Salad might not be as healthy as you think. London-based Gadi Frankel, a scientist at Imperial College, says that the increasing popularity of pre-packed salads could lead to a rise in food-poisoning cases.

Professor Frankel, 50, recently spoke at the Food Micro 2008 conference in Aberdeen, where he presented research on how salmonella bacteria are able to contaminate salads and vegetables.

He tells People: "Salmonella bacteria attach themselves to salad leaves by using stringy appendages. We are working to reduce the risk of the bacteria attaching to salad in the future."

In 2007, a salmonella outbreak in the UK was traced to imported basil; and in 2006, an E. coli outbreak in the US was traced to bagged baby spinach.

While he acknowledges that consumers should not be scared to buy pre-packed salads, he says the increased uptake in the salads is likely to be reflected in a future rise in food-poisoning. "If there is a small chance that someone will contract the infection, then the more products sold, the more chance of contracting it. Even though the chance is small, it's there."

A former scientist at Israel's Weizmann Institute in Rehovot, Professor Frankel adds: "We shouldn't be scared to buy pre-washed bags. Our motive is to ensure that it stays safe and prevent this kind of contamination."

He is now seeking funding to continue his research.