Iowa State University

Inside Iowa State
January 7, 2000

Food scientist specializes in "injured" bacteria

by Brian Meyer, Agriculture Information
It's the start of a new year and in a lab in the Food Sciences Building, Aubrey Mendonca is putting bacteria on a new diet -- a starvation one.

"We take them from nutrient-rich media and put them in nutrient-poor media," said Mendonca, a food safety microbiologist in the food science and human nutrition department. "For bacteria, the change is like living on feasts every day, then one day waking up in a famine."

Mendonca (pronounced Men-DON-sah) is studying how disease-causing bacteria, like E. coli 0157:H7and Listeria monocytogenes,adapt to stress, such as starving conditions. Stress-toughened pathogens may pose new food safety problems.

"Understanding more about these pathogens may provide us with an extra margin of food safety," he said. "This is particularly important to the growing segment of our population that have weak immune systems. These include the elderly, the very young, those battling cancer or AIDS and those who've received organ transplants. What may be a mild food-related illness to healthy people could be a life-threatening one to them."

Mendonca has found that what doesn't kill microbes can make them stronger.

"Often what we microbiologists study are pampered lab microbes grown under optimum conditions," he said. "We've assumed such conditions provide us with strong organisms that are resistant to food-preservation methods. But this assumption is dubious because bacteria that have adapted to stresses, like mild heating or starvation, show more resistance than microorganisms that aren't adapted."

The starvation diet mimics a potential scenario in processing plants. "There may be bacteria that cling to equipment for several days without food," Mendonca said. "If they adapt to the stress and come in contact with food again, it's likely they'll be tougher to destroy."

Mendonca joined the ISU faculty in 1998. He previously worked at North Carolina A&T State University, where he developed a microbial food safety program, and as a postdoctoral scientist at Penn State, where he became interested in injured microbes -- those that survive food processing.

"Injured bacteria are a problem because they aren't easily detected by conventional methods for ensuring foods are pathogen-free," he said.

Mendonca explained why this is true for heat-injured bacteria. Microorganisms produce toxic byproducts derived from their use of oxygen. Microbial cells make enzymes that detoxify these byproducts.

Heating foods during processing, such as milk during pasteurization, kills most bacteria. The survivors' detoxifying enzymes have been destroyed. Without them, injured microbes can't handle oxygen, and will die.

After processing, methods to detect pathogens involve aerobic incubation. In the now-deadly open air, injured microbes won't grow. So they can slip past the radar.

Now here's the insidious part.
"Many meat products today are vacuum-packaged," said Mendonca. "The airless conditions shield the injured microbes from the oxygen that would kill them. The packaging provides a perfect environment for the pathogens to repair themselves and regain their virulence. When the packaging is opened, the bugs are once again dangerous."

Mendonca has developed a detection system to uncloak heat-injured Listeria monocytogenes. His system creates an oxygen-free zone that allows convalescing microbes to repair themselves. Now he is working with researchers at the USDA's National Animal Disease Center on molecular-based detection methods to more rapidly detect injured Listeriain heat-processed pork and turkey products.

That project is part of Mendonca's work with the federally funded Food Safety Consortium, involving Iowa State, Kansas State University and the University of Arkansas.

In another consortium project, he's studying added ingredients in pork products. Some additives, such as salt, sodium lactate and phosphate, have antimicrobial activity. The work may help predict more effective combinations of ingredients and heat to eliminate pathogens.

Mendonca also continues work begun at North Carolina A&T on testing solutions to destroy pathogens on the skins of cantaloupes and apples.

"We need to be vigilant for emerging food-borne pathogens because of changes in the way we process, package and handle food," he said. "Those changes are in response to consumers' demands for convenience, taste and flavor, extended shelf-life and safety.

"It's also becoming more challenging to develop new ways to protect food because consumers want fewer chemicals and may prefer minimally processed foods," Mendonca said. "Microbiologically safe food has got to look good and taste good. Consumers look for those attributes first."

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