Optimization of probiotic-mediated selenium nanoparticles for superior antibacterial action against methicillin-resistant Staphylococcus aureus

The need for new bactericidal agents is becoming increasingly crucial as current antibiotic treatments are becoming less effective against methicillin-resistant Staphylococcus aureus (MRSA). The present study involves the biosynthesis of selenium nanoparticles (SeNPs) using the cell-free supernatant of Limosilactobacillus fermentum (OR553490). To achieve the two objectives of optimizing the biosynthesis of SeNPs and enhancing their antibacterial activity, response surface methodology (RSM) coupled with the Box-Behnken design was employed to vary the components to achieve the desired outcomes. For the maximum production of biosynthesized SeNPs (OD_max), the optimum conditions were found to be a Na₂SeO₃ concentration of 30 mM at pH 7, incubation temperature within the 30-40 °C range for 72 h, and a metabolite-to-precursor ratio within the range of 1:1 to 1:4 (v/v%). Conversely, for maximum antibacterial activity against MRSA (strain ATCC 43300), the optimum conditions are a higher precursor concentration (50 mM) at a 1:1 ratio (30 °C for 48 h). The SeNPs were well characterized; most were coated with a protein layer and had a uniform rod-shaped structure with a mean diameter of 30.94-43.94 nm. The presence of the protein coat was confirmed by the presence and identification of an amine and C-N band in the FT-IR spectrum. The synthesized protein-coated SeNPs exhibited a zeta potential of - 20.93 ± 5 mV, which indicates good stability. Both optimized SeNPs and linezolid were tested against MRSA, with the SeNPs demonstrating greater anti-MRSA activity than linezolid, which served as the clinical benchmark. SeNPs demonstrated identical minimum inhibitory and bactericidal concentrations (MIC/MBC) of 50 µg/ml, effectively doubling the potency of linezolid (MIC/MBC: 100/110 µg/ml). Probiotic-mediated rod-shaped SeNPs may offer an environmentally viable solution for treating drug-resistant bacterial infections.