The BBSRC-funded study provides clear evidence that livestock were the original source of an MRSA (meticillin-resistant staphylococcus aureus) strain which is now widespread in people.
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 scientists, who are based at The Roslin Institute at The University of Edinburgh.
Researchers say 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.
After switching to human hosts, the Staphylococcus aureus¬ ¬bacterium became resistant to the antibiotic methicillin and developed into methicillin-resistant Staphylococcus aureus¬ or MRSA. In addition, the bacteria acquired the ability to avoid attack by the human immune system. However, these bacteria that originated in cattle do not appear to be more aggressive or more resistant to antibiotics than other MRSA affecting humans.
Lead researcher, Professor Ross Fitzgerald, of The Roslin Institute, said: "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 Staphylococcus aureus¬ which jumped from cattle and developed the capacity to transmit widely among human populations'.
The study is published in the journal mBio.
Laura Spoor, of The Roslin Institute and first author on the research paper, added: "This research provides insight into how some strains of MRSA have evolved and help us better understand how they have adapted to cause disease in different host species."