Ocimum Tenuiflorum L. As a Potential Antibiotic Adjuvant: Comparative in Vitro Evaluation of Antibacterial and Antibiotic Potentiating Activity

Background: Antimicrobial resistance has intensified the need for strategies that improve the activity of existing antibiotics. Medicinal plants contain chemically diverse metabolites that may exert antibacterial effects and modify bacterial susceptibility to conventional antimicrobial agents. Ocimum tenuiflorum L. (Tulsi) is rich in phenolics, flavonoids, phenylpropanoids, and terpenoid constituents and has demonstrated antimicrobial activity. Methods: Cold aqueous and ethanolic extracts of O. tenuiflorum leaves were prepared and subjected to preliminary phytochemical screening and liquid chromatography-mass spectrometry (LC-MS) profiling. Antibacterial activity was evaluated by agar disc diffusion against Escherichia coli, Staphylococcus epidermidis, and Pseudomonas aeruginosa at concentrations ranging from 6.25 to 100 mg/mL. Amoxicillin-clavulanic acid, gentamicin, and ciprofloxacin were tested alone and in combination with the extracts. Experiments were performed in triplicate, and results were expressed as mean ± standard deviation. Percentage enhancement was calculated relative to the antibiotic-alone zone of inhibition. Results: Both extracts demonstrated concentration-dependent antibacterial activity, with the ethanolic extract consistently producing larger inhibition zones. At 100 mg/mL, the ethanolic extract produced zones of 18.67 ± 0.58, 21.33 ± 0.58, and 15.67 ± 0.58 mm against E. coli, S. epidermidis, and P. aeruginosa, respectively. The corresponding aqueous-extract zones were 13.67 ± 0.58, 16.33 ± 0.58, and 11.33 ± 0.58 mm. Combination testing showed increased inhibition with all three antibiotics. With ethanolic extract, the greatest percentage enhancement was observed for amoxicillin-clavulanic acid: 42.9% against E. coli, 38.7% against S. epidermidis, and 48.4% against P. aeruginosa. LC-MS profiling indicated a diverse phytochemical composition, including putative phenolic acids, flavonoids, phenylpropanoids, and triterpenoids. Conclusion: O. tenuiflorum demonstrated appreciable in vitro antibacterial activity and enhanced the inhibitory activity of selected antibiotics in disc-diffusion screening, with the ethanolic extract showing greater activity than the aqueous extract. The findings support further investigation of O. tenuiflorum as a potential plant-derived antibiotic adjuvant. However, the observed enhancement should be considered preliminary because disc diffusion alone cannot establish pharmacological synergy.