Phytochemical Analysis and In Vitro Antibiofilm Activity of Ethanolic Leaf Extract from Quercus alnifolia Poech Against Staphylococcus aureus

Antibiotic resistance is on the rise, rendering discovery of new antibacterial sources essential. Biofilms drive resistance and cause complications in healthcare settings, emphasizing that preventing pathogenic biofilms is vital. Quercus species, with medicinal potential, might provide novel approaches against pathogens. Cyprus hosts four understudied Quercus species—Q. alnifolia Poech, Q. × campitica Hadjik. & Hand, Q. coccifera var. calliprinos (Webb) Boiss., and Q. infectoria subsp. veneris (A.Kern.) Meikle—where Q. alnifolia and Q. × campitica are endemic. This study assessed the antibacterial, antibiofilm, and preformed biofilm reduction effects of their ethanolic leaf extracts on Staphylococcus aureus (ATCC 6538) and performed phytochemical analysis. Because biofilm formation often drives recalcitrance, sub-minimum inhibitory concentrations (sub-MIC) of Quercus extracts were tested on planktonic and biofilm S. aureus. At a sub-MIC of 0.156 mg/mL, Q. alnifolia and Q. × campitica extracts displayed notable antibiofilm activity. Liquid chromatography–mass spectrometry of Q. alnifolia revealed several bioactive compounds where these compounds may support wider antibacterial effects. This is the first report of Q. alnifolia and Q. × campitica ethanolic leaf extracts with antibiofilm activity against S. aureus and associated phytochemical analyses. These results support further practical research into the potential applications of these Quercus extracts as antibacterial materials.