Combinatorial Antibacterial and Antibiofilm Activities of Medicinal Plant Extracts Against Oral Bacterial Pathogens

This study investigated the antibacterial properties and biofilm disruption mechanisms of five medicinal plants traditionally used in Limpopo communities for the treatment of oral infections, namely Sesamum senecioides Klotzsch, Euclea undulata Thunb, Kleinia longiflora DC, Aloe greatheadii Schönland, and Drimia elata Jacq. Five plants were analysed using four solvents (methanol, dichloromethane (DCM), petroleum ether (PE), and water) of different polarities to quantify their phytochemical constituents. Antibacterial, combinational effects, and antibiofilm activities were evaluated through broth microdilution and crystal violet staining assays. Phytochemical analysis revealed that E. undulata had the highest levels of tannins (2400.74 ± 8.31 mg GAE/g in petroleum ether), phenolic compounds (377.63 ± 2.25 mg GAE/g), and flavonoids (31.26 ± 2.76 mg QE/g). It also demonstrated low minimum inhibitory concentrations (MICs) of 0.15 µg/mL against Streptococcus mutans and Porphyromonas gingivalis. Combination studies showed that E. undulata and K. longiflora exhibited a synergistic antibacterial effect (FICI = 0.15) at the tested concentrations. In the biofilm assays, E. undulata inhibited more than 60% of biofilm formation, while A. greatheadii exhibited strong antibiofilm activity, with inhibition exceeding 60% across several concentrations tested. During the initial biofilm prevention stage, inhibition was exhibited against S. mutans at 1.26 and 2.50 µg/mL and against P. gingivalis at 1.24 µg/mL. At the initial cell attachment stage, A. greatheadii exhibited over 60% inhibition of P. gingivalis at 1.24 µg/mL. During the biofilm eradication stage, inhibition exceeding 60% was observed against Pseudomonas aeruginosa at 1.24 µg/mL, S. mutans across 0.63–2.52 µg/mL and P. gingivalis across 0.62–1.24 µg/mL. Overall, these findings demonstrate the antibacterial and antibiofilm potential of the investigated medicinal plants against dental pathogens and support their further investigation as potential sources of bioactive compounds for alternative approaches to managing dental infections.