Development and Evaluation of a Topical Antibacterial Gel from Rhizophora apiculata Blume Leaves: Standardization, Phytochemical Quantification, and Bioactivity Assessment

Introduction Rhizophora apiculata Blume is a mangrove species rich in polyphenolic secondary metabolites, including flavonoids, phenolic acids, and tannins, which are known to exhibit antibacterial activity. The development of topical gel formulations containing these bioactive compounds may provide a promising alternative approach for the management of skin and wound infections. This study aimed to develop and evaluate a standardized topical gel formulation containing R. apiculata Blume leaf extract and to assess its physicochemical characteristics, phytochemical content, and antibacterial activity. Methods Ethanolic leaf extract of R. apiculata Blume was incorporated into gel formulations at concentrations of 3%, 5%, and 7% (w/w). The resulting formulations were evaluated for physicochemical properties, including organoleptic characteristics, homogeneity, spreadability, adhesiveness, pH, and viscosity. Quantitative determination of flavonoids (Quercetin Equivalent Flavonoids, QEF), total phenolics (Gallic Acid Equivalent Phenolics, GAEP), and tannins (Tannic Acid Equivalent Tannins, TAET) was performed using UV–Vis spectrophotometry. Antibacterial activity was assessed against Staphylococcus aureus , Staphylococcus epidermidis , Escherichia coli , and Pseudomonas aeruginosa using the agar well diffusion method, with Bioplacenton ® gel as a positive control. Data were analyzed using one-way analysis of variance (ANOVA) followed by Tukey’s HSD post hoc test, with p < 0.05 considered statistically significant. Results All formulations exhibited acceptable physicochemical characteristics, including homogeneous yellowish-green appearance, characteristic odor, spreadability ranging from 4.36–4.90 cm, adhesiveness of 31.67–38.50 s, skin-compatible pH values (6.36–6.70), and viscosity ranging from 125.83–175.00 Pa·s. Quantitative phytochemical analysis demonstrated a concentration-dependent increase in flavonoid, phenolic, and tannin contents, with the 7% formulation showing the highest QEF (0.5898 ± 0.0025 mg QE/g gel), GAEP (0.4395 ± 0.0067 mg GAE/g gel), and TAET (0.2845 ± 0.0025 mg TAE/g gel) values ( p < 0.001). All formulations exhibited antibacterial activity against the tested bacterial strains. The 7% gel formulation produced the largest inhibition zones against E. coli (18.02 ± 0.41 mm), S. aureus (14.94 ± 0.30 mm), S. epidermidis (13.82 ± 0.27 mm), and P. aeruginosa (24.76 ± 0.21 mm), indicating enhanced antibacterial efficacy with increasing extract concentration. Discussion The observed concentration-dependent increase in antibacterial activity may be associated with the higher levels of flavonoids, phenolics, and tannins present in the formulations. The favorable physicochemical properties of the gels further support their suitability for topical application. Among the tested formulations, the 7% gel demonstrated the most promising overall performance in terms of phytochemical content and antibacterial activity. Conclusion R. apiculata Blume leaf extract was successfully formulated into a topical gel with desirable physicochemical characteristics and significant antibacterial activity against both Gram-positive and Gram-negative bacteria. The findings support the potential application of R. apiculata leaf extract as a natural source for topical antibacterial formulations, with the 7% formulation showing the most favorable characteristics and warranting further investigation.