A new study shows that Salmonella thrives on salad bags

Food poisoning is often associated with hurried trips to the bathroom and wistful reflections on what you ate the day before. The meat main course is usually the culprit – was your chicken undercooked, or was your steak too rare?

Food safety experts are increasingly pointing out that we should look at other sources of food poisoning. The suspicion is growing that the side dish that accompanied your suspect meat dish may contain more than dietary fiber.

Researchers have found that salads containing spinach and lettuce are susceptible to colonization by bacteria responsible for food poisoning, most commonly Salmonella, E. coli, and Listeria. In the US in 2014, salmonella-contaminated beansprouts infected over 100 people, with a quarter being hospitalized. In February 2016, over 50 people developed salmonellosis in Victoria, Australia, after consuming bagged salad leaves. In July 2016, 161 people became ill after eating mixed lettuce leaves in the UK, and two people died. In the EU league table, green salads are now ranked as the 2nd most common cause of food-borne illness.

Sinister salad

Salad leaves, for example, are prone to infection because they’re usually consumed raw and only minimally processed. It is not surprising that a lot of research has been done to improve the microbial security of salad leaf cultures and optimize protocols for processing.

Up until now, little was known about the behavior of the bacteria that cause food poisoning in salad bags.

Salmonella can grow in salad bags. Shutterstock

In our most recent study, we focused on Salmonella because it is a aggressive pathogen that has been implicated with salad-associated illnesses. The juices from the cut ends of the leaves allowed Salmonella to grow even in the water that was kept refrigerated. This was surprising because Salmonella prefers a temperature of 37C.

In the five days that a bagged lettuce is typically stored, 100 Salmonella bacteria multiplied to more than 100,000. Salmonella bacteria were also firmly attached to the leaves of the salad by the juices. Even vigorous washing with water was not enough to remove them. The pathogen was also able to adhere to plastic containers and bags used to store salads. We found the most alarming finding that the exposure to the liquids from the leaves of the salads appeared to increase Salmonella’s ability to infect the consumer.

The results of our project do not suggest that eating salad leaves is a risky activity. However, it provides a better understanding of the factors contributing to food poisoning associated with salads and highlights the importance of maintaining good practices in production and preparation. Public Health England recommends washing all soil-grown vegetables and salad leaves thoroughly.

Leafy salads are an important part of a healthy diet. They should be prepared, stored, and consumed according to the instructions on the package, including the refrigeration and use-by dates. Avoid any bags with mushy leaves. Also, do not use any salad containers or bags that appear swollen. Store the salad in the refrigerator and consume it as soon as possible to prevent the growth of pathogens.

Salads are not likely to make you sick, but because they’re eaten raw, you should be careful.

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