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J Bacteriol. Mar 2008; 190(6): 1879–1890.
Published online Jan 4, 2008. doi:  10.1128/JB.01796-07
PMCID: PMC2258875

CmeR Functions as a Pleiotropic Regulator and Is Required for Optimal Colonization of Campylobacter jejuni In Vivo[down-pointing small open triangle]

Abstract

CmeR functions as a transcriptional repressor modulating the expression of the multidrug efflux pump CmeABC in Campylobacter jejuni. To determine if CmeR also regulates other genes in C. jejuni, we compared the transcriptome of the cmeR mutant with that of the wild-type strain using a DNA microarray. This comparison identified 28 genes that showed a ≥2-fold change in expression in the cmeR mutant. Independent real-time quantitative reverse transcription-PCR experiments confirmed 27 of the 28 differentially expressed genes. The CmeR-regulated genes encode membrane transporters, proteins involved in C4-dicarboxylate transport and utilization, enzymes for biosynthesis of capsular polysaccharide, and hypothetical proteins with unknown functions. Among the genes whose expression was upregulated in the cmeR mutant, Cj0561c (encoding a putative periplasmic protein) showed the greatest increase in expression. Subsequent experiments demonstrated that this gene is strongly repressed by CmeR. The presence of the known CmeR-binding site, an inverted repeat of TGTAAT, in the promoter region of Cj0561c suggests that CmeR directly inhibits the transcription of Cj0561c. Similar to expression of cmeABC, transcription of Cj0561c is strongly induced by bile compounds, which are normally present in the intestinal tracts of animals. Inactivation of Cj0561c did not affect the susceptibility of C. jejuni to antimicrobial compounds in vitro but reduced the fitness of C. jejuni in chickens. Loss-of-function mutation of cmeR severely reduced the ability of C. jejuni to colonize chickens. Together, these findings indicate that CmeR governs the expression of multiple genes with diverse functions and is required for Campylobacter adaptation in the chicken host.

Campylobacter jejuni is a gram-negative enteric organism causing gastroenteritis in humans (38). As a major food-borne pathogen, C. jejuni is well adapted in its mammalian and avian hosts, as well as in food animal production environments. So that it can survive in different conditions, C. jejuni has evolved multiple strategies for adaptation, including high rates of genetic variation (mediated by mutation and horizontal gene transfer) and differential gene expression (6, 11, 30). Indeed, previous analyses of the genomic sequences of C. jejuni revealed the presence of multiple genes encoding regulatory functions (9, 14, 32, 35). The majority of the transcriptional regulators have not been functionally characterized, but the two-component regulatory (TCR) systems in C. jejuni have recently received attention. In C. jejuni NCTC 11168 and RM1221, each system has nine response regulators and six histidine sensor kinases (9, 32). Several of the regulators or TCR systems, including DccRS (28), PhosSR (49), FlgSR (50), CbrR (36), RacRS (4), and CheY (53), have been studied, and all were found to be required for Campylobacter colonization in vivo. The RacRS system is responsive to temperature and controls the expression of multiple proteins in C. jejuni, while the PhosSR system senses phosphate conditions and modulates the expression of 12 genes that are involved in phosphate transport and utilization (4, 49). DccRS controls the expression of several genes encoding probable membrane-associated proteins and is required for Campylobacter colonization of mice and chickens, but the signals to which it responds and the functions of the DccR-regulated genes have not been defined (28). CbrR modulates the Campylobacter response to bile, but its cognate sensor kinase and the target genes controlled by it are unknown (36). The FlgSR system controls the flagellar regulon and affects the motility of Campylobacter (50). It was also found that FlaR undergoes phase variation due to the presence of homopolymeric tracts of adenine and thymine in the coding gene (13). These examples illustrate that C. jejuni utilizes multiple TCR systems for adaptation to different environments.

In addition to the TCR systems, several non-TCR system regulators, including Fur, SpoT, HspR, and CmeR, have also been characterized in Campylobacter. Fur functions as a transcriptional repressor and controls iron homeostasis in C. jejuni (31, 45). Mutation of Fur affected the expression of 53 genes and significantly reduced the colonization of chickens by Campylobacter (31). HspR is a negative regulator for the heat shock response system in C. jejuni, and inactivation of HspR led to increased expression of several genes involved in the heat shock response and decreased expression of 17 genes (1). The HspR mutant showed decreased motility, increased sensitivity to temperature, and reduced adherence to and invasion of cultured epithelial cells (1). SpoT functions as a regulator for the stringent response in C. jejuni and is important for the survival of Campylobacter under various stress conditions (11). Deletion of spoT resulted in differential expression of multiple genes and reduced Campylobacter adherence, invasion, and intracellular survival in cell cultures (11). In a previous study by workers in our laboratory (21), a transcriptional regulator designated CmeR was characterized. CmeR belongs to the TetR family of transcriptional regulators and functions as a repressor of CmeABC, a resistance-nodulation-division-type efflux pump (23). The CmeABC pump is composed of three membrane components (CmeA, CmeB, and CmeC) and contributes to Campylobacter resistance to various antimicrobial agents and bile compounds present in the intestinal environment (23, 24). Inactivation of CmeABC abolished the ability of C. jejuni to colonize chickens (24), indicating that bile resistance is an important physiological function of CmeABC and that this efflux pump plays an important role in facilitating Campylobacter adaptation to the intestinal tract.

CmeR is encoded by a gene located immediately upstream of the cmeABC operon and has two distinct domains, an N-terminal helix-turn-helix (HTH) DNA-binding motif and a potential ligand-binding domain in the C-terminal region (21). An in vitro electrophoretic mobility shift assay showed that CmeR binds specifically to the inverted repeat (TGTAAT) in the promoter region of cmeABC and represses the transcription of this efflux operon. Deletion of CmeR or mutation in the CmeR-binding site impedes the repression and results in overexpression of CmeABC (21). Importantly, the expression of cmeABC is strongly induced by bile salts in culture media (22). Since bile compounds are normally present in animal intestinal tracts, it is likely that CmeABC is upregulated during in vivo infection. Indeed, the DNA microarray work conducted by Stintzi et al. showed that cmeABC was significantly upregulated in rabbit ileal loops (40). Real-time quantitative reverse transcription-PCR (qRT-PCR) conducted in our laboratory also showed that there was a 14-fold increase in the expression of cmeABC in the chicken cecum compared with an in vitro culture (Y. W. Barton and Q. Zhang, unpublished data). Bile salts inhibit the binding of CmeR to the promoter DNA of cmeABC and thus release the inhibition of cmeABC by CmeR (22), resulting in overexpression of cmeABC. Recent protein crystallization studies confirmed the two-domain structure of CmeR and showed that CmeR functions as a homodimer (12). A striking feature of CmeR revealed by crystallization is the presence of a large ligand-binding pocket, which has the capacity and flexibility to accommodate diverse ligands, including bile salts. This finding suggests that CmeR may interact with multiple ligands in modulating gene expression in C. jejuni.

