Antibacterial Effectiveness of Zinc Oxide and Magnesium Oxide Nanoparticles Against Enterococcus faecalis: An In Vitro Study

Abstract Background: Persistent endodontic infections, often attributed to Enterococcus faecalis , pose a significant challenge to successful root canal treatment. Conventional disinfection methods have limitations in eradicating bacteria within complex root canal anatomy. Nanoparticles (NPs) have evolved as favorable antibacterial agents due to their distinctive properties. Aim: To compare the antimicrobial activity of ZnO and MgO NPs (nanorods and nanospheres) with 3% sodium hypochlorite (NaOCl) against E. faecalis (American-type culture collection [ATCC] 29212 and a clinical oral isolate) using agar well diffusion, minimum inhibitory concentration (MIC), and time-kill assays. Methods: ZnO and MgO NPs (nanorods and nanospheres) were synthesized and characterized using ultraviolet-visible (UV-visible) spectrophotometry to determine the optical properties of materials and transmission electron microscopy for measuring nanoparticle size, distribution, morphology, and grain size. Agar well diffusion was used to measure antimicrobial activity at various volumes (50–150 µL). MIC was calculated using the broth microdilution method. Time-kill assays were performed by quantifying colony-forming units at 15 min intervals for 90 min. One-way analysis of variance with Tukey’s post hoc test ( P < 0.05) was used for statistical analysis. Results: The peak acquired for ZnO nanorods was 360 nm, ZnO nanosphere was 350 nm, MgO nanorods were 250 nm, and MgO nanosphere was 270 nm in UV-visible spectrophotometry. The transmission electron microscope confirmed an average NP size of 20 nm for all morphologies. In the agar well diffusion assay, ZnO nanorods exhibited better activity than other NPs except for 3% NaOCl against both E. faecalis strains at all volumes (75–150 µL) ( P = 0.001). MgO nanorods also demonstrated significant activity, although generally at higher volumes (150 µL) than ZnO ( P = 0.001) and MgO ( P = 0.002) nanospheres. The MIC values (mg/mL) against the ATCC strain were as follows: MgO-S (1.17), MgO-R (18.75), ZnO-R (18.75), and ZnO-S (37.5). Against the clinical isolate, the MIC values were as follows: MgO-R (18.5), MgO-S (18.5), ZnO-R (37.5), and ZnO-S (75). In the time-kill assay, all NPs, except MgO nanorods against the ATCC strain (effective at 30 min), achieved rapid bactericidal activity within 15 min, comparable to 3% NaOCl. Conclusion: Both ZnO and MgO NPs, particularly ZnO nanorods, demonstrated promising antimicrobial activity against E. faecalis in vitro , comparable to 3% NaOCl in certain assays. ZnO NPs of various shapes have distinct active facets that increase their antimicrobial action such as rods and wires that have more facets than spherical NPs. ZnO nanorods penetrate easily into the cell walls of bacteria than nanosphere. Additionally, ZnO nanorod and sphere release more reactive oxygen species (ROS) that interacts with proteins, resulting in bacterial cell lysis. The main mechanism of action of MgO NPs (nanorod and nanosphere) is the formation of ROS that interacts with the proteins peptide linkages in microorganisms, ultimately resulting in cell death. These results highlight the potential of these NPs in endodontic disinfection, such as incorporation into sealers or gutta-percha coatings. Future research should focus on evaluating their biocompatibility and long-term efficacy in biofilm models.