Silver nanoparticles biosynthesized by Citrobacter farmeri effectively inhibit growth and biofilm formation in Acinetobacter baumannii.

The emergence of carbapenem-resistant Acinetobacter baumannii (A. baumannii) has severely limited available therapeutic options, posing a significant clinical challenge. This study aimed to evaluate the antibacterial and antibiofilm efficacy of biosynthesized silver nanoparticles (AgNPs) against clinical multi-drug resistant (MDR) A. baumannii isolates. Silver nanoparticles were biosynthesized using Citrobacter farmeri (C. farmeri) (A121) and subsequently characterized for their physicochemical properties. Antibacterial activity against A. baumannii was assessed using the disc diffusion assay, while biofilm-forming ability and antibiofilm efficacy were evaluated using the microtiter plate method. Molecular analysis revealed a high prevalence of resistance-associated genes, with blaOXA-48 (70%), blaCTX-M (43%), and blaDHA (13.3%) being the most frequently detected ESBL-, AmpC-, and carbapenemase-encoding genes, respectively. Notably, 65.3% of the isolates exhibited strong biofilm-forming capacity. The biosynthesized AgNPs ranged in size from 20 to 60 nm and demonstrated a pronounced, concentration-dependent antibacterial effect at 1.25, 2.5, and 5 mg/mL. Moreover, AgNP treatment resulted in a substantial inhibition of biofilm formation, achieving up to 95% reduction (2.5 mg/mL). Overall, these findings highlight the potent antibacterial and antibiofilm activities of biosynthesized AgNPs against MDR A. baumannii, highlighting their potential as a promising alternative or adjunctive strategy for combating infections caused by highly drug-resistant pathogens.