Despite the recent advances in our understanding of the structure and function of CmeR and its transcriptional regulation of cmeABC, it is still unclear if CmeR regulates the expression of other genes in C. jejuni. It is also not known if CmeR is important for Campylobacter colonization in vivo. In this study, we compared the transcriptomes of C. jejuni NCTC 11168 and an isogenic CmeR mutant of this strain using a DNA microarray along with other molecular methods and identified multiple genes regulated by CmeR. We also characterized a gene (Cj0561c) that is highly repressed by CmeR and determined the role of CmeR in Campylobacter colonization in the chicken model system. Our new findings indicate that CmeR is a pleiotropic regulator modulating the expression of multiple genes with diverse functions in Campylobacter and is required for optimal colonization of chickens.

MATERIALS AND METHODS

Bacterial strains, plasmids, and culture conditions.

The various Campylobacter strains, mutants, and plasmids used in this study and their sources are listed in Table Table1.1. The isolates were routinely grown in Mueller-Hinton (MH) broth (Difco) or on MH agar plates at 42°C under microaerobic conditions. Escherichia coli was grown in Luria-Bertani (LB) broth with shaking (250 rpm) or on LB agar plates at 37°C overnight. When necessary, culture media were supplemented with ampicillin (100 μg/ml), kanamycin (30 μg/ml), or chloramphenicol (20 μg/ml).

TABLE 1.
Key bacterial plasmids and strains used in this study

Bacterial RNA isolation.

C. jejuni NCTC 11168 and the isogenic cmeR mutant 11168ΔcmeR (21) were grown in MH broth for 17 h to the mid-log phase (optical density at 600 nm, ~0.13) and were immediately treated with 2 volumes of RNAprotect bacterial reagent (Qiagen, Valencia, CA) to stabilize the total bacterial RNA. After incubation for 5 min at room temperature, the culture was centrifuged at 10,000 × g for 5 min. Bacterial RNA was isolated using an RNeasy mini kit (Qiagen) according to the manufacturer's instructions. Isolated RNA was treated with an on-column DNase digestion kit (Qiagen), which was followed by removal of the DNase using an RNeasy mini kit (Qiagen). The absence of DNA contamination in the RNA samples was confirmed by reverse transcription-PCR (RT-PCR). The concentration of total RNA was determined by measuring the absorbance at 260 nm with a spectrophotometer, and the integrity and size distribution of the purified RNA were determined by denaturing agarose gel electrophoresis and ethidium bromide staining. The purified RNA was kept at −80°C until it was used. RNA samples extracted from three independent experiments were used for the microarray hybridization experiments.

cDNA synthesis and labeling.

cDNA synthesis and fluorescence labeling were performed using the SuperScript indirect cDNA labeling system (Invitrogen, Carlsbad, CA) according to the manufacturer's instructions. Briefly, 12 μg of total RNA was reverse transcribed using SuperScript III reverse transcriptase and random hexamers in the presence of an aminoallyl-modified nucleotide and an aminohexyl-modified nucleotide together with other deoxynucleoside triphosphates. The RNA was then hydrolyzed by incubating the reaction mixture with sodium hydroxide at 70°C for 10 min. The sodium hydroxide was neutralized by addition of hydrochloric acid. After purification using S.N.A.P. columns (Invitrogen) to remove unincorporated nucleotides, the amino-modified cDNAs were labeled by coupling with a monoreactive, N-hydroxysuccinimide-ester fluorescent dye, Cy3 or Cy5 (Amersham Biosciences, Piscataway, NJ). A final purification step using S.N.A.P. columns (Invitrogen) removed unreacted dyes, and the fluorescently labeled cDNA was used to hybridize microarray slides.

Microarray hybridization.

C. jejuni NCTC 11168 microarray slides were purchased from MWG Biotech, Inc. (now Ocimum Biosolutions). The array contained 1,632 oligonucleotide (50-mer) probes covering the entire transcriptome of NCTC 11168. To compare the gene expression of 11168ΔcmeR with that of NCTC 11168, equal volumes of Cy3- or Cy5-labeled cDNAs from the wild type and the cmeR mutant strain were combined and dried with a SpeedVac and then resuspended in 120 μl of hybridization buffer (MWG). The probes were denatured at 95°C for 3 min, briefly cooled on ice, and then hybridized to the microarray slides using microarray gene frames (MWG). A second microarray slide was hybridized with the dyes reversed to the two samples to ensure dye balance in the experimental design. The slides were placed in a wet hybridization chamber and incubated on a shaker for 20 h at 42°C. After hybridization, the slides were washed with 2× SSC-0.1% sodium dodecyl sulfate (SDS) for 5 min, with 1× SSC for 5 min, and with 0.1× SSC for 5 min (1× SSC is 0.15 M NaCl plus 0.015 M sodium citrate). The washing buffers were prewarmed at 30°C, and all of the washing steps were performed at room temperature. Finally, the slides were dried by centrifugation at 500 × g for 2 min. The hybridization experiment was repeated three times by using total RNA isolated in three independent experiments. Thus, six technical replicates from three biologically independent experiments were used for data analysis.

Data collection and analysis.

Hybridized slides were scanned at a wavelength of 650 nm for Cy5 and at a wavelength of 550 nm for Cy3 using a General Scanning ScanArray 5000 (PerkinElmer, Boston, MA) at 10-μm resolution. The fluorescence intensities were collected with the ImaGene software (BioDiscovery, El Segundo, CA). Genes with fluorescence signals within less than 2 standard deviations of the background were considered insignificant. Background subtraction, Lowess normalization, scale normalization, and median centering were performed using the R statistical package (version 2.0.1; The R Foundation for Statistical Computing). Briefly, the spot-specific median background for each probe was subtracted from the spot-specific signal mean intensity to obtain a background-corrected signal intensity for each probe. The natural logarithm of the signal mean with the background subtracted was normalized using a locally weighted polynomial regression (Lowess) procedure to control for intensity-dependent dye bias (8, 51). The resulting values were then median centered by addition of an additive constant so that the medians of all array-dye combinations equaled zero. Finally, scale normalization was used to normalize the scales of all channels to a constant value by multiplying each value by C/MADj, where MADj is the median of the absolute deviations from the median for a given channel and C is the geometric mean of the collective MADj (51).

