Current Campylobacter Testing Methods Inadequate for Foodborne Illness Attribution, Say Researchers
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A recent study conducted by researchers at the Quadram Institute has demonstrated that current sampling and testing methods for Campylobacter may overlook epidemiologically important strains. Based on their findings, the researchers stress the importance of using optimized culture methods and analyzing multiple isolates per sample.
In the UK, Campylobacter is the most common cause of bacterial gastroenteritis, causing an estimated 500,000 cases each year; although infections are largely underreported, and the burden of outbreaks may be underestimated. The diversity of Campylobacter in sources like chicken meat, which is a high-risk food for contamination and frequently harbors multiple strains, complicates source attribution and outbreak investigations.
In this context, the present study aimed to maximize the recovery and sequencing of Campylobacter from retail chicken samples and assess the effects of sample diversity on outbreak detection.
Using a combination of culture methods, researchers tested 45 chicken samples purchased at retail in Norwich, UK. Campylobacter was isolated from 39 samples (87 percent). Multiple species were identified, including C. jejuni, C. coli, and C. lari, with 33 percent of samples containing two species.
Isolates exhibited substantial sequence type (ST) diversity, ranging from one to eight STs per sample. A simulation was conducted to explore how ST diversity impacts outbreak detection, finding that up to 87 isolates per sample would be necessary to detect 95 percent of the sample’s ST diversity. At the same time, 26 isolates would be required to achieve 95 percent probability of identifying a specific outbreak-related ST.
The presence of multiple Campylobacter populations in retail chicken suggests the occurrence of various contamination events during meat processing, which increases the complexity of tracing foodborne illness outbreaks.
Additionally, the study highlighted the importance of sample size in identifying STs, as smaller sample sizes may result in an incomplete picture of the contamination, complicating source attribution. Isolation method (i.e., broths like CAT and Bolton) may also improve Campylobacter recovery from retail chicken, especially for C. coli, which was more difficult to isolate through direct plating.
The findings affirm that standard protocols that rely on testing a single isolate per sample may underestimate Campylobacter diversity, potentially missing critical outbreak strains, leading to the majority of campylobacteriosis cases to be considered sporadic.
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