Investigation and control of an outbreak of methicillin-susceptible Staphylococcus aureus skin and soft tissue infections in a neonatal intensive care unit.

Background: In the neonatal intensive care unit (NICU), outbreaks caused by methicillin-susceptible Staphylococcus aureus (MSSA) are less commonly described than outbreaks caused by methicillin-resistant Staphylococcus aureus (MRSA) despite the increased burden of MSSA infections. Objective: To investigate a NICU MSSA outbreak utilizing whole-genome sequencing (WGS) and multi-locus sequencing typing (MLST) to identify transmission events. Methods: An investigation was initiated in a level IV NICU after four patients developed MSSA skin and soft tissue infections (SSTI) within three weeks. MLST and WGS were performed on MSSA isolates obtained from clinical and surveillance specimens. Results: During the outbreak, 16 infants developed MSSA infections including SSTIs (n = 15) and bacteremia (n = 1). Thirteen SSTIs presented on neonates' faces, all of whom were on non-invasive respiratory support. During 7 rounds of surveillance, an additional 31 patients were found to be colonized with MSSA. MLST identified a predominant cluster (ST-121). WGS found that all ST-121 isolates were closely related (≤10 genetic variants between isolates) suggesting likely transmission events, harbored the mupA gene, exhibited mupirocin MIC values ≥1,024 g/mL, and were associated with infection. Multiple infection control measures were implemented including the bare below the elbows practice. No further mupirocin-resistant isolates were recovered or ST-121 SSTIs identified after Week 26. Conclusions: WGS analysis furthered the MLST analysis and identified a single MLST as the outbreak-related strain. Successful control of this outbreak was achieved with a multitude of infection prevention and control methods.