Image copyright Science Photo Library Image caption Listeria monocytogenes bacteria

Two more hospital patient deaths have been linked to an outbreak of listeria in pre-packed sandwiches and salads.

Friday's announcement from Public Health England (PHE) takes the number of confirmed cases from six to nine and the deaths from three to five.

Last week PHE confirmed two patients from Manchester Royal Infirmary and one at Aintree Hospital had died.

Sandwiches and salads from the Good Food Chain linked to the outbreak have been withdrawn and production stopped.

Evidence suggested all individuals ate the affected foods before the product withdrawal took place in hospitals on 25 May, PHE said.

The chain - which supplied 43 NHS trusts across the UK - had been supplied with meat produced by North Country Cooked Meats, which subsequently produced a positive test result for the outbreak strain of listeria.

'Swift action'

PHE said it had been analysing previously known cases of listeria from the past two months to see if they were linked.

"To date, there have been no patients linked to this incident outside healthcare organisations, but we continue to investigate," Dr Nick Phin, of Public Health England, said.

"Swift action was taken to protect patients and any risk to the public is low."

He added: "PHE is continuing to analyse all recent and ongoing samples of listeria from hospital patients to understand whether their illness is linked to this outbreak."

A listeria infection can cause a small amount of discomfort but is more likely to seriously affect pregnant women, the elderly and those with a weakened immune system.

In a statement, the Good Food Chain said it was co-operating "fully and transparently with the Food Standards Agency and other authorities" and said it hoped the inquiry would be pursued with "urgency so the wider industry can learn any lessons as soon as possible".

"Our thoughts and deepest sympathies are with the families of those who have died and anyone else who has been affected by this outbreak.

"The underlying cause of it remains unclear," the statement adds.

It is not yet known where the latest two victims were receiving treatment.

Manchester University NHS Foundation and Aintree University NHS Foundation Trust said the new cases did not relate to them.

What is listeria?

Listeria is a bacterium that can cause a type of food poisoning called listeriosis.

Normally, the symptoms are mild - a high temperature, chills, feeling sick - and go away on their own after a few days.

But in this outbreak, the cases occurred in people who were already seriously ill in hospital and they are most at risk of severe infection.

Listeria can then cause damage to organs, spread to the brain or bloodstream and be fatal.

In 2017, figures show there were 33 deaths linked to listeriosis in England and Wales.

Many types of food can become contaminated with listeria such as soft cheeses, chilled ready-to-eat foods like pre-packed salads, sandwiches and sliced meats, and unpasteurised milk products.

Pregnant women are advised to steer clear of soft cheese for this reason.

To reduce the risk, the NHS advises people keep chilled food in the fridge, heat food until it is piping hot and not eat food after its use-by date.

The Good Food Chain, based in Stone, Staffordshire, had been supplied with meat produced by North Country Cooked Meats, which subsequently produced a positive test result for the outbreak strain of listeria.

This business - along with North Country Quality Foods which it distributes through - has also voluntarily ceased production.

Last week North Country Cooked Meats said it was "co-operating fully" the investigations.