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 means scientists must figure out how much damage the injured or starved bacteria can do if left unchecked.
Unfortunately, the answer is that the bacteria remain virulent even after several months in a starved state according to Ramakrishna Nannapaneni, a food science research associate for the University of Arkansas, who works with a research team led by professor Mike Johnson doing research for the Food Safety Consortium.
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 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 killed 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.”
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 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.”