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1 Introduction

Campylobacter jejuni is estimated to cause approximately 400 million cases of human enterocolitis per year. In developing countries, Campylobacter is the most commonly isolated bacterial pathogen from young children with diarrhea (Coker et al. 2002). At older ages, most infections are usually mild or asymptomatic, probably because of immunity that may follow frequent exposure to contaminated food or water (Allos and Blaser 1995; Havelaar et al. 2009). In industrialized nations, C. jejuni is the leading bacterial foodborne pathogen and one of the most important causative agents of traveler’s diarrhea. Ingestion of as few as 500 bacteria is sufficient to develop symptomatic disease (Black et al. 1988). The bacteria colonize the distal small intestine and the colon, and induce mucosal edema, cellular infiltrates, small abscesses, and focal ulcerations (Colgan et al. 1980). Clinical manifestations are fever, abdominal cramps, and bloody or watery diarrhea (Allos and Blaser 1995). Although the symptoms generally resolve within 5–7 days, the economical burden caused by C. jejuni is estimated to be up to 8 billion dollars per year in the US alone (Buzby and Roberts 1997). This is partly attributed to severe complications that can follow C. jejuni infection, such as reactive arthritis and the paralyzing autoimmune neuropathies, Guillain-Barré syndrome and Miller Fisher syndrome (Yuki et al. 2004). The association between C. jejuni infection and the occurrence of irritable bowel syndrome (Spiller 2007) and immunoproliferative intestinal lymphomas (IPSID) (Lecuit et al. 2004) is still under investigation.

In comparison to other intestinal pathogens of global importance, C. jejuni pathogenesis is still poorly understood. Recent genomics, proteomics, and advanced infection biology approaches, however, have led to the discovery of important bacterial traits including the presence of a polysaccharide capsule and sophisticated protein glycosylation machineries. Furthermore, metabolic adaptation in response to changing environments, flagella-driven motility, chemotaxis, protein secretion, colonization of mucus, bacterial infection of mucosal cells, and toxin production appear key steps in the establishment of infection. Here, we will discuss the state-of-the-art of the molecular pathogenesis of C. jejuni infection of the gut.

2 C. jejuni Genetics and Diversity

2.1 Genome Variation

C. jejuni is a Gram-negative spiral-shaped bacterium that needs a microaerophilic growth environment of 30–44°C under laboratory conditions. The bacterium is highly motile due to the presence of a single flagellum at each pole. The C. jejuni genome is relatively small (1.6–1.7 Mb) (Parkhill et al. 2000), but shows considerable genetic diversity among individual C. jejuni isolates. Horizontal gene exchange and natural competence for DNA uptake likely contribute to the largely nonclonal nature of the species. Genome-wide analysis of multiple C. jejuni isolates suggests that about 20% of the C. jejuni genome varies between strains with the presence of unique sets of genes in different isolates (Pearson et al. 2003; Poly et al. 2005; Hofreuter et al. 2006). Approximately 50% of the variable gene pool is located in hypervariable loci involved in the biosynthesis and posttranslational modification of flagellin, in capsule and lipo-oligosaccharide (LOS) production, and in DNA restriction/modification. A relatively large number of C. jejuni genes within the variable DNA regions contain tracts of repetitive nucleotide repeats (Wassenaar et al. 2002; Parkhill et al. 2000). These homopolymeric tracts are prone to undergo high rate slipped-strand mispairing resulting in high frequency on-off switching of gene function.

The C. jejuni genome does not contain typical pathogenicity islands. However, individual strains may contain one or more C. jejuni-integrated elements (CJIEs) with phage characteristics (Fouts et al. 2005; Parker et al. 2006; Clark and Ng 2008) and/or different types of cryptic plasmids (Miller et al. 2007). Conjugative plasmids are frequently found, but generally poorly characterized. A small subset of isolates carries the pVir plasmid (Bacon et al. 2002; Tracz et al. 2005). This plasmid contains elements of a putative type IV secretion system that in other bacterial species is involved in DNA export, conjugation, and protein secretion. In C. jejuni, the pVir plasmid is not required to establish an infection and does not appear to be associated with the development of bloody diarrhea (Louwen et al. 2006) in contrast to earlier suggestions (Tracz et al. 2005).

2.2 Phenotype Diversity in C. jejuni

The variable gene content in C. jejuni isolates generates differences in bacterial phenotype and adaptation potential. At the metabolic level, this is nicely exemplified by the variable ability of strains to utilize glutamine and asparagine as nutrients due to variable presence or allelic variation in the genes encoding gamma-glutamyltranspeptidase (GGT) and a periplasmic asparaginase, respectively (Hofreuter et al. 2008). This may influence bacterial colonization (Barnes et al. 2007). Similarly, strains may secrete isoforms of the FspA protein that differ in their ability to induce host cell apoptosis (Poly et al. 2007), while variable presence of CJIEs contributes to the difference in natural transformability between C. jejuni strains due to encoded DNase activity (Gaasbeek et al 2009). Strain variations in the composition of the flagellar locus can lead to both differences in the flagellin protein backbone and in variable post-translational modifications of flagellin. This variation influences antigenicity and flagella function, i.e., autoagglutination behavior. Even more marked strain diversity originates from the variable composition of the capsular and LOS biosynthesis loci. This results in the presence of many capsule types (Karlyshev et al. 2005) and a huge repertoire of produced surface lipo-oligosaccharides (Karlyshev et al. 2005; Parker et al. 2008). The capsule surface variation is often accompanied by a change in different antigenic properties (Karlyshev et al. 2005) and may contribute to the variable susceptibility of C. jejuni to bacteriophages (Coward et al. 2006). The clinical importance of the LOS diversity is illustrated through the association between the distinct LOS glycoforms that mimic host cell gangliosides and the development of Guillain–Barré syndrome (Yuki et al. 2004). The LOS structures may also differentially interact with host lectin receptors and thus influence the pathogen–host interaction. Thus far, phenotype diversity of C. jejuni is rarely taken into account in molecular pathogenesis studies, most of which are performed with a limited set of strains (e.g., strains 11168, 81–176, and 81116).

2.3 Intrastrain Phenotype Variation

In addition to differences in gene content that may explain diversity in behavior between C. jejuni isolates, individual strains display extensive phenotype variation. Two major mechanisms contribute to the intrastrain phenotype diversity, genetic variation and gene regulation. The genetic variation is largely based on the large number of homopolymeric DNA repeats in the genome. This often leads to uncontrolled variation in promoter activity or a shift in open reading frames. The seemingly random on-and-off switching of genes in the population yields a bacterial progeny that is heterogeneous in the production and/or structure of major surface components including the capsular polysaccharide, LOS and flagellin (Bacon et al. 2001; Linton et al. 2000; Guerry et al. 2002; van Alphen et al. 2008b). This diversity can be highly beneficial to the C. jejuni isolate as a heterogeneous set of bacterial phenotypes can prepare the bacterial population to survive changing environmental conditions.

