THURSDAY, Feb. 5, 2004 (HealthDayNews) -- Scientists have long known Listeria monocytogenes can cause serious -- even deadly -- infection when people eat food contaminated with the bacterium.
But in a new finding that illustrates just how clever this germ may be, researchers say Listeria can hunker down in the gall bladder as well as the liver. And it can grow outside of cells. This all means that a chronically infected person could pass the bacterium to others without showing symptoms.
"Rather than worrying about food contamination coming from soil or animals, it could come from chronically infected food service workers," explains lead researcher Christopher Contag, an assistant professor of pediatrics, microbiology and immunology at Stanford University.
Listeria can cause severe illness or death in people with weakened immune systems and may cause miscarriage in pregnant women. And 20 percent to 40 percent of those affected die even after antibiotic treatment.
The germ is to blame for about 500 deaths and 2,500 illnesses annually in the United States. Pregnant women are about 20 times more likely become infected, compared with other healthy adults.
What's more, unlike many bacteria, Listeria can still grow at low temperatures and live for long periods outside its many animal hosts. It's most commonly found in foods such as soft cheeses and cold cuts, products pregnant women are often told to avoid.
In their experiments with mice, the researchers used a unique imaging technique that looks at the whole body to track the path of Listeria infection. The bacterium was highlighted with a luminescent molecule that can be detected in living tissue.
Using this technique, they unexpectedly found Listeria takes up residence in the gall bladder as well as in the liver.
The study appears in the Feb. 6 issue of Science.
In addition, Listeria, which normally grows inside the host's cells, was found to be reproducing outside cells in the gall bladder. It reproduced in long extracellular chains that could conceivably have filled the gall bladder, but did not cause the mice to have symptoms.
"Growth inside cells was thought to be necessary for Listeria. This is why this was such a surprising finding," Contag says.
Contag's team also found that even mice who showed no signs of Listeria infection had the germ in their gall bladder, he says.
Based on this finding, "we assume that people could be walking around healthy, not showing any symptoms, and still be carrying the bacteria in the gall bladder."
One way Listeria infection can occur is the fecal-oral route, where infection-laden stool from one person touches food that is then eaten by another. Good hygiene, particularly hand washing, is paramount for anyone handling food, Contag stresses.
The next step is to find out how Listeria grows outside of cells and what it needs to survive. "If you know that, then you can begin to develop the tools that will prevent it in that environment," Contag says.
Brian J. Wilkinson, a professor of biology at Illinois State University, says the new study offers a "whole new angle on Listeria infection."
"This finding raises the possibility of a Typhoid Mary-type spread of the bacteria," he adds. "There is a possibility that humans are carriers of Listeria in the gall bladder and can contaminate food and spread the organism."
Michael G. Schmidt, a professor of microbiology and immunology at the Medical University of South Carolina, adds, "Microorganisms are constantly doing the unexpected. You cannot lock them into one behavior. Once you think they are going in one direction, they immediately do something unexpected."
"This is a life-threatening disease and all of a sudden we have a new way of spreading the disease," Schmidt says.
Schmidt also speculates that Listeria infection in the gall bladder may trigger the formation of gall stones, in much the same way that Helicobacter pylori bacteria in the gut causes ulcers and stomach cancer.
This is something that should be looked into, Schmidt says. "We may find something remarkable."
More and more, scientists are learning that the chronic diseases for which there have been no explanations often turn out to be initiated by bacteria or the result of a problem with the immune system, Schmidt adds.
More information
For more on Listeria, check the U.S. Centers for Disease Control and Prevention or the U.S. Food and Drug Administration.