The normalized data were subjected to a mixed-linear model analysis using the SAS statistical package. This analysis was performed on a gene level with fixed effects of treatment (cmeR mutant versus the wild type) and dye (Cy3 versus Cy5) and random effects of replications, samples, and slides. As part of this analysis a t test for differential expression across treatments was performed for each probe, and the associated P values were converted to Q values to correct for multiple testing using a previously described method (42). The Q values could be used to estimate the false discovery rate (FDR) associated with a set of genes declared to be differentially expressed. For example, the FDR for a list of genes with Q values less than or equal to 0.1 is estimated to be 10%. The differential data were obtained by using the inverse natural log of the mean treatment difference. For this study, we chose a P value of <0.05 and a change equal to or greater than 2.0-fold as the cutoff for significant differential expression in comparisons of the cmeR mutant and wild-type strain NCTC 11168.

PCR, RT-PCR, and real-time qRT-PCR.

Key PCR primers used in this study are listed in Table Table2.2. PCR was performed in a 50-μl mixture containing each deoxynucleoside triphosphate at a concentration of 200 nM, each primer at a concentration of 200 nM, 2.0 mM MgCl2, 100 ng of Campylobacter genomic DNA, and 2.5 U of Platinum Taq DNA polymerase (Invitrogen) or Pfu Turbo DNA polymerase (Stratagene). RT-PCR was conducted using the SuperScript III one-step RT-PCR system with Platinum Taq DNA polymerase (Invitrogen). An RT-PCR mixture lacking the reverse transcriptase was included as a negative control. Real-time qRT-PCR was performed as described previously (22) using gene-specific primers.

TABLE 2.
Key oligonucleotide primers used in this study

Primer extension.

The transcription start of the Cj0369c-cmeR operon was identified by using a nonradioactive primer extension method described by Lloyd et al. (25). A 6-carboxyfluorescein-labeled primer, CJ0369FAM (Table (Table2),2), was used in the reaction. The previously described method was used without any modifications except that an ABI 3730 DNA sequencer (Applied Biosystems) with GeneScan −500 ROX internal lane standards (Applied Biosystems) was used to analyze the samples. The sizes of the cDNA fragments were determined using the GeneScan analysis software (version 3.1; Applied Biosystems).

Expression of cmeR using a shuttle vector.

Primers 370F and 368R3 (Table (Table22 and Fig. Fig.1A)1A) were used in PCR to amplify a 2.2-kb fragment containing cmeR, Cj0369c, and the upstream sequence of Cj0369c. This fragment was cloned into shuttle vector pRY107 (52) using the restrictions sites in the primers, resulting in plasmid pRYC7 (Table (Table11 and Fig. Fig.1B).1B). To determine if there is an internal promoter within Cj0369c that drives the expression of cmeR, primers 369F and 368R (Table (Table22 and Fig. Fig.1A)1A) along with VentR DNA polymerase (New England Biolab) were used in PCR to amplify the operon without the upstream sequence of Cj0369c. The amplified fragment was subsequently cloned into the SmaI site of pRY107 to create pRYC1 (Table (Table11 and Fig. Fig.1B).1B). Compared with pRYC7, pRYC1 lacks the promoter region of Cj0369c (Fig. (Fig.1B).1B). Both pRYC7 and pRYC1 were sequenced, and no mutations were detected in the cloned DNA. By conjugation, the plasmid constructs were introduced into 11168ΔcmeR, resulting in C. jejuni strains 11168ΔcmeR+ and 11168ΔcmeRC1, respectively (Table (Table1).1). The pRYC7 plasmid was also conjugated into 11168W7ΔcmeR, generating 11168W7ΔcmeR+ (Table (Table1).1). The expression of cmeR from these C. jejuni constructs was determined by SDS-polyacrylamide gel electrophoresis (PAGE) and immunoblotting using rabbit anti-recombinant CmeR (rCmeR) polyclonal antisera.

FIG. 1.
Identification of the transcription unit of the Cj0369c-cmeR operon. (A) Genomic organization and sequence features of the Cj0369c-cmeR operon. ORFs are indicated by large arrows. The gene designations are indicated under the arrows. The bent arrows indicate ...

Production of rCmeR and rCj0561c and their antibodies.

Full-length six-His-tagged rCmeR of NCTC 11168 was produced in E. coli by using the pQE30 (Qiagen) vector and the method described by Lin et al. (21). The pQE30 vector was also used to express six-His-tagged recombinant Cj0561c (rCj0561c). Briefly, a portion of the coding sequences of Cj0561c was amplified by using the Pfu polymerase with primers 561clF and 561clR (Table (Table2).2). The PCR product was digested with BamHI and SmaI, which recognized the restriction sites embedded in the primers, and ligated to the pQE30 plasmid predigested with the same enzymes. The recombinant plasmids were electroporated into E. coli M15 (Qiagen). Transformants were selected on LB agar plates containing 30 μg/ml of kanamycin and 100 μg/ml of ampicillin. The plasmids were sequenced, and no mutations were detected in the cloned sequence. rCj0561c was purified using an Ni-nitrilotriacetic acid column (Qiagen) by following the manufacturer's instructions. The purified rCmeR and rCj0561c were resuspended in phosphate-buffered saline and emulsified with an equal volume of incomplete Freund's adjuvant (Sigma). Each antigen was subcutaneously injected into two New Zealand White rabbits (100 μg of protein/rabbit). Each rabbit received two additional booster immunizations at 2-week intervals. The rabbits were bled 21 days after the last injection. Pre- and postimmune serum samples were analyzed by immunoblotting.

Construction of an isogenic ΔCj0561c mutant.

Two separate fragments of the coding sequence of Cj0561c and its flanking genes were amplified using the Taq polymerase (Promega) with primer pairs 561UF/561UR and 561LF/561LR (Table (Table2).2). A chloramphenicol resistance cassette (cat) was amplified with the Pfu polymerase (Stratagene) from the pUOA18 plasmid (46) using primers ChlU and ChlL (Table (Table2).2). The amplified DNA fragments of Cj0561c and the cat cassette were digested with BamHI and XbaI (Table (Table22 shows the restriction sites in the primers) and were purified with a PCR clean-up kit (Qiagen). The digested Cj0561c DNA fragments were ligated to the cat cassette by using T4 DNA ligase (Promega). As a result, the cat cassette was flanked by the Cj0561c fragments. The ligated product was purified by agarose gel electrophoresis and then cloned into the pGEM-T vector (Promega). The plasmid construct was transformed into E. coli JM109. Then the plasmid construct was purified from E. coli and introduced into C. jejuni 11168W7 via electroporation. Insertional mutants were selected on MH agar plates with 4 μg/ml of chloramphenicol, and the double crossover in the 11168W7Δ561 mutant was confirmed by PCR. This mutation resulted in deletion of 609 bp of the coding sequence from Cj0561c and simultaneous insertion of the cat gene into the same location.

