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ekZentralinstitut für Ernährungs- und Lebensmittelforschung (ZIEL), Abteilung Mikrobiologie, Technische Universität München, Weihenstephaner Berg 3, 85354 Freising, Germany
Correspondence
Thilo M. Fuchs
thilo.fuchs{at}wzw.tum.de
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The GenBank/EMBL/DDBJ accession number for the Flag-2 sequence of Yersinia enterocolitica is AM600695.
A supplementary table listing the primers used is available with the online version of this paper.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). This Gram-negative, rod-shaped micro-organism is widely distributed in nature and able to survive for extended periods in terrestrial and aquatic environments at ambient temperature. The bacterium has been isolated from numerous species including flies (Fukushima et al., 1979; Rahuma et al., 2005). Swine have been identified as a major source of human infection (Bottone, 1997; Fredriksson-Ahomaa et al., 2006). A multiphasic life cycle, which comprises a free-living phase, a potential insect-associated phase and a host-dependent phase, may be characteristic of biotypes 2–4 of this pathogen (Bresolin et al., 2006b).
In Y. enterocolitica, low temperature is a key environmental factor for the expression of several genes important for survival outside mammalian hosts (Bresolin et al., 2006a; Kapatral et al., 2004) as well as for the coordinate expression and assembly of the flagellar structure (Kapatral & Minnich, 1995; Kapatral et al., 2004; Rohde et al., 1994). So far, all functional flagellar genes of Y. enterocolitica have been found to be located in the common flagellar cluster 1 (Flag-1), which is also required for full virulence (Young et al., 2000). A second, but inactive, flagellar cluster, termed Flag-2, has been found in around one-fifth of Escherichia coli strains tested and also in Yersinia pestis and Yersinia pseudotuberculosis, but not in Y. enterocolitica 8081 (Ren et al., 2005). The distribution of Flag-2 in the Y. enterocolitica biotypes has not been investigated.
Except for some distinct differences, the flagellar hierarchy in Y. enterocolitica resembles that of E. coli and Salmonella spp. (Horne & Pruss, 2006), in which the regulation and function of flagellar genes are well understood. Their genomes contain more than 50 flagellar genes, which can be categorized into three promoter classes. These correspond to the temporal requirements for flagellar gene products during the morphological development of the flagellum, which consists of a membrane-spanning hook–basal-body and an external structure (Aldridge & Hughes, 2002; Chilcott & Hughes, 2000). The regulation of flagellar gene expression is coordinated by the flhDC master operon, which is transcribed in a temperature-independent manner from a class 1 promoter (Kapatral et al., 2004). FlhD and FlhC act as transcriptional activators for class 2 promoters including the transcriptional regulators FliA (
28) and FlgM (anti-28) and genes required for the formation of the hook–basal-body. Most genes necessary for late flagellar morphogenesis, motor rotation and chemotactic signalling are transcribed from class 3 promoters. Rohde et al. (1994) reported 23 °C as being the temperature for maximal flagellar gene expression. However, little is known about the induction and transcription of flagellar genes in Y. enterocolitica at temperatures below 25 °C, and in what way their expression differs from that in related species.
In this study, screening of a luxCDABE-reporter mutant library of Y. enterocolitica W22703 at low temperature identified transposon insertions within 12 genes involved in motility and chemotaxis. Sequence analysis of two mutants revealed a strain-specific flagellar region, which is homologous to the flagellar cluster Flag-2 of Y. pestis and Y. pseudotuberculosis. FlhC-dependent expression of Flag-2 genes is demonstrated, and evidence for their functionality is provided. Furthermore, we show that Flag-1 and Flag-2 genes are maximally expressed at approximately 20 °C, indicating that enhanced motility is part of the psychrotolerant life style of Y. enterocolitica, which includes survival and growth at environmental temperatures.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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and 3. All cultures were grown in LB broth (10 g tryptone l–1, 5 g yeast extract l–1 and 5 g NaCl l–1) or on LB agar (LB broth supplemented with 1.5 %, w/v, agar). E. coli was grown at 37 °C and Y. enterocolitica at the temperatures indicated below. If appropriate, the media were supplemented with the following antibiotics: 20 µg chloramphenicol ml–1, 50 µg kanamycin ml–1, 20 µg nalidixic acid ml–1 or 18 µg tetracycline ml–1 (for E. coli) and 12 µg tetracycline ml–1 (for Y. enterocolitica).
