According to a study, the Salmonella prevalence on tested foods on sale in the United Kingdom was low but highest for imported frozen chicken.

Researchers from the Quadram Institute and the University of East Anglia in England isolated Salmonella from 42 food samples.

Salmonella isolates collected from food using whole genome sequencing (WGS) were compared to isolates from humans in the UK.

Raw foods were collected at retail in Norfolk, including 311 samples each of chicken, leafy greens, and pork, 279 prawns, and 157 salmon between May 2018 and November 2019.

Positive chicken results

Work was funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and the Food Standards Agency (FSA) and published in the journal Microbial Genomics.

A previous study by some of the same scientists found that imported chicken and salmon were more likely to be contaminated than domestic products.

In the latest work, 17 percent of 88 imported chicken samples contained Salmonella related to human-derived isolates, but in domestic chicken, the figure was only 2.3 percent of 214 samples. However, most imported chicken samples were frozen while domestic chickens were mainly chilled, so that the differences may be due to unsafe cooking practices associated with frozen chicken. Chicken samples that contained Salmonella Enteritidis originated from multiple countries, including Poland.

Salmonella was isolated from 30 chicken, eight prawns, and four pork samples and included 14 serovars, of which Salmonella Infantis and Salmonella Enteritidis were the most common. Salmonella Enteritidis was only isolated from imported chicken.

Salmonella Newport twice and Salmonella Enteritidis (nine times) were only isolated from imported chicken samples. Salmonella Kedougou and Salmonella Mbandaka were found once and Salmonella Ohio twice from domestic samples. Salmonella Infantis was isolated 14 times from both domestic and imported chicken.

Linking food and human samples

Monophasic Salmonella Typhimurium was the only type found in multiple commodities. Isolates were collected from two domestic pork and one domestic chicken sample from three supermarkets.

All Salmonella Weltevreden-positives in the study were four samples of black tiger prawns from Vietnam, one from Indonesia, and one of unknown origin. Other samples were positive for Salmonella Bovismorbificans, Brunei, Derby, Newport, Reading, and Schwarzengrund.

Closely related human isolates were collected up to three years before or a year after those from food samples. According to researchers, further epidemiological data are required to assess the source of human cases.

Only monophasic Salmonella Typhimurium, Salmonella Enteritidis, and Salmonella Infantis found in food were similar to isolates from sick people.

A quarter of contaminated foods harbor diverse Salmonella strains that wouldn’t have been detected if only a single isolate were sampled.

“Whole genome sequencing identified foods associated with clinically important Salmonella and foods with genetically diverse Salmonella, which may hinder outbreak investigations and source attribution,” said Dr. Samuel Bloomfield from the Quadram Institute and lead author of the study.

Researchers looked at each sequence for genes that confer resistance to antibiotic drugs. They found 5.1 percent of chicken and 0.64 percent of pork samples had genes that would make them resistant to multiple antibiotics. This information could be useful for directing treatment.

”Food sources, farming and production practices, and consumer behavior are constantly changing, altering the types of foods associated with foodborne disease. Preventing future salmonellosis outbreaks relies on continued surveillance of Salmonella on retail food with the high resolution of WGS to relate food and human isolates.”

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