Background

Campylobacter jejuni and Campylobacter coli are important zoonotic pathogens that are recognised as the principal causative agents of bacterial gastroenteritis [1, 2]. C. jejuni and C. coli are common commensals of poultry [3] with broiler chickens being the primary reservoir accounting for up to 80% of human infection [4]. These organisms are also common inhabitants of the gastrointestinal tract of other food producing animals such as cattle, pigs and sheep [5]. Dominant Campylobacter genotypes, belonging to the ST-21 clonal complex, ST-45 clonal complex and ST-828 clonal complex, exhibit a multi-host, generalist lifestyle [6,7,8]. By contrast, other C. jejuni lineages exhibit a host-adapted population structure in which certain genotypes are associated with a particular host species or ecological niche [9]. Similarly, certain lineages of C. coli have been linked to the swine production environment as well as the non-agricultural, environmental niche [10].

C. jejuni and C. coli show significant phenotypic diversity [11,12,13,14,15], and vary considerably in their ability to both adhere to and invade human intestinal epithelial cells in vitro [15]. Furthermore, C. jejuni genotypes vary in their infection ecology of the chicken host [16]. C. jejuni and C. coli show high mutation rates and are known to recombine with DNA obtained by natural transformation [17], a trait that drives population heterogeneity and can impact upon pathogenicity. For example, single nucleotide polymorphisms in porA, encoding the major outer membrane protein, have been shown to give rise to hyper-virulence in ruminants [18]. Many key surface molecules of Campylobacter are phase variable which may also impact upon variation in infection [19,20,21,22]. Large scale recombination within the Campylobacter genome, often associated with niche adaption has also been observed to impact upon infection potential [23].

Autotransporter proteins are the largest and most diverse class of secretory virulence determinants in Gram-negative bacteria [24, 25]. These surface-exposed or secreted proteins share a mechanism of export, conferred by their C-terminal β-barrel structure whilst virulence properties are conferred by their N-terminal functional or “passenger” domain [24]. We recently described the CapC autotransporter in the commonly utilised reference strains C. jejuni 81,116 [26] and C. jejuni M1 [27], which is absent in the reference isolates C. jejuni NCTC 11168 and C. jejuni 81–176 [28]. Advances in sequencing technology have resulted in the public availability of large collections of genome sequences of C. jejuni and C. coli [29], which have been used to show distinct distribution patterns of gene families involved in pathogenesis, metabolism and stress responses [23, 30,31,32]. Autotransporter proteins often occur in families within a bacterial species or genus [33], and the distribution of such autotransporter families in isolates from distinct backgrounds may aid our understanding of phenotypic variation in Campylobacter species, and shed light on host specificity and niche adaption of different Campylobacter genotypes.

In this study we used publicly available Campylobacter genome sequences to demonstrate that the CapC autotransporter of C. jejuni 81,116 is a representative of a larger family of Campylobacter autotransporters. Furthermore, we identify a related family of autotransporters, CapD, that are related to, but distinct from CapC, and have determined the distribution, genotype associations and extent of gene decay of the capC and capD genes within the genus Campylobacter, focusing on C. jejuni and C. coli.

Results

Identification of the capC and capD autotransporter families in Campylobacter species

Initial screenings with the CapC protein sequence from C. jejuni 81,116 (C8J_1278) against C. jejuni and C. coli genomes from Genbank showed that there were several sequence variants present in addition to CapC in the C. jejuni and C. coli genome sequences. These autotransporter genes exhibited considerable sequence divergence in the N-terminal “passenger” domain yet share significant identity in their C-terminal domains (Fig. 1a) [25, 28]. The phylogenetic tree in Fig. 1b shows that the newly identified CapC-like autotransporters separate into two, defined clusters; one which we named CapC as it includes the originally described capC autotransporter described in C. jejuni 81,116 and C. jejuni M1 [28], designated capC1. Another cluster was named CapD and this encompasses the capD autotransporter family. In addition to the divergence in protein sequence, a major difference between the capC and capD autotransporter families is the location of a homopolymeric G-tract. In capC autotransporters, the poly-G tract is located upstream of the coding sequence in the putative promoter region whilst in the capD autotransporter family the poly-G tract is located in the coding sequence or is absent entirely (Fig. 1). Autotransporter genes belonging to the capC family were identified in C. peloridis, C. ornithicola, C. lari, C. upsaliensis, C. subantarcticus and C. cuniculorum (Fig. 1c). Autotransporter genes belonging to the capD family were detected in C. ornithicola, C. volucris and C. subantarcticus (Fig. 1c). Alignment of the complete amino acid sequences of those autotransporters as well as alignment of only the C-terminal region of each autotransporter (Fig. 1c) illustrates the division of all autotransporters detected in Campylobacter into the distinct capC and capD families. The position of the poly-G tract for capC and capD is conserved throughout the genus Campylobacter (Fig. 1c).

