Potentially Fatal Heart Infection Caused by Food-Borne Bacteria

A new study claims that particular strains of Listeria monocytogenes--a bacterium commonly found in soft cheeses and chilled ready-to-eat products--are able to invade the heart, leading to serious and difficult-to-treat endocarditis.

Researchers at the University of Illinois (Chicago, USA) obtained a strain of Listeria that had been isolated from a patient with endocarditis, which develops in about 10% of serious Listeria infections. The researchers were interested in determining whether patient predisposition led to the heart infection, or whether something different about the specific Listeria strain caused it to target the heart. These cardiac infections are difficult to treat, with more than one-third proving fatal, and have not been widely studied or understood.

The researchers found that when they infected mice with either the cardiac isolate or a lab strain, 10 times as much bacterium were found in the hearts of mice infected with the cardiac strain. In the spleen and liver, organs that are commonly targeted by Listeria, the levels of bacteria were equal in both groups of mice. The researchers also found that while the lab-strain-infected group often had no heart infection at all, 90% of the mice infected with the cardiac strain had heart infections.

The researchers repeated the experiment with 10 more strains of Listeria, and found that only one other strain also seemed to also target the heart. Molecular genetic studies and cardiac cell cultures suggested that these cardiac-associated strains display modified proteins on their surface that enable the bacteria to more easily enter cardiac cells, targeting the heart and leading to bacterial infection. The study was published ahead of print on January 25, 2011, in the Journal of Medical Microbiology.

"Listeria is actually pretty common in foods, and because it can grow at refrigerated temperatures, as foods are being produced with a longer and longer shelf life, Listeria infection may become more common,” said lead author Nancy Freitag, PhD, an associate professor of microbiology and immunology. "In combination with an aging population that is more susceptible to serious infection, it's important that we learn all we can about these deadly infections.”

L. monocytogenes is a bacterium commonly found in soil, stream water, sewage, plants, and food, and are responsible for listeriosis, a rare but potentially lethal food-borne infection, with a case fatality rate for those with a severe form of infection approaching 25% (Salmonella, in comparison, has a mortality rate estimated at less than 1%). Listeria has been found in uncooked meats, uncooked vegetables, processed foods, unpasteurized milk, and foods made from unpasteurized milk. While pasteurization and sufficient cooking kill Listeria, contamination may occur after cooking and before packaging.

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