Synergistic antibacterial potential of the Philippine oregano (Coleus aromaticus Benth.) essential oil with tetracycline against Aeromonas species

The rise in antibiotic resistance among pathogenic bacterial isolates like Aeromonas species has been linked to the continued use of antibiotics for treatment in aquaculture. This study investigated the antibacterial potential of the Philippine oregano (Coleus aromaticus Benth.) essential oil and its synergy with tetracycline. Minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and agar-well diffusion assays against Aeromonas sobria and Aeromonas hydrophila were evaluated, and synergism potential was assessed using fractional inhibitory/bactericidal concentration (FIC/FBC). Results showed no significant difference in the zone of inhibition between tetracycline and the combined essential oil extract with tetracycline, suggesting similar antibacterial activity, while there was a significant difference in the zone of inhibition between tetracycline and essential oil alone. The combined essential oil with tetracycline exhibited an additive, synergistic activity against A. sobria and indifferent-additive activity against A. hydrophila. These findings suggest that Philippine oregano (Coleus aromaticus Benth.) essential oil is not an effective treatment for Aeromonas infection as monotherapy but as an adjunct to tetracycline. Keywords: antibiotics, antibacterial synergism, Aeromonas species, Philippine oregano, zone of inhibition