Apart from via (random) genetic variation, the C. jejuni can switch phenotype by controlled regulation of gene expression. This type of regulation usually acts at the level of the entire population rather than of individual bacteria and typically occurs in response to distinct environmental cues. Illustrative examples are the availability of iron and phosphate, which regulate the biosynthesis of iron and phosphate acquisition systems (Palyada et al. 2004; Wösten et al. 2006). Other traits of C. jejuni that appear to be subject to gene regulation are capsule production, flagella synthesis, flagella-mediated protein secretion, and biofilm formation. The molecular mechanisms that drive these events and their importance for C. jejuni colonization and virulence largely remain to be determined.

2.4 Metabolic Adaptation

C. jejuni encounters a variety of environmental niches ranging from surface water to the gut of animals and humans. Survival under these conditions requires intricate adaptation machineries that enable C. jejuni to switch between, e.g., different nutrient sources, and to respond to alterations in oxygen availability and temperature such as exist in the intestine of different hosts. Microarray analysis of C. jejuni cultured under different environmental conditions demonstrates major differences in gene expression after growth at 37 and 42°C, simulating the body temperatures of human and chicken, respectively (Stintzi 2003). Similarly, in a limited oxygen environment, C. jejuni can switch to alternative electron acceptors including fumarate and nitrite (Sellars et al. 2002), and utilize alternative amino acids as preferred carbon source (Guccione et al. 2008; Wright et al. 2009). The change in metabolic state in different environmental niches may alter C. jejuni virulence properties. In chickens, which are a major reservoir of C. jejuni, colonization of the cecum occurs without apparent intestinal pathology. The bacteria preferentially reside in the mucus in close proximity to the epithelial cells but apparently do not adhere to or invade the intestinal tissue (Beery et al. 1988; Meinersmann et al. 1991). The altered body temperature and the much more abundant presence in the chicken cecum of the amino acids serine, proline, aspartate, and glutamate, that are preferentially metabolized by C. jejuni, may influence C. jejuni behavior such as bacterial growth and chemotaxis and thereby alter bacterial virulence.

The environmental changes in C. jejuni behavior appear mainly driven via sophisticated two-component signal transduction systems that control the expression of distinct metabolic regulons (reviewed in Wösten et al. 2008) and posttranscriptional regulatory mechanisms (Yun et al. 2008; Fields and Thompson 2008). Infection experiments in chickens with C. jejuni with genetically defined defects in different two-component signal transduction systems demonstrate that they are essential for bacterial colonization and/or persistence in the intestine (Brás and Ketley 1999; Svensson et al. 2009; MacKichan et al. 2004; Wösten et al. 2004). Future expression profiling of human C. jejuni intestinal isolates may reveal which adaptation machineries are activated in the human intestine and whether these systems affect C. jejuni virulence traits.

3 C. jejuni Virulence Repertoire

3.1 Campylobacter Infection in Humans

Human volunteer studies using clinical isolates confirm that C. jejuni causes dysenteric symptoms with high numbers of leukocytes in the feces (Black et al. 1988). Pathology on intestinal biopsies and experimental animal models show damage to columnar epithelial cells, increased exfoliation, necrosis, diffuse neutrophil infiltration of the lamina propria with superficial crypt abscesses, and histopathological features similar to Salmonella- and Shigella-induced colitis (Black et al. 1988; Russell et al. 1989). C. jejuni is also found inside colonic mucosal cells (van Spreeuwel et al. 1985), indicating that Campylobacter is able to invade human epithelial cells in vivo. The molecular basis of C. jejuni intestinal pathology is not completely understood. The major bacterial traits that are thought to contribute to C. jejuni colonization and pathogenesis are outlined below.

3.2 Flagella and Flagella-Mediated Motility

Flagella-mediated motility is highly important for the successful C. jejuni colonization of the gastrointestinal tract of experimental animals and human volunteers (Morooka et al. 1985; Walker et al. 1986). Bacterial motility is conferred through a single unsheathed flagellum that is present at each pole. Over 40 genes are involved in C. jejuni flagella biogenesis and assembly (Wösten et al. 2004, 2008). C. jejuni produces two different (approximately 59-kDa) flagellin subunits, FlaA and FlaB, that are both incorporated into the flagellum although not in equal amounts. The expression of FlaA and FlaB is controlled by different transcription factors, namely the alternative sigma28 (FlaA) and sigma54 (FlaB) transcription factors (Wösten et al. 2004; Hendrixson and DiRita 2003). The biological advantage of the differential regulation of the expression of the two flagellins is unknown. Both types of flagellin can be assembled into a filament structure and differ in only 9–10 amino acids in their otherwise conserved domains that form the backbone of the flagella fiber. It cannot be excluded that the amino acid differences influence the properties and/or structure of the hollow fiber or perhaps the protein secretion through the filament, but this awaits further study.

Flagella assembly in C. jejuni is unusual as the filament consists of 7 rather than 11 protofilaments (Galkin et al. 2008) and because flagella formation requires glycosylation of the flagellin monomers (Linton et al. 2000; Karlyshev et al. 2002; Goon et al. 2003) (Fig. 1). The molecular basis for the altered architecture of the flagellum and the need for glycosylation are unknown. The altered packaging of the flagellum may have co-evolved with the inability of C. jejuni flagellins to activate the innate Toll-like receptor TLR5 (Galkin et al. 2008). The glycosylation of the flagellins may enforce structural requirements important in the export and/or polymerization of flagellins, although this would not explain the existing variation in glycans used to decorate the flagellins. Alternatively, the sugar coat on the flagellum may provide strength, rigidity, and charge that facilitate the C. jejuni to access more viscous environment.

Fig. 1
figure 1_7

Schematic structure of the flagellar secretion apparatus and the different types of carbohydrates structures of C. jejuni. The flagellum is composed of a basal body embedded in the membrane and an extracellular fiber consisting of thousands of flagellin polymers. The C. jejuni flagellins are decorated with variable O-linked carbohydrates. Other variable carbohydrate surface structures are the outer membrane lipooligosaccharide (LOS) and the polysaccharide capsule (CPS). The largely conserved N-linked glycans are attached mainly to proteins in the periplasm. In the absence of a type III secretion system, the flagellar secretion apparatus appears to secrete several putative virulence proteins including FspA, FlaC and up to eight Cia proteins

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The composition in the surface-exposed regions of both FlaA and FlaB is highly variable among isolates due to horizontal exchange and recombinatorial events (Wassenaar et al. 1995) and heterogeneity of the attached glycan moieties. The glyco-modifications are encoded by the O-glycosylation locus (Guerry et al. 2006). This locus varies in gene composition between strains and contains several contingency genes that contribute to intrastrain carbohydrate heterogeneity (Guerry et al. 2006). State-of-the-art chemical analysis indicate that the attached carbohydrates predominantly consist of pseudaminic acid or legionaminic acid derivatives that are attached to distinct serine and threonine residues in the variable domain (Thibault et al. 2001; Logan et al. 2002, 2009; McNally et al. 2006; Guerry and Szymanski 2008). Variable substitution of the acetamido group of pseudaminic acid with acetaminido or hydroxyproprionyl groups causes further microheterogeneity in the carbohydrates moieties (Thibault et al. 2001). While the variation in amino acid sequence of the variable domain does not seem to influence flagella-mediated motility, the attached glycans do influence flagella-mediated auto-agglutination and microcolony formation (Guerry 2007; Guerry et al. 2006; van Alphen et al. 2008b) and thus are functionally relevant. The diversity in the surface-exposed region of C. jejuni flagellins may also serve to escape attack from the immune system (Andersen-Nissen et al. 2005) and bacteriophages (Coward et al. 2006).

