Michael Johnson

Listeria monocytogenes, a bacterial scourge of food producers and processors, can live too well even when it’s hurt. It can rise up after months of inactivity and potentially contaminate raw or processed food. The pending question is whether it’s still virulent enough to cause serious liseriosis infections as its uninjured counterparts could.

Cells of L. monocytogenes and other pathogens such as Salmonella and Campylobacter may be left for dead after they’ve been apparently starved from a lack of nutrients such as food, water or oxygen. Not necessarily so, says emeritus food science professor Michael Johnson of the University of Arkansas System’s Division of Agriculture, who has researched the cells’ reaction to stress for the Food Safety Consortium.

“We’re trying to understand starvation and how that would affect the ability of cells to survive in different food processing environments,” Johnson said. A cooperative study with Iowa State University a few years ago showed that L. monocytogenes cells that had been injured, but not killed, by irradiation treatments used for foods were able to bounce back as virulent pathogens. Johnson said the work at Arkansas is showing that the bacteria can recover not only from a rough blow delivered by irradiation but also from prolonged deprivation of nutrients.

“The starved cells can survive without any added nutrients for nine months to a year in a physiological phosphate-salt water solution,” Johnson said. “These bacteria have figured out ways to survive without additional nutrients. They’re not growing but they’re still alive. When they get a new set of nutrients, they can grow again.”

The mystery that still occupies the Arkansas researchers’ efforts is how the inactive but still viable bacteria use the breakdown waste proteins released by other cells that die from the starvation treatment. The bacterial cells did not die during the stage when they were not getting any new outside nutrients that they were usually accustomed to receiving.

“What key things in the life of a cell are still intact in the starvation stage so that they keep a cell alive when the nutrients come along?” Johnson asked, posing the question the researchers seek to solve. “These bacteria have figured out a way whereby they go into a completely rested stage of life in which they’re apparently not metabolizing, they’re not doing anything.”

There could be serious implications for people if they consume food that contains these reawakened bacteria. Johnson offered the example of someone eating a piece of soft cheese with recently re-energized L. monocytogenes on it, an especially dangerous prospect for an immunocompromised person. Listeriosis can cause serious illness and possibly death for high-risk groups such as pregnant women, newborns and the elderly.

It’s not known yet whether the pathogenic cells that had been starved before their rejuvenation are more virulent, as virulent or less virulent than those that have been actively growing and unstarved. What’s known for certain is that the formerly stressed pathogens can at least come back to life.

“We expect bacteria to do well where there are nutrients and moisture,” Johnson said. “The big question in food processing is how do they persist when we take away all the life support systems, when we take away the nutrients and take away the water. Do they still remain alive? The answer is yes.”