Green biosynthesis of selenium nanoparticles by Ralstonia insidiosa which demonstrate effectiveness against human cancer cells, Candida species and multidrug-resistant Acinetobacter baumannii.

Acinetobacter baumannii is a major global concern due to its multidrug resistance and persistence multidrug-resistant (MDR) pathogens pose a serious threat in hospital environments, particularly among immunocompromised patients. In this study, selenium nanoparticles (SeNPs) were biosynthesized using Ralstonia insidiosa isolated from petroleum-contaminated soils in Iraq. SeNP formation was confirmed by UV-visible spectroscopy, AFM, TEM, FE-SEM, and EDX analyses, which revealed predominantly spherical, well-dispersed nanoparticles in the nanoscale range. The antimicrobial activity of SeNPs was evaluated against Gram-positive and Gram-negative bacteria, Candida spp., and ten MDR Acinetobacter baumannii clinical isolates. SeNPs exhibited strong antimicrobial activity, with a uniform minimum inhibitory concentration (MIC) of 16 µg mL-1 against all MDR A. baumannii isolates and concentration-dependent inhibition against other bacterial and fungal pathogens, showing notable activity against Candida guilliermondii. To explore potential resistance-related interactions, MexB efflux pump gene expression was analyzed in two representative MDR A. baumannii isolates. SeNP treatment resulted in strain-dependent modulation of MexB expression, indicating variable bacterial responses rather than consistent efflux inhibition. In addition, cytotoxicity assays demonstrated dose-dependent antiproliferative effects of SeNPs against PC3 prostate cancer cells, with lower toxicity toward normal WRL68 liver cells.