Researchers at the University of Leicester have found that juices in salad bags are a breeding ground for Salmonella.
Even tiny amounts of juices leaching from salad leaves can encourage the pathogen to grow, and the juices also allow particularly dangerous strains of the microbe to thrive.
Dr Primrose Freestone and her team found that the microbes can stick to both the salad leaves and the bag. And if you this washing the leaves would work, think again - once the Salmonella has grown on the leaves, it can be impossible to wash off with water. It even survives in the fridge.
Freestone said: "Salad leaves are cut during harvesting and we found that even microliters of the juices which leach from the cut-ends of the leaves enabled Salmonella to grow in water, even when it was refrigerated.
"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."
She said that people should eat bagged salad as soon as possible after opening.