Biosynthesis and Characterization of Selenium Nanoparticles by <i>Kocuria</i> Strain Bm3: Evaluating Their Synergistic Antimicrobial Activity with <i>Chlorella</i><i>vulgaris</i> Crude Extract Against Multidrug-Resistant Bacteria

Abstract

Background: Mangrove ecosystems are rich in halotolerant microorganisms capable of synthesizing antimicrobial nanoparticles. Exploring these microorganisms for the development of novel antimicrobial agents is increasingly important, particularly given the escalating problem of multidrug resistance. Objectives: This study aimed to confirm the biosynthesis of selenium nanoparticles (SeNPs) by the Kocuria strain Bm3, isolated from the Hera mangroves in Iran, and to evaluate the antimicrobial efficacy of SeNPs in combination with Chlorella vulgaris crude extract (C. vulgaris CE) against various bacterial strains. Methods: The synthesized SeNPs were characterized with respect to morphology and size. Antimicrobial activity was assessed by measuring inhibition zones; minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values were determined. Synergistic effects were calculated using the Fractional Inhibitory Concentration Index (ΣFIC). Additionally, cytotoxic effects on MCF-7 cell lines were evaluated. Results: The combination of C. vulgaris CE and SeNPs significantly enhanced antibacterial activity against various strains, most notably a clinical isolate of Escherichia coli, which exhibited marked growth inhibition and reduced MIC values (P < 0.001). While Staphylococcus aureus did not demonstrate significant differences among treatments, Staphylococcus epidermidis and Staphylococcus saprophyticus showed improved sensitivity with the combination treatment, achieving substantial growth inhibition zones (P < 0.001). In contrast, the clinical isolate of Acinetobacter baumannii displayed potential resistance, with no growth inhibition observed when treated alone. The MTT assay revealed that C. vulgaris CE and SeNPs each exhibited low cytotoxicity individually, resulting in MCF-7 cell viability percentages of 49.75 ± 3.75% and 44.46 ± 4.26%, respectively; however, their combination significantly reduced cell viability to 29.36 ± 2.64% (P < 0.001). Conclusions: The SeNPs synthesized by the Kocuria strain Bm3 significantly enhance the antimicrobial properties of C. vulgaris CE, indicating their potential as a therapeutic strategy against multidrug-resistant infections. Further investigation into their cytotoxic effects is warranted to evaluate safety and efficacy for clinical applications.