The role of asymptomatic enteric carriage in maintaining a long-term hospital outbreak of ST228 methicillin resistant Staphylococcus aureus

Whole-genome sequencing (WGS) of 228 isolates was used to elucidate the origin and dynamics of a long-term outbreak of MRSA ST228 SCCmec-I that involved 1600 patients in a tertiary care hospital between 2008 and 2012. Combining the sequence data with detailed metadata on patient admission and movement confirmed that the outbreak was due to the transmission of a single clonal variant of ST228, rather than repeated introductions of this clone into the hospital. We note that this clone is significantly more frequently recovered from groin and rectal swabs than other clones (p<0.0001), and is also significantly more transmissible between roommates (p<0.01). Unrecognized MRSA carriers, together with movements of patients within the hospital, also seem to have played a major role. These atypical colonisation and transmission dynamics can help explain how the outbreak was maintained over the long-term. This “stealthy” asymptomatic colonisation of the gut, combined with heightened transmissibility (potentially reflecting a role for environmental reservoirs), means the dynamics of this outbreak share some properties with enteric pathogens such as vancomycin resistant Enterococci or Clostridium difficile.