Tests are being carried out on up to 90 children at the centre of an E.coli outbreak at a nursery.

Two children from Menai Nursery at Bangor, Gwynedd, are suffering from the most serious, 0157 strain of the stomach bug bacteria, health officials confirmed on Tuesday.

Serious strain : Dr Davey Jones

The pair, aged about 12 months, did not require hospital treatment and are described as "currently well".

But now the other children who attended the nursery in are undergoing tests which will establish whether the disease has spread.

Parents have been asked not to take their children to any nursery until screening has finished.

Many E.coli strains are harmless, but the more serious ones can cause serious illness, including vomiting, diarrhoea and, in severe cases, intestinal bleeding. Most people recover within two weeks.

This strain can cling onto the inside of the intestines,

Dr Davey Jones

Bangor University E.coli researcher Dr Davey Jones said the 0157 strain seen in the nursery children was especially nasty.

"This particular strain has a very good capacity to cling onto the inside of the intestines, where it releases a toxin into the bloodstream," he said.

"That causes the intestines to bleed and the main symptom is bloody diarrhoea."

The E.coli bacteria exist everywhere in the environment but are commonly ingested by contaminated food.

Letters were sent to parents stating that all children who have been at the nursery since the beginning of July are to be screened for infection - even if they have since moved somewhere else.

Nursery manager Evelyn Parry declined to comment on the outbreak, saying staff had to protect the confidentiality of the children.

Standards of hygiene were satisfactory at the nursery, health officials said, but it remains closed while inquiries continue.

North Wales Health Authority and Gwynedd County Council formed a taskforce to deal with the outbreak.

In Scotland, a current outbreak of E.coli 0157 at a caravan park in the Highlands has affected a total of 15 people.

The outbreak was confirmed on 1 August and experts said they believed a private water supply may have caused it to spread.