Investigation of the Role of the Campylobacter jejuni Type VI Secretion System in Bacterial Secretion of Virulence Factors and Interactions with Host Cells

Campylobacter jejuni is a leading cause of bacterial gastroenteritis worldwide. An increasing number of C. jejuni strains possess a Type VI Secretion System (T6SS) capable of delivering bacterial effector proteins across a bacterial or host membrane. Despite this increasing prevalence of T6SS-positive C. jejuni isolates, the role of the T6SS is still not well understood. Phenotypic comparisons were performed between 488 (a T6SS-positive human clinical isolate from Brazil), a number of 488 isogenic defined mutants in genes encoding essential T6SS components and also 81-176 (a laboratory reference T6SS-negative strain). The T6SS serves a major role in bacterial competition in some bacteria. However in this study, competition assays performed with T6SS-positive against either T6SS-negative C. jejuni or E. coli did not reveal any significant differences. Expression of T6SS genes in the 488 strain by RT-PCR and secretion of the TssD needle-like component into the culture supernatant were indicative that the T6SS is functional. qRT-PCR analysis of T6SS gene expression demonstrated that tssD expression is up-regulated in the presence of 0.1% (w/v) sodium deoxycholate, a secondary bile salt. The 488 wild-type strain was significantly more resistant to the effects of oxidative stress, more interactive and invasive in a chicken cell line, more cytotoxic in the Galleria mellonella infection model and was able to colonise chickens in higher numbers compared to a 488 tssD mutant and the T6SS-negative 81-176 strain. Whole genome sequencing of the 488 strain was performed to analyse the T6SS cluster. Comparisons with previously sequenced strains identified a highly conserved T6SS cluster in strains isolated from humans and chickens. In order to identify potential T6SS effectors in C. jejuni, mass spectrometry was performed to compare the secretome of the 488 strain with that of a 488 tssBC contractile sheath double mutant. Bioinformatic analyses revealed the presence of multiple VgrGs in C. jejuni strains and identified a putative effector-immunity module associated with the C. jejuni T6SS. This study expanded on the knowledge of the function of the C. jejuni T6SS and highlighted the importance of the T6SS during in vivo survival of T6SS-positive C. jejuni strains.