Antimicrobial Activity of Essential Oils of Lamiaceae Aromatic Spices Towards Sheep mastitis-Causing Staphylococcus aureus and Staphylococcus epidermidis

Mastitis in ewes is responsible for massive economic losses. Antibiotics are routinely used for mastitis control but its excessive use leads to development of antibiotic resistance with deleterious outcomes both for animal and public health. Essential oils (EOs) show antibacterial proprieties and no resistance has been reported after prolonged exposure; however their efficacy depends on their chemical composition. In this study EOs chemical composition from four autochthonous aromatic herbs, from Alentejo region, southern Portugal, Calamintha nepeta subsp. nepeta, Lavandula stoechas subsp. luisieri, Rosmarinus officinalis and Thymus mastichina was accessed. EOs of R. officinalis showed predominance in monoterpene hydrocarbons (63 %) whereas EOs of T. mastichina, L. luisieri and C. nepeta were rich in oxygenated monoterpenes (71-95 %). The antimicrobial activity of selected EOs was investigated towards Staphylococcus aureus (n=24) and Staphylococcus epidermidis isolates (n=24) from ovine mastitic milk origin. Results of disk diffusion assay revealed that C. nepeta, L. luisieri and T. mastichina EOs are highly active against both S. aureus and S. epidermidis strains, whereas R. officinalis EO is highly active against S. aureus strains but inactive against several S. epidermidis isolates. EOs concentration causing bacterial growth inhibition ranged from 500 to 4,000 mu g mL(-1) in liquid microassays.