In Vitro Antibacterial Activity of Plant Ethanolic Extracts against Fish Pathogens

Fifteen commercial ethanolic extracts from medicinal plants were in vitro screened for antibacterial activity against Listonella anguillarum (serotypes O1 and O2), Yersinia ruckeri, Photobacterium damselae subsp. piscicida, and Lactococcus garvieae. Their antimicrobial potential was assessed by the disc diffusion assay, then minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were established by the broth microdilution method. The extracts of lavender, Lavandula officinalis; lemon balm, Melissa officinalis; basil, Ocimum basilicum; oregano, Origanum vulgare; rosemary, Rosmarinus officinalis; common sage, Salvia officinalis; and red bilberry, Vaccinium vitis-idaea, showed a broad spectrum of inhibitory effects. The extracts of yarrow, Achillea millefolium; arnica, Arnica montana; marigold, Calendula officinalis; Icelandic lichen, Cetraria islandica; horsetail, Equisetum arvense; grindelia, Grindelia robusta; java tea, Orthosiphon stamineus; and thyme, Thymus vulgaris, were less or not active. P. damselae subsp. piscicida was the most susceptible bacterial strain, while Y. ruckeri was the most resistant. These results can be considered for further investigations aimed to identify novel natural antimicrobial compounds that could be used in aquaculture for the control of bacterial infections.