Evaluation of the Antibacterial Activity of Gentamicin in Combination with Essential Oils Isolated from Different Cultivars and Morphological Parts of Lavender (Lavandula angustifolia Mill.) against Selected Bacterial Strains

The aim of the study was to investigate the antibacterial effects of essential oils isolated from different cultivars and morphological parts of lavender (Lavandula angustifolia Mill.) in combination with the aminoglycoside antibiotic gentamicin. This in vitro study analyzed the effectiveness of the combinations of gentamicin and lavender essential oils against the following strains: Staphylococcus aureus ATCC 25923, Staphylococcus aureus MRSA and Pseudomonas aeruginosa ATCC 9027. The effect of the combination of lavender oils with gentamicin was tested using the checkerboard method. A synergistic effect against S. aureus strain ATCC 25923 was found when gentamicin was combined with lavender essential oils isolated from flowers and leafy stalks (flowers: ‘Blue River’ FICI—0.192; ‘Ellagance Purple’ FICI—0.288; leafy stalks: ‘Blue River’ FICI—0.192; ‘Ellagance Purple’ FICI—0.320). A synergistic effect was also observed for the combination of gentamicin with lavender essential oils from flowers against the resistant strain of S. aureus (MRSA) (‘Blue River’ FICI—0,191; ‘Ellagance Purple’ FICI—0.263), as well as for the essential oils from leafy stalks (‘Blue River’ FICI—0.076; ‘Ellagance Purple’ FICI—0.089). No interaction was observed for the combination of studied essential oils with gentamicin against P. aeruginosa strain ATCC 9027 (FICI = 1.083–1.300).