Bioinformatic analysis of the Campylobacter jejuni Type VI secretion system

Campylobacter jejuni is one of the leading bacterial causes of human gastroenteritis worldwide and is a common coloniser of avian species too. The Type VI secretion system (T6SS) has been demonstrated as an important secretion apparatus, delivering effectors to mediate interbacterial competition and host colonisation. The T6SS was recently discovered in C. jejuni, prevalent among human, chicken, and environmental isolates, although its primary role(s) remain undetermined. To better understand the genotypic makeup and role of the T6SS in C. jejuni, we assessed its prevalence, as well as the C. jejuni Integrative Element 3 (CJIE3), in 513 publicly available sequenced strains and carried out an in-depth comparative genomic and bioinformatic analysis of a newly re-sequenced and assembled T6SS+ strain 488. We propose to reclassify the 'T6SS-containing CJIE3' identified in 488 as a pathogenicity island termed C. jejuni Pathogenicity Island – 1 (CJPI-1), after comparative analysis of the gene content, sequence similarity and synteny to strain RM1221 and plasmid pCJDM202. Two alternative VgrG proteins, CJ488_0978 and CJ488_0998, were detected within T6SS+ C. jejuni genomes, harbouring distinct C-terminal domains. Detailed analysis also showed that the C. jejuni T6SS is multifunctional, identifying several putative T6SS-associated effectors possessing anti-bacterial and anti-eukaryotic activity. CJPI-1 was also found to possess a putative toxin-antitoxin (TA) module, conserved across pCJDM202 and the genomic island CJIE3, as well as several other functionally predicted proteins including conjugal transfer protein TraG. This in-silico study represents the first comprehensive bioinformatic analysis of T6SS-containing C. jejuni, providing foundations that should contribute to future exploratory and experimental characterisation of T6SS effectors and TA modules within C. jejuni.