For the first time, researchers have shown how transmission of methicillin-resistant Staphylococcus aureus (MRSA) from one person to another can be precisely tracked in a hospital setting. The team has developed a remarkable new method that can 'zoom' from large-scale inter-continental transmission events to the much finer detail of person-to-person infection of MRSA within a single hospital.
The ability to track strains in this way will enable researchers to understand how strains can spread so rapidly, and should lead to novel infection control strategies, not only for MRSA but also for other emerging superbugs.
The team used new very high-throughput DNA sequencing technologies to compare individual MRSA isolates from patients to precisely show their genetic relatedness. Very quickly they were able to spot single-letter changes in the genetic code and to identify differences between even the most closely related of MRSA isolates.
"We looked at two very different sets of samples," explains Dr. Simon Harris, from the Wellcome Trust Sanger Institute and co-lead author on the study. "We have 42 samples taken from people across the globe, who became infected with MRSA between 1982 and 2003. The second set is from a single hospital in northeast Thailand, and consists of 20 samples from patients who developed MRSA infection within seven months of each other, all possibly linked by a chain of person-to-person transmission." Harris added, “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."
The team sequenced the whole genomes of all the samples using a next-generation DNA sequencing technology. This technique reveals the minutiae of single-letter genetic changes in the hospital samples, and showed that no two infections were caused by entirely identical bacteria. Based on these subtle genetic differences, the researchers divided the Thai hospital samples into two groups. In the larger group of 13 bacteria they found five which were extremely similar and differed altogether by just 14 single-letter changes.
