Antibacterial, Synergistic, and Antibiofilm Activities of Liquidambar orientalis Mill. Essential Oil Against Clinically Relevant Bacterial Pathogens

Background/Objectives: The increasing prevalence of antimicrobial resistance has reduced the effectiveness of conventional antibiotics, highlighting the need for alternative or adjunct antimicrobial agents. This study aimed to investigate the antibacterial, synergistic, and antibiofilm activities of Liquidambar orientalis Mill. essential oil against clinically relevant bacterial pathogens, using 24 bacterial strains representing 12 bacterial species. Methods: The volatile constituents of L. orientalis essential oil obtained by hydrodistillation of storax balsam were characterized by gas chromatography-mass spectrometry (GC-MS) and gas chromatography-flame ionization detection (GC-FID). Antibacterial activity was evaluated using minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), disk diffusion, and agar well diffusion assays against a panel of Gram-positive and Gram-negative bacterial pathogens. Synergistic interactions with erythromycin, amoxicillin, gentamicin, and enrofloxacin were assessed using fractional inhibitory concentration index (FICI) analysis. Antibiofilm activity was also evaluated against representative biofilm-forming strains. Results: A total of 90 volatile constituents were identified by GC-MS, of which styrene was the most abundant (68.6%), followed by (E)-ethyl cinnamate (6.8%), α-pinene (5.9%), and β-pinene (2.9%). The essential oil exhibited broad-spectrum antibacterial activity, with an overall inhibition rate of 60.4%. Activity was higher against Gram-positive (77.77%) than Gram-negative (73.33%) bacteria. MIC and MBC values ranged from 15.62 to 250 µg/mL and 31.25 to 250 µg/mL, respectively. FICI analysis revealed strain- and antibiotic-dependent interactions, ranging from synergistic to antagonistic effects. Variable antibiofilm activity was observed, with the strongest inhibition against Brucella melitensis and the weakest against Pseudomonas aeruginosa. No significant differences in MIC or MBC values were observed between bacterial groups (p > 0.05). Conclusions: L. orientalis essential oil demonstrated in vitro antibacterial activity against several clinically relevant pathogens, with particularly notable activity against B. melitensis. The essential oil also showed synergistic interactions with selected antibiotics and variable antibiofilm effects. These findings suggest that L. orientalis essential oil may have potential as an antibacterial agent or adjunct to antibiotic therapy; however, further in vivo and clinical studies are required to confirm its therapeutic applicability.