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). To isolate chromosomal DNA, 1.5 ml of bacterial culture was centrifuged, and the sediment was resuspended in 400 µl lysis buffer (100 mM Tris, pH 8.0, 5 mM EDTA, 200 mM NaCl). After incubation for 15 min on ice, 10 µl 10 % SDS and 5 µl proteinase K (10 mg ml–1) were added, and the sample was incubated at 55 °C overnight. The chromosomal DNA was then precipitated with 500 µl 2-propanol, washed in ethanol, dried and dissolved in 500 µl TE buffer (10 mM Tris/HCl, 1 mM Na2EDTA, pH 7.4) containing 1 µl RNase (10 mg ml–1). Transposon mutagenesis using pUT mini-Tn5 luxCDABE Km2 and mating with E. coli S17.1 
pir as the donor strain has been described elsewhere (Bresolin et al., 2006a). PCRs were carried out with Taq polymerase (Fermentas) using the following programme: one cycle at 95 °C for 2 min; 30 cycles at 95 °C for 10 s, at the appropriate annealing temperature for 30 s, and at 72 °C for 45–180 s depending on the expected fragment length; one cycle at 72 °C for 10 min. All primers are listed in Supplementary Table S1, available with the online version of this paper. Chromosomal DNA (100 ng µl–1) was used as a template for PCR amplification.
Inverse PCR and DNA sequencing.
Identification and characterization of transposon insertion sites was performed by inverse PCR (Bresolin et al., 2006a). Briefly, 400 ng chromosomal DNA of each transposon mutant was completely digested with ClaI, HindIII or SspI (Fermentas), enzymes were heat-inactivated and fragments were treated with T4 DNA ligase (Invitrogen) to allow self-ligation, resulting in circular molecules. Subsequent inverse PCR (Ochman et al., 1990) was performed using transposon-specific primers derived from the O-end or the I-end of mini-Tn5 (Table S1). The PCR fragments obtained were sequenced by MWG-Biotech with primers hybridizing to a 100 bp transposon region near the O-end or the I-end.
Sequencing of strain-specific DNA was performed by inverse PCR (as described above) using the restriction enzymes HaeIII (USB), HhaI, HindIII, HpaI, MspI, MunI, RsaI, SspI and VspI (Fermentas) and primers listed in Table S1. Sequencing was performed by 4base lab.
Bioinformatics.
Mapping of mini-Tn5 luxCDABE insertions was performed with the Y. enterocolitica BLAST Server from the Sanger Institute (http://www.sanger.ac.uk/cgi-bin/blast/submitblast/y_enterocolitica). Sequences without similarities to the sequence of Y. enterocolitica 8081 (accession no. AM286415/AM286416) were classified as specific for strain W22703, used in this study. Sequence assembly was done with Vector NTI Advance (Invitrogen). The resulting sequence was annotated using the NCBI ORF-Finder (http://www.ncbi.nlm.nih.gov/gorf/gorf.html). Homology searches of the single ORFs were performed with BLASTP from NCBI (http://www.ncbi.nlm.nih.gov/BLAST/). Promoter sequences located upstream of the identified genes were deduced using a promoter prediction program (http://www.fruitfly.org/seq_tools/promoter.html). The accession number of the Y. pseudotuberculosis genome sequence is NC_006155.
Measuring expression profiles using a luxCDABE reporter.
