Damaged salad leaves in bagged salads help the bacteria Salmonella enterica grow, according to a new study.

Salmonella is the bacterium that can cause a food-borne infection called Salmonellosis, a type of food poisoning.

While salad is often exposed to salmonella in the field, either from insects, bird droppings, manure or other sources, contamination-caused infection outbreaks among humans are rare.

But they can still happen despite health and safety practices processors use to minimize bacteria, like irrigating and washing the salad greens with clean water, said the study’s coauthor Primrose Freestone, Associate Professor in clinical microbiology from the University of Leicester.

The study, published in the journal of the American Society for Microbiology, looked at salad juices in water, which mimicked the environment inside the bagged salads that are available in stores.

Researchers found that, after five days in the fridge, 100 salmonella bacterium multiplied to approximately 100,000.

“In general it is thought that high numbers (between 105-106 cells) of salmonellae need to be consumed to cause illness,” wrote Richard Lawley, author of The Food Safety Hazard Guidebook, on Food Safety Watch.

They also found that salad juices encouraged the formation of salmonella biofilms, or thin layers of the bacteria that cling to the leaf and are “powerfully resistant” to being washed off.

On top of that, juices from the damaged leaves also helped salmonella stick to the salads’ plastic containers.

According to Freestone, one type of leaf was more prone to contamination than others. “It seems the pathogen prefers spinach,” said Freestone.

The study helps understand the potential risks to consumers, but more research is needed to find ways of reducing the risk of food poisoning in bagged salad.

In the meantime, Government of Canada currently recommends keeping pre-cut or ready-to-eat fruits and vegetables “properly refrigerated at 4°C or below” and re-washing any leafy greens sold in open bags or containers to minimize the risks.