A type of MRSA found in humans originated in cattle at least 40 years ago, finds research.
The study provides clear evidence that livestock were the original source of an MRSA strain which is now widespread in people.
University researchers studied the genetic make-up of more than 40 strains of a bacterium - called Staphylococcus aureus - that can build up antibiotic resistance to develop into MRSA.
At least two genetic subtypes of the bacterium, which have become endemic in people, have been traced back to cattle by The Roslin Institute scientists.
Human infections caused by bacteria being transmitted directly from livestock are well known to occur.
However, this is the first clear genetic evidence of subtypes of Staph. aureus which jumped from cattle and developed the capacity to transmit widely among human populations.
Researchers said that the most likely scenario is that the bug crossed over from cattle to people through direct contact - perhaps through people working with farm animals.
The research will help scientists find out how the bacteria are able to spread and cause disease in humans and to prevent further strains from jumping from livestock.
The study is published in the journal mBio.