Implications of ad-hoc molecular typing for infection control measures in a multi-cluster, multi-phenotypic Serratia marcescens outbreak in a neonatal intensive care unit

Abstract

Serratia marcescens is known to cause outbreaks in Neonatal Intensive Care Units (NICU). Traditionally epidemiological data, antimicrobial resistance patterns and epidemiological typing are being used to guide infection prevention methods. Whole genome sequencing (WGS) applications such as core-genome multi-locus sequence typing (cgMLST) applied during an outbreak would potentially give more detailed information on the source and spread of bacteria enabling more efficient control measures. CgMLST was used during an S. marcescens outbreak at a NICU. Neonates admitted to the NICU of the Leiden University Medical Center (LUMC) during an outbreak between September 2023-January 2024, with S. marcescens being cultured were included. Environmental samples were taken to search for a common source, antibiotic susceptibility testing was performed, and antimicrobial resistances genes were analysed. S. marcescens strains from 17 of the 20 positive patients were available for molecular typing. The cgMLST scheme revealed five different complex types consisting of four separate clusters. Multiple clusters made an unidentified persistent environmental source as cause of the outbreak less likely, leading to a quick downscaling of infection prevention measures. Differences were shown in aminoglycoside resistance patterns of isolates within the same complex types and patients. The use of ad-hoc cgMLST provided timely data for rational decision making during an S. marcescens outbreak at the NICU. Antibiotic phenotyping alone was found not to be suitable for studying clonal spread during this outbreak with S. marcescens.