Bioluminescence measurements were performed in white 96-well plates with clear bottoms (Matrix Technologies). Transposon mutants were grown overnight at 30 °C with shaking (500 r.p.m.) in deep-well microtitre plates filled with 800 µl selective LB broth, and then 1 : 4000 diluted in 200 µl LB broth with kanamycin. For investigation of the expression profiles, the plates were incubated at 6, 10, 15, 20, 25, 30 and 37 °C. Bioluminescence (at 490 nm) and OD405 of all plates were measured in parallel using a Wallac VICTOR2 1420 multilabel counter (Perkin Elmer Life Sciences). Bioluminescence was recorded as relative light units (RLU), and background activity of 0.15 RLU was subtracted. Cells were measured in the late exponential phase (OD405 
0.8–0.9). To allow a direct comparison of results obtained at all temperatures applied, RLU were related to the growth of cells (RLU/OD).
Complementation of W22703-fliS1 : : Tn5lux and W22703-fliT : : Tn5lux.
The complete coding sequence of fliS1 (YE2525) and 411 nucleotides of its upstream sequence, as well as the coding sequence of fliT (YE2526) and 207 nucleotides located upstream, were amplified at an annealing temperature of 50 °C and with an elongation time of 2 min using the oligonucleotides HAPF1/fliTR1 and fliSF1/ybcMR1 (Table S1), respectively. Each fragment was digested with EcoRI and cloned separately into the EcoRI site of pACYC184, resulting in the recombinant plasmids pACYC184/fliS1 and pACYC184/fliT. Cloning was performed in E. coli DH5
MCR and constructs were confirmed by PCR and restriction analysis. The direction of gene transcription corresponds to that of the disrupted plasmid gene encoding chloramphenicol acetyltransferase. Plasmid constructs were transformed by electroporation into the respective Y. enterocolitica mutant strains.
Construction of insertional knockout mutants.
Knockout mutants of the putative flagellin gene flaA (orf9), the putative regulatory gene (orf10) and the hook-length-control gene fliK (orf5) of the new flagellar region Flag-2 as well as the flagellar regulatory gene flhDC were generated by plasmid insertion via homologous recombination. Short intragenic fragments of the target genes were amplified from Y. enterocolitica chromosomal DNA using primers listed in Table S1. Fragments were digested with XbaI and SacI (Fermentas) and ligated into the XbaI/SacI-restricted suicide plasmid pKRG9. The recombinant plasmids were transformed into E. coli S17.1 pir by electroporation and transferred into Y. enterocolitica W22703 via plate mating. For this purpose, five colonies of donor strain and approximately 20 colonies of recipient strain were mixed on an LB agar plate and incubated for 6 h at 30 °C. The complete lawn was scraped off the plate and resuspended in 1 ml LB medium. Serial dilutions were prepared and conjugants were selected on plates containing nalidixic acid and chloramphenicol. To exclude illegitimate recombination, the correct insertion of the recombinant plasmid was confirmed by PCR using a gene-specific primer or a cloning primer, and a plasmid-derived primer (Table S1).
Motility assay.
