While we’ve already discovered how to dress up a standard grocery-store bagged salad and even make the greens last a little longer, we’ve been at a loss for making sure the leaves are free from food-borne illnesses. But now a new study has shed some light on one very telling sign.

According to the Guardian, researchers at the University of Leicester in England discovered that the juices released from the broken leaves of lettuce, spinach, and other greens increase the risk of the bacteria salmonella 2,400-fold. Not only that, those broken leaves and their juices also increases the virulence — or how much damage the bacteria can do — and makes it easier for the salmonella to cause infection.

A lead researcher at the university’s Department of Infection, Immunity and Inflammation, Dr. Primrose Freestone, explained in a press statement: “These juices also helped the salmonella to attach itself to the salad leaves so strongly that vigorous washing could not remove the bacteria, and even enabled the pathogen to attach to the salad bag container.”

The team of microbiologists also found that salmonella could grow at a lower refrigeration temperature (39.2 degrees) than previously believed (41.3 degrees). Plus, even when kept in the fridge, open the bags of salad allowed the bacteria to grow much faster than sealed ones.

So what can you do? First of all, pay attention to the news and be sure no greens — especially the likely culprit, spinach — have been recalled. Besides that, be smart when you shop. Look for bags where the leaves are still intact. And once you take them home, consume them immediately. Don’t let an open bag sit in the fridge for too long or it may start to harbor some very unwanted illnesses.