Nationwide P. aeruginosa Outbreak in Norway: ST244 and Distinct Clusters
Background: A nationwide collection of Pseudomonas aeruginosa bacteraemia isolates was assembled during a wild-type P. aeruginosa ST3875 outbreak affecting ~400 patients across 40 hospitals in Norway in 2021–22.  Aim: To investigate the molecular epidemiology of P. aeruginosa in Norway through a multi[1]centre retrospective study.  Methods: All available isolates collected over 18 months were retrieved from 20 microbiology laboratories and underwent whole-genome sequencing at five university hospitals. Metadata, including demographics, sampling dates, and locations, was collated. Phylogenetic and comparative genomic analyses were performed to identify sequence types (STs), clusters, resistance determinants, and virulence factors.  Findings: We included 376 unique P. aeruginosa bacteraemia patients from 33 hospitals (69% male; median age 76 years), representing >90% of the national catchment area. Incidence varied regionally (5.1–16.3 per 100,000). ST was resolved for 362 isolates, revealing 164 distinct types. Global high-risk and epidemic clones (e.g., ST233, ST244, ST277, ST298, ST308) comprised 30% of the collection. No isolate carried carbapenemases or extended-spectrum ß-lactamases. Aside from the ST3875 outbreak clone (N = 56), nine unrecognised phylogenetic clusters (2–20 cases each) spanning multiple hospitals and regions were identified, including a widespread ST244 cluster harbouring a novel genomic island with putative virulence determinants, indicating a selective advantage for dissemination.  Conclusions: This comprehensive genomic surveillance study uncovered multiple unexpected clusters of P. aeruginosa bacteraemia isolates in Norway, along with widespread presence of susceptible epidemic clones. Routine multi-level surveillance integrating sequencing with epidemiological data could enable earlier recognition of emerging high-risk clones and outbreaks, strengthening infection prevention efforts