Clinical impact of the type VI secretion system on virulence of Campylobacter species during infection.

BACKGROUND: The clinical course of Campylobacter infection varies in symptoms and severity depending on host factors, virulence of the pathogen and initiated therapy. The type VI secretion system (T6SS) has been identified as a novel virulence factor, which mediates contact-dependent injection of enzymes and toxins into competing bacteria or host cells and facilitates the colonisation of a host organism. We aimed to compare the clinical course of Campylobacter infection caused by strains with and without the T6SS and identify possible associations between this putative virulence factor and the clinical manifestations of disease.

METHODS: From April 2015 to January 2017, patients with detection of Campylobacter spp. were identified at the University Hospital of Basel and the University Children's Hospital of Basel and included in this case-control study. Presence of the T6SS gene cluster was assayed by PCR targeting the hcp gene, confirmed with whole genome sequencing. Pertinent clinical data was collected by medical record review. Differences in disease- and host-characteristics between T6SS-positive (case) and -negative (control) were compared in a uni- and multi-variable analysis. Hospital admission, antibiotic therapy, admission to intensive care unit, development of bacteraemia and in-hospital mortality were considered as clinical endpoints.

RESULTS: We identified 138 cases of Campylobacter jejuni infections and 18 cases of Campylobacter coli infections from a paediatric and adult population. Analyses were focused on adult patients with C. jejuni (n = 119) of which 16.8% were T6SS-positive. Comparisons between T6SS-positive and -negative C. jejuni isolates did not reveal significant differences regarding clinical manifestations or course of disease. All clinical endpoints showed a similar distribution in both groups. A higher score in the Charlson Comorbidity Index was associated with T6SS-positive C. jejuni isolates (p < 0.001) and patients were more likely to have a solid organ transplant and to be under immunosuppressive therapy.

CONCLUSIONS: Our study does not provide evidence that T6SS is associated with a more severe clinical course. Interestingly, T6SS-positive isolates are more commonly found in immunocompromised patients: an observation which merits further investigation.