Conditional Commensalism of Campylobacter in Chickens

Two species of Campylobacter, C. jejuni and C. coli, dominate the epidemiology of human infection. Both species are characterised by their broad host range, and spectrum of effects that they have on the hosts that they colonise. In non-immune humans, both cause severe gastrointestinal symptoms, as they can do in companion animals. Farmed animals, and in particular poultry, appear to carry these organisms as intestinal commensals. Given the severe disease seen in some host species this may be because the bacterium is adapted to different hosts. For example, poultry adapted strains may cause severe disease in humans which are an accidental host. Alternatively, the apparent commensalism of Campylobacter in poultry may in fact be controlled parasitism, with the host able to control, but not prevent, colonisation by the bacterium. Here we review the evidence for commensalism of Campylobacter in poultry and ask the questions: are C. jejuni and C. coli really avian commensals, or are they just well controlled pathogens?