MRSA transmission in ICUs: Genomic analysis of patients, their environments and healthcare workers.

BACKGROUND MRSA-and now USA300 MRSA-is a significant ICU pathogen; healthcare worker (HCW) contamination may lead to patient cross-transmission. METHODS From 9/2015-2/2016, to study spread of MRSA, we enrolled HCWs in 4 adult ICUs caring for patients on MRSA contact precautions. Samples were collected from patient body sites and high-touch surfaces in patient rooms. HCW hands, gloves, and personal protective equipment were sampled pre/post-patient encounter. Whole genome sequencing (WGS) was used to compare isolates from patients, HCW, and environment. RESULTS There were 413 MRSA isolates sequenced (38% USA300, 52% USA100) from 66 patient encounters. Six of 66 HCWs were contaminated with MRSA prior to room entry. Isolates from a single patient encounter were typically either USA100 or USA300; in 8 (12%) encounters both USA300 and USA100 were isolated. WGS demonstrated that isolates from patient, HCW, and environment often were genetically similar, while there was substantial between-encounter diversity. Strikingly, there were 5 USA100 and 1 USA300 clusters that contained similar strains (<22 SNVs, with most <10 SNVs) within the cluster despite coming from different encounters, suggesting intra- and inter-ICU spread of strains, i.e., 4 of these genomic clusters were from encounters in the same ICU; 5 of 6 clusters occurred within 1 week. CONCLUSION We demonstrated frequent spread of MRSA USA300 and USA100 strains among patients, environment, and HCWs. WGS identified possible spread within and even between ICUs. Future analysis with detailed contact tracing in conjunction with genomic data may further elucidate pathways of MRSA spread and points for intervention.