Genomic epidemiological analysis of a single-centre polyclonal outbreak of Serratia marcescens, Belgium, 2022 to 2023

<i>Serratia marcescens</i> is an opportunistic pathogen with a propensity to cause nosocomial outbreaks, particularly in neonatal intensive care units (NICUs). We present a sustained outbreak spanning over 18 months (1 January 2022-29 August 2023) in a NICU in Antwerp, Belgium, affecting 61 neonates, identified through samples taken for diagnostic purposes and by rectal screening. Ten neonates were infected: five with lower respiratory tract infection, four with conjunctivitis and one fatal case with sepsis. In a logistic regression analysis, nursing in an incubator was significantly associated with acquisition of <i>S. marcescens</i> (odds ratio (OR): 2.99; 95% confidence interval (CI): 1.14-8.25; p < 0.05). Whole genome sequencing-based multilocus sequence typing (wgMLST) and core genome single nucleotide polymorphism (cgSNP) analysis of isolates from clinical (n = 4), screening (n = 52) and environmental samples (n = 8), identified eight clusters and five singletons not associated with the clusters. Although outbreak measures were successful in containing further spread within the ward during sudden surges when > 4 cases per week were identified (peak events), several peaks with different clonal clusters occurred. The emergence of similar outbreaks in Belgian hospitals underscores the need of continuous surveillance and NICU-specific infection prevention and control (IPC) measures.