3.3 Chemotaxis

With their flagella, C. jejuni can specifically swim towards a variety of substances or away from harmful environments. The directed motility appears to be driven by at least two types of taxis systems, namely chemotaxis that responds to environmental stimuli, and energy-taxis that drives motility towards environments promoting optimal electron transport. Major chemoattractants include mucin and the amino acids serine, proline, aspartate, glutamate (Hugdahl et al. 1988). Identified activators of the C. jejuni energy taxis system are fumarate and pyruvate, which yield high levels of electron transport and ATP (Hendrixson et al. 2001). In silico analysis of the C. jejuni genome sequence predicts the presence of at least nine putative chemotaxis sensing receptors (methyl accepting receptors, MCPs), the energy-taxis proteins CetA and CetB, one chemotaxis protein with potential histidine kinase activity (CheA), and multiple proteins with a CheY-like response regulator domains (CheY, CheA and CheV), as well as CheZ, CheV, CheB and CheR orthologs that may be involved in signal amplification and/or adaptation to chemotactic stimuli (Marchant et al 2002).

At the functional level, C. jejuni CheA and CheY are required for directional motility in soft agar (Golden and Acheson 2002; Hendrixson et al. 2001; Yao et al. 1997) as noted for other bacterial species. A C. jejuni CheY mutant is hyperinvasive in cultured cells (Golden and Acheson 2002; Hickey et al. 1999) possibly because this mutant is still capable of flagella-based movement but is no longer directional (Yao et al. 1997). Both CheY and CheA mutants are deficient in colonization of the mouse or chicken gastrointestinal tract (Takata et al. 1992; Yao et al. 1997; Hendrixson and DiRita 2004), indicating that chemotaxis is essential for colonization.

The energy-taxis system of C. jejuni involves the sensory protein complex CetB and CetA (Hendrixson et al. 2001). The complex shares features with the energy-taxis receptor Aer of E. coli including the presence of domains of Aer (divided between CetA and CetB), a sensory PAS domain in CetB, and a predicted transmembrane region, HAMP domain and HCD domain in CetA. Interestingly, CetA and CetB are co-transcribed independently of the flagellar regulon (Elliott and DiRita 2008). While CetA and CetB mutants display normal colonization of the chicken intestine (Hendrixson and DiRita 2004), inactivation of CetA but not CetB causes a moderate (five-fold) reduction in invasion of cultured human epithelial cells (Elliott et al. 2009). Thus, the energy-taxis system may be important for virulence rather than colonization and/or may display host-specific activation. The signals that activate the C. jejuni CetA/CetB energy-taxis complex in vivo and the molecular basis of the altered invasive properties are still unresolved.

3.4 C. jejuni Capsule and LOS

Like most Gram-negative bacterial species, C. jejuni produces surface glycolipids including capsular polysaccharide (CPS) and lipo-oligosaccharide (LOS) (Fig. 1). The polysaccharide capsule was first discovered after analysis of the C. jejuni genome sequence (Parkhill et al. 2000) and confirmed by electron microscopy (Karlyshev et al. 2001). The capsule consists of repeating oligosaccharide units attached to a dipalmitoyl-glycerophosphate lipid anchor (St Michael et al. 2002; Corcoran et al. 2006). The CPS is extensively substituted with variable O-methyl phosphoramidate, methyl, ethanolamine, and N-glycerol groups (McNally et al. 2007). The structural variation in CPS is consistent with the noted genetic diversity in the cps gene cluster and is the basis of the Penner serotyping used to distinguish C. jejuni isolates (Karlyshev et al. 2005). Environmental regulation of capsule biosynthesis as present in many other bacterial pathogens has not been investigated in great detail. However, capsular biosynthesis undergoes frequent phase variation, suggesting that the C. jejuni capsular phenotype is variable (Bacon et al. 2001). Capsule-deficient C. jejuni show increased surface hydrophobicity and serum sensitivity, and reduced invasion of INT-407 cells and virulence, in ferrets (Bacon et al. 2001).

The majority of the C. jejuni cell wall consists of oligosaccharides attached to a lipid A anchor (LOS). The structure of C. jejuni lipid A follows the same architectural principle as in most other Gram-negative species, although the lipid A backbone of C. jejuni is composed of a phosphorylated disaccharide containing diaminoglucose and glucosamine as the major molecular species with slightly longer N- and O-linked acyl chains (Moran 1995). The disaccharide is variably substituted with phosphate or phosphoethanolamine (Moran 1997). The biological significance of the different core structure is unknown. C. jejuni lipid A seems to have a lower fluidity at 37°C than Salmonella LPS, possibly because of the different acyl chain characteristics. C. jejuni LOS activates the proinflammatory TLR4/MD2 pathway (de Zoete and van Putten, unpublished results), but is less biologically active than Salmonella LPS with regard to toxicity, pyrogenicity, and the induction of TNFα (Moran 1995).

The LOS inner core region consists of two heptose residues attached to one KDO molecule. The distal heptose can contain O-linked glycine (Dzieciatkowska et al. 2007). The outer core of C. jejuni LOS is highly variable in structure. Frequent horizontal gene exchange and rearrangements of LOS genes has resulted in mosaic-like organization of the LOS gene cluster with different strains each have their own LOS gene repertoire (Parker et al. 2008; Gilbert et al. 2002). Additional intrastrain LOS heterogeneity stems from the variable expression of genes with homopolymeric nucleotide tracts that are prone to slipped-strand mispairing (Parkhill et al. 2000). Other mechanisms contributing to the LOS diversity are mutations leading to gene inactivation or different acceptor specificities of glycosyltransferases (Gilbert et al. 2002). Of particular biological importance is the ability of most C. jejuni strains to variably incorporate sialic acids into its LOS (Godschalk et al. 2007; Guerry et al. 2002; St Michael et al. 2002). This can result in the formation of ganglioside-like structures as reported for Helicobacter pylori, Haemophilus influenzae, Haemophilus ducreyi, and the pathogenic Neisseria species. The ganglioside mimics may elicit pathogenic antibodies that cross-react with host cell glycolipids and contribute to the development of the autoimmunity-based Guillain-Barré syndrome (Yuki et al. 2004). C. jejuni LOS may promote adhesion to and invasion of host cells (Fry et al. 2000; Guerry et al. 2002) and may target host cell glycan receptors with immunomodulatory functions (Louwen et al. 2008; Avril et al. 2006). The variable oligosaccharide structure may thus aid C. jejuni to colonize different hosts or intestinal niches.