SDS-PAGE and immunoblotting.

C. jejuni proteins were separated by SDS-PAGE and immunoblotted using anti-CmeR and anti-Cj0561c antibodies as described previously (23).

Construction of transcriptional fusion.

The promoter region of Cj0561c was amplified from C. jejuni 11168W7 genomic DNA by using the Pfu polymerase with primers 561PF and 561PR (Table (Table2).2). The amplified DNA was digested with BamHI and XbaI and cloned into the pMW10 plasmid (48), leading to transcriptional fusion of the Cj0561c promoter with the promoterless lacZ gene. The ligation mixture was transformed into E. coli DH5α cells. The plasmid with the transcriptional fusion was designated pMW561 (Table (Table1).1). PCR and DNA sequencing were performed to verify the desired fusion and the lack of mutations in the cloned promoter DNA in pMW561, which was subsequently transferred to C. jejuni 11168W7 and 11168W7ΔcmeR using the triparental conjugation method reported previously (29).

β-Galactosidase assay and induction of Cj0561c by bile salts.

The β-galactosidase assay was performed as described previously (21). To compare the promoter activities of Cj0561c in 11168W7 and 11168W7ΔcmeR, these two strains carrying pMW561 were grown in MH broth overnight, and the cultures were harvested in order to measure β-galactosidase activity. To measure the induction of Cj0561c by bile salts, C. jejuni 11168W7 carrying pMW561 was grown under microaerobic conditions in MH broth or MH broth supplemented with cholic acid (2.0 mg/ml) and taurocholic acid (4.0 mg/ml) for 16 h. The same growth conditions were also used for 11168W7ΔcmeR carrying pMW561 to determine if Cj0561c was further induced by bile salts in the cmeR mutant. The β-galactosidase activities in the cultures were then measured to determine if bile salts affected transcription of Cj0561c. The concentrations of the bile salts used in the cultures were below the minimal concentrations that inhibit 11168W7. 11168W7 containing empty vector pMW10 was used as the background control for β-galactosidase activity.

Chicken colonization experiments.

One-day-old broiler chickens were obtained from a commercial hatchery. The chickens were negative for C. jejuni as determined by culturing cloacal swabs prior to use in the study. Nonmedicated feed and water were given ad libitum to the chickens. To determine if Cj0561c contributes to Campylobacter colonization in chickens, three groups of chickens were inoculated when they were 3 days old with 11168W7 (group 1), 11168W7Δ561 (group 2), and 1:1 mixture of these two strains (group 3). Inoculation was performed via oral gavage using a dose of approximately 106 CFU of bacteria per bird. For groups 1 and 2, five birds from each group were euthanized and cecal contents were collected on days 3, 6, and 9 postinoculation (p.i.). For group 3, the 12 birds inoculated were sampled on days 3, 6, and 9 p.i. using cloacal swabs. The feces from each bird were weighed and diluted in MH broth. The diluted samples were cultured on MH agar plates with Campylobacter-specific selective supplements (Oxoid, United Kingdom) for colony counting. For the group 3 fecal samples, duplicate plating of each sample on MH agar plates with supplements and on chloramphenicol (4 μg/ml)-containing plates was performed to determine the total Campylobacter number and the 1168W7Δ561 number for each sample. The plating media were tested prior to use to ensure that they supported the growth of the mutant strain. The number of CFU per gram of feces was calculated for each chicken and was used as an indicator of the colonization level.

To determine the role of CmeR in chicken colonization, four groups of birds (15 birds per group) were inoculated with 11168W7 (group 1), 11168W7ΔcmeR (group 2), 11168W7ΔcmeR+ (group 3), and 1:1 mixture of 11168W7 and 11168W7ΔcmeR (group 4). Each bird received approximately 106 CFU of C. jejuni via oral gavage when it was 4 days old. Five chickens from each group were sacrificed on days 3, 6, and 9 p.i., and the cecal contents were collected for culturing Campylobacter. For groups 1 to 3, the fecal material was weighed, serially diluted, and plated onto MH agar plates containing Campylobacter-specific growth supplements and selective agents. For group 4, each fecal sample was cultured using two types of plates: the conventional selective plates for counting total Campylobacter colonies and the same plates containing 4 μg/ml of chloramphenicol for counting colonies of 11168ΔcmeR. All of the plating media were pretested for suitability to recover the various strains inoculated into chickens. The level of colonization was expressed as CFU/g of feces. Representative Campylobacter colonies recovered from each group of chickens were analyzed by PCR and by growth on appropriate antibiotic-containing media. The chicken experiment was conducted twice with the same design.

In both experiments, the detection limit of the plating methods was 100 CFU/g of feces. A bird from which no Campylobacter colonies were detected was considered negative and assigned a value of 0 for the purpose of calculating means and for statistical analysis. The significance of differences in the levels of colonization between the group inoculated with a mutant strain and the group inoculated with the wild-type strain was determined using Student's t test, Welch's t test to allow for nonconstant variation across treatment groups, and the Wilcoxon rank-sum test to allow for nonnormality. For all comparisons discussed in Results, the conclusions for all three tests were the same at a significance level of 0.05. For clarity and brevity, we present only the P values for the Wilcoxon rank-sum test in Results.

Microarray data accession number.

The microarray data have been deposited in the NCBI Gene Expression Omnibus (http://www.ncbi.nlm.nih.gov/geo/) under accession number GSE5412.

RESULTS

Identification of the native promoter of cmeR.

To facilitate the characterization of cmeR, we identified the promoter that drives the transcription of this gene in C. jejuni. As illustrated in Fig. Fig.1A,1A, Cj0369c and cmeR are tandemly positioned on the C. jejuni chromosome. The two open reading frames (ORFs) overlap by 14 nucleotides, suggesting that they are likely cotranscribed. To examine this possibility, RT-PCR was performed using total RNA from NCTC 11168 and primers MGCJ0368-F1 and MGCJ0369-R2 (Table (Table2)2) that span the junction of Cj0369c and cmeR. A positive RT-PCR product was obtained from the RNA sample (data not shown), while control RT-PCRs (with no reverse transcriptase) yielded no products, suggesting that cmeR and Cj0369c are cotranscribed. There is a single predicted RpoD promoter in front of Cj0369c, and there is no predicted promoter in front of cmeR (34). To determine the location of the promoter that drives the expression of cmeR, pRYC7 and pRYC1 (Fig. (Fig.1B)1B) were introduced into 11168ΔcmeR, and expression of cmeR from the plasmid constructs was detected by immunoblotting using anti-rCmeR antibodies. As shown in Fig. 1C, a CmeR-specific product was detected in 11168ΔcmeR transformed with pRYC7 (lane 3) but not in 11168ΔcmeR transformed with pRYC1 (lane 4). This result indicated that there is no promoter in the coding sequence of Cj0369c and that cmeR is expressed from the promoter in front of Cj0369c.

