Minimum Inhibition Concentration of surface-modified ZnO nanostructures on Streptococcus mutans

In this study, minimum inhibition concentration (MIC) and minimum bactericidal concentration (MBC) were estimated for three types of zinc oxide (ZnO) nanoparticles on a Gram-positive bacterial species: Streptococcus mutans. The structural properties of these nanoparticles, designated as ZnO-A, ZnO-K, and ZnO-Ax, were characterized using the techniques of field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence spectroscopy. Both MIC and MBC were evaluated using a series of dilutions (serial dilution) in a 96-microtiter plate following the standard method CLSI M100-Ed32. The commercial ZnO-K nanoparticle had the largest average crystallite size, i.e., 42 nm followed by ZnO-A (37.5 nm), and ZnO-Ax (37.8 nm). ZnO-Ax was synthesized via post-oxygen annealing, while ZnO-A was prepared via energy combustion. All three ZnO nanoparticles yielded a similar MIC value, i.e., 0.156 mM, in S. mutans. However, the colony-forming unit (CFU/mL) at 0.156 mM varied among the ZnO particles. The ZnO-Ax nanoparticle had the lowest colony number in S. mutans, suggesting that the ZnO-Ax gave better inhibition towards the bacteria, probably due to its high surface area and O: Zn ratio (1.09) that enhanced the generation of reactive oxygen species generation for antibacterial activity compared to ZnO-A and ZnO-K.