Antimicrobial Susceptibility Patterns and Genetic Determinants of Carbapenem and Aminoglycoside Resistance Among Serratia marcescens Bloodstream Isolates

Objective: Serratia marcescens (S. marcescens) is an important opportunistic pathogen associated with bloodstream infections, particularly in intensive care units (ICUs). This study aimed to evaluate antimicrobial susceptibility patterns and to investigate genetic determinants associated with carbapenem and aminoglycoside resistance among S. marcescens bloodstream isolates.Methods: A total of 98 non-duplicate S. marcescens isolates recovered from blood cultures between 2019 and 2023 were retrospectively analyzed. Species identification was performed using MALDI-TOF MS. Antimicrobial susceptibility testing was carried out by disk diffusion and interpreted according to EUCAST version 14.0 criteria. Phenotypic assays were used to detect carbapenemase activity. Carbapenem-resistant isolates were screened by polymerase chain reaction for blaKPC, blaNDM, blaOXA-48, blaVIM, blaIMP, and the 16S rRNA methyltransferase gene rmtB.Results: Adult patients accounted for 75.5% of the isolates, and approximately 40% originated from ICUs across all age groups. High susceptibility rates were observed for third- and fourth-generation cephalosporins, carbapenems, aminoglycosides, fluoroquinolones, and trimethoprim–sulfamethoxazole, whereas resistance rates to amoxicillin–clavulanate (96.9%) and cefoxitin (54.1%) were high, reflecting intrinsic resistance mechanisms. Carbapenem resistance was detected in three isolates (3.1%), two of which exhibited phenotypic carbapenemase activity. Polymerase chain reaction analysis revealed blaNDM in one isolate and blaVIM in another, whereas rmtB was not detected.Conclusion: Although the overall antimicrobial susceptibility profile of S. marcescens bloodstream isolates remains favorable, the emergence of NDM- and VIM-producing S. marcescens strains highlights the necessity for continuous local surveillance and strict infection-control practices, particularly in ICUs.