In vitro antimicrobial effect of essential tea tree oil(Melaleuca alternifolia), thymol, and carvacrol on microorganisms isolated from cases of bovine clinical mastitis

Both Gram-negative and Gram-positive bacteria have recently developed antibiotic resistance to treatments for bovine mastitis, creating a serious concern for public and animal health. The objective of this study was to analyse in vitro microbicidal activity of tea tree oil, thymol and carvacrol (composed of oregano and thyme essential oils) on bacteria isolated from clinical mastitis. Field isolates and ATCC strains of the Staphylococcus spp, Streptococcus spp, Escherichia coli, Klebsiella pneumoniae, and Candida albicans genera were analysed. The agar diffusion technique was used to test bactericidal susceptibility and plate microdilution was utilized to determine the minimum inhibitory, bactericidal, and fractional inhibitory concentrations. Thymol alone and the combinations of thymol-carvacrol and thymol-TTO obtained the highest inhibition diameters for Gram-negative bacteria, while for Gram-positive bacteria and C. albicans, thymol and the combination thymol-carvacrol obtained the highest indices. TTO, thymol, and carvacrol had MIC values of 1.56-25 mg/ml, 0.05-0.4 mg/ml, and 0.02-0.2 mg/ml, respectively. CMB results for the Gram-negative and gram-positive groups were 0.39-0.78 mg/ml, and for C. albicans, 0.78-1.56 mg/ml. Results for the fractional inhibitory concentrations show that the TTO+thymol and thymol+carvacrol combinations had additive activity against groups of Gram-negative bacteria and C. albicans. These natural components, evaluated individually and in combinations, have an effectiveness above 70%.