In addition to capsule and LOS, C. jejuni may produce another type of polysaccharide that may be involved in biofilm formation (Kalmokoff et al. 2006; Joshua et al. 2006). The nature of this surface polysaccharide is unknown but its biosynthesis may require carbamoylphosphate synthase (McLennan et al. 2008). Biofilm formation is upregulated under anaerobic conditions and in a C. jejuni SpoT mutant. This mutant is defective for the stringent response that is important for survival of environmental stress (McLennan et al. 2008). The formation of C. jejuni biofilms may contribute to survival in aquatic environments. Its role in C. jejuni virulence remains to be determined.

3.5 Surface Proteins and Protein Glycosylation

The number of identified surface-exposed membrane proteins in C. jejuni is limited. One principal protein is the major outer membrane porin, MOMP (Moser et al. 1997; Dé et al. 2000). C. jejuni MOMP is a β-barrel protein with surface exposed loops that are hypervariable in amino acid composition between isolates (Clark et al. 2007), suggestive of selective pressure by the immune system. A second porin protein, Omp50, is upregulated during in vivo growth of C. jejuni (Stintzi et al. 2005). Other C. jejuni surface proteins include the fibronectin binding protein CadF (Konkel et al. 1997) and the lipoproteins Omp18 (Burnens et al. 1995; Konkel et al. 1996) and JlpA (Jin et al. 2001). A separate class of proteins are the PEB proteins implicated in amino acid and phosphate transport as well as bacterial adherence to eukaryotic cells (Pei et al. 1998; Leon-Kempis Mdel et al. 2006). How these proteins exert this dual function remains to be established.

C. jejuni produces two putative autotransporters, CapA and CapB (Ashgar et al. 2007). In other pathogens, autotransporters represent an extensive and rapidly growing family of secreted virulence-associated proteins. The identified C. jejuni autotransporters each contain a homopolymeric tract and are therefore predicted to undergo phase variation. Insertional inactivation of CapA results in reduced adhesion and invasion of Caco-2 cells and loss of the ability to colonize chickens, suggesting that the protein may play a role in C. jejuni colonization and virulence (Ashgar et al. 2007).

In addition to the O-glycosylated flagellins, C. jejuni contains at least 35 different N-linked glycoproteins (Young et al. 2002). Genes involved in the N-linked protein glycosylation pathway are encoded by the protein glycosylation (pgl) locus (Szymanski et al. 1999). In contrast to the O-glycosylation locus, this locus is conserved among C. jejuni strains which explains why all N-glycoproteins appear to carry the same heptasaccharide moiety, GalNAc-α1,4-GalNAc-α1,4-(Glcβ1,3)-GalNAc-α1,4-GalNAc-α1,4-GalNAc-α1,3-Bac, where Bac is bacillosamine (Young et al. 2002). The oligosaccharide is attached to asparagine residues that are part of the specific glycosylation consensus sequence, Asp/Glu-Y-Asn-X-Ser/Thr, where Y and X is any amino acid except proline (Kowarik et al. 2006). Important here is that virtually all N-glycosylated proteins appear to be located in the periplasm (Fig. 1). Their limited (if at all) surface exposure in the intact bacterium may explain the conserved nature of the oligosaccharide. The function of the N-linked protein glycosylation is still an enigma. Disruption of the glycosylation pathway reduces C. jejuni adherence and invasion in INT-407 cells and the colonization of the intestinal tracts of animals (Szymanksi et al. 2002; Karlyshev et al. 2004). The molecular basis of the attenuated C. jejuni behavior remains to be defined, but may reflect a general dysfunction of the C. jejuni membrane.

3.6 Secreted Factors

C. jejuni secretes several putative virulence factors into its environment. Most identified factors are secreted through the flagellar secretion apparatus (Fig. 1). Apart from flagellar components, this machinery secretes the FlaC, FspA, and at least eight C. jejuni invasion antigens (Cia). The 26-kDa FlaC protein is predicted to resemble FlaA and FlaB except that it lacks the variable central domain of the flagellins (Song et al. 2004). Structurally, the protein FlaC is not required for flagellum formation or motility, but was shown to bind to HEp-2 cells, both when secreted by the bacteria during in vitro infection of HEp-2 cells and as purified recombinant protein (Song et al. 2004).

The approximately 18-kDa FspA protein is present in C. jejuni in either of two variant forms, FspA1 or FspA2. The FspA proteins display considerable heterogeneity between strains. FspA2, but not FspA1, binds eukaryotic cells and induces apoptosis in epithelial cells (Poly et al. 2007).

In contrast to FlaC and FspA, secretion of the Cia proteins requires contact with host cells or the presence of mucin or serum (Rivera-Amill et al. 2001). Deoxycholate induces the transcription of the ciaB gene, and C. jejuni harvested from Muller-Hinton agar plates supplemented with deoxycholate also secrete Cia proteins (Malik-Kale et al. 2008). The apparent diversity in signals that drive Cia secretion suggests that the secretion event may be related to environmental changes rather than a distinct environmental cue. In some C. jejuni strains (but not strain 129108), inactivation of ciaB prevents secretion of other Cia proteins and blocks bacterial invasion of cultured epithelial cells (Konkel et al. 1999).

Like several other enteropathogens, C. jejuni secretes a cytolethal-distending toxin (CDT) (Smith and Bayles 2006). This toxin is comprised of three subunits, CdtA, CdtB, and CdtC, all of which are essential for toxin activity (Lara-Tejero and Galán 2001). CdtA and CdtC are essential for binding to host cells (Lee et al. 2003), while CdtB internalization by the eukaryotic cells is essential for toxicity (Lara-Tejero and Galán 2001). CdtB, which has DNase I-like activity, is translocated to the nucleus and induces eukaryotic cell cycle arrest in the G2 phase (Whitehouse et al. 1998). Cytotoxicity or cell cycle arrest can be achieved by adding a combination of the three purified toxin proteins to cultured epithelial cells (Lee et al. 2003; Lara-Tejero and Galán 2001). In addition to its cytotoxic effect, CDT also appears to stimulate the proinflammatory NF-kappaB pathway and elicits IL-8 secretion (Hickey et al. 2000; Zheng et al. 2008).

C. jejuni also secretes several nonproteinaceous molecules. One important factor may be the auto-inducer AI-2. This compound influences C. jejuni swarming motility, autoagglutination, biofilm formation, sensitivity to hydrogen peroxide, and the transcription of the cytolethal-distending toxin genes (He et al. 2008). C. jejuni AI-2 production, which is dependent upon luxS, is maximal during mid- to late-exponential growth, but rapidly decreases at high cell concentrations during entry into the stationary growth phase (Quinones et al. 2009).

