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Infection and Immunity, February 2006, p. 1368-1372, Vol. 74, No. 2
0019-9567/06/$08.00+0 doi:10.1128/IAI.74.2.1368-1372.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Biotechnology and Environmental Biology, School of Applied Sciences, Royal Melbourne Institute of Technology University, Melbourne, Victoria, Australia
Received 16 June 2005/ Returned for modification 1 August 2005/ Accepted 27 October 2005
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The C. jejuni LOS is partly encoded by the wlaII gene cluster that has been shown to exhibit a high degree of variation among strains (6, 14) (Fig. 1). Presently, the function of individual LOS genes is not fully understood; however, the wlaND, cgtA, cgtB, cstII, neuB, neuC, neuA, and waaF genes are essential for the formation of human ganglioside-like LOS structures which can induce GBS (7-10, 12, 18). Upstream of the waaC gene, the wlaI gene cluster is found which is highly conserved in this bacterium (4, 15) (Fig. 1). The wlaI locus is mainly involved in protein glycosylation, although at least one gene, galE, is also involved in LOS synthesis (3).
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FIG. 1. wlaI locus (wlaM-Cj1132c), wlaII locus (waaC-waaF), and its downstream region (waaF-dnaX), approximate DNA fragment sizes that were deleted from WT 81116 (numbers within parenthesis), and integration points (arrows). WT 81116, host cell (GM1, accession no. Y11648 and AF343914); O:4, donor cell (GM1+, accession no. AF215659); A to J, 81116 transformants exhibiting ganglioside GM1 (except transformant A); boldface letters, homologous gene regions; a, essential genes for the formation of ganglioside-like LOS structures; dash between genes, unknown DNA regions; vertical arrows, the most upstream and downstream integration points; triangle point (left), integration point in or upstream of wlaM; elbow arrows and triangle points (right), integration points in or downstream of waaF and dnaX, respectively; N, not determined as unavailable PCR product; asterisk, integration point for kanamycin resistance cassette in O:4 wlaVA.
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wlaVA) could be successfully constructed by introducing the selection marker for kanamycin resistance and still reacted with CTB, and (iii) we previously showed that mutating the wlaVA gene in a strain HB 93-13 (O:19) did not affect the LOS structure, the LOS sugar composition, and the ganglioside mimicry (unpublished data). C. jejuni 81116 was grown on 5% defribinated horse blood-supplemented Columbia agar at 37°C under microaerobic conditions (5% O2, 10% CO2, and 85% N2) for 16 h. The bacterial cells were harvested and suspended in 1 ml of heart infusion broth, and 50 µl of bacterial suspension was preincubated in an Eppendorf tube (1.5 ml in size) containing 1 ml of heart infusion agar under microaerobic conditions for 2 h. Subsequently, 10 µg of isolated chromosomal DNA of O:4
wlaVA was added, and the mixture was incubated under the same conditions for 3 h. After incubation, the bacteria were grown as previously mentioned on selective medium supplemented with kanamycin (50 µg/ml) for 48 h. The transformation efficiency of WT 81116 was approximately 2 x 102 CFU per 10 µg of genomic DNA. One hundred and fifty kanamycin resistance colonies were randomly picked and immunologically probed with CTB to screen for GM1-positive transformants. Surprisingly, 145 of 150 colonies reacted with CTB, whereas only 5 colonies were CTB negative (Fig. 2, selected transformants). Therefore, these results showed that C. jejuni 81116 could be transformed into a number of potential GBS-inducible transformants.
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FIG. 2. Dot blot ELISA probed with CTB. 1, WT O:4; 2, O:4 wlaVA; 3, WT 81116. A, L, M, O, and Q are all CTB-negative transformants. B and C are representative CTB-positive transformants. +, CTB positive; , CTB negative.
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wlaVA genes had been acquired by the CTB-positive transformants, PCR-restriction fragment length polymorphism (PCR-RFLP) with HindIII as the restriction enzyme was performed. Using the genome sequence of C. jejuni NCTC 11168 (15), the primer WaaC-F (5'-CCGTGGTTTTGCAATTTATC-3'), which is located in the waaC gene (nucleotides 53 to 72), and the primer WaaF-R (5'-AAGTTCTTGTTCGGCTTTTC-3'), which is located in the waaF gene (nucleotides 594 to 575), were designed. These primers were used to amplify the entire wlaII gene cluster of WT O:4 (13.558 kb), O:4
wlaVA (14.055 kb), WT 81116 (16.308 kb), 13 selected CTB-positive transformants (B, C, D, E, F, G, H, I, J, K, N, P, and R), and all 5 CTB-negative transformants (A, L, M, O, and Q). PCR was carried out by using the Expand Long Template PCR system (Roche). For the CTB-positive transformants, the entire wlaII gene cluster (16.308 kb) was replaced by the wlaII locus of O:4
wlaVA (14.095 kb) as the PCR-RFLP patterns of these transformants were identical to the pattern of O:4
wlaVA and markedly different from that of WT 81116. For the CTB-negative transformants L, M, O, and Q, a partial exchange of LOS synthesis genes was observed. Presumably, some essential gene(s) for the synthesis of the GM1-like LOS and, hence, CTB binding is missing from their wlaII gene clusters. Surprisingly, CTB-negative transformant A had received the complete wlaII locus of O:4
wlaVA (Fig. 3, see also Fig. 2). A possible explanation could be a change in the length of some of the homopolymeric tracts found in this gene cluster or mutations in the LOS synthesis genes (9) or other genes that are essential in the expression of the CTB-binding epitope. These results showed that part and even the entire wlaII gene cluster was easily taken up and integrated in the genome of C. jejuni 81116, resulting in new strains carrying GM1-like LOS structures.
