Two more cases of E.coli have been identified among a group of children following an outbreak at a nursery school in north Wales.

The new cases - which brings the total to four - were confirmed after screening tests carried out on 150 children from the from Menai Nursery at Bangor, Gwynedd.

E.coli exists everywhere in the environment

On Tuesday, health officials said two young children, aged about 12 months, had contracted the most serious strain of the bacteria - 0157.

So far they have not been admitted to hospital but an outbreak control team set up tests to determine if the disease had spread.

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.

A statement from the North Wales Health Authority said: "There is still no evidence of how the infection was introduced to the nursery but the most likely explanation is that a child acquired the infection in the community, which then subsequently spread by direct contact from child to child."

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.

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 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.

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

Householders and businesses in the area have been advised to use boiled or bottled water while attempts to isolate the outbreak continue.