A national outbreak of Serratia marcescens in Norway, June 2021 – February 2023

Report on a national outbreak investigation of Serratia marcescens complex type 755 in Norway, with 74 cases identified between June 2021 and February 2023. Extensive data collection was performed through review of patient's journals, interviews, and whole genome sequencing. No epidemiological links, common exposures or common risk factors were identified among cases. Cases displayed a pattern of opportunistic infections. No source of the outbreak was identified. Phylogenetic analysis revealed an overall highly diverse bacterial population containing multiple distinct clusters. This challenges the distinction between an outbreak and a pseudo-outbreak and highlights dilemmas in the interpretation of large genomic datasets in outbreak investigations.