Identification of Multiple Subtypes of <i>Campylobacter jejuni</i> in Chicken Meat and the Impact on Source Attribution

Most source attribution studies for <i>Campylobacter </i>use subtyping data based on single isolates from foods and environmental sources in an attempt to draw epidemiological inferences. It has been suggested that subtyping only one <i>Campylobacter</i> isolate per chicken carcass incurs a risk of failing to recognise the presence of clinically relevant, but numerically infrequent, subtypes. To investigate this, between 21 and 25 <i>Campylobacter jejuni</i> isolates from each of ten retail chicken carcasses were subtyped by pulsed-field gel electrophoresis (PFGE) using the two restriction enzymes <i>Sma</i>I and <i>Kpn</i>I. Among the 227 isolates, thirteen subtypes were identified, the most frequently occurring subtype being isolated from three carcasses. Six carcasses carried a single subtype, three carcasses carried two subtypes each and one carcass carried three subtypes. Some subtypes carried by an individual carcass were shown to be potentially clonally related. Comparison of <i>C. jejuni</i> subtypes from chickens with isolate subtypes from human clinical cases (<i>n</i> = 1248) revealed seven of the thirteen chicken subtypes were indistinguishable from human cases. None of the numerically minor chicken subtypes were identified in the human data. Therefore, typing only one <i>Campylobacter</i> isolate from individual chicken carcasses may be adequate to inform <i>Campylobacter</i> source attribution.