Background <p>This study aimed to evaluate the antibacterial efficacy of silver and titanium dioxide nanoparticle coatings applied to stainless steel and nickel-titanium orthodontic arch wires against the Streptococcus mutans bacterium.</p> Methods <p>A total of 198 arch wires were divided into two groups based on the type of arch wire material: 99 stainless steel (SS) wires and 99 nickel titanium (NiTi) wires. Every group of these wire materials was evenly subdivided into three subgroups according to the nanocoating material: non-coated group (control), silver nanoparticles (Ag-NPs) and titanium dioxide nanoparticles (TiO₂NPs). The antibacterial effect was then assessed using the colony-forming unit (CFU), minimum inhibitory concentration (MIC), and agar diffusion technique “minimum zone of inhibition” against S. mutans. The sample included arch wire lengths of 5 millimeters for CFU-MIC tests and 10 millimeters for agar diffusion tests. The collected data were compared using a two-way ANOVA and the Bonferroni test was applied for multiple pairwise comparisons. In the CFU test all samples had been incubated in 10 milliliters of artificial saliva, and the data were collected at intervals of immediately(baseline), after one week, and after two weeks, respectively, then compared using a three-way mixed ANOVA.</p> Results <p>The nano-coating effect on arch wires coated with Ag-NPs and TiO₂NPs demonstrated a strong protective effect against S. mutans (<i>p</i> &lt; .001) compared to uncoated wires in the agar diffusion test and in the CFU test. Ag-NP-coated arch wires demonstrated the highest antibacterial activity across all tests. SS arch wires showed significantly greater antibacterial efficacy than NiTi wires (<i>p</i> = .008) in the agar diffusion test. Although antibacterial activity decreased over time, Ag-NP-coated wires maintained a detectable effect after one week, whereas after two weeks, the activity of all coatings was markedly reduced.</p> Conclusions <p>Coatings of Ag-NPs and TiO₂NPs significantly enhance antibacterial effect against <i>S. mutans in vitro</i>, which significantly contribute to reducing the incidence of dental caries, particularly in SS arch wires coated with Ag-NPs. Although the antibacterial activity decreased over time, this reduction may be associated with potential nanocoating degradation.</p>