Genomic characterization of a clinical multidrug-resistant Serratia marcescens ST6 isolate from Ecuador

This study reports the genomic and phenotypic characterization of a multidrug-resistant Serratia marcescens ST6 isolate recovered from a tracheal aspirate in an intensive care unit in Ecuador. Whole-genome sequencing revealed a 5,027,153 bp genome with 59.50% GC content. Antimicrobial susceptibility testing showed resistance to β-lactams, aminoglycosides, and tetracycline, with intermediate susceptibility to meropenem and gentamicin. Resistome analysis identified a limited set of determinants, including the intrinsic genes aac(6′)-Ic, blaSRT, and tet(41). Virulence profiling revealed a complete flagellar and chemotaxis system, iron acquisition systems (enterobactin, aerobactin), a type VI secretion system, biofilm-associated factors (ompA, luxS, rcsB), and stress response regulators (rpoS, phoP). No plasmids or integrons were detected. Phylogenetic analysis placed ST6 within a genetically diverse regional population with no clustering by geography or sequence type, and no evidence of recent clonal expansion. These findings suggest that clinical persistence of this isolate may be supported primarily by chromosomally encoded intrinsic resistance and virulence traits, without usual horizontal gene acquisition. This study highlights the need for continued genomic surveillance of S. marcescens in resource-limited settings.