4 Mucosal Infection by C. jejuni

4.1 Penetration of the Mucus Layer

The first barrier C. jejuni encounters in the gut is the mucus layer (Fig. 2). C. jejuni can effectively penetrate this barrier. Pathology studies frequently identify large numbers of bacteria in the mucus layer and in intestinal crypts without apparent attachment to the microvillus (Beery et al. 1988; Lee et al. 1986). Mucin is a strong chemoattractant for C. jejuni (Hugdahl et al. 1988) and, instead of being trapped and transported out of the intestinal tract, the bacterium freely moves in parallel streams along the mucus strands (Lee et al. 1986; McSweegan et al. 1987). This efficient mobility in mucus may be due to its spiral-shaped morphology and the presence of putative mucin-degrading enzymes. In addition, C. jejuni swims at higher speeds in environments of high rather than low viscosity and under conditions that immobilize conventional rod-shaped bacteria (Ferrero and Lee 1988). In media with high viscosity, C. jejuni shows longer periods of straight swimming with increased velocity followed by pauses, resembling the swimming pattern of spirochetes rather than of other monotrichous bacteria (Shigematsu et al. 1998). Whether the change in swimming mode is related to the observed downregulation of the flagellin (flaA) promoter in viscous conditions (Allen and Griffiths 2001) awaits further study.

Fig. 2
figure 2_7

Colonization and invasion strategies of C. jejuni. The pathogen can actively swim into the mucus layer and survive in intestinal crypts. Uptake and transport across M cells may enable C. jejuni to migrate into the subcellular environment and to invade epithelial cells at the cell basis. Transient disruption of tight junctions between intestinal cells may enable penetration of the epithelial lining via the paracellular route. Bacteria-directed uptake into epithelial cells may lead to transcellular transport and exocytosis at the cell basis, as well as to trafficking to a unique intracellular compartment in proximity of the Golgi complex. The contribution of the various invasion strategies of C. jejuni to the establishment of a natural infection is unknown

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The use of mucus or high-viscosity medium during in vitro infection tends to increase C. jejuni adherence and invasion of epithelial cells. On other hand, mucus of rabbits previously colonized with C. jejuni impede the bacterial adherence to INT-407 cells due to the presence of Campylobacter-specific IgA, which causes aggregation of C. jejuni within the mucus layer (McSweegan et al. 1987). This observation may at least partially explain why frequent C. jejuni exposure protects against intestinal pathology.

4.2 Cellular Infection

After mucus penetration, C. jejuni can come in close contact with the intestinal epithelial cells. Despite excellent work, the molecular interaction of C. jejuni with eukaryotic cells is still poorly understood. Diverse behavior of the various C. jejuni phenotypes and/or the different strategies that C. jejuni exploits to adhere, invade, and survive in different cell types may explain, but also complicate, comparison and interpretation of reported results. Methodological issues may further blur scientific progress. A typical example is the gentamicin survival assay often used to estimate bacterial invasion into eukaryotic cells. Results obtained with this assay do not accurately measure the number of intracellular bacteria at a given time. Instead, they in fact reflect the outcome of a series of events including bacterial internalization, the resistance against the hostile intracellular milieu, and the adaptive capabilities of C. jejuni that are needed to survive the transition from the intracellular environment to growth on nutrient-rich agar plates in a different gas atmosphere. Recently, discovery of novel C. jejuni invasion and intracellular trafficking pathways was successful because these issues were taken into account (Watson and Galán 2008; van Alphen et al. 2008a). Despite these technicalities, several key steps in the C. jejuni infection of eukaryotic cells have been identified and are outlined below.

4.3 Adherence of C. jejuni to Mucosal Cells

Once close to the epithelial cells, C. jejuni can adhere to the cell surface through a number of adhesins (de Melo and Pechere 1990; Konkel et al. 1997; McSweegan and Walker 1986; Kervella et al. 1993). One identified adhesion is the 42-kDa protein JlpA. which mediates adherence to HEp-2 cells. This event can be inhibited with JlpA-specific antibodies or preincubation of the eukaryotic cells with purified JlpA protein (Jin et al. 2001). JlpA interacts with surface-exposed heat shock protein Hsp90α and is blocked by the Hsp90 inhibitor geldamycin (Jin et al. 2003). A JlpA-GST fusion protein triggers nuclear translocation of the transcription factor NF-kappaB and phosphorylation of p38 MAP kinase. This suggests that JlpA not only confers C. jejuni adherence but also elicits a proinflammatory response in the infected host cell (Jin et al. 2003).

A second protein with adhesive properties is CadF (Konkel et al. 1997). This protein, which may belong to the OmpA-like protein family, confers bacterial adhesion via binding of host cell fibronectin (Fn). The Fn binding domain of C. jejuni CadF has been identified as a single exposed amino acid domain of four residues (Konkel et al. 2005). Apart for CadF, Fn binding has also been proposed for C. jejuni flagellin, the major membrane protein MOMP, and LOS (Moser and Schroder 1997). The significance of this Fn binding for C. jejuni adherence is unknown.

Other putative C. jejuni adhesins are PEB1 (Pei et al. 1998) and certain LOS glycoforms (McSweegan and Walker 1986; Avril et al. 2006). A PEB1 mutant shows a 50- to 100-fold reduction in bacterial adherence to epithelial cells and a reduced colonization of mice (Pei et al. 1998). The protein binds to HeLa cell membranes (Kervella et al. 1993). More recently, PEB1 has been shown to be a conserved asparate/glutamate-binding protein that belongs to the family of cluster three binding proteins of bacterial ATP transporters (Leon-Kempis Mdel et al. 2006). Indeed, purified recombinant PEB1 binds L-aspartate and L-glutamate which may indicate that the protein is important in the utilization of in vivo carbon sources. Biochemical studies demonstrate that the majority of PEB1 protein resides in the periplasmic space and only a small portion is transported across the outer membrane (Leon-Kempis Mdel et al. 2006). The crystal structure of the protein further strengthened its role as periplasmic amino acid binding protein by demonstrating a ligand binding cleft, which could explain the high binding affinity for L-aspartate and L-glutamate (Muller et al. 2007). Whether the adherence-promoting properties of PEB1 relate to its importance in the uptake of amino acids necessary for bacterial growth is unknown. The function of C. jejuni LOS (McSweegan and Walker 1986) and capsule polysaccharide (Bachtiar et al. 2007) in bacterial adherence has not been systematically investigated, partly because their extensive intra- and interstrain structural heterogeneity.

At this time, the relative contribution of the various C. jejuni adhesins to the infection of mucosal cells is difficult to discern. In many studies, inactivation of each of the putative adhesins strongly reduces the association of C. jejuni with eukaryotic cells, indicating that they all are dominant adhesins. This suggests that either they act in a complex or display cell-type specificity. To our knowledge, tissue- or host-specific C. jejuni adhesins have thus far not been identified, but receptor identification for each of the adhesins may resolve this issue.

