You drop a piece of cucumber on the floor. Do you immediately throw it in the bin or reassure yourself of the age-old “five-second rule” and reckon it’s fine to pop it in your mouth after a quick rinse?

If you fall into the latter camp, John Tregoning, professor of vaccine immunology at Imperial College London, has some bad news. He refers to three studies into bacteria transfer that all point towards the rule being false.

In the first, scientists looked at what happens when a range of foods (bread, buttered bread, watermelon and gummy bears) were dropped on a range of surfaces (tile, steel, wood, and carpet) that had been coated with bacteria. “They transferred almost immediately,” he says, adding that the worst combination for transfer happened when wet food hit a solid surface (watermelon on tile or steel).

Another study – where cooked sausage was dropped on to surfaces – showed that bacteria transferred on to the meat even if they had been applied to the surface hours earlier. It showed that “if you put a piece of contaminated chicken on to a work surface and then, two hours later, drop your piece of bread on to it, you can still pick up bacteria from it. It’ll be there for about 24 hours,” he says.

The final paper looked at the “five-second rule” for medical objects in the operating room. Perhaps unsurprisingly, it found that if surgeons drop something on the floor, they need to wash that instrument with detergent for it to be safe to use again.

What does all this mean for your fallen cucumber? “I think you have to accept it’s gone,” says Tregoning. Rinsing it with water won’t be enough to guarantee it’s clean. That’s especially the case if you’re particularly susceptible to infections, or if your dog might have walked something terrible through the house (even hours earlier).