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. 2025 Feb;30(5):2400291.
doi: 10.2807/1560-7917.ES.2025.30.5.2400291.

A national outbreak of Serratia marcescens complex: investigation reveals genomic population structure but no source, Norway, June 2021 to February 2023

Arne Michael Taxt  1   2 , Vegard Eldholm  1 , Nicola Isabelle Kols  3 , Maria Schei Haugan  4 , Niclas Raffelsberger  5 , Anne Mette Asfeldt  6   7 , André Ingebretsen  8   9 , Anita Blomfeldt  10 , Kristin Stenhaug Kilhus  11 , Paul Christoffer Lindemann  12 , Horst Bentele  1 , Jeanette Stålcrantz  1 , Liz Ertzeid Ødeskaug  1 , Thale Cathrine Berg  1 ; Norwegian Serratia study group  13
Collaborators, Affiliations

A national outbreak of Serratia marcescens complex: investigation reveals genomic population structure but no source, Norway, June 2021 to February 2023

Arne Michael Taxt et al. Euro Surveill. 2025 Feb.

Abstract

We report a national outbreak of Serratia marcescens complex type 755 (ct755) in Norway, with 74 cases identified between June 2021 and February 2023. Careful reviews of patient journals and interviews were performed, involving 33 hospitals throughout Norway. All available clinical isolates of S. marcescens collected between January 2021 and February 2023 (n = 455, including cases) from all involved hospitals were whole genome sequenced. Cases displayed a pattern of opportunistic infections, as usually observed with S. marcescens. No epidemiological links, common exposures or common risk factors were identified. The investigation pointed to an outbreak source present in the community. We suspect a nationally distributed product, possibly a food product, as the source. Phylogenetic analysis revealed a highly diverse bacterial population containing multiple distinct clusters. The outbreak cluster ct755 stands out as the largest and least diverse clone of a continuum, however a second cluster (ct281) also triggered a separate outbreak investigation. This report highlights challenges in the investigation of outbreaks caused by opportunistic pathogens and suggests that the presence of identical strains of S. marcescens in clinical samples is more common than previously recognised.

Keywords: Serratia marcescens; WGS; clonality; ct755; outbreak; unknown source; whole genome sequencing.

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Conflict of interest statement

Conflict of interest: None declared.

Figures

Figure 1
Figure 1
Phylogenetic analyses of Serratia marcescens clinical isolates showing the three major clusters, Norway, June 2021 to February 2023 (n = 455)
Figure 2
Figure 2
Overview of Serratia marcescens complex ct755 epidemiological parameters, Norway, June 2021 to February 2023
Figure 3
Figure 3
Distribution of cases of Serratia marcescens complex ct755 infection, by age group and sex, Norway, June 2021–February 2023 (n = 74)
Figure 4
Figure 4
Within-outbreak diversity and temporal analyses, Serratia marcescens outbreak, Norway, June 2021 to February 2023
See this image and copyright information in PMC

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