Fig. 1
figure 1

a Schematic representation of the alignment of capC3 and capD2 genes which are representative of the larger capC and capD families. The C-terminal β-barrel domain (red) between capC and capD genes is strongly conserved yet the N-terminal passenger domain sequence (grey) is highly divergent. The homopolymeric tract (denoted by yellow arrow heads) associated with capC autotransporters is upstream of the start codon, in the putative promoter region. The homopolymeric tract associated with capD autotransporters is located within the coding sequence. b Alignment trees generated using MEGA7 based on full length protein sequences (left) and the conserved C-terminal sequence (right) displaying the relatedness of CapC and CapD autotransporters identified in this study. Clustering of each of these two, distinct families is clear. Highlighted in yellow are autotransporter genes that lack a homopolymeric tract. c Alignment trees generated using MEGA7 based on full length protein sequences (left) and the conserved C-terminal sequence (right) displaying the relatedness of autotransporters belonging to the CapC and CapD families identified in a range of Campylobacter species. Highlighted in yellow are autotransporter genes that lack a homopolymeric tract

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Genetic characterisation of capC and capD autotransporters in C. jejuni and C. coli

In order to fully characterise the extent and distribution of autotransporter genes in C. jejuni and C. coli, each capC and capD variant was used to screen a collection of 5829 C. jejuni and 1347 C. coli genomes (Additional file 1). The capC and capD autotransporters share a degree of similarity (Fig. 1a, b, Additional file 2) in their signal peptide and C-terminal β-barrel domain, but are highly dissimilar in the N-terminal domain. Genes belonging to the CapC family were tentatively designated capC2, capC3 and capC4, respectively, in addition to the original capC1 gene from C. jejuni 81,116. A high degree of sequence similarity was observed between capC1 and capC2, and capC3 and capC4 (Fig. 1b). Genes belonging to the CapD family were designated as capD1, capD2 and capD4 in C. jejuni, and capD3 in C. coli. In C. jejuni and C. coli, the capC1-C4 genes were all present at the same genomic position, in between the ppk gene (encoding a polyphosphate kinase) and the ssrA gene encoding a transfer-messenger RNA. These capC genes are mutually exclusive as they occupy the same genomic position, suggesting recombination and genotype compatibility as the major driver of heterogeneity. We did not detect any genomes containing multiple capC genes in their intact forms. The extended regions upstream and downstream of the capC locus were largely conserved between strains except for the cj1365c gene in capC-negative strains. The capD1 and capD2 genes are also mutually exclusive in C. jejuni and C. coli and are present between the murA gene, involved in peptidoglycan synthesis and fspA2, encoding a flagella-related protein [34]. This location is not conserved in C. coli Clade 3 which encodes the capD3 gene between the moeA gene, involved in molybdenum metabolism [35], and a tRNA/ATPase gene. In the single genome containing capD4, the gene is next to an ABC transporter encoding gene and a contig end.

As the N-terminal part of autotransporters often determines specific targets or functionality, we used predictive software algorithms to investigate the CapC1-C4 and CapD1-D4 proteins. Autotransporter proteins display similarities and differences in their signal peptides, protein size and localisation (Additional file 5), which justifies their differentiation into separate families. CapC proteins have identical signal peptides and similar predicted protein sizes. However, CapC2 and CapC4 are predicted to have dual localisation sites in the outer membrane and secreted extracellularly. CapD autotransporters vary in their signal peptide composition and cleavage site as well as protein size. CapD1 and CapD2 are predicted to be secreted extracellularly, whereas CapD3 and CapD4 are predicted to localise to the outer membrane proteins. This indicates a high degree of structural conservation within the C-terminal of CapC and CapD autotransporter proteins, and a high degree of variation in the N-terminal domains, but does not provide further information on functionality of these domains.

