C. canimorsus and, second, that ermF is expressed and can be used as a selection marker. This result also suggested that pCP29 from F. psychrophilum did not replicate in C. canimorsus 5. We then cloned the ermF gene, including its own promoter, into pBBR1MCS4, giving pMM5, and used E. coli S17-1 as a donor strain to transfer pMM5 to C. canimorsus 5. No Em^r colonies of C. canimorsus appeared after conjugation, demonstrating that the pBBR replicon is not functional in C. canimorsus.

Generation of replicating shuttle vectors for C. canimorsus.
As we identified a functional selection marker, we next aimed to construct plasmids that could replicate in C. canimorsus. We screened eight C. canimorsus strains (Table 1) for the presence of endogenous plasmids. Two plasmids were identified in C. canimorsus strain 7 (not shown). The smaller plasmid, designated pCC7, was sequenced (4,579 bp) (Fig. 1A). A BLAST homology search (nonredundant database; July 2008) revealed a gene encoding a putative replication protein with homology to replicases of C. ochracea (AAY78540; score, 304; E value, 5e–81), Bacteroides vulgatus (CAA60389; score, 300; E value, 1e–79), and B. fragilis (CAA60390; score, 299; E value, 3e–79) (repA; 1,074 bp).

The gene product of a 1,125-bp-long open reading frame (designated ORF CC7p_3) showed homology to an ISPg1 transposase from Porphyromonas gingivalis (NP_904520).

We then generated shuttle vectors by amplifying this repA gene, including 408 bp of its upstream region, and inserting it into pLYL03, which contains ermF and the origin of transfer of RK2. The resulting vector, pMM105.A, could be mobilized by the RP4-mediated conjugation machinery from E. coli S17-1 to C. canimorsus 5 with transfer frequencies of around 10^–4 per recipient (Table 3). This plasmid could also be transferred to C. canimorsus 12, but the frequency of transfer was significantly lower than in C. canimorsus 5 (Table 3). The replicase gene and upstream region that we isolated were thus sufficient for autonomous plasmid replication in C. canimorsus.

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TABLE 3. Frequencies of transfer of E. coli-Capnocytophaga shuttle vectors in matings with E. coli S17-1

Finally, we constructed similar shuttle vectors for C. canimorsus with a Tc selection marker (tetQ; pMM104.A) or a Cf^r marker (cfxA; pMM45.A) (Fig. 2 and Table 3).

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FIG. 2. Maps of Capnocytophaga-E. coli shuttle vectors. Shown are genetic and restriction maps of shuttle vectors pMM45.A (A), pMM104.A (B), and pMM105.A (C). Elements that are functional in Capnocytophaga are indicated in blue: the repA replicon of pCC7, the Cfr marker cfxA, the Tcr marker tetQ, and the Emr marker ermF. In white are elements that are functional in E. coli: oriT for conjugative transfer by RP4 machinery and the selection markers Apr and Kmr. Unique restriction sites are shown in red. The vectors shown in panels B and C were engineered by cloning repA into pLYL001 (25) and pLYL03 (16), respectively.

Construction of an E. coli-C. canimorsus shuttle expression vector.
To generate an expression vector for C. canimorsus, a 257-bp fragment upstream of ermF containing the canonical –33 and –7 boxes of Bacteroides promoters was amplified by PCR. Additionally, the primers used for amplification of the promoter region incorporated unique NcoI, XbaI, and XhoI restriction sites for cloning purposes, as well as six histidine codons, which allow the insertion of a coding sequence in or out of frame with a C-terminal His tag (Fig. 1B and C). The PCR product was digested with appropriate restriction enzymes and inserted into pMM41.A, creating the shuttle expression vector pMM47.A (Fig. 1B). To test this vector, we cloned the promoterless siaC gene encoding a sialidase from C. canimorsus 5 (18) into pMM47.A, resulting in a full-length siaC downstream from the –33 and –7 boxes and in frame with a C-terminal His tag (pMM52). Sialidase could be detected in crude extracts of a sialidase-deficient Tn4351 mutant of C. canimorsus 5 (siaC) harboring pMM52 or of a site-directed mutant of siaC (siaC::ermF) complemented with pMM52, indicating the functionality of the expression vector pMM47.A (see Fig. 4B).

