Emergence of carbapenem-resistant <i>Serratia marcescens</i> co-harboring <i>bla</i> _NDM-1 , <i>bla</i> _KPC-2 , and <i>bla</i> _SRT-2 in bloodstream infection

<i>Serratia marcescens</i> is an emerging opportunistic pathogen with high genetic diversity. The emergence and prevalence of carbapenem-resistant <i>S. marcescens</i> poses a major health threat due to its intrinsic resistance to multiple antibiotics, which severely restricts the selection and treatment of antibiotics for <i>S. marcescens</i> infection. This study presents the first documented case in China of a bloodstream infection caused by <i>Staphylococcus epidermidis</i> and <i>S. marcescens</i> strain (designated S96) co-producing <i>bla</i>_NDM-1, <i>bla</i>_KPC-2, and <i>bla</i>_SRT-2. Strain S96 exhibited resistance to nearly all categories of β-lactam antimicrobials, β-lactam/inhibitor combinations, aminoglycosides, quinolones, and other clinical antibacterial agents, with the exception of tigecycline. Our main objective was to characterize the genetic mechanisms underlying its carbapenem resistance and plasmid transfer potential. Whole-genome sequencing revealed <i>bla</i>_KPC-2 on a 44,047 bp "IncX6-like" plasmid and <i>bla</i>_NDM-1 on a 100,081 bp IncFII(Yp)-type plasmid, alongside chromosomal <i>bla</i>_SRT-2 and <i>aac</i>(6')-<i>Ic</i>. "IncX6-like" and IncFII(Yp)-type plasmids are widely distributed among carbapenem-resistant Enterobacteriaceae strains globally. Conjugation experiments demonstrated that the <i>bla</i>_NDM-1-carrying plasmid could be successfully transferred to recipient <i>Escherichia coli 600</i>, with no significant fitness cost observed (<i>P</i> > 0.05). The experimental results demonstrate that carbapenem-resistant genes can disseminate among Enterobacteriaceae via plasmid-mediated horizontal transfer between bacterial cells. Comparative genomic analysis revealed plasmid structural homology with global counterparts, demonstrating IS-mediated recombination and horizontal gene transfer. The low adaptive cost of plasmid carriage and multidrug resistance phenotype pose significant challenges for clinical management. This study highlights the need for enhanced clinical surveillance and antibiotic stewardship to curb the spread of such multidrug-resistant pathogens.IMPORTANCECarbapenem resistance in <i>Serratia marcescens</i> is primarily mediated by <i>Klebsiella pneumoniae</i> carbapenemase (KPC), with New Delhi metallo-β-lactamase (NDM) being a relatively uncommon alternative resistance mechanism. KPC-2 and NDM-1 coexisting in <i>S. marcescens</i> is extremely rare clinically. This study reports the first clinical isolate of <i>S. marcescens</i> in China co-harboring <i>bla</i>_NDM-1, <i>bla</i>_KPC-2, and <i>bla</i>_SRT-2. The isolate exhibits multidrug resistance to nearly all β-lactam antibiotics and β-lactam/inhibitor combinations, with low adaptive costs and high dissemination potential. The potential spread of resistance genes through mobile genetic elements poses a serious public health risk. The study underscores the need for enhanced surveillance, rational antibiotic use, and novel strategies to combat resistance. It also provides insights into the evolutionary mechanisms of bacterial resistance, emphasizing the urgent need for interventions to address the growing threat of antimicrobial resistance.