Salmonella's Tricky Attack Plan Revealed

Maggie Koerth-Baker

Date: 19 June 2008 Time: 07:51 AM ET

Healthier Tomatoes Grown in Seawater
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As the tomato scare spreads across the country, scientists have discovered how the salmonella bacteria silently builds to formidable numbers while lurking inside your body for days.

More than 380 people have been infected with a rare strain of salmonella bacteria in the recent outbreak, most likely spread by shipments of tainted tomatoes, according to the Centers for Disease Control.

Most of the victims became ill between April 10 and June 5, and could have eaten the toxic food up to three days before they actually got sick, the CDC said.

This lag time between infection and the onset of symptoms is extremely significant, according to a new study by researchers at the University of Rochester Medical Center in New York.

Dr. Jun Sun, assistant professor of gastroenterology and hepatology, said during the lag, salmonella is copying itself, waiting to strike until its numbers are large enough to put up a fight.

And what a fight. Salmonella rarely kills, but victims can end up wishing they were dead. Vomiting, diarrhea, fever and cramps are common symptoms, usually lasting between 4 and 7 days. Most people get better without treatment, but the disease can be dangerous for children, the elderly, and those with compromised immune systems. The recent outbreak hasn't officially caused any deaths, but has put at least 48 people in the hospital.
Sun and her team figured out how salmonella is able to get your immune system to ignore it while it grows.

The culprit is a protein called AvrA. Here is how it works: Usually, when your body is invaded by a bacteria, the bug causes tissue damage, releasing chemicals that lead to swelling and inflammation. Physically, inflammation can isolate the invaders, making it difficult for them to travel to other parts of the body. Chemically, it attracts phagocytes, white blood cells that kill bad bacteria.

Salmonella has the ability to punch through the tight links of cells that make up the intestinal wall, using an arsenal of proteins and toxins it can inject into cells. Sun said scientists always thought AvrA was one of these, but, as her team reported June 4 in the online journal PloS One, AvrA actually has an opposite function.

The study found that AvrA can maintain the tight structure of cell junctions in the intestinal cells, she said.

AvrA temporarily stops salmonella from breaking apart the cell links. Because the bug doesn't damage tissue during this phase, there's no inflammatory response. Instead, salmonella is mostly left alone, free to grow and multiply into a formidable invasion force. Only then does it break through the intestinal walls, beginning its reign of terror and making you sick.

"This changes the way we look at bacteria," Sun said. "We're beginning to realize that salmonella is a creature that has existed many years longer than us and they have skills we don't understand fully. It's trickier than we thought. This helps us better understand how bacteria and we, their hosts, interact."