Oregano oil, carvacrol, and thymol as novel ophthalmic antimicrobials: ocular safety, phenolic interactions, and antibacterial efficacy against clinically relevant ocular pathogens

Bacterial ocular infections are a major cause of vision loss, with antimicrobial resistance limiting treatment efficacy. This study investigated oregano oil and its phenolic components, carvacrol and thymol, as alternative antibacterial agents for ophthalmic use. Gas chromatography-mass spectrometry analysis identified 27 volatile compounds, dominated by carvacrol (~ 75%), O-cymene (~ 7%), and thymol (~ 3%). Antibacterial activity against Neisseria gonorrhoeae, Staphylococcus aureus, and Pseudomonas aeruginosa was assessed using disc diffusion and broth microdilution assays. Minimum inhibitory and bactericidal concentrations (MICs/MBCs) ranged from 0.003 to 0.1% (v/v) for oregano oil and carvacrol, and from 0.003 to 0.1% (w/v) for thymol. Fractional inhibitory concentration (FIC) analysis revealed additive antibacterial interactions (FICI > 0.5-1) between carvacrol and thymol, suggesting these phenolic constituents could serve as adjunct antibacterial agents. Ocular safety was evaluated using the bovine corneal opacity and permeability (BCOP), hen's egg chorioallantoic membrane (HET-CAM), and red blood cell haemolysis assays. At concentrations exceeding MICs (0.5% v/v oregano oil, 0.1% v/v carvacrol, 0.1% w/v thymol), no corneal irritation occurred in the BCOP model, although mild to moderate irritation appeared in conjunctival and membrane assays. Safety index analysis showed oregano oil had the widest safety margin, supporting its potential as an antibacterial for ocular and wider use.