Antibacterial Activity of Essential Oils as Phytotherapeutic Alternatives Against Aeromonas spp. from Ornamental Fish

Aeromonas species are among the most important Gram-negative pathogens in aquaculture and represent a One Health concern, as antibiotic-resistant strains circulate at the interface between ornamental fish, the aquatic environment and humans. This study evaluated the in vitro antibacterial activity of four commercial essential oils from tea tree (Melaleuca alternifolia), peppermint (Mentha piperita), rosemary (Rosmarinus officinalis) and lavandin (Lavandula hybrida) against sixteen oxytetracycline-resistant Aeromonas hydrophila and A. veronii isolated from moribund ornamental fish. All tested bacteria were resistant to oxytetracycline (MIC 8-16 µg/mL). Minimum inhibitory (MIC) and minimum bactericidal (MBC) concentrations were determined by broth microdilution. All EOs inhibited every isolate, but their bactericidal capacity differed markedly. Rosemary oil showed the lowest MIC and MBC ranks and the most consistent values, followed by tea tree and lavandin; all three were bactericidal. Peppermint oil, despite a low MIC, showed a high MBC and was bacteriostatic. These findings indicate that the EOs of rosemary, tea tree and lavandin are promising in vitro candidates for use against oxytetracycline-resistant Aeromonas in aquaculture, in line with the One Health approach, and warrant further evaluation.