In order to locate the transcription start of the Cj0369c-cmeR operon, primer extension was performed using primer CJ0369FAM (Table (Table2)2) and total RNA extracted from strain NCTC 11168. An electropherogram revealed a single distinct 6-carboxyfluorescein peak corresponding to a cDNA fragment consisting of 283 bases (data not shown). This result mapped the transcription start site to the guanine 30 nucleotides upstream of the ATG start codon of Cj0369c (Fig. (Fig.1A).1A). Based on the results of the primer extension analysis and the predicted promoter consensus sequences (48), the −10, −16, and −35 sites of the promoter were identified (Fig. (Fig.1A).1A). Interestingly, there was an inverted repeat between the −10 site and the ribosome-binding site (Fig. (Fig.1A),1A), suggesting a potential regulatory mechanism for this operon.

Identification of genes regulated by CmeR.

It has been well established that CmeR functions as a repressor for cmeABC (21), but it was not known if CmeR regulates other genes in C. jejuni. To examine this possibility, we compared the transcriptome of strain NCTC 11168 with that of 11168ΔcmeR using a DNA microarray. Mutation of cmeR did not affect C. jejuni growth in MH broth, and the mutant and wild-type strain showed comparable growth kinetics (data not shown). Based on the results of the DNA microarray analysis, 27 genes had P values of <0.05 and were differentially expressed ≥2-fold in the wild type and 11168ΔcmeR (Table (Table3).3). Cj0087 showed a 1.97-fold increase (P = 0.011) and is also listed in Table Table3.3. The Q values for these 28 genes indicated that the FDR was approximately 20%. However, real-time qRT-PCR experiments using independent RNA samples confirmed 27 of the 28 genes identified by the DNA microarray (Table (Table3).3). Thus, the actual proportion of false-positive results appeared to be less than 20% in this case. The expression of Cj0583, for which the microarray showed a 6.6-fold increase, was not confirmed by real-time RT-PCR (Table (Table3).3). To confirm that the changes were associated with mutation of cmeR, we compared the expression of 14 representative genes in 11168ΔcmeR+ (11168ΔcmeR complemented with pRYC7) and wild-type strain NCTC 11168. The complementation either partially or completely restored the levels of expression of the tested genes to the wild-type levels (Table (Table33).

TABLE 3.
Differentially expressed genes (≥2-fold changes) in 11168ΔcmeR identified by DNA microarray and real-time qRT-PCR

Of the 28 affected genes, 11 were upregulated and the others were downregulated in 11168ΔcmeR (Table (Table3).3). The identified genes encode membrane transporters (Cj0035c, Cj0366c, Cj0561c, and Cj1619), proteins involved in C4-dicarboxylate transport or utilization and metabolism (Cj0087, Cj0088, Cj0671, Cj0437, Cj0264c, and Cj0119), enzymes for capsular polysaccharide biosynthesis (Cj1424c, Cj1426c, Cj1427c, Cj1428c, Cj1429c, Cj1430c, and Cj1432c), periplasmic proteins or lipoproteins (Cj0089, Cj0091, Cj0092, Cj0628/Cj0629, and Cj0854c), a putative transcriptional factor (Cj1563c, a MerR homolog), a putative iron-binding protein (Cj0241c), a putative purine nucleoside triphosphate pyrophosphatase (Cj1374c), and hypothetical proteins (Cj0040 and Cj0583). Cj0628 and Cj0629 were originally annotated as two separate genes (32), but recent work showed that these two genes form a single ORF encoding an autotransporter protein (2).

Differential expression of the identified genes in 11168W7 and 11168W7ΔcmeR.

The DNA microarray analysis was performed with strain NCTC 11168, whose genomic sequence was used to manufacture the microarray slides used in this study. However, this sequenced strain is a poor colonizer in animals. To determine the role of CmeR and the genes that it regulates in colonization, we introduced the cmeR mutation into 11168W7 (Table (Table1),1), a highly mobile variant of NCTC 11168 and an excellent colonizer of the chicken host. Strains NCTC 11168 and 11168W7 were indistinguishable by pulsed-field gel electrophoresis (data not shown). The cmeR mutation in 11168W7 did not change the growth characteristics because 11168W7 and11168W7ΔcmeR grew similarly in MH broth (data not shown). Using real-time RT-PCR, we confirmed that several representative genes were differentially expressed in 11168W7ΔcmeR and 11168W7 (Table (Table4).4). For example, all six upregulated transporter genes in 11168ΔcmeR (Table (Table3)3) were also upregulated in 11168W7ΔcmeR (Table (Table4).4). In both cases, Cj0561c showed the greatest increase in expression in the cmeR mutant background. This result not only further validates the findings of the DNA microarray analysis but also indicates that the null mutation of cmeR resulted in similar patterns of changes in NCTC 11168 and 11168W7.

TABLE 4.
Differential expression of selected genes in 11168W7ΔcmeR and 11168W7

Confirmation of Cj0561c regulation by CmeR.

As shown by the DNA microarray and real-time RT-PCR analyses (Tables (Tables33 and and4),4), the expression of Cj0561c was greatly upregulated in the CmeR mutant, suggesting that CmeR represses the expression of this gene. Cj0561c encodes a probable periplasmic protein and is transcribed divergently from the flanking genes (Fig. (Fig.2A).2A). Although the Cj0561c protein does not show significant sequence homology to proteins with known functions, it has a single N-terminal hydrophobic domain and a large generally hydrophilic C-terminal region, a structural feature of periplasmic membrane fusion proteins involved in transport across membranes (7). Analysis of the intergenic sequence of Cj0561c revealed the presence of the known CmeR-binding site (Fig. (Fig.2A),2A), an inverted repeat of TGTAAT (21). In contrast to the promoter of cmeABC, which has one inverted repeat (21), there are two inverted repeats (IR1 and IR2) (Fig. (Fig.2A)2A) in the promoter region of Cj0561c, suggesting that CmeR may bind to more than one site in the promoter of Cj0561c.

FIG. 2.
Regulation of Cj0561c by CmeR. (A) Genomic location and sequence features of the promoter region of Cj0561c. The inverted repeats (IR1 and IR2), representing the known binding site of CmeR, are indicated by uppercase letters and arrows. The start codon ...

