MRSA Transmission in Intensive Care Units: Genomic Analysis of Patients, Their Environments, and Healthcare Workers

Background: Methicillin-resistant Staphylococcus aureus (MRSA)-and now USA300 MRSA-is a significant intensive care unit (ICU) pathogen; healthcare worker (HCW) contamination may lead to patient cross-transmission.

Methods: From September 2015 to February 2016, to study the 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, HCWs, 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 patients, HCWs, and environment often were genetically similar, although there was substantial between-encounter diversity. Strikingly, there were 5 USA100 and 1 USA300 clusters that contained similar strains (<22 single-nucleotide variants [SNVs], with most <10 SNVs) within the cluster despite coming from different encounters, suggesting intra- and inter-ICU spread of strains, that is, 4 of these genomic clusters were from encounters in the same ICU; 5 of 6 clusters occurred within 1 week.

Conclusions: 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.