Researchers have suggested ways to improve analysis to help with source attribution and Campylobacter outbreak investigations.
The standard method of Campylobacter detection is through culture, though feasibility constraints usually result in selecting or characterizing as few as one isolate per sample.
Only taking a few isolates can lead to the underestimation of the diversity of Campylobacter on epidemiologically relevant sources like chicken meat, and infection-associated populations can be missed.
Multiple culture method combinations were used to recover and sequence Campylobacter from 45 fresh retail chicken samples purchased across Norwich in 2021, selecting up to 48 isolates per sample.
Scientists said the choice of isolation method could affect Campylobacter recovery from chicken samples, potentially hindering source attribution and outbreak investigations. They added that an optimized culture approach and selecting multiple isolates per sample were essential for a complete recovery.
Most samples positive
The study, published in the journal Epidemiology and Infection, assessed the diversity of recovered isolates and determined the potential impacts on source attribution and outbreak investigation.
Campylobacter is the most common cause of bacterial food poisoning, with more than 500,000 estimated annual cases in the United Kingdom. However, reported outbreaks are rare, and the majority of cases are thought to be sporadic.
Simulations based on resampling were used to assess the impact of Campylobacter sequence type (ST) diversity on outbreak detection.
Scientists used three enrichment broth conditions, two agar plate types and two temperatures, resulting in 12 different culture method combinations.
Campylobacter was recovered from 39 of 45 samples, although only one was positive through all broth, temperature, and plate combinations. Only two samples were culture-positive in all broth and temperature combinations.
Fifteen samples were positive for more than one Campylobacter species. Campylobacter jejuni was identified in 36 samples, Campylobacter coli in 17 samples, and 14 samples contained both species. One sample contaminated with Campylobacter jejuni was also positive for Campylobacter lari.
A lack of outbreaks
The presence of multiple Campylobacter reports can indicate numerous contamination events throughout the meat processing chain.
Sixty-two different STs were identified among the three Campylobacter species, 14 of which were novel. Positive samples contained 1 to 8 sequence types.
The simulation revealed that up to 87 isolates per sample would be required to detect 95 percent of the observed ST diversity, and 26 isolates would be required for the average probability of detecting a random outbreak ST to reach 95 percent.
Extensive sampling of isolates from food samples implicated in outbreaks is not often done.
“This could be a potential reason why the majority of Campylobacter cases are considered to be sporadic; if the source contains multiple strains, then cases exposed to the source may be infected with different strains that do not appear epidemiologically related,” said researchers.
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