Molecular Epidemiology of Carbapenem-Resistant Serratia marcescens Revealing Distinct High-Risk Clones Within a Hospital Setting

Purpose: Carbapenem-resistant Serratia marcescens (CRSM) poses an increasing threat in healthcare settings. This study aimed to investigate the molecular epidemiology of CRSM and the drivers of its dissemination within a hospital setting. Patients and Methods: Twelve non-duplicate clinical CRSM isolates were collected from a tertiary hospital between 2010 and 2024. Clinical data were retrospectively obtained from electronic medical records. Antimicrobial susceptibility testing was performed using the Vitek-2 system. Whole-genome sequencing (WGS) was conducted to determine resistance and virulence gene profiles, sequence types, and phylogenetic relationships. Pulsed-field gel electrophoresis and conjugation assays were performed to assess clonal relatedness and plasmid transferability. Results: CRSM infections predominantly occurred in neonates (50.0%) and adults (50.0%), with the respiratory tract as the most common infection site (66.7%). All isolates exhibited multidrug-resistant phenotypes; resistance to β-lactams ranged from 50.0% to 100.0%, while no amikacin-resistant isolates were detected. Two dominant clonal lineages were identified: ST595 (5 strains) uniformly carrying blaKPC-2 , and ST490 (6 strains, 4 of which harbored blaNDM-5 ). SNP phylogeny identified four distinct, well-supported clades, with ST595 strains showing short branch lengths indicative of clonal expansion events during the study period. The blaNDM-5 gene was successfully transferred via conjugation from four donor strains, and most transconjugants exhibited growth advantages. Core virulence genes were present in all isolates, while accessory virulence factors showed strain-specific variations. Conclusion: CRSM dissemination in this hospital is likely driven by two co-circulating high-risk clones (ST595- blaKPC-2 and ST490- blaNDM-5 ), the high horizontal transfer potential of blaNDM-5, and specific patient demographics. Despite the small sample size and single-center design, these findings provide actionable insights for infection control and antimicrobial stewardship.