.......There are also suggestions that the bacteria become more dangerous in the bags.
A genetic analysis of the Salmonella showed they had gained the mutations that would help them to infect people.
Dr Freestone told the BBC: "We did see bacteria whose behaviour had turned more to virulence.
"I think the bacteria are making a molecular mistake and mistaking chemicals in the salad leaf for ones in the host."
Dr Jeri Barak, from the Food Research Institute at the University of Wisconsin-Madison, said: "It would be fair to conclude that if Salmonella is present in salads, it might grow to infectious doses.
"The rates of produce that have been found to be contaminated are between 0%-3%.
"Consumers should treat bagged salads as temperature-sensitive food products, like milk and ice-cream."
Dr Kimon Karatzas, from the University of Reading, said: "Avoiding fresh produce is not a solution, but if possible, it would be preferable to buy uncut fresh produce over chopped, and to always wash it before you eat - even the ones that are already washed.
"Furthermore, keeping these foods in the refrigerator is important."