In Vitro Antibacterial Activity of Commercial Essential Oils Against Lactococcus garvieae

Lactococcosis, caused by Lactococcus garvieae, is a persistent problem of farmed rainbow trout, and the reduced efficacy of conventional antibiotics has prompted interest in plant-derived alternatives. The in vitro antibacterial activity of five commercial essential oils from Melaleuca alternifolia, Mentha piperita, Citrus limon, Rosmarinus officinalis and Lavandula hybrida was evaluated against six L. garvieae isolates from rainbow trout. Minimum inhibitory and minimum bactericidal concentrations (MIC and MBC) were determined by broth microdilution, and the mode of action was defined by the MBC/MIC ratio; susceptibility to oxytetracycline (OTC) and trimethoprim-sulfamethoxazole (SXT) was also assessed. The major constituents were terpinen-4-ol (tea tree), menthol (peppermint), limonene (lemon), 1,8-cineole (rosemary) and linalool (lavandin). Activities were strain-dependent, with MIC values mostly from 6.25 to 100% (v/v) (single lowest, 1.5625% for rosemary), and lemon was among the least active, but the Kruskal-Wallis test showed no significant differences among oils (p > 0.05), so these differences are descriptive only. Where effective, the oils acted mainly through a bactericidal mechanism (MBC/MIC ≤ 4), but only at high concentrations, so their practical significance appears limited. All six isolates were resistant to OTC (MIC 8-16 µg/mL) and SXT (MIC 8-32 µg/mL). In vivo studies are required before practical use.