February 1, 2011

Lethal Listeria Bacteria Makes Its Way Into Packaged Foods

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While many Americans are familiar with disease-causing bacteria like salmonella, Listeria may sound more foreign. The food-borne pathogen, which can cause brain infection, blood poisoning, abortion and sometimes even death, claims approximately 500 American lives every year.

By comparing the deadly strains of Listeria to benign strains, University researchers strive to not only identify what causes certain strains to be so dangerous but also develop ways to prevent Listeriosis, a food-borne illness caused by Listeria.

Prof. Martin Wiedmann, food science, has shown that Listeria originate in soil and are transmitted to animals through plant materials. Inside the bodies of mammals, Listeria thrive because there is almost no competition for food. They can invade human cells and even relocate from one cell to the next, avoiding the antibodies the human body produces, Wiedmann explained.

“It has an uncanny ability to survive very, very well. In one case, we found a strain that had survived in a processing plant for 12 years. This may be because it is in the best interest of the bacteria that their hosts are kept alive,” Wiedmann said.

Only two of the existing six strains of Listeria cause disease in humans: L. monocytogenes and L. ivanovii. Wiedmann believes that while all of the strains of Listeria were poisonous in the past, certain strains have become less virulent, or actively poisonous, over time.

Strains continue to evolve, and even among L. monocytogenes, there is a strain that lacks a gene that would make the strain potentially fatal.

Wiedmann studies Listeria’s evolution over a span of 47 million years.

“To better understand genome evolution and evolution of virulence characteristics in Listeria, we used a next generation sequencing approach,” Wiedmann said.

The approach is the SOLiD System, developed by Craig Cummings and his team at Life Technologies Corp. The system is a highly accurate, massively parallel sequencing platform that supports a wide range of applications.

“It allows us to discover the differences between large sets of bacterial strains in a relatively fast and affordable way,” Cummings said.

If infected with Listeriosis, patients typically experience symptoms similar to those of meningitis, including fever and neck stiffness. About 20 percent of people diagnosed with Listeriosis die (compared to less than one percent of those diagnosed with salmonella poisoning).

Listeria is a serious concern because it can be found almost anywhere. “Due to its severe consequences, Listeriosis is a major public health concern. L. monocytogenes is also commonly found in nature and in food processing environments and has the ability to survive for extended time periods outside animal hosts,” Wiedmann said. “Thus, control of this organism represents a serious challenge for the food industry.”

­Listeria has been found in raw meats and vegetables, unpasteurized milk and processed foods. Though pasteurization and thorough cooking techniques usually kill the pathogen, contamination often happens during packaging, so “extensive sanitation is required at processing plants,” Wiedmann said.

In the past, Wiedmann research team has helped control Listeriosis outbreaks by tracing the origin of contamination through the use of a web-based pathogen tracker database that they developed. Wiedmann’s most recent finding is a unique strain of L. monocytogenes responsible for an outbreak among dairy cattle.

Wiedmann said he hopes that the food industry will incorporate his research into their plans for improving packaging processes. According to Cummings, Life Technologies Corp. “is planning to use the information Wiedmann collected to develop assays that specifically detect pathogenic Listeria strains.”

“We create the basic knowledge, which industries can then develop and hopefully put to practical use,” Wiedmann said.

Original Author: Maria Minsker