Lineage-specific associations of intact and inactive autotransporters

The 7176 C. jejuni and C. coli genome sequences (Additional file 1) were screened for the presence of capC and capD genes to determine whether the genes detected are intact and therefore predicted to encode a full-length protein, or whether the genes detected are inactive and predicted not to encode a functional protein (Figs. 2 and 3, Table 1, Table 2, Additional file 1). Autotransporter genes, in both intact and inactive formats, are present in most clonal complexes in C. jejuni and C. coli although there were notable associations with specific C. jejuni and C. coli genetic backgrounds. For example, whilst there are instances of capC1 in genomes belonging to numerous clonal complexes, it is predominantly associated with the ST-283 clonal complex and a sub-group of the ST-45 clonal complex (Fig. 2). Moreover, the distribution of intact and inactive autotransporter genes was associated with specific MLST genotypes of C. jejuni and C. coli. For instance, inactive capC3 is highly pervasive in C. jejuni and is present in a wide range of MLST genotypes including the ST-658, ST-52, ST-574, ST-354, ST-443, ST-353, ST-464, ST-573 ST-61, ST-206 and ST-48 clonal complexes. However, the complete, intact gene is mostly present in the ST-45 clonal complex and the ST-573 clonal complex. Similarly, the capC4 gene is associated with numerous clonal complexes in its complete, intact form, but is inactive in the ST-257 clonal complex (Fig. 2, Additional file 1). This apparent linkage of inactive and intact autotransporter genes with genetic background is also observed in C. coli which has a more defined genomic population structure. The capC1-C4 autotransporters are closely associated with C. coli Clade1a/ST-828 and are absent from Clade 2 and 3, whereas the capD3 autotransporter is exclusively associated with C. coli Clade 3.

Fig. 2
figure 2

Prevalence and genotypic associations of autotransporter genes in C. jejuni. A total of 5829 genomes were phylogenetically clustered using Feature Frequency Profiling with a word length of 18. This clustering was depicted in a phylogenetic tree using Figtree. The first row beneath the resulting tree labelled isolation source indicates the source of isolation for each genome within the collection via colour coding with labels directly beneath this row. Rows labelled “capC1”, “capC2”, “capC3”, “capC4”, “capD1”, “capD2” and “capD4” indicate whether the corresponding genomes possesses either intact (dark blue colouring) or inactive (red colouring) formats of each of these genes. No colouring in these rows indicates the absence of a particular autotransporter gene. The box in the middle of the figure labelled “capC G-tract” indicates the length of the homopolymeric tract in the putative promoter region of the capC gene detected within a particular genome. Dark blue colouring indicates the capC or capD gene is intact whereas red colouring indicates whether the capC or capD gene is inactive. G-tract length ranges from 4 to ≥10. The final row shows the associated MLST clonal complex of the corresponding C. jejuni genomes

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Fig. 3
figure 3

Prevalence and genotypic associations of autotransporter genes in C. coli. A total of 1347 genomes were phylogenetically clustered using Feature Frequency Profiling with a word length of 18. This clustering was depicted in a phylogenetic tree using Figtree. The first row beneath the resulting tree labelled isolation source indicates the source of isolation for each genome within the collection via colour coding with labels directly beneath this row. Rows labelled “capC1”, “capC2”, “capC3”, “capC4” and “capD3” indicate whether the corresponding genomes possesses either intact (dark blue colouring) or inactive (red colouring) formats of each of these genes. No colouring in these rows indicates the absence of a particular autotransporter gene. The box in the middle of the figure labelled “capC G-tract” indicates the length of the homopolymeric tract in the putative promoter region of the capC gene detected within a particular genome. Dark blue or Red colouring indicates whether the capC or capD gene is intact or inactive, respectively. G-tract length ranges from 5 to ≥10. The final row shows the associated phylogenetic clade of the corresponding C. coli genomes

