Genome-oriented outbreak management reveals the first detection of concurrent transmissions of Serratia sarumanii and Serratia bockelmannii in a neonatal department

Aim Non-pigmented Serratia marcescens – bacteria otherwise known for occasional reddish pigmentation and frequent outbreaks in neonatal intensive care units (NICUs) – were recently reclassified to novel species: S. sarumanii and S. bockelmannii. This raised questions about possible differences regarding clinical significance. Here, we report the first detected concurrent transmission of these novel species in a neonatal department. Methods Between March and October 2023, non-pigmented S. marcescens (according to mass spectrometry) were repeatedly found in 40 patients of a neonatology department. Proactive short-read (N = 42, including two environmental) and for three large clusters additional long-read (nanopore sequencing, N = 23) whole-genome sequencing (WGS) was performed, followed by ad-hoc core-genome multi-locus sequence typing (cgMLST, SeqSphere+ software), taxonomic analysis with Type Strain Genome Server (TYGS), and virulence factor/resistance prediction with Abricate. Results WGS revealed a polyclonal Serratia spp. population comprising 14 genotypes, including three large clusters (N = 6, 8, and 9, respectively) alongside two pairs of twins and a small cluster of three isolates. In contrast to initial MALDI-ToF classification as S. marcescens, WGS-driven taxonomic analysis reclassified isolates within the largest cluster as S. bockelmannii (including one environmental isolate), whereas the remaining two large clusters were assigned to S. sarumanii. Conclusion WGS-based analysis revealed a prolonged outbreak involving newly classified non-pigmented Serratia spp. – S. bockelmannii and S. sarumanii – through conventional mass spectrometry. This highlights the importance of routinely implementing WGS to accurately track transmission and implement effective infection control measures.