Fri, 22 Jan 2010 15:41 UTC
A new genetic method of tracking infection suggests that the superbug emerged five decades ago in Europe
Scientists used DNA-mapping technology to compare the genetic relatedness of bugs isolated from individual patients.
By identifying letter changes springing up in the bacteria's genetic code, they were able to track MRSA transmission between continents and from patient-to-patient within a single hospital.
The technique, pioneered by scientists at the Wellcome Trust Sanger Institute in Hinxton, Cambridgeshire, is expected to help improve infection control strategies aimed at keeping superbugs at bay.
The scientists focused on 63 samples of MRSA - methicillin resistant staphylococcus aureus - from a particular lineage of the bacterium called ST239.
The strain accounts for a large proportion of MRSA outbreaks in hospitals around the world.
Two thirds of the samples came from hospitals in North and South America, Europe, Australia and Asia collected over a period of more than 20 years between 1982 and 2003.
The remainder came from patients at a single hospital in Thailand over a period of seven months.
Analysis of the samples yielded a "family tree" showing how the strain had spread around the world and branched into clusters of sub-strains.
The European samples were concentrated around the base of the evolutionary tree. Working backwards, the scientists established that the strain probably emerged in Europe in the 1960s.
The finding lends support to the theory that the introduction of widespread antibiotic use in the 1960s may have spawned MRSA.
Natural selection would have favored resistant strains that could survive the antibiotic onslaught.
Another discovery was that one MRSA outbreak in a London hospital intensive care unit was probably due to a bacterial strain imported from south-east Asia - possibly brought in by a single infected patient.
The research is published in the journal Science.
Dr Stephen Bentley, one of the Sanger Institute scientists, said: "Telling the difference between isolates within one species is fundamentally important in the development of public health strategies. It allows researchers and public health officials to see how infections are spread - from person-to-person, from hospital-to-hospital, from country-to-country."
The success of the new technique relies on comparing whole genomes - or blueprints of the genetic code - rather than small sections of DNA.
It can equally be applied to other kinds of bacteria that present a public health menace, such as Clostridium difficile.
Dr Sharon Peacock, co-author from Cambridge University, said: "This new method has allowed us to gain insights into fundamental processes of evolution in S. aureus, one of the most important bacterial pathogens in healthcare in the world.
"We are now able to discriminate between one strain and another, even where they are very closely related. Our research should inform global surveillance strategies to track the spread of MRSA.
"The implications for public health are clear: this technology represents the potential to trace transmission pathways of MRSA more definitively so that interventions or treatments can be targeted with precision and according to need."