Salmonella: The Trojan Horse of Germs

The food-borne bacteria seems to take over and infect host cells, researchers say

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By Amanda Gardner
HealthDay Reporter

FRIDAY, Aug. 4, 2006 (HealthDay News) -- The extremely uncomfortable symptoms of salmonella are something no one ever wants to live through, much less repeat.

Yet, there may be as many as 150,000 cases of this bacterial infection in the United States every year, most of them undiagnosed and unreported.

How the salmonella germ multiplies in undercooked or raw food may be through what scientists compare to the ancient Trojan Horse, by taking over and infecting host cells.

This unusual look into the machinery of this sometimes deadly food-borne bacteria, courtesy of an electron microscope and other technologies, may one day help researchers design drugs to thwart this and other germs.

According to the U.S. Centers for Disease Control and Prevention, about 40,000 cases of salmonella infection are reported in the United States each year, although the actual number of cases may be 30 times higher or more. Although most people recover without treatment, some 600 people die every year.

Most people infected with salmonella develop diarrhea, fever and abdominal cramps within 12 to 72 hours of infection. The illness usually lasts four to seven days. But for some people, the diarrhea may be so severe that they may need to be hospitalized and treated with antibiotics, the CDC says.

The use of the electron microscope in determining the salmonella germs migration may be the beginning of an enlightening journey. "This is one small component of a very big puzzle," said Edward Egelman, co-author of a study into salmonella's unique properties. Egelman is a professor of biochemistry and molecular genetics at the University of Virginia Health System.

Using an electron microscope along with X-ray crystallography and 3-D reconstruction, researchers at Rockefeller University in New York City and the University of Virginia managed to get the first high-resolution picture of part of the SipA protein, a key component of the salmonella bacteria's machinery.

"Salmonella secretes a number of proteins that are involved in taking control of the host cell to engulf the bacterium and in that way infecting the host cells," Egelman told HealthDay. "This particular protein, SipA, is one of a number that are produced, and it has been shown previously to bind to actin."

Actin is one of the most abundant proteins in the human body and is involved in the cytoskeleton -- the framework of cells -- and in cell movement. "It's like a protein scaffold that holds the cell together," he said.

Although scientists knew salmonella proteins bind to actin, they did not know precisely how this occurred.

As it turns out, SipA essentially "staples" actin into long filaments, which then reorganize the cytoskeleton of the host cell. "This reorganization causes the host cell to engulf the bacterium, something that it would not normally do," Egelman explained. "Once the bacterium is engulfed by the host cell, it can multiply within the host cells."

Contrary to scientists' expectations, SipA turned out to be compact and heart-shaped with a globular core. Two "arms" project from either side, the researchers found.

It's not yet clear how this knowledge will translate into protective or therapeutic gains.

"This is just a fragment," Egelman said. "We obviously want to look at larger and larger pieces of this protein. There are other salmonella proteins that are part of the same secretion system, and several of them bind to actin, and we want to look at those."

More information

For more information about salmonella, visit the U.S. Centers for Disease Control and Prevention.

SOURCES: Edward H. Egelman, Ph.D., professor, biochemistry and molecular genetics, University of Virginia Health System, Charlottesville

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