Genomic testing helps ID MRSA outbreak

Genomic testing helps ID MRSA outbreaks

HealthDay News -- Whole-genome sequencing may have helped identify and control a outbreak of methicillin-resistant Staphylococcus aureus (MRSA) in a special-care baby unit (SCBU) much sooner than conventional methods, according to British researchers.
Infection-control specialists suspected a link between 12 MRSA-colonized infants identified over a 6-month period in a SCBU unit at a National Health Service Foundation trust in Cambridge, U.K., but were unable to confirm a persistent outbreak.
However, whole-genome sequencing eventually made the association between the original cases, and subsequently identified 26 related cases of MRSA carriage by tracking MRSA transmission among infants' mothers, a hospital employee in the special care unit and individuals in the community.
"Whole-genome sequencing holds great promise for rapid, accurate and comprehensive identification of bacterial transmission pathways in hospital and community settings, with concomitant reductions in infections, morbidity and costs," Sharon J. Peacock, FRCP, of the Wellcome Trust Sanger Institute in Cambridge, and colleagues reported in Lancet Infectious Diseases.
They validated and expanded findings from the infection-control team's conventional analysis, performing whole-genome sequencing on isolates from all colonized patients in the SCBU and all MRSA isolates with the same antibiotic susceptibility profile from patients in the hospital and community.
Starting in 2011, the hospital's infection control team identified three cases of MRSA carriage in the SCBU, which they grouped as an outbreak based on close temporal relationships and near-identical antibiotic susceptibility among the three isolates.
Thirteen additional infants in the SCBU that tested positive for MRSA more than once in a period spanning the previous 6 months were also identified. The researchers then performed antibiogram comparisons on isolates from these cases, revealing that eight infants had isolates with susceptibility patterns identical or nearly identical to those of the three original cases, bringing the outbreak count to 11.
Deep cleaning, reinforcement of infection-control policy and practices and weekly MRSA screens were then implemented in the SCBU. Four days after deep cleaning another infant tested positive for MRSA, and the isolate matched those implicated in the previous 11 cases. The MRSA type from the outbreak was identified as a new sequence type ST2371, closely related to ST22.
Sequencing data and the timing of the cases suggested that repeated introduction from an external source had allowed the outbreak to persist during periods without known infection on the SCBU and after a deep clean.
"The re-emergence of ST2371 on the SCBU after the deep clean led us to suspect that one or more members of staff might be carrying and transmitting the outbreak MRSA strain," the researchers wrote.
After screening all 154 SCBU staff members, the infection control team identified one staff member who tested positive for MRSA, but did not have visible lesions, and initiated decolonization.
"Whole-genome sequencing of MRSA could make an important contribution to infection-control investigation and practice," the researchers concluded.