Tests results have given the all clear for 73 children from a north Wales nursery at the centre of an E.coli outbreak.

Earlier this month, four children were diagnosed with the most serious strain of the infection - 0157 - which can cause serious illness, but are thought to be doing well.

Health officials examined 150 pupils at the nursery, following the poisoning outbreak last week.

E.coli exists everywhere in the environment

All those tested will be able to return to the Menai Nursery in Bangor, Gwynedd, next Tuesday.

North Wales Health Authority said it still did not know how the bug reached the nursery.

However, experts suspect that one child acquired the infection which was subsequently spread by direct contact from child to child.

On 6 August, it was revealed that two young children, aged about 12 months, had contracted the bacteria.

Parents were advised of the testing procedure and to be particularly careful with hygiene at home.

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.

The bacteria exist everywhere in the environment but are commonly swallowed in contaminated food.

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."

Fifteen are ill at the Highlands caravan park

Health officials have said that standards of hygiene were satisfactory at the nursery but it remains closed while inquiries continue.

In Scotland, a recent 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.

All of those stricken by the bug have been in contact with the Rothiemurchus Caravan Park at Coylumbridge, near Aviemore.