What exactly is Salmonella?

Salmonella is a rod-shaped bacterium. It takes its name from the discovery of the first strain in 1885 by an American vet, Daniel Salmon. Today more than 2,200 varieties have been identified, but the majority of poisonings are caused by just two strains: S. enteritidis and S. typhimurium.

What does Salmonella do to you?

Salmonella lives in the stomach and intestines of humans and animals. It can lead to fever, abdominal pain, diarrhoea, nausea or vomiting. In a small number of cases the condition can become life threatening - especially if the infection spreads to other parts of the body such as the blood or bone. Any of a wide range of mild to serious infections caused by Salmonella is termed salmonellosis.

Where does it live?

Salmonella is an extremely robust organism. It can survive in a wide range of environments, including water, soil, on kitchen surfaces, in animal faeces, and on raw meat. A Salmonella infection is usually acquired by eating food that has been contaminated by the bacterium. But the infection may also be spread by bodily contact.

Why are eggs always mentioned with Salmonella?

Salmonella came to prominence in the UK in 1988 when Edwina Currie MP, a junior health minister, said most egg production in Britain was infected with the bacterium. Her comments sparked a public outcry and two weeks later she was forced to resign. But by early 1989, the House of Commons Select Committee on Agriculture had investigated the issue and concluded there was a link between eggs and Salmonella poisoning. Two million chickens were slaughtered, but this had little effect. Today, one egg in every 700 bought by the public is Salmonella-infected. At the height of the egg scare, the infection rate was marginally worse with one in every 650 carrying Salmonella.

Do chickens have a particular problem?

Most types of Salmonella live in the intestinal tracts of animals and birds. Clean procedures on farms and in abattoirs will help to prevent meat or eggs from becoming contaminated. But the salmonella affecting chickens (S. enteritidis) lives in the ovaries of hens and contaminates the eggs before the shells are formed.

How do you know if an egg is safe?

It is impossible to know if an egg carries the bacterium or not without testing it. The British Egg Information Service says production outlets in the UK which follow the best practice are allowed to put a lion logo on their packs. The eggs are also stamped with a "best before" date that expires 21 days after an egg is laid. But the best advice is to:

• keep eggs refrigerated
• discard cracked or dirty eggs
• avoid cross-contamination of foods by washing knives, cutting surfaces, and plates after contact with uncooked eggs.
• refrigerate unused or leftover egg-containing foods.
• avoid eating raw eggs