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Last Updated: Friday, 6 February, 2004, 01:53 GMT
Lethal bug 'hides in the healthy'
Listeria can cause serious illness and death
Apparently healthy people may be carrying a bacterium which can kill pregnant women and people with weak immune systems, say US researchers.

Listeria, they have found, has the ability to grow and divide in the acid conditions within the gall bladder - without causing illness.

This could be a source of infection as if these "carriers" handle food.

There are more than 100 reported cases of Listeria infection in England and Wales each year.

Listeria is a remarkably resilient bacterium - it can flourish at refrigerator temperatures, and is found in soft cheeses and meats.

In humans, infections are relatively uncommon, but frequently devastating - between one and two in five patients who develop listeriosis die, even with antibiotic treatment.

Pregnancy threat

Pregnant women are particularly vulnerable, being more than 20 times more likely than other healthy adults to become infected.

This represents a new face of the pathogen - growing in a different place and a different way
Dr Christopher Contag, Stanford University
Researchers from Stanford University School of Medicine in California wanted to find out where the bacteria ends up in the body.

They infected mice with Listeria which had been "tagged" with a molecule that shows up on scans - to see where the bacteria ends up in the body.

When the mice were scanned, there was an unexpected result.

Bile beater

A large concentration of bacteria were found in the gall bladder - a sac connected to the digestive system which produces and stores bile, a fluid that helps the gut digest dietary fats.

Bile actually forms part of the gut's defence against colonisation by bacteria.

However, not only were the Listeria bacteria living there, they were dividing and accumulating in large numbers.

If people improved their standard of hygiene, the number of cases would drop overnight
Professor Colin Hill, University College Cork
Dr Christopher Contag, assistant professor of paediatrics at Stanford, said: "This represents a new face of the pathogen - growing in a different place and a different way.

"We were surprised to see the intensity of the signal in the gall bladder."

He added: "To have discovered a chronic carrier state in the gall bladder of an animal model, suggesting a potential source of food contamination, is important."

The bacterium appears to express at least one gene that allows them to break down salt in the bile - aiding its survival in such hostile conditions.

Big killer

Professor Mike Peck from the Institute of Food Research in Norwich told BBC News Online that Listeria represented an important threat.

He said: "The Food Standards Agency has identified five infections which it is aiming to cut by 20% over a fixed period of time - Listeria is one of them.

"It is responsible for 17% of deaths associated with food-borne infections. The consequences associated with developing the infection are far more serious."

Professor Colin Hill, a Listeria expert from University College Cork in Ireland, told BBC News Online that preventing Listeria infections was "hardly rocket science".

He said: "If people improved their standard of hygiene, the number of cases would drop overnight.

"The faecal-oral route remains the main method of transmission."

He said that while it was not known exactly how contagious the infection was, it was possible that in some susceptible patients, even being exposed to a single bacterium could lead to illness.

The study was published in the journal Science.

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