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Chipotle Mexican Grill has temporarily closed 43 restaurants in Washington state and Oregon while authorities investigate an outbreak of E. coli.

People who ate in six Chipotle restaurants are among 20 cases of E. coli that are under investigation.

Chipotle said it was acting with "an abundance of caution" as most of the 43 outlets were unaffected.

Chipotle has 1,700 restaurants selling burritos, tacos and salads and promotes its healthy, fresh ingredients.

"We are working with health department officials to determine the cause of this issue. We offer our deepest sympathies to those who have been affected by this situation," Chris Arnold, Chipotle spokesman said in a statement.

E. coli is short for Escherichia coli. It is a type of bacterium present in the gut of humans and other animals.

Most strains are harmless but some can produce toxins that cause sickness in humans.

The severity of the illness varies considerably, but some types of E. coli lead to severe cramps and diarrhoea.