Prepackaged salads have made it easier than ever to add an assortment of leafy greens to your family’s diet. But one recent study published in Applied and Environmental Microbiology found that pre-cut bagged salads may carry a higher risk for salmonella and other bacteria.

The researchers found that even a trace amount of juices from cut or damaged leaves in the salad bag promoted the growth of salmonella bacteria. The juices also helped the salmonella attach more strongly to the leaves, making it nearly impossible to wash it off with water.

This doesn’t mean you have to avoid salad greens. “Leafy greens are part of a healthy diet, so we want to make sure we’re doing all we can to limit the likelihood that they will cause foodborne illness,” says Matthew LeMaitre, M.D., of the Center for Bariatrics at Bayshore Medical Center. “To reduce the risk of foodborne illness, such as salmonella, listeria or E. coli, consider buying your greens fresh and chopping them yourself.”