Electrotransformation as a method to transfer DNA.
The shuttle vectors described above allowed us to test if electrotransformation could be applied as an alternative way of introducing DNA into C. canimorsus. Competent bacteria were prepared by washing them in ice-cold water and 10% glycerol and giving them a final cold shock by freezing them in liquid N₂. In this way, plasmid DNA (pMM47.A) isolated from an E. coli host strain could be transformed into C. canimorsus 5 with an efficiency of 2.1 x 10³ (±2.3 x 10³) clones per µg of DNA; 4.4 x 10^–7 (±4.0 x 10^–7) transformants per viable C. canimorsus 5 bacterium could be obtained in this way. A 2-min heat treatment at 56°C before electroporation was tested in order to prevent the degradation of DNA by intracellular restriction systems, but this treatment proved to reduce the transformation efficiency. The same decrease was observed when using MgCl₂ or NaCl during the washing steps (data not shown). Hence, sufficient transformation efficiency could be reached using electrotransformation of DNA isolated from E. coli.

E. coli-C. canimorsus shuttle vectors can be introduced to C. cynodegmi, C. ochracea, and C. gingivalis by RP4-mediated conjugation machinery of E. coli.
E. coli S17-1 carrying plasmid pMM45.A (Cf^r), pMM104.A (Tc^r), or pMM105.A (Em^r) (Fig. 2) was mated with C. cynodegmi, C. ochracea, or C. gingivalis (Table 3). All three shuttle vectors (ermF, tetQ, and cfxA) were functional in C. cynodegmi. In contrast, only tetQ and ermF were functional in C. ochracea, and tetQ and cfxA were functional in C. gingivalis. Although conjugation frequencies varied from 10^–4 to 10^–8 transconjugants per recipient cell, depending on the species (Table 3), tools developed for C. canimorsus can thus be used for other species in the genus.

Tn4351 transposition in C. canimorsus.
As shown before, Tn4351, derived from B. fragilis, could be introduced into C. canimorsus 5 using E. coli BW19851 to mobilize the delivery vector pEP4351 by conjugation. Em^r colonies of C. canimorsus 5 appeared at a frequency of 10^–6 to 10^–8 per recipient. Genomic DNA was thereafter analyzed by Southern blotting after HindIII restriction for transposon integration (Fig. 3). For the C. canimorsus 5 mutants W2E9, X7B9, and Y2F12, two bands hybridized with DIG-labeled IS4351, while for mutant X2E4, three bands hybridized (Fig. 3A). In mutant X2E4, the cat gene from the delivery vector could also be detected by PCR amplification, indicating that a cointegration event took place (Fig. 3B). We conclude that clones W2E9, X7B9, and Y2F12 contained one copy of Tn4351 flanked by the IS4351 sequences, while X2E4 contained one copy of Tn4351 but cointegrated with the vector, resulting in three copies of IS4351, as schematically represented in Fig. 3C.

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FIG. 3. Integration of Tn4351 into the chromosome of C. canimorsus 5. (A) Southern blot hybridization. Genomic DNA from the wild type (C. canimorsus 5 [Cc5]) and four insertion mutants (W2E9, X7B9, Y2F12, and X2E4), as well as DNA from pMM13 and pEP4351, was digested with HindIII and hybridized with DIG-labeled IS4351. Plasmids pMM13, containing one copy of IS4351, and pEP4351, containing Tn4351 (two copies of IS4351), served as positive controls. (B) PCR amplification of the 633-bp cat gene from the vector pEP4351 to identify vector cointegration events. (C) (Top) Schematic representation of Tn4351 integrated into the chromosome (open bars) with the IS4351 insertion sequences flanking ermF and tetX. (Bottom) Schematic representation of a cointegrate with the cat gene and the mobilization (mob) site of the vector.

