Pathogen Protection

Pathogen Protection

Most people take the act of eating for granted. They bite into the turkey sandwich with gusto and virtuously munch on their spinach salad — or they chow down on the cheeseburger and fries. But whatever the food, the consumer inevitably pays the price at some point in their lives, in the currency of foodborne illness.

The Centers for Disease Control estimates that 76 million people succumb to some form of foodborne illness annually. Cramps, nausea, vomiting, diarrhea — these common symptoms often keep a child out of school or an adult home from work for a few days — a mild dose of discomfort. But out of these cases, about 325,000 people end up in the hospital and about 5,000 people die annually from complications resulting from these foodborne diseases — most often the very old, the very young, and people with existing illnesses.

Why so much illness from the simple act of eating? We live in a microbial world, said Steven C. Ricke, the director of the Center for Food Safety and Microbiology in the Institute of Food Science and Engineering, and the holder of the Donald “Buddy” Wray Food Safety Endowed Chair.

“Most of the pathogens that cause foodborne illness exist in the environment,” Ricke said. “What we need to understand is what triggers the microbes to become pathogens.”

Doctoral student Lisa Bielke of Houston, Texas, streaks Salmonella culture on a plate treated with lactobacilli, a beneficial bacterium that can be used to treat bacterial infections in poultry gastrointestinal tracts.

Food has many opportunities to become contaminated on its journey from the farm to processing plant to grocery store to kitchen table. First of all, healthy animals harbor microbes, such as Salmonella and Campylobacter, and these can potentially contaminate meat during processing.

“Animals are not grown in a sterile environment,” said Mike Johnson, professor of food science in the Dale Bumpers School of Agricultural, Food and Life Sciences. In addition to harboring potential pathogens, animals are exposed to air, water, earth and other animals that could potentially spread disease.

Fresh fruits and vegetables are not immune to invasion by pathogens — they can become carriers if washed with contaminated water. Later in processing, foodborne microbes can be introduced by infected humans or by cross-contamination through contact with another product. Finally, the way food is handled in the home or restaurant can determine whether or not disease ensues. Under the right conditions, one bacterium can produce about 17 million progeny in 12 hours.

At the University of Arkansas, researchers seek to reduce the number of foodborne illnesses in three ways: prevention, containment and reduction. While researchers have for years studied containment and reduction, prevention remains a relatively new approach. Ricke looks at ways to predict pathogen behavior so researchers can then prevent problems with foodborne illness down the line.

“Foodborne pathogens are a moving target. They’re genetically dynamic,” Ricke said. “The more we understand the biology of the organisms, the more we will know what they are capable of doing.”

The organisms’ names read like a “who’s who” line-up in Latin: Campylobacter jejuni, Escherichia coli, Listeria monocytogenes and Salmonella. These four microorganisms hold the distinction of being the major bacteria of concern in food safety and public health. The Center for Food Safety has researchers working on some aspect of all of them.

Ricke studies Salmonella in the guts of chickens. Salmonella causes somewhere between 2 to 4 million cases of salmonellosis in the United States annually. Most people contract it from raw meats, poultry, eggs and unpasteurized dairy products.

This microorganism typically shuns acidic environments, but it has evolved genes that allow it to tolerate stomach acid. And while the bacteria reside in the intestinal tracts of these birds, they do not always cause infections. Knowing the circumstances that trigger an infection can help researchers determine how to prevent them from occurring.

Escherichia Coli


Campylobacter Jejuni

To look at virulence, Ricke studied the prevalence of Salmonella in the gastrointestinal tracts of chickens fed on different dietary regimens. They found that diet modification could lead to changes in the gastrointestinal tract that might encourage virulence and infection.

Killing some of the bacteria through food modification may seem like a good idea, but the ones that remain behind may become more militant, turning into “Ninja” bacteria that multiply and potentially become an infectious strain, Ricke said.

“It’s the same scenario if you suddently take the pathogen outside of the animal — how does it respond to that rapid change?” he said.

Microbes are the ultimate opportunists, looking for ways to reproduce and continue their existence. Johnson studies ways to protect food — and humans — from one of the hardiest pathogens — Listeria monocytogenes.

Listeria infection, while rare, can lead to meningitis, septicemia, encephalitis and spontaneous abortion in pregnant women. It has been associated with raw meat and dairy products. But worst of all from the standpoint of those who wish to combat the growth of Listeria in food products, the microbe can survive at refrigeration temperatures.

“Most of the hurdles we use to keep food safe from microbes don’t work with Listeria,” Johnson said. “So we had to come up with something that would provide protection.”

