Scientists have found a completely new form of the MRSA superbug in two Dublin hospitals.
The Irish research team believes the new organism probably made the jump from animals into humans.
Professor David Coleman from Trinity College Dublin, who led the research, said this strain had never been seen before in any living organism.
A new strain of the superbug has also been found in the milk of British cows.
Experts say there is circumstantial evidence of the bacteria passing between cattle and humans.
However, they ruled out any general threat to the safety of milk and other dairy products.
MRSA is a drug-resistant form of a usually harmless common bacterium that can be deadly when it infects wounds.