Hospitals in large cities act as breeding grounds for the superbug MRSA prior to it spreading to smaller hospitals, a study suggests.
Researchers found evidence that shows for the first time how the superbug spreads between different hospitals throughout the country.
The University of Edinburgh study involved looking at the genetic make-up of more than 80 variants of a major clone of MRSA found in hospitals.
Scientists were able to determine the entire genetic code of MRSA bacteria taken from infected patients.
They then identified mutations in the bug which led to their emergence of new MRSA variants and traced their spread around the country
Dr Ross Fitzgerald, of The Roslin Institute at the University of Edinburgh, who led the study said: "We found that variants of MRSA circulating in regional hospitals probably originated in large city hospitals. The high levels of patient traffic in large hospitals means they act as a hub for transmission between patients, who may then be transferred or treated in regional hospitals."
MRSA - methicillin-resistant Staphylococcus aureus - first started to appear around 50 years ago following the introduction of antibiotics, to which the bacteria has become increasingly resistant.
Paul McAdam, of The Roslin Institute and first author of the paper, said: "Our findings suggest that the referral of patients to different hospitals is a major cause of MRSA transmission around the country. This knowledge could help in finding ways to prevent the spread of infections.
The paper published in the journal PNAS, also found that the MRSA strain studied evolved from antibiotic-sensitive bacteria that existed more than 100 years ago.
The research funded by the Chief Scientist Office, Scottish Government Health and Social Care Directorates, the Medical Research Council and the Biotechnology and Biological Sciences Research involved collaboration between The Roslin Institute,Wellcome Trust Sanger Institute and University of Bath in the United Kingdom and the Broad Institute and the University of Mississippi Medical Centre in the United States.