Antibacterial Potential of Extracts and Phytoconstituents Isolated from Syncarpia hillii Leaves In Vitro

(1) Background: Rapidly increasing antibiotic resistance is one of the greatest threats to global health, affecting individuals regardless of age. Medicinal plants are widely used in traditional medicine to prevent and attenuate infectious conditions with minimal adverse effects. However, only a few have been phytochemically investigated for their medicinal properties and subsequent biological activities. Syncarpia hillii, a plant traditionally used by Indigenous Australians to treat sores, wounds, and skin infections, is no exception. (2) Methods: Primary extracts obtained from mature S. hillii leaves were evaluated for their antibacterial potential against 19 bacterial strains. The methanol extract was subjected to compound isolation and identification due to its preliminary bactericidal efficacy. (3) Results: Staphylococcal species were the most susceptible bacterial strain with a MIC value of 0.63 mg/mL to the S. hillii methanol extract. Quercetin-3-O-β-D-glucuronide and shikimic acid isolated from S. hillii methanol leaf extracts exhibited enhanced antibacterial effects against the tested bacteria with quercetin-3-O-β-D-glucuronide eliciting a MIC value of 0.78 µg/mL against E. faecalis. (4) Conclusions: S. hillii leaves are comprised of bioactive compounds that are bactericidal against several Gram-positive and Gram-negative bacteria.