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Foundations for Intervention

Reducing foodborne illness is goal of CALS microbiologist’s research.

Each year, foodborne microbes make millions sick, lead to hundreds of thousands of hospitalizations and kill more than 3,000 people in the United States alone. In her Schaub Hall laboratory, N.C. State University’s Dr. Sophia Kathariou works to reduce that toll by unraveling the molecular mysteries of two particularly problematic pathogens.

The bacteria Listeria monocytogenes and Campylobacter jejuni act in distinctly different ways – ways that have intrigued Kathariou, a professor in the Department of Food, Bioprocessing and Nutrition Sciences, for more than 25 years.

Her objective: getting to know the bacteria in ways that lay the foundation for interventions that lower or eliminate their ability to contaminate food and infect people. Using molecular epidemiologic tools and genetic, physiologic and pathogenesis approaches, she identifies and characterizes bacterial lineages that are especially relevant to food safety.

A native of Greece, Kathariou has researched Listeria since she was a postdoctoral researcher at the University of Würzburg in Germany. The bacteria, she explains, can be found in raw foods such as uncooked meats. They are also found in soil and water, and once they get into food processing plants, they can persist there for years.

Heat kills Listeria, so it doesn’t survive proper cooking. But, Kathariou says, ready-to-eat foods such as deli meats, smoked seafood and soft cheeses made with unpasteurized milk can become contaminated before being packaged, if the bacterium is present in the plant.

When Listeria winds up in the food supply, it can cause a rare but potentially deadly illness called listeriosis. It can start with mild flu-like symptoms or gastrointestinal symptoms such as diarrhea, but it spreads to other parts of the body, including the blood and brain. In a pregnant woman, it can cross the placenta and infect the fetus, which can lead to miscarriages, stillbirths and seriously ill infants.

Dr. Sophia Kathariou
Dr. Sophia Kathariou began investigating Listeria as a postdoctoral researcher in Germany.

While listeriosis infects only some 1,600 people in the nation annually, about 260 die, according to the U.S. Centers for Disease Control and Prevention.



is “full of surprises,” Kathariou says. Recently a large outbreak – more than 120 cases and 25 deaths – was linked to cantaloupes, a food that had never before been associated with an outbreak.

Finding out how Listeria contaminates processing plants could be key to reducing the incidence of listeriosis as well as the economic losses that occur when food is recalled. So that’s one of the main focus areas of research conducted by Kathariou, her lab manager Robin Siletzky and visiting scientists and students who come from around the world to work in the lab.

They are looking, for example, at the molecular basis that underlies Listeria’s persistence, its cold and freeze tolerance and its ability to form tight-knit microbial communities known as biofilms. They also have made headway in understanding how the bacteria evolve to resist viruses and heavy metals and to survive disinfectants that are routinely used in processing plants to get rid of pathogens.

“Some of those organisms have high levels of resistance to the quaternary ammonium compounds that are used extensively in processing as disinfectants,” Kathariou says. “We are trying to see how they do that, and it appears there is more than one way. For example, if they get exposed to the disinfectant, they can pump it out; that’s called an efflux mechanism,” she adds. “And the other way is they can have specific genes dedicated to detoxifying that disinfectant – genes that they have picked up from another type of bacterium.

“It seems that someplace in the processing plant or perhaps in the sewage – wherever disinfectants were present and other bacteria were present – they picked up from another organism a piece of DNA that they can now use to withstand disinfectant.”

One of the visiting scientists in Kathariou’s lab, Mira Rakic-Martinez, recently looked at what happens when Listeria becomes resistant to disinfectants, and she found that the bacteria also become more resistant to toxic dyes and to antibiotics.

“And she found it the other way around, too: When you treat something with an antibiotic and it becomes more tolerant to the antibiotic, it also becomes more tolerant to disinfectant,” Kathariou says.

Once they’ve characterized traits such as these at the genetic level, Kathariou and her colleagues share that information with collaborators who use animal and cell culture models to determine if such changes affect Listeria’s ability to cause human disease.

Such research is important to state and national public health officials working to resolve outbreaks of foodborne illnesses. Dr. Leslie A. Wolf, laboratory director for the N.C. State Laboratory of Public Health – turned to Kathariou for help in developing a molecular method to determine subtypes of Listeria monocytogenes.

“In public health, we are concerned with the molecular epidemiology of these bacteria to help resolve foodborne outbreaks and identify sources of transmission of foodborne pathogens,” Wolf says. “Having foundational understanding of the evolution of these pathogens is important background information to understanding sources of outbreaks and developing prevention strategies.”

While Kathariou is generating a greater understanding of Listeria, she’s also expanding our knowledge of Campylobacter. This bacterium causes many more human infections than does listeria – in fact, it is one of the most common causes of diarrheal illness in the United States – but it results in fewer deaths.

Still, says Kathariou, campylobacteriosis is significant not only because of the acute gastroenteritis it causes but also because in about one in 1,000 cases it leads to the severe Guillain-Barré syndrome, which can lead to neuromuscular paralysis and sometimes result in lifelong disabilities. Other infectious agents can lead to GBS, but Campylobacter infection is the most common antecedent, she says.

Most cases of campylobacteriosis result from eating raw or undercooked poultry or from cross-contamination of other foods by these items. But the bacteria are found in all animals that people raise for food and can also contaminate milk and water.

For Campylobacter, Kathariou explores such questions as how did virulent strains evolve, how have they adapted to different animal hosts, what makes the bacteria so susceptible to dehydration, which genes make it resistant to antibiotics and what kinds of genes does it need to be able to colonize poultry flocks.

“When animals are infected, we have no easy way of knowing that because the animals typically won’t have symptoms. But even though they do not result in symptoms that we can see, these bacteria are living and growing and the bird does respond to them,” she says. “So we are trying to understand that process, because if we did that, it might give us a new set of interventions to prevent colonization of the birds. And if we could reduce colonization of the birds before they are killed, it would go a long way to reducing illness.”

And reducing illness is, bottom line, what Kathariou’s studies are all about.

“When we work with bacteria that cause diseases, we do so with the goal of finding ways to reduce the public health burden,” Kathariou says, “because at the end of the day, we would like to be able to say that we’ve made a difference, for the better.”

– Dee Shore

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