Genomic surveillance uncovers clusters of MRSA causing bloodstream infections
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Genomic surveillance identified clusters of MRSA among strains cultured from bloodstream infections in two Philadelphia hospitals, researchers reported.
The clustering suggests “that transmission resulted from a mix of community spread and health care exposures long before” the bloodstream infections were diagnosed, they wrote.
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“Nosocomial MRSA infections, including bloodstream infections, have actually decreased in incidence quite a bit since 2010, but many people are still getting MRSA infections outside of hospitals,” Michael Z. David, MD, PhD, assistant professor of medicine in the division of infectious diseases at the University of Pennsylvania, told Healio.
“We were wondering if we looked at the genomes all of the MRSA isolates from all bloodstream infections at two hospitals for a year, would we be able to detect clusters of closely related strains causing these serious infections in more than one person?” David said.
According to David, they hypothesized that a transmission event between patients may happen long before they showed up with MRSA infections and that the infections may not happen at the same time.
David and colleagues performed whole-genome sequencing on 106 MRSA bloodstream infection isolates collected from inpatients in two hospitals over 12 months. They used electronic medical records to collect clinical data and hospitalization history and assessed the similarity of the infections.
Overall, they found that there were nine clusters of isolates among 104 MRSA strains in 104 different patients at the two hospitals meaning that at least nine different strains of MRSA spread between patients in the hospitals or the community and then went on to cause bloodstream infections in at least two people, David said.
According to David, they determined that many of the clustered patients had overlapping hospital stays in the year before their bloodstream infections were diagnosed — although in others, patients were hospitalized at the same hospital but did not have overlapping stays.
David said this suggests that “MRSA transmission between study subjects was indirect, perhaps carried by a health care worker and transmitted to both patients, or perhaps colonizing a health care worker who then passed it on to the second patient.”
He added that there were also instances where patients who shared clustered MRSA strains had no hospital overlap, suggesting that transmission occurred in the community.
The researchers also found that if there were only 13 single nucleotide polymorphism differences among clustered MRSA strains, there was a 50% chance that the patients had an overlapping hospital stay in the past year, David explained.
“It is likely that MRSA spread in the hospital and in the community and then goes on to cause the most serious type of infections in people bloodstream infections,” David said. “The spread from person to person can often occur long before the onset of the infection as people can be colonized for a long time.”
David added that because the transmission event happens so long before the infections in many of the clusters detected, by the time the infections occur, it is “far too late to prevent transmission,” and that transmission detection may be most effectively assessed when people are still asymptomatically colonized by screening.
“Perhaps patients at high risk of infection should be screened for colonization with virulent MRSA strains, and evidence of recent spread can be detected and preventive measures can be performed,” David said.
“In summary, transmission events leading to invasive, antibiotic-resistant infections may occur long before infections are diagnosed, and in the community,” he said. “If you wait until the infections occur, you may be far too late to prevent others from infection with the same strains.”
Perspective
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Alexander J. Sundermann, DrPH, MPH
Talbot and colleagues add to the mounting evidence that whole-genome sequencing surveillance is a critical tool for infection prevention in finding outbreaks and transmission that otherwise goes undetected with current infection prevention practices. The MRSA clusters detected by Talbot were also of high impact, given all isolates were collected from bloodstream infections. Moreover, there were likely other isolates clustered, given only blood cultures were sequenced. To make whole-genome sequencing surveillance more widely adopted, we need to further establish the cost-effectiveness of this approach, facilitate real-time applications and better define genomic pipeline analysis to ensure data can be compared across institutions. My prediction is that genomic surveillance will soon be a standard practice is U.S. health care systems and will be an invaluable tool. Application of this approach described by Talbot will be a paradigm shift in infection prevention to further improve patient safety.
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