Researchers in China tested 340 packages of ready-to-eat chilled pot skewers (a popular food product) and found that about 1 in 5 contained harmful bacteria called Listeria monocytogenes. This bacteria can make people sick, especially pregnant women, elderly people, and those with weak immune systems. The study discovered that the contamination came from multiple sources in food factories and that some dangerous strains persisted over time. The findings highlight the need for better safety practices in food production facilities to protect consumers from this serious health risk.

The Quick Take

  • What they studied: How often a dangerous bacteria called Listeria monocytogenes appears in ready-to-eat chilled pot skewer products sold in stores, and what types of this bacteria are most common.
  • Who participated: 340 packages of commercially available chilled pot skewers (meat-based, vegetarian, and mixed varieties) purchased from stores in China during 2019 and 2021.
  • Key finding: About 20.6% (roughly 1 out of every 5 packages) contained the harmful Listeria bacteria. The contamination rate was similar across different types of skewers, but mixed meat-vegetable products had slightly higher contamination (26.5%) compared to meat-only (15.5%) or vegetarian (19.5%) options.
  • What it means for you: If you eat these products, there’s a roughly 1 in 5 chance of exposure to this bacteria. While most healthy people won’t get seriously ill, pregnant women, elderly people, young children, and those with weakened immune systems face serious health risks. Proper food handling and cooking can reduce risk.

The Research Details

Researchers collected 340 packages of ready-to-eat chilled pot skewers from stores across China in two separate years (2019 and 2021). They tested each package in a laboratory to see if it contained Listeria monocytogenes bacteria. When they found the bacteria, they identified which specific types or strains were present using advanced genetic testing methods. This approach is called a cross-sectional study because researchers took a snapshot of products at specific points in time rather than following the same products over months or years.

The genetic testing involved two main techniques: MLST (a method that looks at specific genes to identify bacterial types) and cgMLST (a more detailed genetic analysis that can detect very small differences between bacteria). These tests helped researchers understand which strains were most common and whether the same dangerous strains appeared in multiple products or at different time points.

This research approach is important because it shows what’s actually happening in stores right now—the real-world contamination risk consumers face. By testing products from different years, researchers could see if certain dangerous bacteria strains stick around in food factories over time, which suggests ongoing contamination problems that need fixing. The genetic analysis helps identify which specific strains are most dangerous and whether they’re the same ones that have caused human illness outbreaks.

The study’s strengths include testing a reasonably large number of samples (340) and using advanced genetic testing methods that are considered the gold standard in microbiology. The researchers also compared their findings to bacteria that actually made people sick in Beijing, which helps confirm the real-world health threat. However, the study only looked at products from China and only in two specific years, so results may not apply to other countries or time periods. The study doesn’t explain exactly how the contamination happened or provide detailed information about specific manufacturers, which limits how much we can learn about prevention.

What the Results Show

The overall contamination rate was 20.6%, meaning about 1 in every 5 packages tested positive for Listeria monocytogenes. This rate was consistent across different product types: meat-based products had 15.5% contamination, vegetarian products had 19.5%, and mixed meat-vegetable products had the highest at 26.5%. While these differences exist, they weren’t statistically significant, meaning they could have happened by chance.

The researchers identified two main types of bacteria that were most common: serogroups 1/2b, 3b, 7 (found in 47.4% of contaminated samples) and 1/2a, 3 (found in 35.5% of contaminated samples). These are important because they’re the types most often linked to human illness. The genetic analysis revealed that two particularly dangerous strains—ST87 and ST121—were the most common, making up 27.6% and 22.4% of contaminated samples respectively. These specific strains have been associated with listeriosis outbreaks in the past.

A concerning finding was that the genetic analysis showed 59 different types of bacteria across 76 samples, suggesting multiple contamination sources within food production facilities rather than a single problem. However, some manufacturers showed the same dangerous strains in products tested in both 2019 and 2021, indicating that certain contamination problems persisted over the two-year period. This persistence suggests that some factories weren’t successfully eliminating the bacteria from their production environments.

