Compound Discovered to Stop Listeria Contamination and Infection

Listeria
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Although the Listeria cantaloupe contamination and infection outbreak of 2011 is officially over according to officials from the CDC, this does not mean that Listeria is no longer a concern the next time you choose produce from your local supermarket. Listeria is a particularly hardy microbe that persists in spite of harsh environmental conditions such as freezing during refrigeration, heating from cooking and acid from your stomach. However, there is new hope. Scientists recently report they have found a chink in Listeria’s armor via a non-toxic compound they say can stop listeria contamination and infection.

The microbe Listeria monocytogenes is responsible for the fatal food-borne disease called listeriosis (Listeria) that swept like wildfire across the U.S. fueling fears of whether or not the produce in our kitchens was safe to eat or not. Listeriosis is responsible for approximately 10 percent of all deaths from food-borne diseases in the United States.

In 2011, a Listeria outbreak resulted in 146 reported infections and 30 deaths and one miscarriage in 28 states. The Listeria infection was traced to a contamination problem with cantaloupe produce from Jensen Farms in Colorado.

One of the things that makes Listeria especially interesting is that it can transition from a saprotrophic existence under a wide range of environmental conditions to become a pathogenic organism that attacks the cells of a human host. A saprotroph is an organism that normally obtains its nutrients by absorbing soluble organic compounds from dead and decaying plant or animal matter. Cantaloupe picked from a field is one example of a food that Listeria can survive on.

The ability of Listeria to transform itself from a saprotroph to an intracellular pathogen is mediated by regulatory networks that enable bacterial survival and virulence in response to environmental signals such as cold, heat or acidic conditions in the stomach.

Researchers from Cornell University have recently discovered that a key component of this regulatory network is a "stress-responsive alternative sigma factor" called sigma B that controls more than 150 genes that contribute to the bacterium’s survival and virulence—including genes necessary for the bacteria to cross through the human gastrointestinal tract.

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According to a press release by Cornell University, "We were the first to characterize sigma B in L. monocytogenes," says Kathryn Boor, Cornell professor of food science, the Ronald P. Lynch Dean of Agriculture and Life Sciences and the paper's senior author. "It's the linchpin in the transition of this organism from a harmless environmental microbe to a human pathogen. It allows these single-celled pathogens to survive environmental assaults associated with transmission in foods, followed by transit in the human body."

Following their discovery of sigma B, the Cornell researchers and other collaborating institutions began to look for compounds that could take advantage of their discovery and use it to inhibit the regulatory network that gives Listeria its pathogenicity.

Use robotic technology, they screened 57,000 compounds that were potential inhibitors of sigma B. They eventually found 41 compounds that inhibited sigma B, one of which is called FSPP that was found to be nontoxic to mammalian cells and inhibited sigma B in Listeria as well as in a hardy soil bacterium called “Bacillus subtilis.”

According to the authors of the paper the results of their discovery of FSPP shows therapeutic promise and suggests that not only will this aid future studies toward understanding the regulatory networks of other hardy pathogenic organisms, but that it may also represent a compound that can be developed into a therapeutic agent.

"This is a newly emerging approach in the search for antibiotics that are not dangerous to mammals but stop such pathogens as Listeria, and could be a possible treatment against other organisms," says Boor.

For a list of “Foods that Kill” that contributed to food-borne illness or death in 2011, follow this link at

References:
1. Cornell University Chronicle Online

2. mBio a journal of the American Society for Microbiology.
Palmer ME, Chaturongakul S, Wiedmann M, and Boor KJ. 2011. The Listeria monocytogenes σB regulon and its virulence-associated functions are inhibited by a small molecule. mBio 2(6):e00241-11. doi:10.1128/mBio.00241-11.

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