New 'zoom' tool tracks MRSA evolution and transmission
22 January 2010
The tool, developed by researchers from the Wellcome Trust Sanger Institute and colleagues, can 'zoom in' from large-scale transmission events between different continents to the much finer detail of person-to-person infection within a single hospital.
This will enable researchers to understand how strains spread so rapidly and to develop novel infection control strategies for MRSA (methicillin-resistant Staphylococcus aureus) and other emerging superbugs.
To test their new method, the researchers looked at two sets of samples. The first contained 42 samples taken from people infected with MRSA worldwide between 1982 and 2003. The second was taken from 20 patients in a hospital in northeast Thailand who developed MRSA infection within seven months of each other.
The team sequenced the whole genomes of all the samples. This allowed them to identify single-letter changes in the genetic code, identifying differences between even the most closely related of MRSA isolates.
Their analysis showed that no two infections were caused by the same bacteria, though they did find a group of related bacteria that differed by just 14 single-letter DNA changes.
"The group of five related MRSA strains caused infections in patients who were resident in intensive care units in adjacent blocks of the hospital. All were isolated within a few weeks of each other. By contrast, bacteria from patients housed in other parts of the hospital were much less similar," said Dr Ed Feil from the University of Bath and co-lead author on the study.
"This cemented our theory that there were two different groups of isolates that had had been introduced to the hospital separately."
"Telling the difference between isolates within one species is fundamentally important in the development of public health strategies," said Dr Stephen Bentley, from the Sanger Institute and senior author on the study.
"We are now able to discriminate between one strain and another, even where they are very closely related. It allows researchers and public health officials to see how infections are spread: from person to person, from hospital to hospital, from country to country."
The team was also able to determine the rate at which MRSA mutated, providing an insight into the rate of evolution of the bug. The particular MRSA strain studied acquired about one single-letter change every six weeks.
Working with teams and samples from North and South America, Europe, Australia and Asia, the researchers derived a mutation rate and evolutionary tree, suggesting where and when this type of MRSA might have emerged.
Analysis of European isolates also suggests that this MRSA emerged in the 1960s in Europe, supporting theories that MRSA emerged with the introduction of widespread antibiotic use in Europe at that time.
Image: Clusters of methicillin-resistant Staphylococcus aureus (MRSA) bacteria. Credit: Annie Cavanagh, Wellcome Images
Reference
Harris S et al. Evolution of MRSA during hospital transmission and intercontinental spread. Science 2009.
