UK – Researchers in the UK have highlighted that while the overall prevalence of Salmonella in retail foods across the UK may be low, the genetic diversity among these lurking pathogens is posing a new challenge to outbreak investigations and food safety.

Between May 2018 and November 2019, researchers embarked on an undercover mission in Norfolk, UK, collecting 1,268 food samples from the shelves of retail stores. 

Their targets included 311 raw chickens, 311 leafy greens, 311 pork cuts, 279 prawns, and 157 salmon fillets. The objective? To uncover the hidden diversity of non-typhoidal Salmonella (NTS) in these seemingly innocent food items.

The results were nothing short of astonishing. Out of the 42 food samples that tested positive for NTS, 199 isolates were sequenced. 

Raw chicken, often a staple in many households, turned out to be the main suspect, with 9.6 percent of samples harboring the NTS culprit. Pork, often a partner in crime in our favorite recipes, had 1.3 percent of its samples implicated.

However, there was no need to worry about your favorite leafy greens or salmon fillets; they were squeaky clean, with no NTS to be found. Prawns, on the other hand, had a bit of a dark side, with 3.7 percent of raw prawn samples testing positive for NTS.

Serovar saga

What makes this discovery even more intriguing is the diverse cast of Salmonella serovars involved. Researchers uncovered 14 different serovars, each seemingly specific to a particular food item. 

One intriguing twist was the discovery of S. I 4,[5],12:i:- in both pork and chicken samples, suggesting cross-contamination between these two popular types of meat.

Chicken samples, whether imported or domestic, were particularly interesting. The imported chicken had an 85 percent likelihood of being frozen, but one chilled imported sample managed to sneak in the S. Enteritidis serovar, making us question our cooking practices for frozen meats.

Out of the NTS-positive chicken samples, a staggering 80 percent contained at least one antibiotic-resistant NTS isolate. Pork samples, on the other hand, showed a 100 percent resistance rate. Prawns, surprisingly, appeared to be the “good guys” in this story, with no antibiotic resistance found.

Perhaps the most alarming revelation was the genetic relatedness of NTS isolates from food to clinical cases in humans. 

Chicken samples took the lead, with 6.1 percent closely related to human clinical isolates. Pork and raw prawn samples lagged behind at 0.6 percent and 0 percent, respectively. Imported chicken, despite its high likelihood of being frozen, showed a worrying 17 percent genetic similarity to human isolates, raising questions about its safety.

Infamous S. infantis

Among the serovars involved, S. Infantis took the spotlight. It was the most common serovar found, present in both domestic and imported chicken samples. This serovar has been on the rise worldwide due to its antibiotic resistance genes, heightened pathogenicity, and knack for forming biofilms.

With 64 percent of NTS-positive samples containing closely related isolates, it’s clear that more attention is needed to ensure food safety. Continued surveillance, using advanced whole genome sequencing, is crucial to prevent future outbreaks of salmonellosis.

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