Point mutations in the major outer membrane protein drive hypervirulence of a rapidly expanding clone of Campylobacter jejuni

Abstract

Infections due to clonal expansion of highly virulent bacterial strains are clear and present threats to human and animal health. Association of genetic changes with disease is now a routine, but identification of causative mutations that enable disease remains difficult. Campylobacter jejuni is an important zoonotic pathogen transmitted to humans mainly via the foodborne route. C. jejuni typically colonizes the gut, but a hypervirulent and rapidly expanding clone of C. jejuni recently emerged, which is able to translocate across the intestinal tract, causing systemic infection and abortion in pregnant animals. The genetic basis responsible for this hypervirulence is unknown. Here, we developed a strategy, termed "directed genome evolution," by using hybridization between abortifacient and nonabortifacient strains followed by selection in an animal disease model and whole-genome sequence analysis. This strategy successfully identified SNPs in porA, encoding the major outer membrane protein, are responsible for the hypervirulence. Defined mutagenesis verified that these mutations were both necessary and sufficient for causing abortion. Furthermore, sequence analysis identified porA as the gene with the top genome-wide signal of adaptive evolution using Fu's Fs, a population genetic metric for recent population size changes, which is consistent with the recent expansion of clone "sheep abortion." These results identify a key virulence factor in Campylobacter and a potential target for the control of this zoonotic pathogen. Furthermore, this study provides general, unbiased experimental and computational approaches that are broadly applicable for efficient elucidation of disease-causing mutations in bacterial pathogens.clonal expansion | bacterial pathogenesis | pathogen evolution | population genetics | Campylobacter

Whole-Genome Sequencing in Epidemiology of Campylobacter jejuni Infections

“…There is no reason to think that clonal dispersal is limited to certain sublineages of ST-45. Indeed, in a recent publication, the clonal expansion during the 1970s of the highly virulent ST-8 lineage, which is now causing vast numbers of ovine abortions in the United States, was reported (9). The presence of clonal populations makes genomic distinction between epidemiologically associated and nonrelated isolates difficult and can adversely affect the use of WGS methodology in surveillance and outbreak investigations for public health purposes; more studies exploring the genomic diversity and possible occurrence of clonal subpopulations in other C. jejuni lineages are needed to resolve this predicament.…”

Key Role of Capsular Polysaccharide in the Induction of Systemic Infection and Abortion by Hypervirulent Campylobacter jejuni

“…This notion is supported by findings from a very recent study (37), which identified the key role of specific mutations in porA, encoding the major outer membrane protein (MOMP), in abortion induction by C. jejuni clone SA. Specifically, transferring specific mutations in porA of C. jejuni IA3902 (HS:1,8) to NCTC 11168 (HS:2), a nonabortifacient strain, converted it into a fully virulent strain (37), despite the fact that IA3902 and NCTC 11168 have different CPS sequences (Table 1). These findings indicate that the function of CPS is required but is not sufficient for abortion induction.…”

“…To determine whether other clone SA isolates possessed the same CPS genes, we carried out a sequence-based comparison of the CPS locus of IA3902 and those of other C. jejuni strains from multiple sources, including clone SA isolates from sheep ( Table 2). Of note, the CPS sequences of these strains were previously determined to a near-complete stage with paired-end reads (2 by 100 bp) on an Illumina HiSeq 2000 machine (37). The results showed that all the sequenced clone SA isolates (n ϭ 63) had nucleotide sequences in their CPS loci that were virtually identical to that of IA3902 regardless of their isolation sources (sheep abortion, bovine abortion, goat abortion, sheep feces/bile, chicken feces, and human gastroenteritis) or years (1991 to 2013) ( Table 2).…”

“…CP001876.1). Comparative genomic analysis of the CPS locus was carried out by utilizing the nearcomplete genome sequences of a large number of clone SA isolates determined previously (37). The distribution of the CPS loci among clone SA genomes and relevant Campylobacter strains were ascertained by using the available sequence data.…”

The Periplasmic Chaperone Network of Campylobacter jejuni: Evidence that SalC (Cj1289) and PpiD (Cj0694) Are Involved in Maintaining Outer Membrane Integrity

“…Moreover, the OM is an essential permeability barrier (thus affecting antibiotic sensitivity) and a key player in nutrient acquisition, natural competence and biofilm formation. Most of these functions are protein-mediated; in C. jejuni the importance of a number of OM proteins (OMPs) have been determined, including porins such as the Major Outer Membrane Porin (MOMP; PorA), the fibronectin binding protein CadF, other adhesins such as PEB1a, CjaA and JlpA and the autotransporter CapA (Rubinchik et al, 2012; Mahdavi et al, 2014; Wu et al, 2016). Highly antigenic OMPs have already been proposed as vaccine candidates, in both chickens and humans (Tribble et al, 2008).…”