Motility was tested by assessing swimming phenotypes on motility agar (LB medium containing 0.3 % agar without antibiotics). Streak cultures of Y. enterocolitica wild-type, mutant and complementing strains were prepared on LB agar plates containing the appropriate antibiotics and incubated overnight at 30 °C. From these plates single colonies were stabbed onto motility agar plates and incubated initially for 2 h at 37 °C to start the assay with non-motile bacteria. The plates were subsequently incubated at 15 °C (44 h), 20 °C (21 h), 25 °C (21 h) and 30 °C (42 h).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). Among mutants with strongly elevated light emission, we identified 21 clones harbouring transposon insertions in 12 different genes associated with motility and chemotaxis (Table 1
; Fig. 1a), six of which are part of the Flag-1 cluster. Interestingly, five insertion sites were located within a different genome region of W22703 showing significant homologies to flagellar genes of Y. pestis (Fig. 1b) and Y. pseudotuberculosis. The sequences of these five transposon mutants were assembled into a 2367 bp fragment that was extended by inverse PCR to a total of 11 526 bp. BLASTN analysis revealed that 290 bp at the very 5'-end of this fragment were identical to the sequence of YE3610 within the Y. enterocolitica 8081 genome sequence. No further sequence identities were observed, indicating that the 11.2 kb region is absent in strain 8081. YE3610 is located within the 199 kb plasticity zone of Y. enterocolitica 8081, which is the largest region of species-specific genomic variation found within this species (Thomson et al., 2006). Further sequence analysis resulted in the annotation of 12 ORFs (Fig. 1b) with identities of deduced amino acid sequences to proteins from other pathogenic Yersinia species and from E. coli of between 31 % and 94 % (Table 2). All the ORFs were highly similar to the Flag-2 genes of Y. pestis CO92 (Parkhill et al., 2001) and Y. pseudotuberculosis IP32953 (YPTB3329–YPTB3322) as well as to the Flag-2 flagellar cluster from E. coli 042 (Ren et al., 2005).
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. The strains included W22703 and 8081 as a positive- and negative control, respectively, and 47 strains that had been isolated on five continents from humans and animals (Table 3). Distinct PCR fragments were obtained when chromosomal DNA of biotypes 2–5 served as a template. In each of these reactions, the PCR product showed the expected length of 1554 bp (I), 1834 bp (II), 1529 bp (III), 1817 bp (IV), 2171 bp (V), and 1517 bp (VI). In contrast, none of the six primer pairs produced amplified fragments when genomic DNA of biotype 1A or 1B was used, with the exception of fragment III, representing flaA, which was amplified from genomic DNA of strains 213-36/89 and 214-36/84 (Table 3). The presence of Flag-1 in each strain was confirmed by the amplification of fliT using primers fliSF1 and ybcMR1. No PCR amplification was observed for two strains, 237 and SZ4501/97, and the latter strain was found to be less motile than the wild-type strain in the swarming assay (results not shown).
Expression of motility and chemotaxis genes is maximal at approximately 20 °C
Most experiments addressing temperature-dependent transcription of virulence or flagellar genes of Y. enterocolitica are performed within a relatively narrow temperature range (24–37 °C). We therefore decided to investigate the transcriptional response of all mutants with insertions into chemotaxis and flagellar genes located in Flag-1 and Flag-2 at 6, 10, 15, 20, 25, 30 and 37 °C at an OD405 between 0.8 and 0.9. Very low light emission was observed for all mutants at 37 °C, with bioluminescence signals only slightly above background level (Fig. 2a, b). This finding is in accordance with the non-motile phenotype of Y. enterocolitica cells at human body temperature. The reporter activity of all mutants measured increased with decreasing temperature with a maximum at 20 °C, with the exception of W22703-YE2575 : : Tn5lux (Fig. 2b) and W22703-fleC : : Tn5lux (Fig. 2a), which had maximum activity at 25 °C. The highest expression was observed for fleC, which encodes a flagellin (Fig. 2a). Mutants W22703-fliD2 : : Tn5lux and W22703-fliS2 : : Tn5lux showed another temperature optimum at 30 °C (Fig. 2c), suggesting a putative role of the Flag-2 genes in virulence towards mammals. The temperature-dependent activity of enzymes involved in bioluminescence was also taken into account by using a correction factor derived from our own experimental data and the Arrhenius prediction (Bresolin et al., 2006a). This approach gave an optimal temperature of 15–20 °C for the expression of all genes investigated (data not shown).
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). To investigate their regulatory dependence on the flagellar activator FlhC, flhC was mutated by insertional knockout in strains W22703-fliD2 : : Tn5lux, W22703-fliS2 : : Tn5lux, W22703-fliT : : Tn5lux and W22703-fleC : : Tn5lux. The latter two served as control strains. The luciferase activity was then measured at 6–37 °C. The results obtained demonstrate that not only fliT or fleC, but also the Flag-2 genes fliS2 and fliD2 are under the control of the flagellar master operon at lower temperatures (Fig. 2c).
