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Foodborne pathogen sheltered by harmless bacteria that support biofilm formation

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Pathogenic bacteria that stubbornly lurk in some apple-packing facilities may be sheltered and protected by harmless bacteria that are known for their ability to form biofilms, according to Penn State researchers, who suggest the discovery could lead to development of alternative foodborne-pathogen-control strategies. 

That was the key finding that emerged from a study of three tree-fruit-packing facilities in the Northeast where contamination with Listeria monocytogenes was a concern. The research, done in collaboration with the apple industry, was an effort to better understand the microbial ecology of food-processing facilities. The ultimate goal is to identify ways to improve pathogen control in the apple supply chain to avoid foodborne disease outbreaks and recalls of apples and apple products.

“This work is part of Penn State’s efforts to help producers comply with standards set forth in the federal Food Safety Modernization Act, often referred to as FSMA,” said researcher Jasna Kovac, assistant professor of food science, College of Agricultural Sciences. “The Department of Food Science at Penn State, through research and extension activities, has an ongoing collaboration with the apple industry, led by Luke LaBorde, professor of food science.”

The research was done in collaboration with the apple industry, in an effort to better understand the microbial ecology of food-processing facilities. The ultimate goal is to identify ways to improve pathogen control in the apple supply chain to avoid foodborne disease outbreaks and recalls of apples and apple products. 

In the study, researchers sought to understand the composition of microbiota in apple-packing environments and its association with the occurrence of the foodborne pathogen Listeria monocytogenes. Their testing revealed that a packing plant with a significantly higher Listeria monocytogenes occurrence was uniquely dominated by the bacterial family Pseudomonadaceae and the fungal family Dipodascaceae.

“As we investigated the properties of these microorganisms, we learned that they are known to be very good biofilm formers,” said lead researcher Xiaoqing Tan (upper left), a recently graduated master’s degree student in food science and a member of the Penn State Microbiome Center, housed in the Huck Institutes of the Life Sciences. “Based on our findings, we hypothesize that these harmless microorganisms are supporting the persistence of Listeria monocytogenes because they protect the harmful bacteria by enclosing them in biofilms. We are testing this hypothesis in a follow-up study.”

The findings of the research, published today (Aug. 21) in Microbiome, provide insight into the Listeria contamination problem and may lead to researchers and the apple industry getting closer to solving it, Kovac believes. Equipment in fruit-processing plants — such as brush conveyors — have a poor sanitary design that makes them difficult to clean and sanitize, she pointed out. She and LaBorde plan to work with the apple industry to devise more effective cleaning and sanitizing strategies.

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Researchers collected samples in apple-packing facilities in which Listeria monocytogenes has been persistent. They discovered that harmless bacteria may be sheltering the pathogens.

 “Following up on these findings, we are experimenting with some of the nonpathogenic strains of bacteria that are not harmful to humans to see whether they can be used as biocontrols,” she said. “Once applied on the surfaces of the equipment in these environments, they may be able to outcompete and suppress Listeria, thus reducing food-safety risks and potential regulatory action. We are still exploring that approach in a controlled laboratory environment. If it proves to be feasible, we would like to test it in apple-packing and processing facilities.”

The challenge presented by microbiota possibly sheltering Listeria monocytogenes is not limited to fruit-processing facilities or produce, Penn State researchers suspect. They will soon begin analyzing microbial communities in dairy-processing facilities to determine the microbial composition and ecology of these environments.

Eggs from small flocks more likely to contain Salmonella enteritidis

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Eggs from small flocks of chickens are more likely to be contaminated with Salmonella enteritidis than eggs sold in grocery stores, which typically come from larger flocks that are regulated by the U.S. Food and Drug Administration, according to researchers in Penn State’s College of Agricultural Sciences.

egg-dirty-feb-12That conclusion — which flies in the face of conventional wisdom that eggs from backyard poultry and small local enterprises are safer to eat than “commercially produced” eggs — was drawn from a first-of-its kind, six-month study done last year in Pennsylvania. Researchers collected and tested more than 6,000 eggs from more than 200 selling points across the state.

Salmonella enteritidis is a leading foodborne pathogen in the United States, with many outbreaks in humans traced back to shell eggs. The FDA requires shell-egg producers from farms with more than 3,000 chickens be in compliance with the FDA Final Egg Rule, which is aimed at restricting the growth of pathogens. However, small flocks with fewer than 3,000 layer chickens currently are exempted. Eggs from these producers often are marketed via direct retail to restaurants, health-food stores and farmers markets, or sold at on-farm roadside stands.

From April to September 2015, the researchers purchased two to four dozen eggs from each of 240 randomly selected farmers markets or roadside stands representing small layer flocks in 67 counties of Pennsylvania. Internal contents of the eggs and egg shells were cultured separately for Salmonella using standard protocols. Salmonella recovered were classified by serotype, and any Salmonella enteritidis isolates present were further characterized to evaluate their relatedness to isolates of the bacteria that have caused foodborne illness outbreaks.

Test results revealed that of the 240 selling points included in the study, eggs from five — 2 percent — were positive for Salmonella enteritidis. Eggs sold at one of the positive selling points contained the bacteria in egg shells; the eggs from the other four selling points had Salmonella enteritidis in internal contents.

That is a higher prevalence of the pathogen than that found in studies of eggs from large flocks, noted lead researcher Subhashinie Kariyawasam, microbiology section head at Penn State’s Animal Diagnostic Laboratory. Those eggs, from flocks of more than 3,000 birds, are subject to federal regulations aimed at reducing Salmonella enteritidis contamination.

egg-farmThese regulations require measures such as placement of Salmonella-“clean” chicks, intensive rodent control, cleaning and disinfecting between flocks, environmental monitoring of pullet and layer houses, continuous testing of eggs from any Salmonella-positive houses, and diverting eggs from Salmonella-positive houses for pasteurization.

Kariyawasam — who presented the research findings to the American Veterinary Medical Association and the American Association of Avian Pathologists at their August meeting in San Antonio, Texas — said the study clearly demonstrated that Salmonella enteritidis is present in the eggs produced by small flocks.

“The research highlights the potential risk posed by the consumption of eggs produced by backyard and small layer flocks. And, analysis of the Salmonella enteritidis present in the eggs from small flocks shows they are the same types commonly reported to the Centers for Disease Control and Prevention from human foodborne outbreaks,” she said. “These findings emphasize the importance of small-producer education on Salmonella enteritidis control measures and perhaps implementation of egg quality-assurance practices to prevent contamination of eggs produced by backyard and other small layer flocks.”

Eggs from small flocks make a negligible contribution to the table egg industry in the United States, Kariyawasam noted. But the growing demand for backyard eggs and eggs from nonfarm environments — with small egg-producing flocks managed in cage-free systems and pasture situations — suggests these production systems deserve some scrutiny.

“We were curious about Salmonella contamination of eggs produced by these flocks because the prevalence of this pathogen in smaller flocks was not known. Now we know that the prevalence of Salmonella enteritidis in eggs produced by small flocks is higher than in eggs produced by larger flocks.”