Patient-to-Patient Transfer of Staphylococcus aureus May Not Be Common

Hospital-acquired Staphylococcus aureus infection is generally preceded by colonization, leading to efforts to detect and eradicate such colonization — particularly that involving methicillin-resistant S. aureus (MRSA). S. aureus evolves primarily through point mutations and accumulates single-nucleotide variants (SNVs). Whole-genome sequencing (WGS) detects these SNVs and reveals the genetic relatedness of different isolates at higher resolution than conventional methods, such as spa gene typing.

In a study involving a 16-bed adult intensive care unit (ICU) in the U.K., nasal and perineal swabs to screen for S. aureus colonization were collected from 1136 patients between January 2010 and February 2011. In 1109 patients, screening was done within 24 hours of admission. All patients received chlorhexidine washes, and MRSA-positive ones received nasal mupirocin.

Acquisition of S. aureus colonization occurred in 44 (6.4%) of the 680 patients with subsequent samples. Thirty-seven of the isolates were available for further analysis. Five strains — all MRSA — were of all the same spa type and were associated with overlapping ICU stays, meeting the traditional criteria for patient-to-patient transmission. Only two of these were related by WGS. On the other hand, WGS identified five instances of S. aureus transmission that were not apparent based on spa type and shared ICU time.