TORONTO - Researchers have developed a "revolutionary" genetic profiling tool that allows them to distinguish between closely related strains of the potentially deadly superbug MRSA and even pinpoint when and where in the world the mutated bacteria first arose.
MRSA, or methicillin-resistant Staphylococcus aureus, is a strain of Staph bacteria that is not eradicated by the antibiotics commonly used to treat it.
Most MRSA infections occur in hospitals or other health-care settings, such as nursing homes. Those most at risk are older adults and people with weakened immune systems. But there are also strains typically acquired outside of hospitals. These so-called community-associated MRSA strains can cause serious skin and soft tissue infections, as well as severe pneumonia.
Not only does the genome sequencing equipment help sort MRSA strains that have been transmitted from one continent to another, but it also can determine which strains are infecting patients in one hospital over another or even between wards.
The ability to track MRSA's genetic intricacies will enable researchers to understand how strains can spread so rapidly and should lead to better infection-control strategies -- for this superbug and others that may emerge, the researchers say.
Using the sophisticated DNA sequencing tool, scientists at the Wellcome Trust Sanger Institute in London analyzed more than 63 samples of MRSA.
Of those, 43 from people in North and South America, Europe, Australia and Asia who were infected between 1982 and 2003. Another 20 came from patients at a single hospital in Thailand who contracted MRSA within seven months of each other.
"We wanted to test whether our method could successfully zoom in and out to allow us to track infection on a global scale -- from continent-to-continent -- and also on the smallest scale, from person-to-person," co-lead author Dr Simon Harris of the Sanger Institute said during a teleconference from London.
Based on the DNA differences among the samples, the researchers were able to construct an evolutionary tree of the superbug. That allowed them to pinpoint the emergence of the dominant global strain of MRSA to Europe in the 1960s -- the same period when widespread antibiotic use began on the continent.
The scientists, who report on their work in this week's issue of the journal Science, determined that the European strain was behind a MRSA outbreak in Portuguese hospitals in the early 1990s. But a second wave there late in the decade was caused by a strain from Brazil.
A rash of cases in the U.K. in 2003 were caused by a strain that appeared to have originated in Southeast Asia.
"This illustrates how such intercontinental transmissions can result in seriously and sustained waves of hospital infection throughout the country that receives it," said Harris.
In the Thai hospital, genetic analysis showed patients were infected with two different MRSA strains that had been separately introduced into the facility.
Senior author Dr. Stephen Bentley of the Sanger Institute said the ability to differentiate between isolates is important for developing public-health programs to reduce the number of people infected with and sickened by MRSA.
"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."
Co-author Dr. Sharon Peacock, a professor of clinical microbiology at the University of Cambridge in England, said MRSA is a leading cause of hospital-acquired infection worldwide -- and the problem is getting worse.
While national statistics are sketchy in Canada, the United States saw MRSA cases double to 278,000 in 2005 from 127,000 in 1999, with related deaths rising to 17,000 from 12,000 over the same period, she said. In the U.K., there were more than 1,200 MRSA-related deaths in 2008.
Peacock said being able to genetically sort out strains could help flag transmission hotspots, so measures like frequent handwashing among care providers and environmental cleaning could be beefed up to prevent more patients getting infected.
Transmission control measures "are driving down rates of infection, but it's clear that a single intervention will not be enough," she said.
"I think that this tool will provide extra ammunition to identify routes of transmission for MRSA, so infection control can be concentrated more on areas that appear to be letting MRSA transmit between patients or between hospitals."
