(REUTERS) - Are you washing raw food well enough?

The World Health Organization says food poisoning caused by bacteria kills more than 400,000 people a year.

Researchers say the way the industry washes raw food using chlorine - is supposed to kill or reduce the number of bacteria but it is actually making them undetectable.

Professor Bill Keevil, head of microbiology, University of Southampton, says: "It is good standard practice to chlorinate, disinfect foods to reduce those numbers but our work is showing in fact that this is masking the problem and you may not be killing the pathogens at all."

The work showed some bacteria survive the stress of coming into contact with chlorine by shutting down - a state that makes them invisible to standard lab tests.

The team showed nematode worms could be killed by bacteria after they were washed in chlorinated water.

Prof Keevil adds: "This is absolute proof they are still alive, they are still capable of causing infection. So the implications for this are broad. So for a start we have these many outbreaks of disease that can't be traced. Are they due to stressed, viable but not culturable bacteria that are missed using routine approaches?"

Salads, cucumber and tomatoes are just some of the foods affected - normally washed with chlorine and eaten raw.

Prof Keevil says: "We think the foodchain does need to sit up and take notice because they've been relying on this common disinfection procedure for oh, a hundred years, particularly for drinking water supplies.

"It's stood us in good stead. It's certainly reduced outbreaks of disease particularly from drinking water but with the many unidentified outbreaks it's making us think again. Is this because pathogens are stressed and they're being missed?"

He said everyone needs to wash raw food thoroughly before eating it.