4.4 Mechanisms of C. jejuni Entry into Eukaryotic Cells

Analysis of intestinal biopsies of infected patients and primates (van Spreeuwel et al. 1985; Russell et al. 1993; Babakhani et al. 1993) as well as results from numerous in vitro studies (Newell and Pearson 1984; de Melo et al. 1989; Konkel et al. 1992a,b; Wassenaar et al. 1991; Babakhani et al. 1993; Oelschlaeger et al. 1993) indicate that C. jejuni is internalized by eukaryotic cells. Reported C. jejuni invasion levels, as mostly determined by the gentamycin assay, display huge variation between laboratories and are strongly dependent on multiplicity of infection used as an inoculum. The often-used C. jejuni strain 81–176 typically enters young semiconfluent INT-407 and Caco-2 cells in 2 h with an invasion efficiency of 1–2%, but efficiencies range from 0.001 to 4% (Hu and Kopecko 1999). In most studies, even with the most invasive strains, only one to three bacteria are internalized per cell (Biswas et al. 2000), much less than reported for other enteropathogens. However, when selected for the appropriate phenotype and dependent on the environmental conditions, uptake levels of 70–80% of the inoculum can be obtained within 2–4 h of infection (van Alphen et al. 2008a), suggesting that C. jejuni has the intrinsic ability to efficiently enter eukaryotic cells.

A key factor in virtually all C. jejuni uptake studies is the presence of functional flagella (Grant et al. 1993; Wassenaar et al. 1991; Yao et al. 1994; Szymanski et al. 1995; van Alphen et al. 2008a). C. jejuni that lack flagella, or carry a short flagellum consisting of only FlaB, show reduced invasion of epithelial cells (Wassenaar et al. 1991; Szymanski et al. 1995; Yao et al. 1994). Furthermore, C. jejuni with paralyzed flagella adhere but do not enter eukaryotic cells, suggesting that motility is a prerequisite for bacterial internalization (Yao et al. 1994). The hyperinvasiveness of CheY mutants, that display enhanced directional motility (Yao et al. 1997), also points in this direction.

The signals that drive the actual internalization process are still poorly defined. Upon contact with the cell surface, C. jejuni triggers membrane ruffling and invaginations, and is taken up with its polar tip first (Krause-Gruszczynska et al. 2007; Hu et al. 2008). The uptake process may require de novo protein synthesis (Oelschlaeger et al. 1993) and the flagellar secretion of the aforementioned FlaC, FspA, and Cia as the proteins. Inactivation of FlaC causes reduced invasion of C. jejuni into cultured epithelial cells but does not affect bacterial adherence (Song et al. 2004). The production and secretion of the Cia proteins is triggered by contact with host cells. However, the role of these proteins in the internalization event remains to be elucidated, as CiaB is obligatory for C. jejuni invasion in certain strains only (van Alphen et al. 2008a; Goon et al. 2006) and delivery of the secreted molecules to the host cells has not been demonstrated. Thus far, no function has been assigned to any of these proteins.

Recently, a novel highly efficient C. jejuni invasion mechanism has been identified that acts independently of CiaB or FlaC (van Alphen et al. 2008a) (Fig. 2). The highly invasive C. jejuni phenotype displays a remarkable route of invasion that yields on average 10–15 intracellular bacteria per epithelial cell. The C. jejuni first swims towards the subcellular space of cultured epithelial cells (a process termed “subvasion”) and then accesses the cell at the basal cell side. The subvasion process requires functional flagella. Molecular analysis of the selected highly subvasive bacteria indicated a change in the bacterial taxis system. This led to the discovery that C. jejuni subvasion can be directly controlled by the availability of nutrients (van Alphen and van Putten, unpublished results). The precise mechanism via which subcellular C. jejuni enters the eukaryotic cells, is still under investigation. One possible regulatory factor is the post-transcriptional regulator carbon starvation regulator CsrA. Inactivation of the protein, which is required for resistance of C. jejuni to oxidative stress, causes a strong increase in C. jejuni invasion despite reduced motility (Fields et al. 2008).

Other molecules uniquely involved in C. jejuni invasion are gamma-glutamyltranspeptidase (GGT) (Barnes et al. 2007), polysaccharide capsule (Karlyshev et al. 2000; Kanipes et al. 2004; Bacon et al. 2001), and sialylated LOS (Louwen et al. 2008). The underlying mechanisms, however, remain to be explored. The presence of the virulence plasmid pVir can enhance but is not required to trigger C. jejuni invasion (Bacon et al. 2002).

4.5 Cellular events Accompanying C. jejuni Internalization

Most studies are consistent with the scenario that C. jejuni-directed entry into epithelial cells proceeds via the local depolymerization of cortical actin filaments and the formation of microtubuli-based membrane projections (Krause-Gruszczynska et al. 2007; Watson and Galán 2008; Konkel et al. 1992a, b; Monteville et al. 2003; Oelschlaeger et al. 1993; Hu and Kopecko 1999). Microtubule depolymerizing agents, such as nocodazole, block C. jejuni invasion (Hu and Kopecko 1999; Oelschlaeger et al. 1993). The involvement of the actin cytoskeleton in C. jejuni uptake is not always found. Whether this relates to the presence of cortical cytochalasin D-insensitive actin filaments in certain cell types (Godman et al. 1980; Horvath and Kellie 1990) is unknown. Efficient C. jejuni internalization also requires caveolin-1 (Krause-Gruszczynska et al. 2007; Wooldridge et al. 1996; Hu et al. 2006a, b). However, as the entry process was shown to be dynamin-independent (Watson and Galán 2008), C. jejuni internalization is unlikely to occur via caveolea-mediated endocytosis. Instead, the caveolin-1 containing lipid membrane domains may be important for proper C. jejuni activation of tyrosine kinases (Watson and Galán 2008; Hu et al. 2006 b) and of the small Rho GTPases Rac 1 and Cdc42 (but not RhoA) that could drive the cytoskeletal rearrangements (Krause-Gruszczynska et al. 2007, Wooldridge et al. 1996; Hu et al. 2006a, b). Both the activation of the GTPases and C. jejuni uptake is blocked by the kinase inhibitors genistein, tyrphostin-46, wortmannin, and staurosporin. At least nine proteins become phosphorylated in C. jejuni-infected cells, including phosphoinositol 3-kinase and heterotrimeric G proteins (Wooldridge et al. 1996; Biswas et al. 2004). Their exact role in the uptake process, as well as the requirement of the release of Ca2+ from intracellular stores for C. jejuni entry (Hu et al. 2005), remain to be defined. Cell signaling studies with different mutants indicate that CadF and PEB1 are not essential for activation of the Rho GTPases (Krause-Gruszczynska et al. 2007). CadF does increase tyrosine phosphorylation of paxillin in focal adhesions and thus may contribute to cytoskeletal rearrangements during the Fn-mediated uptake at the basolateral cell surface (Monteville and Konkel 2002).

4.6 Intracellular Trafficking of C. jejuni

Once ingested by eukaryotic cells, C. jejuni resides in a membrane-bound cellular compartment and co-localizes with microtubules and the microtubule motor protein dynein during the entire invasion process (Hu and Kopecko 1999; Konkel et al. 1992b; Oelschlaeger et al. 1993; Russell and Blake 1994). In INT-407 cells, C. jejuni is able to replicate after an initial decline in the number of intracellular bacteria. Replication ultimately results in the deterioration of the epithelial monolayer. Bacterial iron acquisition is essential for intracellular survival in this cell line (Naikare et al. 2006). In HEp-2 epithelial cells, C. jejuni ultimately localize in vacuoles that show signs of phago-lysosome fusion and change from spiral to coccoid forms with a concomitant decrease in viability (de Melo et al. 1989).

