The susceptibility of Campylobacter concisus to the bactericidal effects of normal human serum

Campylobacter concisus is an emerging pathogen of the gastrointestinal tract that has been associated with Barrett's oesophagus, enteritis and inflammatory bowel disease. Despite having invasive potential in intestinal epithelial cells in-vitro, bacteraemic cases with C. concisus are extremely scarce, having only been reported once. Therefore, we conducted a serum resistance assay to investigate the bactericidal effects of human complement on C. concisus in comparison to some other Campylobacter species. In total, 22 Campylobacter strains were tested by incubation with normal human serum and subsequent cultivation in microaerobic conditions for 48 hours. Killing time was evaluated by decrease in total CFU over time for incubation with different serum concentrations. Faecal isolates of C. concisus showed inoculum reduction to less than 50% after 30 min. Campylobacter jejuni was sensitive to serum, but killing was delayed and a bacteraemic Campylobacter fetus subsp. fetus isolate was completely serum resistant. Interestingly, sensitivity of enteric C. concisus to human serum was not associated to different faecal-calprotectin levels. We find that faecal isolates of C. concisus are sensitive to the bactericidal effects of serum, which may explain why C. concisus is not associated to bacteraemia.