MRSA "superbug" genome decoded

On 18 June 2004, the BBC reported fears that deaths from the 'superbug' MRSA (methicillin-resistant Staphylococcus aureus) might double in the next five years. While all reports stress the need for improved cleanliness as a vital preventive measure, medicine is crying out for new ways to tackle MRSA and to monitor its progress.

MRSA is resistant to the majority of antibiotics commonly used in hospitals and there are fears that it might acquire resistance to the remaining effective antibiotic. The number of cases in the UK has grown 600% in eight years.

Today (24 June 2004), researchers from The Wellcome Trust Sanger Institute and their colleagues announce the completion of the genome sequence of an MRSA strain and its antibiotic-sensitive 'cousin' MSSA. The strain of MRSA chosen, which causes half of all UK outbreaks, contains several unique genetic elements that are related to its virulence and drug resistance. It has emerged as a major threat in only the past ten years.

Dr Matt Holden, leader of the S. aureus analysis team, said: "This new atlas shows that the strain that is most prevalent in the UK is very different from previously sequenced superbugs. It has evolved a 'Pick'n'Mix' genome that incorporates favoured genes that improve its resistance and ability to cause disease - genes that are vital to its spread and success."

Both strains show evidence of movement of DNA between organisms, acquiring DNA segments from other isolates. In MRSA, some of these new segments may help the organism to thrive in hospitals. More than 6% of its genes are novel.

Dr Julian Parkhill, Project Manager at the Wellcome Trust Sanger Institute, said: "MRSA and other pathogens are driven to acquire new tricks to evade or defeat the defences we put up. The dramatic and rapid evolution of S. aureus species underlines the importance of this kind of research to understand how bacteria evolve and how they acquire these new tools."

"Our results clearly show that bacteria are capable of very rapid evolutionary change and remain a formidable enemy."

Staphylococcus aureus, the species of bacteria that includes MRSA and MSSA, appear to evolve extremely rapidly and today's report suggests that improved methods will be required to track S. aureus outbreaks. The genome sequence will help in this.

Dr Sharon Peacock, Honorary Consultant in Clinical Microbiology at the John Radcliffe Hospital, Oxford, said: "The rise in hospital-acquired infections and the increase in resistance to antibiotics are major challenges to health service providers around the world. The genome sequence and the light it sheds on evolution in MRSA gives us one more tool to understand how disease occurs and possibly to find new methods to combat these diseases."

DNA elements carrying antibiotic resistance and virulence genes are moved between the sensitive and resistant strains. These exchanges dramatically change the virulence of the organism, adding blocks of genes encoding 'superantigens' such as those that cause toxic shock or food poisoning as well as making MRSA more resistant to antibiotics.

Genome sequences produced six years ago have already been used to develop new vaccines for bacterial meningitis, currently in trials.