Four transposons had inserted into an ORF encoding a putative flagellar hook-associated protein HAP (fliD2), and the fifth transposon insertion was located in the neighbouring downstream gene, termed fliS2, which encodes a flagellin-specific chaperone FliS2 (Fig. 1b). To test the functionality of Flag-2 genes in strain W22703, we generated three insertional mutants with knockouts of the putative flagellar subunit gene flaA (orf9), a predicted regulatory gene (orf10) and fliK (orf5) which is similar to flagellar hook-length-control genes. Their in vitro growth at both 15 and 30 °C was identical to that of the wild-type strain. The swimming phenotypes of the mutants were investigated in motility assays at different temperatures. We observed an increase of motility at temperatures below 30 °C (Fig. 3a). No effect on motility was observed when flaA was mutagenized in strains with a Tn5lux insertion into cheA, fleC, fliB, fliD2, fliS2, ybcM, YE2575 or YE2848 (data not shown).
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). FliS1 probably acts as a cytosolic chaperone, and FliT is involved in the transcriptional regulation of flagellar genes (Auvray et al., 2001; Kutsukake et al., 1999). The motility phenotypes of the remaining transposon mutants, as well as of a fleC/flaA double mutant, did not differ from the wild-type phenotype under the tested conditions (data not shown).
To confirm the results obtained for the fliT and fliS1 mutants, the wild-type strain and the mutants W22703-fliS1 : : Tn5lux and W22703-fliT : : Tn5lux were transformed with the constructs pACYC184/fliS1 and pACYC184/fliT, respectively. Each fragment cloned carried the putative native promoter sequence (Table 1). In swarming assays, the hyper-motility of W22703-fliT : : Tn5lux was drastically reduced in the presence of pACYC184/fliT (Fig. 3b). A similar non-motile phenotype resulted from fliT overexpression in the wild-type, indicating that FliT acts as repressor of flagellar synthesis in Y. enterocolitica. Motility of the non-motile mutant W22703-fliS1 : : Tn5lux was successfully restored with the plasmid pACYC184/fliS1 (Fig. 3c).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). By PCR analysis of 49 strains representing six biotypes, we show that the newly described flagellar region Flag-2 in Y. enterocolitica is specific to strains from the low-pathogenic biotypes 2–5 and is absent from the high-pathogenic biotype 1B strains and from the biotype 1A strains tested. No pseudogenes were identified among the 12 ORFs annotated. The temperature-and flhDC-dependent expression of fliD2 and fliS2 were demonstrated in this study, as well as the functionality of at least three other Flag-2 genes, fliK, flaA and orf10. Their products are predicted to be involved in flagellar hook-length control and in regulation, activities that might explain the slightly hyper-motile phenotype of the respective mutants. Due to the absence of flaA in two Y. enterocolitica strains, as well as the wild-type-like motility of several double mutants in a flaA– background, the role of this additional flagellar subunit gene remains unclear. The distribution pattern of Flag-2 is similar to that of other genome domains, such as the insecticidal pathogenicity island tc-PAIYe (Bresolin et al., 2006b), indicating an evolutionary split of biotypes 1A and 1B from biotypes 2–5. We suggest that biotypes 2–5, but not 1A or 1B, use insects as transmission vectors, and that Flag-2 genes contribute to the fitness of pathogenic Y. enterocolitica outside their mammalian hosts, possibly by increasing the chance of encountering an invertebrate host.
