Harboring the Enemy

Pathogenic bacteria lurking in some apple-packing facilities may be sheltered and protected by harmless bacteria in slimy biofilms, according to researchers in the college who say the discovery could lead to the 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," said Jasna Kovac, assistant professor of food science.

The study revealed that a packing plant with a significantly higher Listeria monocytogenes occurrence was also dominated by certain common bacteria and fungi.

"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," said lead researcher Xiaoqing Tan, 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.

Biofilms are a collection of microorganisms that attach to a surface and then secrete a slimy material that slows down the penetration of cleaners and sanitizers, Kovac explained. "If a pathogenic bacterium is enclosed in a biofilm formed by microbiota, it is more likely that cleaning and sanitizing procedures will be less effective. This is a novel perspective, and it may well explain how Listeria monocytogenes has persisted in food-processing plants despite repeated efforts to kill and remove it."

The findings of the research may help the apple industry find solutions to contamination problems. The team plans to work with the apple industry to devise more effective cleaning and sanitizing strategies.

--Jeff Mulhollem