Synergistic Effect of Antibacterial and Antibiofilm Activity of Oldenlandia corymbosa Combined with Oxytetracycline Against Drug-Resistant Bacteria

Abstract

This study evaluated the antioxidant, antibacterial, synergistic, and antibiofilm activities of Oldenlandia corymbosa, a Thai medicinal plant known for hepatoprotective and anti-breast cancer properties. Antibacterial activity and synergy were assessed using broth microdilution and checkerboard assays, with interactions interpreted by fractional inhibitory concentration index (FICI). Antibiofilm activity was determined using the crystal violet assay.  The ethanol extract from aerial parts of O. corymbosa exhibited strong antioxidant activity, with a Ferric Reducing Antioxidant Power (FRAP) value of 5.98 ± 0.12 mg FeSO₄/g extract at 0.1 mg/mL and a DPPH IC₅₀ of 0.19 mg/mL. The extract showed antibacterial activity against 10 of 11 opportunistic bacterial strains, with minimum inhibitory concentrations (MICs) ranging from 20 to 80 mg/mL, except for Proteus mirabilis, which was resistant.  In combination studies, O. corymbosa extract demonstrated strong synergistic effects with Oxytetracycline against Staphylococcus aureus (FICI = 0.17).  However, no effect was observed against drug-resistant Acinetobacter baumannii or Pseudomonas aeruginosa. Combination with Ampicillin was largely ineffective, except for S. aureus (FICI = 1.016 as an additive effect). The extract significantly inhibited biofilm formation in both standard and drug-resistant strains, including Escherichia coli ATCC 25922, MRSA, P. aeruginosa, and A. baumannii, with inhibition ranging from 64.15% to 75.86% over 4–24 hours. Notably, combinations with Oxytetracycline or Norfloxacin enhanced antibiofilm activity in resistant strains, achieving approximately 68.23%–75.90% inhibition within 8–12 hours. These findings highlight O. corymbosa as a promising multifunctional antimicrobial and antibiofilm agent.