By KRISTA CONGER

Bacteria responsible for a lethal form of food poisoning may escape the immune system by hiding out in the gall bladder of seemingly healthy people. The finding by researchers at the School of Medicine suggests that an unwitting food worker could transmit the bacteria to others by contaminating food products.

The bacteria, Listeria monocytogenes, can cause severe illness or death in people with weakened immune systems and may cause miscarriage in pregnant women. “Twenty to 40 percent of people with listeriosis” – the disease caused by Listeria infection – “die even after antibiotic treatment,” said Jonathan Hardy, PhD, the study’s first author and a research associate in the Department of Pediatrics. The research was published in the Feb. 6 issue of Science.

Listeriosis causes about 500 U.S. deaths annually; pregnant women are about 20 times more likely than other healthy adults to become infected. Unlike many bacteria, Listeria is perfectly happy growing at refrigerator-like temperatures and can survive for long periods outside of its many animal hosts. It is most commonly found in foods such as soft cheeses and deli meats.

“To have discovered a chronic carrier state in the gall bladder of an animal model, suggesting a potential source of food contamination, is important,” said senior author Christopher Contag, PhD, assistant professor of pediatrics and of microbiology and immunology. Until now, it had been thought that the bacteria that tainted the food came directly from infected animals, or from soil or water harboring these hardy bacteria.

Hardy and colleagues at Stanford and Xenogen Corp. used a unique imaging technique to track the ebb and flow of Listeria infection in live mice. They tagged the bacteria with a luminescent molecule that can be non-invasively detected in living tissue, and then analyzed when and where the lunch-meat lurkers popped up. The ability to visualize the whole animal enabled them to identify the gall bladder as an important bacterial bunker – something they hadn’t expected and wouldn’t have found without using whole-body imaging.

“We were somewhat surprised to see the intensity of the signal in the gall bladder,” Contag said. “In contrast to traditional methods of detecting infection, imaging let us follow the same animal day after day and look at the whole body at once, picking up more subtle nuances of infection.”

Another surprise was how the bacteria were dividing in the gall bladder. Normally Listeria tries to evade the immune system by replicating inside the host’s own cells. But in the gall bladder, which serves as a way station for bile in between meals, the bacteria go it alone, reproducing in long extracellular chains that can cram the organ without causing symptoms in the animal.

“The most striking thing about all of this research is that we get tremendous amounts of bacterial growth without disease,” said Hardy. “This represents a new face of the pathogen – growing in a different place and a different way.” Intriguingly, the only other bacteria shown to hunker down in the gall bladder of an unbothered host is the one that causes typhoid fever, which is spread between people via contaminated food.

The researchers speculate that because it’s difficult for immune cells and antibiotics to enter the gall bladder, the organ is a safe zone for bacteria. After escaping detection, the bacteria are released with the flood of bile that enters the small intestine following a meal. Poor hygiene can spread the bacteria excreted in stool to the hands and then to food, leaving it poised to infect the next victim.

Listeria’s predilection for such exotic digs appears to be a result of fortuitous genetics; the bacteria express at least one gene that enables them to break down salt in the bile and possibly make the surroundings more hospitable.

“The bacteria have obviously maintained a set of genes that allows them to grow there,” said Contag. “Since the bacterium was often found in the gall bladder in our study, it implies that they use the genes routinely.” The researchers are now planning to investigate whether various Listeria mutants replicate in the gall bladder and to look for possible molecular targets for therapeutic intervention.

“We’d like to know how Listeria mutants that cannot replicate inside host cells gain access to the gall bladder,” said second author Kevin Francis, PhD, director of technical applications for Xenogen. “This mechanism could be targeted to prevent the carrier state.” Francis engineered the bioluminescent Listeria used in the study.

In vivo bioluminescent imaging was developed at Stanford in the Contag laboratory and has been licensed by Xenogen. Contag chairs the company’s scientific advisory board.