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Table 1 The number and proportion of genomes within major C. jejuni clonal complexes and C. coli Clades from the collection used in this study that encode intact and inactive capC autotransporter genes. The number and proportion of genomes that do not encode capC or capD is also shown
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Table 2 The number and proportion of genomes within major C. jejuni clonal complexes and C. coli Clades from the collection used in this study that encode intact and inactive capD autotransporter genes
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Homopolymeric G-tract length does not influence intact or inactive status of capC

Homopolymeric guanine/cytosine tracts mediate adaptive mutations in Campylobacter species through slipped-strand mispairing of these repetitive sequences [21, 36]. Variation in the homopolymeric tract identified in the coding sequence of capD autotransporters will influence inactivation of capD genes, but whether the poly-G tract upstream of capC genes influences inactivation of the downstream gene was not known. The poly-G tract upstream of the capC1 start codon in the C. jejuni 81,116 reference genome is also present at the equivalent site in capC-C4-positive genomes (Fig. 1a). To determine whether this homopolymeric tract influenced the observed inactivation of capC genes, we compared the length of poly-G tracts with the active/inactive status of the downstream autotransporter gene (Fig. 2 and Fig. 3). In C. jejuni, tract length ranged from G = 4 to G ≥ 10 and the most common tract length was G9 (Fig. 2, Additional file 1). capC autotransporters within the same clonal complex were determined to be intact at a range of poly-G tract lengths; for example, in ST-45 the complete, intact capC1 and capC3 are present with poly-G tract lengths of G4 to G10. Similarly, the G-tract length of inactive capC4 in C. jejuni ST-257 ranges from G8 to G ≥ 10. Furthermore, in C. coli, intact and inactive capC autotransporters were present with tract lengths of G7, G8, G9 and G10. These results indicate that homopolymeric tract length does not correspond with whether capC autotransporter genes are intact or inactive and that intact or inactive status of capC autotransporters is closely associated with clonal complex (Additional files 3 and 4).

Discussion

The autotransporter family is comprised of many important bacterial virulence factors in Gram-negative pathogens [24, 33]. These proteins consist of an N-terminal “passenger” domain which determines the effector function of the autotransporter [24], and a C-terminal β-barrel domain which facilitates insertion into the bacterial outer-membrane [25]. The CapC1 autotransporter has been shown to contribute to virulence in C. jejuni and the CapA autotransporter has been reported to be involved in adhesion to epithelial cells and chicken colonisation [28, 37, 38], although we do not yet know the exact mechanism by which CapC1 contributes to virulence. Bioinformatic analysis of the passenger domains of CapC1-C4 and CapD1-D4 did not result in identification of specific domains that may explain such functionality (Additional file 5).

In this study, we have described two novel autotransporter families in Campylobacter and report the lineage-specific distribution and decay of these autotransporter genes. Notably, we determined that capC autotransporters are shared between C. jejuni and C. coli lineages [39]. The capC and capD autotransporter genes are common throughout C. jejuni and C. coli in either their inactive or intact forms, except for select lineages which do not appear to encode CapC- or CapD autotransporters (Additional file 1). There is a clear, defined sub-population within ST-45 containing capC3 rather than capC1. The degree of demarcation between lineages that encode certain autotransporters is exemplified by this sub-population and is evidence of strong genotype associations rather than with isolation source. Due the linkage of genotype and ecological niche observed in Campylobacter [9], observed associations of an autotransporter with a particular genetic lineage may cause an indirect association with an isolation source. These associations may be exaggerated considering that the collection of publicly available Campylobacter genomes used in this study is heavily comprised of human clinical isolates belonging to ST-21 and Clade 1a C. coli, which are more readily available than isolates from other sources. Human infections are commonly transmitted via poultry or ruminant sources, but for these human isolates the transmission route is not known. The high frequency with which ST-21 and Clade 1a C. coli isolates appear in the dataset can skew interpretations regarding the proportion of autotransporter genes encoded by Campylobacter. Ecological association displayed by certain genotypes does not preclude events leading to transmission of isolates to different niches. Definitive source attribution is difficult in Campylobacter species [40, 41], particularly with multi-host adapted lineages which display poor host specificity markers [6]. Therefore, potential associations of autotransporters with ecological niches via quantitative source attribution, are difficult to accurately infer. Ultimately, possession of capC and capD autotransporters is correlated with the genetic background of C. jejuni and C. coli.

