The Listeria That Won't Die: How Much Damage They Can Do

Meat processors already know that dangerous Listeria monocytogenes bacteria can withstand some major assaults. They sanitize the food processing environment and heat their products to kill the bacteria on cooked and ready-to-eat meats, but a few of the bacteria are merely injured or starved and live to cause trouble another day.

Federal regulations have a zero tolerance standard for Listeria monocytogenes in ready-to-eat products, so it's important for processors to find ways to beat back every last bacterium. That leaves research scientists in the position of figuring out how much damage the injured or starved bacteria can do if left unchecked.

They can do quite a bit of damage even after several months in a starved state, according to new research results. Ramakrishna Nannapaneni, a food science research associate for the University of Arkansas, has been exploring the issue for the Food Safety Consortium with a research team led by professor Mike Johnson.

"There has been a tremendous effort to find out virulence differences within Listeria monocytogenes for risk assessment needs," Nannapaneni said, pointing to the bacterium's damaged cells. "The next logical question is how long they remain virulent. That's been the focus of our work."

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 practices are necessary in food processing environments, but Nannapaneni noted that inadequate chemical sanitizing can leave some bacteria alive and virulent.

The Arkansas 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 percent of a target mouse 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 percent of their target cell population within six hours, then 90 percent of the target after eight hours.

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

The food processing industry has 99 percent of the cases under control, Nannapaneni said, and is aiming at the rest. "They are very serious about this organism," he continued. "They are taking powerful measures for preventing cross-contamination or eliminating it." Even with a limited amount of cross-contamination, the virulence of the surviving bacteria makes them targets forelimination.

This project used mouse hybridoma cells to demonstrate the power of starved Listeria monocytogenes. The next step is to test the bacteria on human cell models to discover if they are equally susceptible and how quickly they can be infected.

Then it's time to determine what controls are necessary to kill the starved pathogens. It's already known, from Food Safety Consortium research conducted by Aubrey Mendonca of the Iowa State University food science faculty, that starved Listeria monocytogenes cells show increasing resistance to stresses such as irradiation.

"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 (spherical shaped), but they still remain viable and virulent."