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

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.