Intact capC autotransporters are predominantly associated with the ST-45 and ST-283 clonal complexes in C. jejuni and the ST-828 (Clade 1a) clonal complexes in C. coli. Considering the high degree of inactive capC genes in other clonal complexes, the high proportion of intact, functional capC in ST-45 and ST-828 is striking and could be indicative of a functional role for these autotransporters in colonisation of the agricultural niche or in the multi-host lifestyle exhibited by these lineages. However, ST-21 is also a generalist lineage that is prevalent within the agricultural niche, yet isolates from this clonal complex do not contain the capC autotransporter gene whilst thriving in these environments [42]. Rather, C. jejuni ST-21 often contains the capA/B autotransporter genes, which may mitigate for the absence of CapC or CapD autotransporters [28, 37]. Previous studies have demonstrated that C. jejuni isolates from generalist lineages readily recombine with each other in vitro, yet despite a considerable degree of niche overlap, the ST-45 and ST-21 lineages do not show any evidence of recombination with each other in nature [7]. Therefore, the ecological barrier that segregates these lineages may also restrict capC autotransporter genes to ST-45 and ST-828.

We have identified shared capC1-C4 autotransporter genes between C. jejuni lineages and introgressed C. coli ST-828 (Figs. 2, 3). In C. jejuni, the CapC autotransporter family is restricted to select genotypes in either its intact or inactive form. However, C. coli ST-828 encodes predominantly intact capC1-C4 autotransporters with no discernible association of each capC allele with sub-population structure of ST-828. Considering the similarity between capC autotransporter genes (Fig. 1) as well as the upstream and downstream genes, this observed incidence is consistent with interspecies sharing of capC autotransporters between C. coli Clade1a/ST-828 and multiple C. jejuni lineages, probably via a shared niche. Recombination between C. jejuni and C. coli ST-828 has been demonstrated previously by the accumulation of C. jejuni alleles by C. coli [39, 43, 44].

Both capC and capD have homopolymeric G-tracts associated with the genes, but their respective position is distinct. The capC genes have poly-G tract upstream of the capC start codon, whereas the capD genes have a poly-G tract in the open reading frame or do not have a poly-G tract at all. Our analysis shows that the length of this poly-G tract, whilst variable, does not correlate with the inactive/intact status of the capC autotransporters and therefore does not influence inactivation of these genes (Figs. 2, 3). Coupled with the association of intact and inactivated formats with specific clonal complexes, we propose that inactivation of these genes is linked with Campylobacter genotype rather than homopolymeric tract length. Exceptions to this are the inactive capC3 genes in the ST-48, ST-206 and ST-61 clonal complexes which predominantly possess a G-tract of 4 consecutive nucleotides and those in ST-443, ST-52 and ST-658 which possess G-tracts of 5 nucleotides. The capC3 gene in these lineages all display highly similar patterns of inactivation (Additional files 3 and 4) and are decayed to such an extent as to make reversion to intact status by addition or deletion of a nucleotides upstream of the coding sequence impossible. The uniform G-tract length in these clonal complexes is likely the result of gene decay of the entire locus including the intergenic regions due to lack of maintenance pressure. It is therefore likely that a progressive process of pseudogene formation is responsible for degradation of autotransporter genes in specific lineages rather than phase variation mediated by poly-G tracts. Pseudogenisation of autotransporters suggests a functional redundancy of these genes in certain lineages, leading to inactivation once their respective functions are no longer required within a specialised niche [45, 46]. This “adaptive loss” scenario has been observed in C. jejuni previously and is a proposed consequence of niche differentiation [45]. Conversely, this would suggest a possible environmental pressure selecting for the maintenance of intact capC and capD in C. jejuni ST-45 and ST-283 and C. coli ST-828 and for capD3 in C. coli Clade 3. Given the location of the poly-G tract, it is conceivable that strand-slippage may impact upon the expression of the capC genes [26, 28]. Furthermore, given the widespread sharing of capC autotransporters, it is possible that the intergenic regions upstream and downstream are also shared by inter-lineage and inter-species recombination making evaluation of the impact of homopolymeric tract length very difficult.

