Transmission of Staphylococcus aureus between health-care workers, the environment, and patients in an intensive care unit: a longitudinal cohort study based on whole-genome sequencing

Price, J. R., Cole, K., Bexley, A., Kostiou, V., Eyre, D. W., Golubchik, T., Wilson, D. J., Crook, D. W., Walker, A. S., Peto, T. E. A., Llewelyn, M. J. and J. Paul (2017)
Lancet Infectious Diseases 17: 207-214. (pdf)

Considerable efforts are made to prevent nosocomial transmission of Staphylococcus aureus. Healthcare-workers (HCWs) have been implicated in outbreaks but lack of evidence from non-outbreak situations means that routine HCW screening and S. aureus eradication are controversial.

We performed a prospective observational study to determine how often S. aureus is transmitted from HCWs or the environment to patients on an intensive care/high dependency unit (ICU/HDU) where standard infection control measures were in place. Over fourteen months we systematically sampled HCWs, the environment and patients. Whole-genome sequencing data were used to define subtypes (<=40 single nucleotide variants (SNVs)) and infer recent transmission.

198 HCWs, 39 environmental locations and 1854 patients were sampled and 1819 isolates sequenced. S. aureus was detected in 29.5-39.7% HCWs at single four-weekly time-points and in 58.1% at least once. Monthly environmental sampling identified S. aureus in 7.7-50.0% of samples. During the study, 605 genetically distinct subtypes were identified (median SNV difference 273 (IQR 162-399)) at a rate of 38 (range 34-42) per month. Only 25 instances of transmission to patients were detected; seven from HCWs, two from the environment, 16 from other patients.

In the presence of standard infection control measures HCWs infrequently act as sources of transmission to patients. S. aureus epidemiology in ICU/HDU is characterised by continuous ingress of distinct subtypes rather than transmission of genetically-related strains.