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FIG. 3. PCR-RFLP patterns of the wlaII gene cluster digested with HindIII. , -DNA digested with PstI; 1, WT O:4; 2, WT 81116; 3, O:4 wlaVA; and A to R, transformants.
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wlaVA. The PCR products of approximately 14 kb were obtained for all strains. PCR-RFLP patterns of transformants showed a high degree of variation within the wlaI gene clusters, and those were different from the patterns of WT 81116 and O:4
wlaVA (Fig. 4). Restriction mapping analysis showed that WT 81116 randomly integrated DNA fragments from the O:4
wlaVA into its genome using various integration points. Most of them were positioned in the region of the galE gene as a 698-bp DNA fragment containing the partial galE gene was missing from the wlaI loci of transformants A, B, C, D, F, H, I, and J. A previous study in a C. jejuni strain GB11 also evidenced the integration point in the galE region (5). Other sites were distributed throughout the wlaI gene cluster such as in the wlaK, wlaF, wlaE, and wlaC genes.
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FIG. 4. PCR-RFLP patterns of the wlaI gene cluster digested with HhaI. , -DNA digested with PstI; 1, WT O:4; 2, WT 81116; 3, O:4 wlaVA; and A to J, transformants.
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wlaVA, and transformants B, C, D, E, F, G, H, and J. A PCR product was not amplified from transformants A and I. RFLP analysis of PCR products showed that all transformants tested had the pattern of the donor DNA, and the site of recombination appears to be located in or downstream of the Cj1155c homologous region.
To identify the integration point downstream of the Cj1155c homologous region, HindIII-PCR-RFLP was performed as described above. Using the genome sequence of C. jejuni NCTC 11168 (15), the primer Cj1155c-F (5'-AGGTATGGTGCAAGCATTAT-3'), which is located in the Cj1155c gene (nucleotides 100 to 119), and the primer DnaX-R (5'-TAGGCTCTCCAAAACAATCT-3'), which is located in the dnaX gene (nucleotides 1516 to 1497), were designed. These primers were used to amplify a 5.185-bp DNA fragment ranging from Cj1155c to dnaX. A PCR product of approximately 5.2 kb was obtained for WT O:4, O:4
wlaVA, WT 81116, and 81116 transformants (B to J). A PCR product was not amplified from transformants A. The integration point for transformants B, D, E, F, and H was located in or downstream of the dnaX homologous region since their RFLP patterns were identical to the RFLP pattern of O:4
wlaVA. For transformants C, G, I, and J, the integration point was located in the Cj1155c homologous region because their RFLP patterns were identical to the pattern of WT 81116. The most upstream and downstream integration points for each 81116 transformant and the approximate sizes of WT 81116 DNA that were deleted during genetic recombination were conclusively shown (Fig. 1).
To determine whether natural transformation with chromosomal DNA resulted in new genotypes of C. jejuni, pulsed-field gel electrophoresis (PFGE) with SacII as the restriction enzyme was performed. It was found that the SacII-PFGE patterns of the transformants A, B, C, D, E, F, H, I, and J were identical to the PFGE pattern of WT 81116 (Fig. 5, see representative PFGE pattern of transformant A). Interestingly, the PFGE pattern of the transformant G was different from that of WT 81116. It seems a large 388-kb DNA fragment and a 135-kb fragment of the O:4
wlaVA were inserted into the chromosomal DNA of WT 81116. Presumably, this insertion is at a location other than the LOS gene cluster since the change involving the LOS genes is apparently too small to be detected by PFGE. This indicated that not only the LOS gene cluster but also other locations throughout the WT 81116 genome had undergone a genetic exchange. These results showed that natural transformation can result in genotypic diversity, as also shown by previous studies (1, 2, 16).
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FIG. 5. SacII-PFGE patterns of the transformants. , molecular size marker (48.5-kb lambda concatemer); 1, WT 81116; 2, O:4 wlaVA; A and G, transformants. Arrows indicate two DNA fragments of O:4 wlaVA found in the transformant G.
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wlaVA, except for LOS isolated from transformant Q that showed a similar LOS pattern to the WT 81116. Interestingly, the CTB-positive transformants (B, C, D, E, F, G, H, I, and J) carrying the LOS gene cluster of O:4
wlaVA showed a different LOS pattern to that of O:4
wlaVA. The cause for this difference is unclear and requires further investigation. As expected, the LOS patterns of WT O:4 and O:4
wlaVA were similar (Fig. 6). These results indicated that not only the wlaII gene cluster but also other genes are involved in LOS synthesis.
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FIG. 6. LOS analysis by Tricine-SDS-PAGE followed by silver staining. 1, WT O:4; 2, O:4 wlaVA; 3, WT 81116; A to Q, transformants exhibiting CTB-positive (+) or -negative () phenotypes.
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