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UK MRSA Genomes Highlight Overlapping Hospital-Community Transmission Clusters

NEW YORK (GenomeWeb) – New research suggests the methicillin-resistant Staphylococcus aureus (MRSA) strains circulating in some UK hospitals in 2012 and early 2013 reached beyond the healthcare setting, contributing to community-acquired MRSA cases that, in turn, influenced hospital infections.

Researchers from the London School of Hygiene and Tropical Medicine (LSHTM), the University of Cambridge, the Wellcome Trust Sanger Institute, and elsewhere prospectively identified nearly 1,500 MRSA-positive individuals sampled at hospitals or at dozens of general medical practices in East England between April 2012 and April 2013. With sequences for almost 2,300 MRSA isolates from these individuals, they uncovered 173 transmission clusters, including 28 clusters that crisscrossed hospital and community cases, as they reported today in Science Translational Medicine.

These and other results offer "definitive evidence for the spread of so-called hospital-associated lineages … through transmission networks that include the community," senior author Sharon Peacock, a researcher affiliated with LSHTM, the University of Cambridge, and the Sanger Institute, and her colleagues wrote. "The repeated introduction of MRSA from the community into hospitals and vice versa signals the need for more robust action to detect and tackle community-associated carriage."

Peacock and her colleagues put together a collection of 1,465 consecutive MRSA-positive cases using samples submitted to a single clinical microbiology and public health lab from three East England hospitals that systematically screen for the bug in newly admitted patients and do ongoing sampling in their critical care units. They also included samples submitted by 75 general practitioner sites.

The team sequenced 2,320 MRSA isolates from these MRSA-positive patients, using the Illumina HiSeq 2000. Following quality control, the group was left with genome sequences for 2,282 of the isolates, representing at least one isolate per person.

Based on multilocus sequence types, SNP patterns, and other clues in the new genomes, the researchers traced transmission networks for 598 of the individuals tested. Across the 173 transmission clusters identified, they found that 118 clusters of MRSA transmitted between 371 individuals in the healthcare setting, another 27 clusters that exclusively encompassed 72 community-acquired cases, and 28 clusters comprised of transmission events affecting 157 individuals in healthcare and community settings.

Teasing out the types of cases in these clusters pointed to situations that may benefit from more vigilant infections control. For example, the authors saw evidence of "persistent carriers" spreading MRSA across hospital wards, sometimes indirectly. They also highlighted a lack of MRSA testing for patients being discharged from hospitals — one potential factor suspected of helping along delayed transmissions that would be missed without genetic analyses.

Nearly 75 percent of the sequenced isolates came from a clonal complex called CC22 that has been implicated in health care-associated MRSA infections in the UK. In the transmission clusters crossing over between healthcare settings and the community, the team saw multiple CC22 clusters, along with clusters that included hospital-associated clonal complexes such as CC30 and community associated strains in several other clonal complex groups.

The investigators also identified cases involving strains typically linked to cases outside of the UK, including lineages from Europe, Taiwan, and the US.

"The large number of patients screened … allowed us to sample MRSA lineages that are not dominant in the UK but are endemic in other areas of the world," the authors explained, noting that "identification of transmission clusters involving these lineages in hospitals, in the community, and at the hospital-community interface suggests that our findings may be applicable to other UK regions and other countries."