Prevalence and impact of ampC and luxS genes in multi-drug resistant Serratia marcescens from clinical specimens in Najaf Governorate

Background: Serratia marcescens is an opportunistic member of the order Enterobacterales and an important cause of hospital-acquired infections. Aim: To determine ampC and luxS gene carriage in clinical S. marcescens isolates from Najaf hospitals and assess associations with antimicrobial resistance, efflux activity, and biofilm formation. Methods: From November 2024 to August 2025, 90 clinical specimens were processed by standard methods. Isolates were identified by conventional bacteriology and GN Vitek-2. Antimicrobial susceptibility testing followed CLSI (2024) disk diffusion. Biofilm formation was quantified by the microtiter-plate assay; efflux activity by ethidium-bromide agar cartwheel; ampC and luxS were detected by PCR. Results: Thirteen isolates (14.4%) were S. marcescens; all satisfied MDR criteria. Strong biofilm production was observed in 12/13 (92.3%); all isolates demonstrated phenotypic efflux activity. luxS was present in 13/13 (100%), while ampC was detected in 2/13 (15.4%). Conclusions: MDR S. marcescens with robust biofilm and efflux phenotypes is circulating in Najaf hospitals. The universal presence of luxS and detection of ampC underscore the need for strengthened infection-prevention measures and antimicrobial stewardship.