Wednesday, March 31, 2010

Tracing the Roots of MRSA

A study published earlier this year used genomic analysis to trace one of the most common strains of MRSA, ST239, back to the introduction of widespread antibiotic usage in 1960's Europe. The genomic data also gave researchers insight into the transmission of MRSA on both local and global levels.
Led by Dr. Simon Harris of the Wellcome Trust Sanger Institute, the authors analyzed 63 ST239 isolates, 43 of those from different locations around the globe over a period from 1982-2003. By determining the presence single nucleotide polymorphisms (SNPs), small variable regions in an organism's genome, in each isolate, they were able to create a phylogeny that maps out the likely origin of each strain (see image).

The completed puzzle told the researchers that the isolates clustered by location, as would be expected, but that some isolates seemed to have been transmitted between two locations, perhaps by a single individual. For example, isolates from a MRSA outbreak in a London hospital were part of the same genetic cluster that belonged to Thai isolates, indicating the likelihood of a "single intercontinental transmission event."

The phylogeny was created to limit the number of common SNPs that would have come about by convergent evolution - when a mutation develops multiple times in different branches of a genetic tree. Of the SNPs that did indicate convergent evolution, over 25 percent of these were genes involved with resistance to currently-used antibiotics, showing that clinical practice of drug use is a major driver of evolution towards resistant bacteria.

Finally, the analysis allowed the researchers to calculate the rate of mutation of ST239 bacteria at one core SNP every six weeks. Using this figure, they were able to trace a common ancestor for all of the isolates to one strain originating in Europe in the mid-to-late 1960's. This coincides with the beginning of the popular use of antibiotics in Europe, and with the first identified MRSA cases there.

This study may be most useful for the technique it uses to trace resistant isolates through time and location. Combined with improved global surveillance of MRSA, these methods may be used to detect introduction of new strains and target the needed forms of diagnostics and interventions, the authors write.

Coauthor Dr. Sharon Peacock told The Telegraph:
"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."

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