Molecular epidemiology of clinical infections caused by Serratia marcescens complex in a tertiary care hospital system

Abstract

Background: Serratia marcescens is a Gram-negative opportunistic pathogen associated with outbreaks in healthcare settings. It produces an AmpC beta-lactamase, conferring resistance to many antibiotics and contributing to morbidity and mortality, particularly in intensive care units. The extent of S. marcescens transmission between hospitalized patients and its implications for infection control are not well understood. Methods: From January to December 2022 and from January to June 2024, we prospectively identified and enrolled consecutive specimens taken from patients for routine bacterial culture at six hospitals. We collected relevant patient characteristics using retrospective chart review. Only the first isolate of Serratia marcescens from a patient that was identified via conventional bacteriologic culture was included, and the identification was confirmed by bioMérieux matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). We performed whole genome sequencing using the Illumina NextSeq 2000 platform and used Genome Taxonomy Database Toolkit (GTDB-Tk) taxonomies to construct a maximum-likelihood phylogenetic tree. We queried assembled genomes against the Comprehensive Antibiotic Resistance Database (CARD) to predict determinants of antimicrobial resistance. Potential transmission was assessed by calculating average nucleotide identity (ANI) between isolates from different patients on the same ward within one month. Results: Of 147 identified isolates, 125 met study inclusion criteria. MALDI-TOF MS and genome-based species identification were discordant in 64 (51.2%) cases, indicating the involvement of multiple species within the recently described S. marcescens complex in both community- and hospital-associated infections. Two isolate pairs had a putative spatiotemporal link, with one pair meeting the ANI threshold for possible transmission. Conclusions: Possible transmission of S. marcescens complex between hospital inpatients was rare. Current MALDI-TOF MS methods cannot reliably distinguish between S. marcescens complex members; laboratory reporting should therefore identify isolates to the complex level. Improved species-level identification may refine infection prevention efforts by informing decisions on whether to investigate temporally clustered cases as potential outbreaks.