Evaluation of the antibacterial activity of selenium nanoparticles and those conjugated with lysozyme against Bacillus cereus spiked in pasteurized skim milk

Abstract

Selenium nanoparticles (SeNPs) and SeNPs conjugated with lysozyme (SeNPs-lysozyme) were synthesized via sodium selenite reduction with ascorbic acid and polysorbate-80 stabilization. These were physiochemically characterized to impart antibacterial properties. X-ray diffraction analysis (XRD) revealed that both formulations had hexagonal crystalline structures. Fourier transform infrared (FT-IR) confirmed the successful conjugation of lysozyme to SeNPs through the emergence of amine peaks at 1542 cm−1. Dynamic light scattering (DLS) showed lysozyme conjugation increased nanoparticle size from 76.6 to 92.4 nm and charge from − 12.6 to + 17.6 mV. Transmission electron microscopy (TEM) images confirmed that spherical morphologies and size increased from 73–79 nm for SeNPs to 84–98 nm for SeNPs-lysozyme after lysozyme capping. Both formulations exhibited identical minimum inhibitory concentrations (MIC) of 125 μg/mL. However, lysozyme conjugation reduced SeNPs cytotoxicity by 2.4-fold based on IC50 values from an MTT (Thiazolyl Blue Tetrazolium Bromide) assay. When tested in phosphate-buffered saline (PBS), SeNPs and SeNPs-lysozyme fully inhibited Bacillus cereus (B. cereus) proliferation up to 6 log10 CFU/mL inoculums over 24 h of incubation. However, at (7 log10 CFU/mL) inoculum, only partial inhibition occurred, with (2.5 ± 0.3 log10 CFU/mL) growth still detected for SeNPs and (2.2 ± 0.2 log10 CFU/mL) for SeNPs-lysozyme. At all inoculum levels (5, 6, and 7) log10 CFU/mL, B. cereus grew a lot in milk (more than 8 log10 CFU/mL). This was probably because SeNPs or SeNPs-lysozyme tend to aggregate around milk components, making them less effective.