Campylobacter coli Clade 3 Isolates Induce Rapid Cell Death In Vitro

Campylobacter are major human enteropathogens. C. coli show less genetic diversity than C. jejuni and cluster into three clades, of which clade 1 includes most human and farm animal isolates while environmental C. coli mainly belong to clades 2 and 3. Recently, we whole genome-sequenced eight C. coli clade 2 and 3 isolates cultivated from water, and here we studied their interaction with human HT-29 colon cancer cells compared to clinical clade 1 isolates. All C. coli clade 3 isolates caused cell necrosis already 1-2 hours after inoculation, whereas none of the clade 1 and 2 isolates analyzed induced cell death. Isolates from clades 2 and 3 adhered better than clade 1 isolates to epithelial cells but all isolates induced similar levels of IL-8. Alignment and phylogenetic analysis of translated putative virulence genes cadF, flpA, iamA, ciaB and ceuE revealed clade-specific protein sequence variations with clade 1 and 2 sequences more closely related and clade 3 sequences further apart in general. Moreover, when RNA levels were measured, clade 3 isolates showed a significantly lower expression of cadF, iamA and ceuE than clade 2 isolates, while flpA levels were higher in clade 3 isolates. The cytolethal distending toxin genes were also expressed in clades 2 and 3 although there was no difference between clades. Our findings demonstrate differences between effects of C. coli clade 1, 2 and 3 isolates on human cells and suggest that C. coli clade 3 might be more virulent than clade 2 due to the observed cytotoxicity. IMPORTANCE Campylobacter coli is a common zoonotic cause of gastroenteritis in humans worldwide. The majority of infections are caused by C. coli clade 1 isolates, whereas infections due to clade 2 and 3 isolates are rare. Whether this depends on a low prevalence of clade 2 and 3 isolates in reservoirs important for human infections or their lower ability to cause human disease is unknown. Here, we studied the effects of C. coli clade 2 and 3 isolates on a human cell line. These isolates adhered to human cells to a higher degree than clinical clade 1 isolates. Furthermore, we could show that C. coli clade 3 isolates rapidly induced cell death suggesting differences in the virulence of C. coli . The exact mechanism of cell death remains to be revealed but selected genes showed interesting clade-specific expression patterns.