UK team tracks evolution of MRSA

For the first time, a Cambridge-led team of scientists has succeeded in tracking the evolution and transmission of methicillin resistant staphylococcus aureus (MRSA) across the world, offering the possibility of affordable tests to identify and block fatal superbugs before they spread. The breakthrough, led by the Wellcome Trust Sanger Institute (WTSI) in Hinxton, Cambridgeshire, UK, and published in the journal Science, means that researchers and public health officials may be better able to understand and control the spread of bacteria around the world, and from person to person.

In the study, the team examined 63 isolates of one MRSA strain, called ST239, from two different samples: 43 from a global strain that infected people between 1982 and 2003, and 20 from a hospital in Thailand where the patients developed MRSA infection within seven months of each other, possibly due to transmission from person to person. The samples were selected to test whether the team's method - a high-throughput genomics approach that allows a high-resolution view of the smallest genetic mutations in the bacteria's DNA - could successfully track infection on both a global and an individual scale. The success of the new technique relies on the use of full genomes rather than small sections of DNA, which allowed the team to see single-letter genetic changes in the samples, and to construct a genetic 'family tree' of the MRSA microbes as a result. In addition, they were able to calculate the rate of mutations in the DNA sequence - about one single-letter change every six weeks - an unprecedented insight into the rate of bacterial evolution in vivo.

In the global sample, the team found that the isolates from Europe were at the base of the evolutionary tree, suggesting that the origins of MRSA are related to the introduction of widespread antibiotic use in Europe in the 1960s. In the Thai sample, the team found that two different groups of isolates had been introduced to the hospital separately, and that of 20 infections, only five were caused by the same bacteria.

The implications of the study for public health were pointed out by Dr Sharon Peacock from Cambridge University: '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.'

However, the team highlighted that the costs associated with sequencing the full genome of an organism are an important limitation on it's widespread use, and the technique still needs refining before it can be cheaply used by the state-funded NHS. Nonetheless, as senior author Dr Stephen Bentley points out: 'ten years ago it took about three years and cost about half a million pounds…now it takes between four and six weeks, and costs about £200… Perhaps if you can get the cost of an individual test down to £20 or £30 then it may be viable to introduce that into a hospital setting as a routine test.'