A novel trafficking pathway was recently identified in semiconfluent T84 intestinal epithelial cells. In these cells, C. jejuni appears to traffick into a unique intracellular compartment and avoid delivery to lysosomes (Watson and Galán 2008) (Fig. 2). Once formed, the C. jejuni-containing vesicle is transported along microtubules to close proximity of the Golgi apparatus in the perinuclear region. Recovery of intracellular bacteria from this compartment requires oxygen-limiting conditions, a finding that could change conclusions from previously described invasion studies. The trafficking of C. jejuni to the non-lysosomal compartment is related to its entry mechanism as infection of Fc receptor-transfected cells with antibody-opsonized C. jejuni results in delivery of the bacteria to endosomes rather than the unique intracellular compartment (Watson and Galán 2008).

In polarized epithelial cells, C. jejuni is able to translocate to the basolateral surface to be released in the subcellular space (Fig. 2). During transit, C. jejuni remains with a membrane-bound compartment and there is only very limited replication intracellularly (Hu et al. 2008). The signals that drive this transcellular route are unknown.

4.7 Translocation of the Intestinal Mucosa

As well as through cellular invasion and the transcellular pathway, C. jejuni is also able to (transiently) disrupt tight junctions between polarized cells and thereby gain access to the subepithelial tissue via the paracellular route (Brás and Ketley 1999; Grant et al. 1993; Konkel et al. 1992b; MacCallum et al. 2005a) (Fig. 2). Infection of polarized T84 cells with C. jejuni decreases transepithelial electric resistance and causes a redistribution of the tight junction transmembrane protein occludin from an intercellular to an intracellular location, indicating alterations in the tight junctions (Chen et al. 2006). This event is enhanced in the presence of IFNγ (Rees et al. 2008). Once the tight junctions have been passed, C. jejuni can enter the eukaryotic cells at the basolateral surface as shown in EGTA-treated polarized cells (Monteville and Konkel 2002). The molecules that drive the transcellular migration and subsequent internalization process are largely undefined, although it can be imagined that the binding of Fn by CadF promotes basolateral uptake of C. jejuni through interaction with integrin receptors.

Several observations suggest that in vivo C. jejuni may exploit M cells to penetrate the intestinal barrier (Fig. 2). M cells are an important port of entry for a variety of pathogens including Salmonella (Siebers and Finlay 1996). In rabbit intestinal loop models, C. jejuni selectively associates with M cells (Everest et al. 1993; Walker et al. 1988). In differentiated polarized Caco-2 cells, the bacteria efficiently invade only approximately 5% of the cells that may represent M-like cells (Hu and Kopecko 1999; van Alphen and van Putten, unpublished results). The observed active penetration of the subcellular space by C. jejuni (van Alphen et al. 2008a) may reflect the spread of the pathogen from infected M cells into the subepithelial layer. These bacteria can subsequently invade the epithelial cells at cell basis. It has been speculated that, at low inocula (500–800 bacteria), M cells are primarily exploited by C. jejuni to traverse the epithelium, while at high inocula, the pathogen may invade villus epithelial cells, perhaps as a secondary event after loss of tight junction integrity (Everest 2005; Chen et al. 2006).

4.8 Interaction of C. jejuni with Professional Phagocytes

Whether C. jejuni can survive within professional phagocytes is still under debate. Both survival and replication inside monocytes and macrophages, but also induced CDT-dependent apoptosis, has been reported (Hickey et al. 2005; Kiehlbauch et al. 1985; Siegesmund et al. 2004). Clinical isolates of C. jejuni survive for several days in murine macrophages, whereby catalase plays an important role in providing resistance to hydrogen peroxide (Day et al. 2000). Others report that C. jejuni survival in human monocyte-derived macrophages is donor-dependent: in cells from most donors, C. jejuni is killed within 24–48 h inside the cells, while 10% of the donors carry monocytes that are unable to kill C. jejuni (Wassenaar et al. 1997). Intriguingly, C. jejuni DNA fragments can be detected in circulating human blood cells of distinct individuals for a period of up to 12 months, although no viable bacteria could be detected (van Rhijn et al. 2002). This may indicate the presence of either viable, non-culturable C. jejuni within these cells, or that the bacterium resides in a thus far unidentified niche within the body that serves as a reservoir for continuous C. jejuni infection of monocytes. In murine bone marrow-derived macrophages, C. jejuni is delivered to a lysosomal compartment and killed within 24 h of infection (Watson and Galán 2008). C. jejuni is also readily internalized by human dendritic cells and induces maturation and cytokine production in these cells (Hu et al. 2006a, b). In mice, this results in a Th1-effector response against C. jejuni (Rathinam et al. 2008).

5 Cellular Response to Infection

Despite its global relevance, the molecular basis of the C. jejuni intestinal pathology is still an enigma. One candidate bacterial factor contributing to the inflammatory pathology is CDT. The toxin not only induces cell cycle arrest but also activates the NF-kappaB inflammatory pathway (Whitehouse et al. 1998; Lara-Tejero and Galán 2001; Zheng et al. 2008). It can be imagined that the CDT-induced growth arrest affects the constant renewal of the epithelial cell lining and thereby disrupts the integrity of the intestinal barrier. This may allow bacterial tissue penetration and the induction of an inflammatory response (Whitehouse et al. 1998). Transcription profiling on Caco-2 cells infected with C. jejuni for 6 h indicated upregulation of genes involved in cell growth, gene transcription, steroid biosynthesis, and inflammation, but also in cell polarity, water movement, and solute transport (Rinella et al. 2006). The C. jejuni -specific altered gene expression was not observed in murine intestinal CT-62 cells, suggesting that the response may be species-specific. On the other hand, C. jejuni stimulates Na+ and Cl− secretion in infected rat ileum in a calcium-dependent and possibly protein kinase C-dependent fashion (Kanwar et al. 1995). Whether this effect is related to observed transcriptional alterations and/or the development of diarrhea awaits more knowledge of the bacterial factors that contribute to these responses

Analysis of the cellular signaling pathways indicates that C. jejuni induces a potent innate immune response that may contribute to the inflammatory pathology. C. jejuni infection of intestinal cells activates the transcription factors NF-kappaB and AP-1, causes phosphorylation of ERK and p38 mitogen-activated protein kinases and of JUN N-terminal protein kinase, and induces the basolateral secretion of proinflammatory mediators (Mellits et al. 2002; Jones et al. 2003; MacCallum et al. 2005b; Chen et al. 2006). Activation of the ERK and p38 mitogen-activated protein kinases requires de novo protein synthesis of C. jejuni factors that are produced upon contact with the host cells (Watson and Galán 2005). However, CDT (Zheng et al. 2008) and boiled extracts of C. jejuni also induce a proinflammatory transcriptional response in intestinal cells (Mellits et al. 2009). Other factors implicated in induction of the inflammatory response are C. jejuni lipoproteins and LOS, which activate the innate TLR2 and TLR4/MD2 signaling pathways, respectively (Zheng et al. 2008; Hu et al. 2006a, b). These effects appear particularly profound with damaged bacteria or isolated compounds (Hu et al. 2006a, b; de Zoete and van Putten, unpublished results). Maximal induction of the innate response with live C. jejuni appears to involve the intracellular Nod1, but not Nod2 innate immune receptor (Zilbauer et al. 2007). C. jejuni flagellin is a poor activator of TLR5 (Watson and Galán 2005; Andersen-Nissen et al. 2005; Johanesen and Dwinell 2006). Reconstitution of a recombinant C. jejuni flagellin that is able to activate TLR5, identified three small conserved regions in the flagellin backbone as the basis of the C. jejuni evasion of the TLR5 response (de Zoete and van Putten, unpublished results).