Site-directed gene replacement using an antibiotic resistance cassette.
Taking advantage of the DNA transfer procedures and selection markers that we had established, we next tried to perform site-directed gene replacement. For the proof of principle, we selected the siaC gene as a target. A replacement cassette consisting of ermF flanked by 500-bp regions homologous to siaC was constructed, as schematically shown in Fig. 4A. The resulting plasmid, pMM106, lacking the replicon for C. canimorsus, was introduced by E. coli S17-1 into C. canimorsus 5. Transconjugants selected on Em were assumed to have integrated pMM106 into the chromosome by a single recombination event at the homologous regions of siaC. Em^r colonies were obtained at a transfer frequency of 3.4 x 10^–4 Em^r colonies per C. canimorsus 5 recipient. The colonies were then immediately replicated on Cf and Em, and Cf-sensitive and Em^r colonies assumed to have undergone an excising event of the vector backbone were picked. The disruption of the sialidase (siaC::ermF) was confirmed by PCR, sequencing, and immunoblotting against SiaC (Fig. 4B), as well as by testing for the loss of sialidase activity using 2'-(4-methylumbelliferyl)--D-N-acetylneuraminic acid as a substrate (Fig. 4C). Activity and sialidase expression could be restored by introducing in trans the full-length gene cloned into the expression shuttle vector pMM47.A (Fig. 4B and C).

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The availability of genetic methods is crucial for the study of molecular mechanisms associated with the pathogenesis of bacterial infections. In this study, techniques that allow the genetic manipulation of C. canimorsus were developed, opening the possibility of genetic analysis of bacteria of the genus Capnocytophaga. We showed that C. canimorsus can serve as a recipient for RP4-mediated conjugation, but we found that the classical broad-host-range replicon pBBR1 is not functional in C. canimorsus. The replicon from a natural plasmid of the closely related F. psychrophilum also did not lead to plasmid replication in C. canimorsus. Therefore, we isolated and sequenced an endogenous plasmid from C. canimorsus 7 and identified a replication region that could be used to engineer shuttle vectors. These vectors could also be used in other species of the genus Capnocytophaga. The selection markers ermF, cfxA, and tetQ, originating from Bacteroides spp., could be used successfully in C. canimorsus, suggesting that the promoter region and the sigma factor resemble those found in Bacteroides spp. rather than those from E. coli (3). This is in line with the assumption that the classical selection markers used in E. coli could not be used in C. canimorsus, presumably due to the lack of promoter recognition. We thus engineered an expression vector for C. canimorsus using the promoter region of IS4351 with the Bacteroides consensus for –33 and –7 boxes located upstream from the ermF gene in Tn4351 (24).

For a transposon mutagenesis approach, we tested Tn4351, a transposon widely used in Bacteroides spp. (29), Flavobacterium spp. (19), and P. gingivalis (8-10). Southern blot analysis showed that Tn4351 integrated into the C. canimorsus genome, either alone or as a cointegrate with its vector. This vector coinsertion has been reported to occur in a strain-dependent manner in bacteria of the phylum Bacteroidetes. It has also been reported that Tn4351 does not integrate in a random manner (11). For these reasons, a mariner-based transposon for Flavobacterium spp. (HimarEm) was constructed by Braun et al. using ermF as a selectable marker (5). Although Himar insertions are reported to occur at positions containing the target nucleotide sequence "TA" and are usually described as being otherwise random (36), Himar insertions were not completely random in F. johnsoniae (5). In spite of these limitations, the mariner transposon could be another approach for transposon mutagenesis of C. canimorsus.

A method for directed gene disruption by allelic exchange with a resistance marker cassette was also developed, demonstrating that homologous recombination occurs in C. canimorsus. Inserting a resistance marker cassette into the chromosome might influence the expression of downstream genes located in an operon and thus limit this method in some instances.

Taken together, a collection of techniques allowing genetic manipulations in C. canimorsus has been established. This will not only provide the basis for new approaches to understanding the mechanisms underlying pathogenesis of C. canimorsus infections, but will also make genetic manipulation available for other species in the genus Capnocytophaga.

 
We thank Mark J. McBride for generously providing plasmids and for invaluable advice, Georges Wauters and Michel Delmee for providing Capnocytophaga strains, and Hwain Shin for critical reading of the manuscript. We are also grateful to Nadia B. Shoemaker and Abigail A. Salyers for helpful suggestions.

This work was supported by the Swiss National Science Foundation (grant 32-65393.01).

 
* Corresponding author. Mailing address: Biozentrum, Klingelbergstrasse 50-70, CH-4056 Basel, Switzerland. Phone: 41 61 267 21 10. Fax: 41 61 267 21 18. E-mail: guy.cornelis{at}unibas.ch

Published ahead of print on 22 August 2008.

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Applied and Environmental Microbiology, October 2008, p. 6369-6377, Vol. 74, No. 20
0099-2240/08/$08.00+0     doi:10.1128/AEM.01218-08
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