The organisation is studying ways to eliminate the problem, reports Ahmed ElAmin.

Regulations in Europe and the US prohibit any Listeria monocytogenes or Salmonella from all ready-to-eat meal products. Food manufacturers therefore must sanitise their equipment and heat their products to kill the bacteria on cooked and ready-to-eat meats. Outbreaks of the two bacterium are common across the world.

This week member states alerted EU authorities to four separate outbreaks of Listeria monocytogenes and four Salmonella outbreaks.

According to research by Ramakrishna Nannapaneni, a University of Arkansas researcher, the food processing industry is able to eliminate 99 per cent of Listeria monocytogenes through current sterilisation procedures.

However a few of the bacteria are merely injured or starved and live to cause trouble another day, he stated in a press release.

He examined starved cells of Listeria monocytogenes and their impact on mouse cells. He found the damaged Listeria can do quite a bit of damage even after several months in a starved state, according to the new research results.

The experiments tested Listeria monocytogenes cells that had been starved for 196 days and those that had not been starved. The healthy cells were strong enough to kill 90 per cent of a targetmouse cell population within two hours of release. The starved and injured cells, after more than six months of languishing, still had enough strength to kill 60 per cent of their target cellpopulation within six hours, then 90 per cent of the target after eight hours, he found.

"Most of the phenomenon is that the starved ones take a little longer to wake up," Nannapaneni said in a statement. "Once they wake up, they have the strength to go forward."

The situation is relevant for food processing facilities in which Listeria monocytogenes cells are depleted of their nutrients but recover sufficiently to become a threat. Good cleaning practicesare necessary in food processing environments, he said. He noted that inadequate chemical sanitizing can leave some bacteria alive and virulent.

"It's important to understand how these starved cells are waking up and how to suppress them," Nannapaneni said. "The long-term starved cells become smaller and coccoid (sphericalshaped), but they still remain viable and virulent."

Meanwhile research by Gregory Phillips of Iowa State University College of Veterinary Medicine found Salmonella Typhimurium had a similar ability to survive the adverse treatments normally used tocontrol it.

He tested the pathogen against two antibiotics, ampicillin and ofloxacin, and found that some of the bacteria were able survive by altering their physiological makeup and increasing theirtolerance. Since some Salmonella survive the antibiotic treatment, they will also survive the heat treatment used in processing plants, he reasons.

"You will very rarely sterilize a culture of bacteria with antibiotics," Philips said. "You can knock down large numbers of them and for therapeutic uses that's usually sufficientto allow your body to clear out bacteria. But there's always the subpopulation that we say persists or is tolerant."

The more that Salmonella can tolerate stress from heat or antibiotics, its population can regrow after the stress stops, Phillips concluded. He noted a further danger than a mutation could resultfrom gene transfer and lead to the emergence of a newly resistant strain of bacteria.

"Studies are continuing to characterize these persistent mutants," Phillips stated. "We have some mutants now and we're looking at them. That's something we can get some numberson and quantify better."

Nannapaneni and Phillips did the studies for the Food Safety Consortium , which says it will now examine what controls are necessary to kill the starved pathogens.