The researchers compared the bacteria they found in food products with bacteria that had actually made people sick in Beijing between 2013 and 2022. They discovered that some of the same dangerous strains (like ST87) appeared in both the food products and in people who got sick, which strengthens the evidence that these contaminated products pose a real health threat. The presence of rare bacterial types (like ST378, found in only 1.3% of samples) suggests that contamination is coming from various sources rather than a single outbreak.

This research adds to growing evidence that Listeria contamination in ready-to-eat foods is a significant problem in China. Previous studies have documented similar contamination in other ready-to-eat products, but this is one of the first detailed studies of chilled pot skewers specifically. The contamination rate of 20.6% is higher than what’s been reported in some other ready-to-eat foods, making chilled pot skewers a particular concern. The identification of outbreak-associated strains (ST87 and ST121) in commercial products confirms that the bacteria found in stores are the same ones that have caused serious illness in people.

The study only looked at products from China, so we don’t know if similar contamination rates exist in other countries. The samples were collected in only two specific years (2019 and 2021), so we can’t say whether contamination is getting better or worse over time. The study doesn’t identify which specific manufacturers had the most contamination, which limits how much consumers can use this information to make purchasing decisions. The researchers didn’t test whether the bacteria were still alive and capable of causing infection—they only detected its presence. Additionally, the study doesn’t explain the exact reasons why contamination occurred or provide detailed information about food handling practices in the factories.

The Bottom Line

If you eat chilled pot skewers: (1) Cook them thoroughly before eating—heating to 165°F (74°C) kills Listeria bacteria. (2) Don’t eat them cold straight from the package if you’re pregnant, elderly, very young, or have a weakened immune system. (3) Store them properly in the refrigerator and don’t keep them longer than the expiration date. (4) Wash your hands and utensils after handling raw or undercooked products. These recommendations are based on strong evidence about how to prevent Listeria infection.

Everyone should be aware of this risk, but it’s especially important for: pregnant women (Listeria can cause miscarriage or serious infection in newborns), people over 65 years old, young children under 5, and anyone with a weakened immune system (including people with HIV, cancer patients, or those taking certain medications). Healthy adults have lower risk but can still get sick. Food manufacturers and restaurant owners should care most about this research because it shows they need better contamination prevention systems.

If you’re exposed to contaminated food, symptoms of Listeria infection typically appear within 1-3 weeks. However, some people may not show symptoms for up to 2 months. If you develop fever, muscle aches, or gastrointestinal symptoms after eating these products, especially if you’re in a high-risk group, seek medical attention immediately. Prevention is much better than treatment, so proper food handling is important.

Want to Apply This Research?

  • Track your consumption of ready-to-eat chilled products and note the brand, purchase date, and expiration date. Log any gastrointestinal symptoms (nausea, diarrhea, fever) that occur within 3 weeks of consumption to help identify potential food safety issues.
  • Set a reminder to check expiration dates before eating chilled ready-to-eat products. Create a habit of cooking these products to proper temperature (165°F/74°C) rather than eating them cold. If you’re in a high-risk group, consider avoiding these products altogether or choosing alternatives that have been thoroughly cooked.
  • If you regularly consume these products, track your health weekly for any unusual symptoms. Set monthly reminders to review your food purchases and disposal practices. If you notice symptoms, log them in the app with the product details to help identify patterns or potential contamination sources.

This research describes the presence of harmful bacteria in food products and potential health risks. This information is for educational purposes and should not replace professional medical advice. If you believe you have been exposed to contaminated food or are experiencing symptoms of food poisoning (fever, muscle aches, nausea, diarrhea), contact your healthcare provider or local health department immediately. Pregnant women, elderly individuals, young children, and immunocompromised persons should consult their doctor before consuming ready-to-eat chilled products. This study was conducted in China and may not reflect contamination rates in other countries or regions.