Johnson and Navam Hettiarachchy, University Professor of food science, together with graduate student Marlene Janes, now an assistant professor at Louisiana State University, developed an edible film, a tasteless, invisible physical barrier between the meat products and the environment. They used zein — a corn protein used as a coating for candy. However, they added an important ingredient — nisin, a bacteriocin that can penetrate the cell membrane and destroy the cell from within. Nisin had been used in liquid form, but the researchers combined it with the zein to form a protein-coating, bacteria-killing edible film.

“You want to see your treatment kill the bugs until they are not detectable,” Johnson said. The researchers used the edible film to prevent Campylobacter jejuni bacteria from growing on chicken carcasses. Researchers estimate that Campylobacter causes more than 4 million cases of illness a year in the United States.

They also used edible films to prevent the growth of the less pervasive but hardier Listeria monocytogenes in processed meat products.

While Listeria has the distinction of being a virulent, hard-to-kill pathogen, most people find themselves more familiar with Escherichia coli, or E. coli. In recent years, outbreaks of E. coli in vegetables such as spinach and scallions have worried consumers and caused economic havoc for producers. According to the CDC, health-related incidents involving fruits and vegetables are on the rise.

Plants pose a particular challenge because the pathogens don’t generally infect the plant — they just exist on the plant. Hettiarachchy and her research team have started to examine the antimicrobial properties of certain types of plant extracts, peptides and organic acids with the idea of using the extracts and selected substances in edible films to protect fruits and vegetables on the way from the field to the table.

“The consumer is very apprehensive about chemicals in foods,” Hettiarachchy said. “But people are used to eating plant material in their diets.”

Ramakrishna Nannapaneni, a food science researcher, examines starved cells of Listeria monocytogenes impact on targeted mouse cells.

Most people are familiar with grape seeds and green tea, two of the items Hettiarachchy and her research team use to make antimicrobial extracts. The extracts are made using liquids water or other food-grade solvents. The researchers have used the extracts alone and in combination with nisin peptide, a protein fragment, and lactic or malic acid — the former found in milk and the latter in apples.

How to apply the extracts, peptides and organic acids to food products? Hettiarachchy and her team used an edible film containing the antimicrobial extracts, peptides and organic acids. This invisible, tasteless, odorless film can be used to coat meat products, vegetables, fruits and eggs to protect the products from pathogens.

They currently have a patent for producing edible films that contain soy proteins and organic acids with antimicrobial activity. The films can contain different amounts of the various natural antimicrobial products. The nature of the film will allow immediate release or long-term release of antimicrobials. The edible film technology is available to companies for development and commercialization under license.

In addition to the antimicrobial properties of the substances in the film, the film itself acts as a barrier to any outside pathogens. The organic acids and peptides protect the product from within. Lactoferrin, for instance, a peptide derived from milk, binds to iron, preventing it from entering the pathogenic cell and creating an iron deficiency within the cell.

“The metabolism of the bacterial cell will be affected, leading to death,” Hettiarachchy said. Although much of their work has been with Listeria, her research team is also focusing its attention on Salmonella and E. coli.

“The continuing recalls due to pathogen contamination in food products show that further research is needed to better control and kill pathogens,” Hettiarachchy said. “New and novel multiple technologies are in demand to effectively kill or control foodborne pathogens.” This is the current focus of Hettiarachchy’s research team.

E. coli may be the most well-known “bad guy” of the bacterial world. First documented in 1982, a particularly virulent form of the bacteria, E. coli O157:H7, which causes hemorrhagic colitis, was first associated with disease outbreaks in people who consumed undercooked ground meat. In September of 2006, three people died and 200 became ill in more than 26 states and one Canadian province from eating E. coli O157:H7–contaminated spinach traced back to a farm in California. In November of 2006, another outbreak of the strain occurred — this time in green onions found in Taco Bell restaurants in the northeast. Dozens of people in New York, New Jersey, Pennsylvania and Delaware became ill.

Steven Ricke examines bacteria with student Vesela Chalova-Zhekova.

“Obviously the public would like to see outbreaks minimized as much as possible,” Ricke said. And while researchers work to make food safer in the marketplace and in restaurants, clean kitchen habits at home will always remain a part of preventing foodborne illness. In fact, it’s one of the first things Johnson talks about with his students. It only takes a few bacteria from a dirty chopping knife to contaminate the lettuce that’s cut with it next.

Keep hot foods hot and cold foods cold — so say the researchers. Johnson also emphasizes washing hands while cooking. He points out that cross-contamination can occur when the cook picks up a bun to toast it on the grill using hands that have handled raw meat.

Pathogens have opportunities they will exploit, he said.

About The Author

University Relations Science and Research Team

University Relations Science and Research Team

Matt McGowan
science and research writer

Robert Whitby
science and research writer

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