Preliminary In Vitro Evaluation of the Growth-Inhibitory Effects of Nigella sativa Oil on Streptococcus mutans

This exploratory study investigated the in vitro growth-inhibitory effects of Nigella sativa (N. sativa) against Streptococcus mutans (S. mutans) UA159 in comparison with selected natural products, over-the-counter products (OTC), and reference antimicrobial agents. Agar disk diffusion assays were conducted under anaerobic conditions, and zones of inhibition were analyzed using one-way ANOVA with Dunnett's and Šídák-adjusted post hoc tests. N. sativa produced larger inhibition zones than erythromycin (Šídák-adjusted ****p < 0.0001) and chlorhexidine gluconate (Šídák-adjusted ****p < 0.0001) under the tested conditions. A microdilution assay indicated concentration-dependent inhibition of S. mutans (0-2.68% v/v), with an estimated IC₅₀ of 1.45% (v/v). These findings indicate that N. sativa exhibits measurable in vitro growth-inhibitory effects, supporting further investigation to more fully characterize its antimicrobial potential.