To confirm the regulation of Cj0561c by CmeR, the promoter region of Cj0561c was fused with the promoterless lacZ gene in the pMW10 plasmid. The transcriptional fusion construct (pMW561) was introduced into C. jejuni 11168W7 and 11168W7ΔcmeR. As shown in Fig. Fig.2B,2B, the base level of transcription of Cj0561c in 11168W7 was low, but its expression was approximately 46-fold greater in 11168W7ΔcmeR. In addition, immunoblotting was performed to compare the production of Cj0561c in 11168W7 and the production of Cj0561c in 11168W7ΔcmeR. As shown in Fig. Fig.2C,2C, Cj0561c was barely detected by anti-Cj0561c antibodies in 11168W7 but was greatly overexpressed in 11168W7ΔcmeR. These results are consistent with the findings of the DNA microarray and real-time RT-PCR analyses and strongly indicate that CmeR inhibits the expression of Cj0561c.

Cj0561c is inducible by bile salts.

Our previous studies showed that bile compounds, which are normally present in animal intestinal tracts, inhibit the binding of CmeR to the promoter of cmeABC and induce the expression of this efflux operon (22). Since CmeR also represses Cj0561c, we suspected that bile salts may also induce the expression of Cj0561c. This possibility was investigated using transcriptional fusion construct pMW561 (Table (Table1).1). Strain 11168W7 carrying pMW561 was cultured in the presence or absence of bile salts. Compared with the base level of expression in MH broth, the transcription of Cj0561c showed 17- and 28-fold increases in the presence of cholic acid and taurocholic acid, respectively (Fig. (Fig.3A).3A). This finding indicates that Cj0561c is strongly induced by bile salts. To determine if the induction is via the CmeR regulatory pathway, 11168W7ΔcmeR carrying pMW561 was also cultured in the presence of bile salts. As shown in Fig. Fig.3B,3B, the base level of expression of Cj0561c in 11168W7ΔcmeR was high due to loss of the repressor. When bile salts were added in the culture media, the transcription of the Cj0561c promoter showed only 1.5- and 2.0-fold increases with cholic acid and taurocholic acid, respectively (Fig. (Fig.3B),3B), which were much less than the changes in the wild-type background. This result indicates that the induction of expression of Cj0561c by bile salts occurs predominantly through the CmeR pathway.

FIG. 3.
Induction of Cj0561c by bile salts. C. jejuni 11168W7 carrying pMW561 (A) and 11168W7ΔcmeR carrying pMW561 (B) were grown in MH broth or MH broth supplemented with cholic acid (CA) or taurocholic acid (TCA). The promoter activity of Cj0561c was ...

Mutation of Cj0561c does not affect the susceptibility to antimicrobial compounds.

Compared with 11168W7, 11168W7Δ561 did not show any changes in the susceptibility to various antibiotics, toxic compounds, and heavy metals tested in this study, including ciprofloxacin, erythromycin, ampicillin, cephalothin, cefoperazone, cefotaxime, gentamicin, ethidium bromide, polymyxin B, vancomycin, tetracycline, rifampin, cholic acid, taurocholic acid, SDS, CuSO4, ZnCl2, NiCl2, MnCl2, and CoCl2. This finding suggests that under in vitro culture conditions, Cj0561c is not involved in Campylobacter resistance to antimicrobials.

Role of Cj0561c in in vivo colonization.

The strong induction of Cj0561c by bile salts suggests that it may be involved in the adaptation of C. jejuni in the intestinal environment. To examine this possibility, chickens were colonized with 11168W7 and its isogenic mutant 11168W7Δ561. When separately inoculated into chickens, the two strains colonized the birds extensively, and no significant differences (P > 0.05) were observed in the levels of colonization between the two groups (Fig. (Fig.4A).4A). The Campylobacter colonies recovered from the chickens inoculated with 11168W7Δ561 were also transferred to plates containing chloramphenicol (4 μg/ml), and all of the colonies tested grew on the plates, indicating that the cat insertion in Cj0561c was stable in chickens. Notably, when the two strains were coinoculated into a group of chickens, the colonization level of 11168W7Δ561 was significantly (P < 0.01) less than that of the wild-type strain on days 3, 6 and 9 p.i. (Fig. (Fig.4B).4B). This result indicates that Cj0561c contributes to Campylobacter colonization and is required for the full fitness of this organism in the intestinal tract of chickens.

FIG. 4.
Effect of the Cj0561c mutation on colonization of chickens by C. jejuni. The experiment included three groups of birds. The first two groups of birds were inoculated with 11168W7 and 11168W7Δ561 (A). The third group was infected with a 1:1 mixture ...

CmeR is required for optimal colonization in chickens.

The pleiotropic effect of CmeR on gene expression in C. jejuni suggests that it may be an important factor in Campylobacter adaptation in the animal host. To examine this possibility, strains 11168W7, 11168W7ΔcmeR, and 11168W7ΔcmeR+ were inoculated into four groups of chickens (15 chickens per group). By day 3 p.i., all four groups of chickens were colonized by Campylobacter. There were approximately 1- to 2-log reductions on days 3 and 6 p.i. in the mean number of CFU/g of feces in the 11168W7ΔcmeR-inoculated group compared to the 11168W7 group (Fig. (Fig.5A).5A). The difference was not statistically significant (P = 0.222) on day 3 p.i. but was significant (P < 0.012) on day 6 p.i. On day 9 p.i., the two groups showed similar levels of colonization (P = 0.421). To examine the stability of the insertional mutation in cmeR, representative Campylobacter colonies recovered from the chickens inoculated with 11168W7ΔcmeR were transferred to plates containing chloramphenicol (4 μg/ml) and analyzed by PCR using primers that detect the cat insertion in cmeR. All of the tested colonies grew well on the plates and harbored the cat cassette inserted into cmeR (data not shown). This result indicated the insertional mutation in 11168W7ΔcmeR was stable during in vivo colonization and was able to confer resistance to 4 μg/ml of chloramphenicol, which was used for differential plating in the group with a mixed infection. Surprisingly, the levels of Campylobacter colonization in the group inoculated with 11168W7ΔcmeR+ were similar to the levels of Campylobacter colonization in the group inoculated with 11168W7ΔcmeR (Fig. (Fig.5A),5A), indicating that the complementation did not restore colonization. However, PCR and plating analysis of the Campylobacter isolates from this group revealed that 13 of 15 randomly selected colonies (5 colonies from each sampling time point) lacked the pRYC7 plasmid, suggesting that the complementing plasmid was lost during colonization. The loss of the plasmid occurred quickly because the majority (4/5) of the tested colonies for day 3 p.i. were already negative with pRYC7. In the group inoculated with a 1:1 mixture of 11168W7 and 11168W7ΔcmeR, the mutant strain was increasingly outnumbered by the wild-type strain from day 3 p.i. to day 9 p.i. (Fig. (Fig.5B).5B). The difference between the two populations was not statistically significant on day 3 p.i. (P = 0.204) but was significant on day 6 p.i. (P < 0.01) and day 9 p.i. (P = 0.011). In fact, 11168W7ΔcmeR was not detected in the majority of the chickens necropsied on days 6 and 9 p.i. The chicken experiment was repeated with the same design, and similar results were obtained (data not shown). These findings, especially the results for the group with a mixed infection, clearly indicate that CmeR is required for optimal colonization of chickens by Campylobacter.

