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

Abstract

Abstract 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.