Penicillin susceptibility in Staphylococcus aureus from blood isolates– a multicentre surveillance study in Denmark, Iceland, Norway and Sweden

BACKGROUND: Staphylococcus aureus bacteraemia (SAB) remains associated with substantial morbidity and mortality. Interest in penicillin-susceptible S. aureus (PSSA) has re-emerged, but robust epidemiological data and reliable susceptibility testing are needed to guide antibiotic choice. METHODS: In this multicentre observational study, clinical microbiology laboratories in Sweden, Denmark, Norway, and Iceland contributed antibiotic susceptibility testing (AST) data for S. aureus blood culture isolates collected over six consecutive months in 2024. All laboratories followed EUCAST breakpoints, with additional confirmatory testing in selected centres. RESULTS: A total of 2,203 isolates were included. SAB incidence ranged from 34.0 to 62 per 100,000 inhabitants annually. Median patient age was 73 years (IQR 59-82), and 63.8% were male. PSSA constituted 32.2% of isolates, methicillin-resistant S. aureus (MRSA) 2.2%, while methicillin-susceptible, penicillin-resistant isolates comprised 65.6%. Among isolates with penicillin zone diameter ≥26 mm, 97.7% were penicillin-susceptible; 2.3% were resistant based on zone edge characteristics and/or blaZ detection. Resistance to non-β-lactams was low overall but higher among MRSA isolates. CONCLUSIONS: PSSA comprises a substantial proportion of SAB isolates in the Nordic countries, supporting routine susceptibility testing and more targeted use of benzylpenicillin.