In vitro antibacterial activity of four nano-encapsulated herbal essential oils against three bacterial fish pathogens

Bacterial pathogens causing diseases in fishes have broad host ranges, and may cause high mortality or ongoing chronic infections. Pathogen treatments using antibiotics are becoming limited due to increasing concerns for the development of resistance and environmental dissemination of bacteria harboring resistance genes. This study aimed to determine whether nano-emulsions of selected plant-derived essential oils (Origanum vulgare, Eucalyptus globulus, Melaleuca alternifolia and Lavendula angustifolia) were bacteriostatic or bactericidal to Aeromonas hydrophila, Streptococcus iniae and Photobacteriumdamselae subspecies damselae. All treatments showed antibacterial activity, and in almost all cases the activity of the nano-emulsions was superior to their essential oil counterparts. Origanum vulgare (oregano) nano-emulsion had the most effective antibacterial activity, with a minimal inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of 3.12 mu g/ml against all three species; substantially better than tetracycline. The present study highlights the potential of new formulations of essential oils to enhance bacterial disease control in cultured fish.