Outbreak of penicillin-susceptible, eta-positive Staphylococcus aureus in a neonatal intensive care unit

Staphylococcus aureus is one of the most common microorganisms causing wound infections, and outbreaks of multi-drug-resistant strains in neonatal intensive care units (NICUs) are a major concern. In Germany, screening patients for colonization with multi-drug-resistant bacteria is strongly recommended in these wards under national infection-control guidelines. Outbreaks caused by penicillin-susceptible strains are less commonly reported. In the first few months of 2024, penicillin-susceptible S. aureus was detected in 20 infants, of whom 10 developed confirmed infections, primarily purulent conjunctivitis. Using a cumulative sum (CUSUM) control chart applied to monthly acquisition counts from January 2021 to December 2025, an outbreak was identified between December 2023 and May 2024. Whilst the detection rate during the non-outbreak phase was 1.17 patients per month, it rose to 3.33 patients per month during the outbreak ( p < 0.001). The number of infections was also significantly higher, with 1.67 infections per month within the outbreak period compared to 0.30 infections per month ( p < 0.001). The median gestational age was lower in patients with a confirmed infection than in patients with colonization only (214.5 vs. 243.5 days, p = 0.029). Four isolates from three affected children had undergone detailed typing. All isolates exhibited the same spa - (t328), ST- (15), and CT- (38152) types. In the cgMLST analysis, these isolates form a single cluster. All four isolates harbored the eta gene, suggesting the potential for expression of exfoliative toxin A. Following implementation of a bundle of enhanced infection-control measures, no further outbreak-associated detections occurred. As part of this response, and because systematic identifiable staff screening was restricted by data-protection requirements, all staff members were offered voluntary anonymous screening for nasal S. aureus colonization and, where appropriate, access to mupirocin decolonization. Although the contribution of individual interventions cannot be determined, the temporal association suggests that the combined infection-control measures likely contributed to containment of the outbreak. Our report shows that antibiotic susceptible pathogens can cause outbreaks in NICUs. Surveillance that also includes susceptible pathogens may facilitate the detection and epidemiological assessment of suspicious clusters.