Penn State food scientists discover new life stage for common pathogen

April 13, 2011

UNIVERSITY PARK, Pa. -- Researchers in Penn State's College of Agricultural Sciences have discovered new information about a common but dangerous germ's life cycle that could change the way foods are processed.

The discovery of a fifth, previously unknown life stage of Listeria monocytogenes has implications for food processors striving to keep the pathogen out of their products and for consumers who may become ill by ingesting contaminated food.

The unexpected discovery came during a study on the effects of high-pressure processing on Listeria monocytogenes, according to Stephen Knabel, professor of food science.

"We wanted to find out whether high-pressure processing would be an appropriate way of killing this often lethal foodborne pathogen," said Knabel, who conducted the research with fellow food science professor Ramaswamy Anantheswaran and doctoral candidate Jia Wen.

"The fifth stage of the life cycle was a surprise," he said. "We thought that Listeria would be killed by high pressure. But in this newly discovered stage, it becomes more resistant to heat and pressure and possibly other processes common in food plants.

"This study is very important because we discovered something new about this organism with potentially great practical significance to the food industry," Knabel noted.

Listeria monocytogenes is a foodborne pathogen that often is found in food-processing plants and that sometimes contaminates raw milk and ready-to-eat dairy and meat products. In the United States, an estimated 2,500 people become seriously ill with listeriosis each year and about 500 of these die, according to the U.S. Centers for Disease Control and Prevention.

Those with weakened immune systems -- such as infants, AIDS patients and the elderly -- are most at risk.

Listeria commonly contaminates food products after pasteurization and before packaging. Pasteurization previously was thought to be enough to completely kill Listeria. However, the Penn State research shows that Listeria actually might survive the pasteurization process in this newly discovered fifth or long-term survival phase of its life cycle.

"Textbooks say there are four phases in a microbe's life cycle," explained Jia Wen. "But in this fifth phase, Listeria changes its shape from rods to round-shaped cocci, and it becomes extremely resistant to heat and pressure. This is unusual behavior for a pathogen like Listeria."

The study could explain why this pathogen can persist for so long in food-processing plants and contaminate foods after pasteurization.

"The possibility that Listeria in the long-term-survival phase may survive pasteurization and possibly cleaning and sanitizing is of great concern to food companies," said Wen. "This study reveals that modern methods of killing this foodborne pathogen may not be totally effective."

Right now, the implications of this discovery, are unknown. It still hasn't been determined whether normal pasteurization and sanitation processes can kill Listeria monocytogenes in this fifth phase. If these processes cannot kill Listeria cells in this newly discovered stage of life, then new processes would have to be developed quickly to ensure food safety.

"We don't know everything about this microorganism," Anantheswaran said. "We need to do more research on this topic and go from there."

While the research reveals Listeria monocytogenes has a unique means for long-term survival and resistance, it also is a significant discovery about microorganisms.

"Look at what nature can do," said Anantheswaran. "It doesn't matter how much we learn about science, nature can always surprise us."

The research was published in the journal Applied and Environmental Microbiology.

  • listeria monocytogenes

    Listeria under a microscope.

    IMAGE: Centers for Disease Control

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Last Updated May 16, 2013