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Methicillin-resistant Staphylococcus aureus (MRSA), is a bacterial strain that is resistant to a range of antibiotics including the drug of choice, methicilllin. These bacteria are easily transmissible, especially among hospital patients who are immunocompromised, and can cause mild to severe infections, including septicemia.

A proof-of-principle study in the journal Science describes the use of high-throughput sequencing to track the evolution and spread of MRSA [Harris SR et al. 2010 Jan 22; 327(5964):469-74]. By sequencing genomes of 63 MRSA ST239 isolates (43 from a global collection recovered over 20 years, and 20 collected in a single hospital in Thailand over 7 months), the team discerned relationships between isolates from different locations, tracked spread from one continent to the next, and gained a better understanding of its transmission within a single health care facility.

The advanced high-throughput form of DNA sequencing used for this research was crucial, allowing comparison of whole MRSA genomes and pinpointing subtle differences between them, including SNPs and small insertions and deletions.

The results suggest that the MRSA isolates typically cluster by geographical location, with European isolates being particularly diverse, consistent with a possible European origin for the ST239 strain. Although intercontinental transmission of seems to be rare, the researchers showed several exceptions. For example, isolates in Portuguese hospitals in the early 1990s came from a European source, but were replaced by isolates more similar to Brazilian ST239 in the late 1990s.

The researchers also found that the ST239 strain appears to be mutating faster than previously recognised, with a new mutation appearing roughly every six weeks. Such mutations frequently affect parts of the genome related to drug resistance. In the Thai hospital, the researchers found even greater genetic diversity between ST239 isolates than detected from the global samples; their results also suggested that genome sequencing could help track the course of infection outbreaks within hospitals.

Comment: The sequencing approach, relying on comparison of whole genomes instead of looking at either single loci or limited regions in the genome, should help scientists to gain insights into fundamental processes of evolution in not only S. aureus, but other clinically important pathogens of the world. It could help to locate MRSA transmission hotspots, whether as part of global surveillance strategies or to target elevated infection control strategies in local settings.