Conclusions

In this study we report on two novel, related autotransporter families in the genus Campylobacter and show that capC and capD autotransporter genes display specific distribution patterns of intact and inactive genes associated with MLST clonal complexes. This widespread, lineage-specific inactivation of capC and capD genes in Campylobacter likely represents gene decay as a consequence of functional redundancy, host/niche adaption or a lack of environmental selection towards maintenance of intact genes, especially in C. jejuni. The select presence of autotransporters highlights that Campylobacter virulence mechanisms vary between strains and genetic backgrounds and that accessory gene distribution and decay is an important consideration when evaluating Campylobacter phenotypic variability. This contrasts with capC genes being exchanged between C. jejuni and C. coli, presumably via a shared environment and recombination. Furthermore, this pattern of genetic exchange highlights the erosion of intrinsic recombination barriers between these species arising through co-habitation. Further studies are required to fully examine interspecies recombination of capC autotransporters, and whether the barriers that prevent recombination of these autotransporters, thus restricting them to certain lineages, are ecological or essential in nature. Other virulence determinants and metabolic genes should also be examined to more accurately define genotype associations and the extent of exchange of genetic material between C. jejuni and C. coli.

Methods

Campylobacter genomes used in this study

A collection of 7176 complete and draft Campylobacter genome sequences (5829 C. jejuni, 1347 C. coli) were used in this study and obtained from PubMLST (http://pubmlst.org/campylobacter) and Genbank. These genomes are listed in Additional file 1, with PubMLST ID, Genbank accession number, isolate source category, MLST sequence type, clonal complex, capC1–4 and capD1–4 status and G-tract length capC genes included where available. For C. coli, the clades 1a (ST-828), clade 1b (ST-1150), clade 1c (non-introgressed), clade 2 and clade 3 [30, 39] were also determined. The assembly quality of genome assemblies was evaluated using Quast V 4.6.3 [47] and poor quality assemblies were excluded, based on aberrant genome size (< 1.5 Mbp or > 2.0 Mbp), low N50 (< 25 kbp), high L50 (> 25), and high number of Ns per 100 kb (> 50).

Determination of the prevalence of intact and inactive autotransporters in C. jejuni and C. coli

Genome sequences were screened for the presence of the capC1–4 and capD1–4 genes by using Abricate version 0.8 (https://github.com/tseemann/abricate) and BLAST+ version 2.9.0 (NCBI). All genomes in the collection were annotated using Prokka [48], and these annotations were screened for complete and truncated versions of the CapC1–4 and CapD1–4 proteins using BioEdit version 7.25 [49]. The G-tract length of the capC1–4 promoters was determined after querying the genome sequences with the promoter of the capC1 gene of C. jejuni 81,116 (C8J_1278). Phylogenetic trees were created for C. jejuni and C. coli genomes using Feature Frequency Profiling with a word length of 18 [50], as used previously for earlier collections of C. jejuni and C. coli genome sequences [31, 32]. Colour-coding of intact and inactive gene encoding isolates within a phylogenetic tree generated using Figtree allowed associations of autotransporters with genotypes to be visualised.

Bioinformatic tools for comparison of CapC and CapD autotransporter families

SignalP 5.0 (http://www.cbs.dtu.dk/services/SignalP/), CELLO V2.5 (http://cello.life.nctu.edu.tw/), NCBI Conserved Domain Database (http://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi), Phyre2 (http://www.sbg.bio.ic.ac.uk/phyre2/html/page.cgi?id=index) and Protein Molecular Weight Calculator (https://www.bioinformatics.org/sms/prot_mw.html) were used to identify signal peptides, conserved domains, autotransporter protein size and subcellular localisation of CapC and CapD autotransporters.