FIG. 5.
Inactivation of CmeR reduces C. jejuni colonization in chickens. The experiment included four groups of birds. The first three groups were inoculated with 11168W7, 11168W7ΔcmeR, and 11168W7ΔcmeR+ (A). The fourth group was inoculated ...

DISCUSSION

This study demonstrates that CmeR, in addition to controlling the expression of the multidrug efflux pump CmeABC, modulates the expression of multiple genes with diverse physiological functions in C. jejuni. This finding indicates that CmeR is a pleiotropic regulator in C. jejuni. We also demonstrated the tight regulation of Cj0561c by CmeR and the important role of CmeR in Campylobacter colonization in chickens. Our findings reveal new functions for this transcriptional regulator and define its importance in governing Campylobacter adaptation to in-host environments. Based on the new results and previous findings (12, 21, 22), we propose a functional model for CmeR in which it binds to target DNA via its N-terminal HTH motif and interacts with various inducing signals via its large ligand-binding pocket located in the C-terminal region. Interaction with the inducing signals presumably triggers a conformational change in the DNA-binding domain and thus affects the expression of CmeR-regulated genes. We have previously shown that bile salts are inducers of CmeR, but it is possible that CmeR also interacts with other unidentified signals in the intestinal tract, which will be examined in future studies. At this stage, we do not know if CmeR interacts with the TCR systems identified in C. jejuni. However, the presence of an inverted repeat between the −10 sequence and the ribosome-binding site of the Cj0369c-cmeR operon (Fig. (Fig.1A)1A) suggests that this operon is regulated by an unidentified mechanism.

The genes identified by DNA microarray analysis (Table (Table3)3) can be directly or indirectly regulated by CmeR. The known binding sequence of CmeR was identified in the promoter regions of cmeABC and Cj0561c but was not present in the intergenic regions of other CmeR-regulated genes. Alignment of the intergenic regions of the CmeR-regulated genes did not reveal a consensus sequence potentially bound by CmeR (data not shown). However, repeat sequences different from the known CmeR-binding site are present in some of the intergenic regions. The absence of a consensus binding sequence in the intergenic sequences does not exclude the possibility that CmeR may have certain flexibility in recognizing target sequences and hence can interact with multiple promoters in C. jejuni. Alternatively, some of the identified genes may be indirectly regulated by CmeR via unidentified secondary regulatory pathways.

In Staphylococcus aureus, the MgrA protein serves a pleiotropic global regulator that modulates the expression of multidrug efflux pumps, type 8 capsular polysaccharide, and extracellular proteins (27, 43, 44). MgrA contains a typical HTH DNA-binding motif and is a member of the MarR family of transcriptional regulators. Similar to CmeR, MgrA also functions as both a repressor and an activator for various genes in Staphylococcus (16, 27, 43). A recent DNA microarray study indicated that MgrA affects the expression of 355 genes in multiple functional categories in S. aureus (26). These findings plus the results from this study suggest that the simultaneous control of multidrug efflux pumps and other physiological pathways by a transcriptional regulator may be a common mechanism that facilitates bacterial adaptation to various environment changes.

In the DNA microarray, we arbitrarily chose a twofold or greater change as one of the cutoff criteria with the intention of selecting for biologically significant changes in gene expression. If the criterion was lowered to ≥1.5-fold, 61 genes (P < 0.05; FDR, ~20%) were identified by the DNA microarray (data not shown). Thus, the gene list in Table Table33 represents the minimum number of genes affected by CmeR, and it is likely that additional genes outside the cutoff are also affected by the mutation in CmeR. One of the limitations of the DNA microarray technology is the lack of simultaneous detection of all genes in an operon. Although it is known that cmeA, cmeB, and cmeC are cotranscribed and are all upregulated in the cmeR mutant (21, 23), only cmeB was identified in 11168ΔcmeR by the microarray method (Table (Table3).3). Subsequent tests using real-time qRT-PCR confirmed that cmeA (10-fold) and cmeC (15-fold) were also upregulated in 11168ΔcmeR. The lack of simultaneous detection of all genes in an affected operon by the DNA microarray has also been observed in other studies (40, 47) and suggests that DNA microarray analysis has a lower sensitivity than real-time qRT-PCR. Interestingly, Cj0583 showed a 6.6-fold change in expression when the DNA microarray was used, but this was not verified by real-time RT-PCR (Table (Table3).3). Further analysis of the microarray data indicated that expression of Cj0583 was consistently upregulated on all six slides (P = 6.88E-05), which argues against the possibility that Cj0583 was identified due to experimental errors. The reason for the discrepancy between the microarray results and the RT-PCR data for Cj0583 is unknown. In general, the levels of changes in gene expression detected by real-time qRT-PCR are higher than those detected by DNA microarrays (Table (Table3).3). Similar findings were reported previously by other investigators, suggesting that a DNA microarray has a lower dynamic range than real-time qRT-PCR (31, 39).

Several genes, including aspA (Cj0087), dcuA (Cj0088), dcuB (Cj0671), sdhA (Cj0437), and Cj0246c, involved in C4-dicarboxylate transport and utilization and Campylobacter adaptation to oxygen-limited conditions were upregulated in the cmeR mutant (Tables (Tables33 and and4).4). The observed changes were shown to be effects of the cmeR mutation and were not due to the differences between NCTC 11168 and 11168W7. In previous work other investigators reported that different variants of NCTC 11168 varied in gene expression and virulence and that several genes involved in C4-dicarboxylate transport and utilization were upregulated in the variants that are more virulent (5, 10). Realizing that the two variants (NCTC 11168 and 11168W7) used in this study may have some differences in gene expression, we generated a CmeR mutant with each variant and compared each cmeR mutant with its isogenic wild-type strain to determine gene expression and phenotypic changes. Mutation of cmeR in both variants consistently resulted in enhanced expression of the genes involved in C4-dicarboxylate transport and utilization (Tables (Tables33 and and4).4). In addition, the expression level of the dcu genes was near the wild-type level when 11168ΔcmeR was complemented in trans by pRYC7 (Table (Table3).3). Together, these data convincingly showed that the dcu system is affected by the cmeR mutation.