The activation of the innate immune response by C. jejuni results in the production of an array of proinflammatory cytokines and chemokines including IL-1α, IL-1β, IL-6, IL-6, and TNFα, but also of the neutrophil chemoattractant IL-8 (Hickey et al. 1999, 2000; Jones et al. 2003; Hu and Hickey 2005; Bakhiet et al. 2004; Johanesen and Dwinell 2006). These factors likely promote tissue damage and the recruitment of neutrophils and monocytes. The influx of these cell types at the site of infection is confirmed by immunohistochemistry on biopsies of infected patients. Produced innate effector molecules such as beta-defensins have potent antibacterial activity against C. jejuni (Zilbauer et al. 2005).

As chickens do not develop intestinal inflammation during C. jejuni colonization, a comparison of the avian and human innate immune system might shed light on molecular basis of inflammation. Analysis of the functional TLR repertoire of chickens indicates that this species has functional TLR4 and TLR5 receptors but lacks a TLR9 orthologue (Keestra and van Putten 2008; Keestra et al. 2008). The TLR2 complex differs from the mammalian species as it consists of a heterodimer of TLR2 and TLR16. TLR16 combines the specificity for di- and triacylpeptides of mammalian TLR1 and TLR6 in a single molecule (Keestra et al. 2007). Remarkably, the avian species appear to lack a functional MyD88-independent pathway needed for β-interferon production in response to LOS. This feature may explain the resistance of chickens to endotoxin shock (Keestra and van Putten 2008). Preliminary analysis of the response of individual chicken TLR receptors to C. jejuni provides, except for the different LOS response, no obvious basis for the lack of inflammation in C. jejuni infected chickens (de Zoete and van Putten, unpublished results).

6 Experimental Campylobacter Infection Models

Despite the identification of a number of putative virulence factors, their biological significance for C. jejuni pathology remains largely unknown. Understanding the pathogenesis of C. jejuni infections in vivo has long been hampered by the lack of suitable infection models. In its natural habitat of the avian species and other warm-blooded animals, C. jejuni colonizes the intestine but rarely causes disease (Beery et al. 1988). Thus, these species are suitable to investigate C. jejuni colonization rather than virulence. Similarly, murine infection models generally yield variable C. jejuni colonization but virtually no intestinal pathology unless the gut flora is limited or the immune system is compromised (Jesudason et al. 1989; Hodgson et al. 1998). In C3H SCID mice with limited gut flora, C. jejuni causes severe inflammation of the colon and cecum, but not diarrhea (Chang and Miller 2006). Intestinal colonization and pathology are observed only for motile, chemotaxis-proficient C. jejuni. NF-kappaB-deficient C57BL/29 mice display moderate intestinal inflammation, but only after infection with CDT-positive C. jejuni strains (Fox et al. 2004). Experimental infection of congenic C57BL/6 IL-10-deficient mice produces pathological lesions similar to those seen in humans, with C. jejuni present at paracellular junctions and at the basolateral surface of the epithelium (Mansfield et al. 2007). Mice deficient in the Toll-like receptor adaptor protein MyD88 can be persistently colonized by C. jejuni but do not develop pathology (Watson et al. 2007). In these mice, colonization requires N-glycosylation of proteins and capsule production (Watson et al. 2007), indicating these traits are required for colonization even when the innate immune response is attenuated.

The C. jejuni animal model closest resembling human infection is the oral infection of nonhuman primates. In experimentally infected macaques, C. jejuni causes an acute enterocolitis with bacteria invading the mucosa before the development of inflammation (Russell et al. 1989, 1993). Microscopy showed bacteria located in intestinal crypts, within surface epithelial cells and in the subepithelial tissue, indicative of mucosal penetration (Russell et al. 1989, 1993). These animals also display several clinical symptoms as observed during human infection, including bloody diarrhea. However, the nonhuman primate animal model is rarely used, mainly on ethical grounds. A promising alternative model involves the use of gnotobiotic piglets. Infection of colostrum-deprived newborn piglets with C. jejuni results in clinical symptoms and histopathology similar to those observed in humans infected with C. jejuni. The bacteria cause gross lesions in the large intestine with edema, neutrophil infiltration, and sloughing of epithelial cells, and hemorrhage or increased mucus production. At the cellular level, bacteria are found inside intestinal epithelial cells as well as in the underlying tissue (Babakhani et al. 1993; Boosinger and Powe 1988).

Overall, the in vivo infection models suggest that in most animals the indigenous gut flora and/or a well-developed host immune system may limit C. jejuni intestinal pathology. The use of animals with limited gut flora and/or immune deficiencies as C. jejuni infection models may shed light on the molecular basis and species-specificity of the C. jejuni-induced pathology, and provide information on the bacterial phenotype and metabolic status of the bacteria compared with those isolated from the human intestine.

7 Conclusions and Outlook

A primary challenge in C. jejuni research remains the discovery of the molecular basis of the pathogenic behavior of C. jejuni in humans compared to the commensal behavior in most other species, in particular poultry. Dissection of the virulence strategies of C. jejuni is seriously hampered by the huge phenotype diversity. Extensive surface variation and the broad adaptation potential appear to provide the pathogen with an array of seemingly redundant tools to exploit host cell biology. Strain differences further complicate functional analysis of C. jejuni pathogenicity. Yet, current knowledge indicates flagella-mediated motility, chemotaxis, and penetration of mucus as key determinants of bacterial colonization in all species. The induction of pathology appears to require damage of the integrity of the epithelium lining the mucosa, either via toxin production, invasion of epithelial cells, weakening of cellular tight junctions, and/or traversal of intestinal M cells. Bacterial products as well as cellular damage may elicit an inflammatory response that contributes to the development of pathology. One key determinant of the course of an infection may be the local microenvironment at the site of infection. C. jejuni appears to be able to quickly adapt its phenotype to changing microenvironments like the availability of oxygen and nutrient sources. Recent progress in the understanding of the C. jejuni genome diversity, gene regulation, and its dynamic behavior in different environments has paved the way to finally understand and attack the virulence strategies of this important pathogen.