Antimicrobial in vitro effects of eight essential oils on Pasteurella multocida and Mannheimia haemolytica from nasopharyngeal swab samples of fattening calves

Essential oils are secondary metabolites of aromatic plants and are used in phytotherapy to treat various diseases. In the present study, eight selected essential oils - ajwain oil (Trachyspermum ammi L.), fennel oil (Foeniculum vulgare Mill. subsp. vulgare var. vulgare), thyme oil chemotype (ct.) thymol (Thymus vulgaris L.), tea tree oil (Melaleuca alternifolia Cheel.), oregano oil (Origanum vulgare L.), mountain savory oil (Satureja montana L.), lemongrass oil (Cymbopogon citratus (DC.) Stapf.) and eucalyptus oil (Eucalyptus globulus Labill.) -were examined for their antibacterial effect against Pasteurella (P.) multocida and Mannheimia (M.) haemolytica isolates from deep nasopharyngeal swab samples of fattening calves using agar diffusion and microdilution. All eight essential oils were effective against the tested isolates. Lemongrass oil proved to be the most potent of all eight essential oils, while fennel oil was only weakly effective. Different antimicrobial effects were observed between the two research methods. The effectiveness of ajwain, thyme, oregano and mountain savory oils was comparable in agar diffusion. However, this could not be reproduced using the microdilution method. P. multocida was found to be more sensitive to all essential oils tested than M. haemolytica. This study shows that the tested essential oils have antimicrobial in-vitro effects on P. multocida and M. haemolytica isolates and that the examination method is associated with the test result.