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Winter 1997

Research Magazine > ARCHIVE > Winter 97 > Article

Salmonella's Fatal Flaw
by Catherine Gianaro

Salmonella is a bug that people just can't stomach. But just how bad a bug is it?

Historically, the view from such health authorities as the Centers for Disease Control and Prevention was that all types of salmonellae are potential killers. Margie Lee disagrees.

By and large, almost any of the large family of salmonella bacteria can cause a case of Montezuma's revenge. But Lee, a microbiologist at UGA's College of Veterinary Medicine, said the real fear lies in septicemia, a disease which occurs when salmonella bacteria begin to grow in your blood.

"Septicemia is bad news. It'll kill you," Lee said. "Most salmonellae will probably cause diarrhea, but the vast majority cannot cause septicemic disease."

In the medical research community, those are fighting words. The CDC has maintained for decades that all salmonellae can enter the bloodstream and cause septicemia.

But Lee's discovery, which was published last September in Applied and Environmental Microbiology, makes a strong case otherwise. She found that the salmonellae that have a specific genetic marker and are capable of causing septicemia can be found in chicken eggs and chicken ovaries. This marker indicates the presence of a virulence plasmid - a "mini" chromosome that contains genes that can cause septicemia.

To conduct the research, Lee tested 245 salmonella varieties from different locations, including waste water, human diarrhea, chicken houses, chicken ovaries, chicken eggs and chicken feces. Only one type, Salmonella enteritidis, which Lee found only in chicken eggs and chicken ovaries, possessed the virulence plasmid.

More importantly, her research showed that most infected chickens do not carry the deadly salmonella, just the type that causes diarrhea. Even if you eat salmonella-contaminated food, you may not get sick.

"[To get diarrhea] it first takes a million salmonella cells to get in your mouth and go down your throat to your stomach to your intestines," Lee said. Even if a person becomes infected with salmonella, the bacteria needs to battle the body's defense system before infection can occur.

If the bacteria make their way to the intestines, they attack the epithelial cells that digest food and nutrients. The epithelial cells signal the body, which dispatches white blood cells to the intestines to kill the salmonella. There, they prevent the organ from absorbing fluid and cause diarrhea. "So actually, the diarrhea is in response to your body doing a good job," Lee said.

If a person is infected with Salmonella enteritidis and if the white blood cells fail to kill the invader, the bacteria may penetrate the tissue and blood vessels, and could cause septicemia. People are vulnerable to septicemia when their white blood cells do not function correctly - for instance, if someone is fighting another illness like AIDS or cancer. Others who also are susceptible to the virulent disease are the very young, because their bodies are still developing, and the very old, whose immune systems are more likely to malfunction.

Even though the threat of septicemia may be limited to only two chicken-borne types of the bacterium, there have been deadly outbreaks of other salmonella-related illnesses in the United States, including one recently in California.

"Many AIDS patients have been eating rattlesnake meat because it's supposed to be some sort of holistic medicine," Lee said. "A number of people died from [salmonella-infected meat] because their bodies were busy fighting off a much bigger virus."

Salmonella infections also have been associated with vegetables. "Because of organic gardening, people use manure," she said, And a lot of it hasn't been composted," which kills most types of salmonellae.

Unusual house pets also have been a recent cause of sickness. According to the CDC, African pygmy hedgehogs and iguanas both have caused salmonella outbreaks, resulting in numerous severe cases of illness.

Each year, there are 23 million reported cases of illness caused by all food-borne bacteria, and of these, 9,000 are fatal. Of those, salmonella is the least threatening, Lee said.

Even though people become sick from salmonella poisoning, it rarely becomes deadly. "[Salmonella] doesn't merit the public's fear. Car accidents kill more people," Lee said. "You have to look at what is really lost, and on the whole, it's seldom life."

But since the genetic markers that indicate the deadly strain of the bacteria are not even visible under a microscope, people should play it safe. "Cook your chicken and eggs well," Lee said. "That's the bottom line."

For more information about the UGA School of Veterinary Medicine, access http://www.vet.uga.edu/.

Margie Lees research suggests that only one type of chicken-borne salmonella can cause septicemia, a potentially fatal blood disease. This type of salmonella, Salmonella enteritidis, which can be found in chicken eggs and ovaries, enters a person's digestive tract (at left) and may invade the blood stream where the yellow-colored bacteria (at right) multiply, causing the potentially deadly disease.


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