By sequencing the entire genome of numerous samples of the notorious MRSA (Methicillin-resistant Staphylococcus aureus) bacteria–a drug-resistant strain of staph responsible for thousands of deaths in the United States each year–researchers at the Wellcome Trust Sanger Institute in the United Kingdom have gained clues as to how the superbug travels both around the globe and in local hospitals. Scientists say the approach will shed light on the epidemiology of the troublesome bacteria and help public health programs target their prevention efforts most effectively.
The research, which would have been impossible just two years ago, was enabled by fast and inexpensive sequencing technology from Illumina, a genomics company based in San Diego. “The work demonstrates the value of applying high-resolution sequencing technology to public health problems,” said Caroline Ash, a senior editor at the journal Science, where the research was published, at a press conference on Wednesday. “Potentially the technology could pinpoint the origin of the outbreak and the origin of its spread.”
About 30 percent of people carry Staphylococcus aureus bacteria on their skin, often harmlessly. But for some people, the microbes can cause severe problems, including serious skin infections, sepsis, and death. Antibiotic-resistant strains of the virus emerged in the 1960s, and these now account for more than half of all hospital-acquired infections in the U.S.
In the current study, researchers sequenced 63 MRSA samples, some collected from across the globe during a 20-year period, and some from a single hospital in Thailand over 20 months. While standard analysis methods, which analyze only a small portion of the microbes’ DNA, classified each isolate as being of the same subtype, sequencing the whole genome allowed scientists to identify very small genetic differences between the microbes.
The researchers constructed an evolutionary tree for the microbes and confirmed that MRSA likely first emerged in Europe in the 1960s, coinciding with the growing use of antibiotics to treat staph infections. The tree also showed that the superbugs evolved drug resistance multiple times over the past 40 years. “That demonstrates there is immense selective pressure caused by antibiotic use worldwide,” said Simon Harris, the lead author on the study, at the press conference.
The researchers also analyzed minor genetic differences in MRSA samples collected in a much more localized setting–a single hospital in Thailand–and discovered greater than expected diversity among the microbes. That suggests that patients were infected by new strains coming into the hospital, rather than patient-to-patient transmissions, says Harris. That finding might affect control measures. “If you institute infection control settings in this hospital, it will only have limited success because, in this case, it looks like patients appear to be getting the infection from other sources,” said Sharon Peacock, a clinical microbiologist at the University of Cambridge who also participated in the research, at the press conference.