The Dcu system is considered an important mechanism for Campylobacter adaptation to low-oxygen environments (19). dcuA encodes an inner membrane protein responsible for the transport of aspartate, as well as other C4-dicarboxylates, across the cytoplasmic membrane (19). Once transported into the cytoplasm, aspartate is converted to fumarate by aspartase (encoded by aspA). When oxygen is limited or absent, fumarate can be used as an alternative terminal electron acceptor in the electron transport chain and supports Campylobacter growth under such conditions (19, 37, 47). DcuB is predicted to function as a transporter for C4-dicarboxylates (succinate and fumarate), exchanging succinate for fumarate, which serves as a terminal electron acceptor (37). It should be pointed out that DcuA and DcuB in E. coli have overlapping functions and that both can transport succinate, fumarate, malate, and aspartate (15). The Cj0264c protein is the sole dimethyl sulfoxide (DMSO)/trimethylamine oxide (TMAO) reductase in C. jejuni and was shown by Sellars et al. (37) to support DMSO- or TMAO-dependent growth under oxygen-limited conditions. It was also speculated by Sellars et al. (37) that the reduction of DMSO and TMAO facilitates Campylobacter survival outside the host, such as in aquatic environments. Notably, aspA, dcuA, dcuB, and sdhA were shown by Woodall et al. (47) to be substantially upregulated in the chicken cecum, suggesting that they are involved in Campylobacter adaptation to the environment in the gut. At this stage, it is not known if CmeR directly or indirectly regulates the C4-dicarboxylate transport and utilization system and if the Dcu system is regulated by multiple mechanisms.

Capsular polysaccharide (CPS) was recently identified in C. jejuni (3, 17, 41). Despite the advances in understanding the structure and function of CPS, little is known about the regulatory mechanisms that modulate the expression of CPS in Campylobacter (18). The CPS gene cluster in strain NCTC 11168 spans 47 kb and contains 40 genes encoding proteins involved in CPS biosynthesis and transport across the membrane (32). It is unclear if these CPS genes are cotranscribed and if they are coordinately regulated by one or more mechanisms. The seven genes affected by the CmeR mutation are located in the middle of the CPS gene cluster and are all involved in CPS biosynthesis (Table (Table3).3). No genes involved in CPS transport showed significant changes in 11168ΔcmeR. This finding suggests that the CPS biosynthesis genes may be regulated separately from the genes with transport functions. How CmeR modulates the expression of CPS production in C. jejuni remains to be determined.

One of the genes affected by the CmeR mutation is Cj1563c, which encodes a probable transcriptional regulator belonging to the MerR family. The MerR proteins in bacteria function as metal-responsive transcriptional factors and modulate the expression of genes involved in the efflux and detoxification of metal ions (33). The fact that Cj1563c was upregulated in 11168ΔcmeR suggests that CmeR represses the expression of this putative regulator. In addition, several genes encoding putative lipoproteins (Cj0089, Cj0091, Cj0628, and Cj0629) and periplasmic proteins (Cj0092 and Cj0854c) were also downregulated in 11168ΔcmeR, but the functions of these products are unknown.

Results of this study clearly showed that CmeR represses the expression of Cj0561c and that this repression is alleviated in the presence of bile salts (Fig. (Fig.22 and and3).3). This regulatory feature is similar to that of CmeABC (21, 22) and is consistent with the findings of other investigators that Cj0561c and cmeABC are upregulated in the intestinal tract (40, 47), where bile compounds are present. Although CmeR is the primary factor controlling the expression of Cj0561c, the transcription of Cj0561c was further induced by bile salts in the CmeR-negative background (Fig. (Fig.3B),3B), suggesting that a secondary CmeR-independent mechanism also modulates the expression of Cj0561c. Unlike CmeABC, Cj0561c does not seem to contribute to bile resistance because inactivation of Cj0561c did not result in a change in the MICs of various bile salts (data not shown). This difference suggests that even though both Cj0561c and cmeABC are inhibited by CmeR and inducible by bile compounds, they have different physiological functions. Although the exact function of Cj0561c is unknown at present, the reduced ability of 11168W7Δ561 to compete with 11168W7 (Fig. (Fig.4)4) in chickens suggests that Cj0561c contributes to Campylobacter adaptation in vivo.

Inactivation of cmeR led to reduced colonization of chickens by C. jejuni, and the reduction was especially obvious in the group with a mixed infection (Fig. (Fig.5).5). This result indicates that CmeR is required for optimal colonization of C. jejuni in vivo. The inability of complementation to restore colonization was likely due to the loss of pRYC7 during the course of infection. The inoculum (11168W7ΔcmeR+) was shown to contain this plasmid (by plating on both MH agar plates and kanamycin-containing plates) before inoculation. In addition, limited passage of 11168W7ΔcmeR+ in antibiotic-free MH broth did not result in loss of the plasmid. However, once the strain was inoculated into chickens, the plasmid disappeared from it, suggesting that pRYC7 is unstable during in vivo infection. Instability of plasmids in C. jejuni has also been reported by other investigators (20) and represents a challenge in characterizing gene functions in Campylobacter. Nonetheless, the colonization defect of the CmeR mutant was reproduced in a replica experiment (data not shown) and was also indirectly confirmed by the results of the unsuccessful complementation due to plasmid loss (Fig. (Fig.5).5). Previously, CmeABC was found to be a key factor in Campylobacter colonization of the intestinal tract by conferring bile resistance (24). In this work, we found that Cj0561c also contributes to intestinal colonization (Fig. (Fig.4).4). Since both cmeABC and Cj0561c were upregulated in 11168W7ΔcmeR (Table (Table3),3), one might have expected that the cmeR mutant strain would colonize chickens better than the wild-type strain. But this was not the case, suggesting that uncontrolled overexpression of cmeABC and Cj0561c is detrimental to Campylobacter during in vivo infection. Alternatively, the colonization defect in the cmeR mutant may be due to a loss of balance in gene expression. CmeR affects the expression of multiple genes with diverse functions (Table (Table3)3) and likely influences multiple physiological processes in C. jejuni. Thus, both the functions of the individual genes and their coordinated expression by CmeR are important for the fitness of C. jejuni during in vivo colonization. How CmeR interacts with its regulated genes and the various signals in the intestinal tract warrants further investigation.

Acknowledgments

This work was supported by National Institutes of Health grant RO1DK063008.

We thank Sonia Pereira for technical assistance with the chicken experiments and Stuart A. Thompson (Medical College of Georgia) for providing helpful information on primer extension.

Footnotes

[down-pointing small open triangle]Published ahead of print on 4 January 2008.

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