We also tested the phenotype of three Flag-1 mutants. The non-motility of the fliF mutant is congruent with the finding that inactivation of fliF in Caulobacter crescentus (Grunenfelder et al., 2003) and Listeria monocytogenes (Bigot et al., 2005) abolishes flagella production and consequently bacterial motility. The increased motility of the fliT mutant indirectly confirms the finding that a Salmonella enterica sv. Typhimurium fliT : : kan mutant, in which flagellar structure and filament length were indistinguishable from those of wild-type flagella, produces twice as many flagella as the wild-type strain. However, no corresponding phenotype of S. enterica sv. Typhimurium with increased motility could be identified (Yokoseki et al., 1995, 1996). The hyper-motile phenotype of the fliT mutant of Y. enterocolitica is in line with the assumption that FliT acts as a negative regulator of the transcription of flagellar genes (Kutsukake et al., 1999). This function is confirmed by our observation that even the wild-type strain showed a decreased motility as a result of fliT overexpression (Fig. 3b). In contrast, Y. enterocolitica motility was abolished in a fliS1 mutant. The motility of the fliS1 strain, but not of the wild-type, after FliS1 overexpression was significantly higher than that of the wild-type alone (Fig. 3c). This effect might be explained by a coupled transcription of fliS1 and fliT, so that strain W22703-fliS1 : : Tn5lux/pACYC-fliS1 genotypically resembles the hyper-motile fliT mutant. Flagellar filaments produced by a fliS mutant in S. enterica sv. Typhimurium were much shorter than those produced by the wild-type strain, indicating that FliS is a cytosolic chaperone involved in controlling flagellin polymerization and preventing FliC degradation (Auvray et al., 2001; Ozin et al., 2003). Motility of a Salmonella fliS mutant was clearly impaired but not completely abolished (Yokoseki et al., 1995).
A causal connection between motility and virulence has been demonstrated in several studies. At 37 °C, Y. enterocolitica cells are non-motile, autoagglutinate and express and secrete Yops. The invasion capabilitiy of this bacterium has been shown to be affected by motility, suggesting that motility is required for migrating to and contacting host cells (Young et al., 2000). The fliA gene, encoding the alternative factor of the flagellar system, is highly induced at 25 °C and repressed at 37 °C (Kapatral et al., 2004). FliA probably indirectly inhibits the expression at 25 °C of seven genes that are encoded by the pYV virulence plasmid, indicating that FliA plays a role in the inverse temperature regulation of flagellar and virulence genes (Horne & Pruss, 2006). This observation is in line with the finding that a non-motile flhDC mutant of Y. enterocolitica also secretes larger amounts of Yops, encoded by the pYV plasmid, than the wild-type bacteria (Bleves et al., 2002). Evidence has also been found for a temperature-dependent synthesis antagonism between type III secretion in Y. enterocolitica, which is necessary for the survival of the bacterium in the mammalian host, and its flagellar assembly system (Bleves et al., 2002). The flagellar export apparatus functions as a secretion system for the virulence-associated phospholipase YplA (Young et al., 1999). Microarray data on S. enterica sv. Typhimurium motility have only recently revealed several FlhDC-controlled virulence genes, among them the virulence operon srfABC, thus demonstrating that the association between motility and virulence is a phenomenon not restricted to Y. enterocolitica (Frye et al., 2006; Wang et al., 2004). Recently, it has been shown that in the insect pathogen Xenorhabdus nematophila the expression of a novel haemolysin, which is required for full virulence of X. nematophila against insects, is also regulated by the flagellar master-operon (Cowles & Goodrich-Blair, 2005). Interestingly, within the scope of identifying genes that are upregulated during prolonged growth of Y. enterocolitica at low temperatures, e.g. during proliferation in insects, we identified a srfA homologue and a haemolysin secretion gene (Bresolin et al., 2006a). It might therefore be speculated that temperature-driven non-motility at 37 °C and maximal motility at approximately 20 °C essentially contribute to the control of the Y. enterocolitica switch between two pathogenicity phases related to mammals and invertebrates.
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ACKNOWLEDGEMENTS |
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Edited by: P. W. O'Toole
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Received 27 April 2007;
revised 28 September 2007;
accepted 5 October 2007.
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