Thymol inhibits Staphylococcus aureus internalization into bovine mammary epithelial cells by inhibiting NF-κB activation

• Staphylococcus aureus growth is inhibited by thymol and thymol has no toxicity to bMEC.
• Internalization of S. aureus into bMEC was reduced by thymol treatment.
• Thymol induces a down-regulation of TAP and BNBD5 gene expression in bMEC.
• NO production by bMEC after S. aureus infection is reduced by thymol.
• Thymol treatment suppresses S. aureus-induced NF-κB activation.

Bovine mastitis is one of the most costly and prevalent diseases in the dairy industry and is characterised by inflammatory and infectious processes. Staphylococcus aureus (S. aureus), a Gram-positive organism, is a frequent cause of subclinical, chronic mastitis. Thymol, a monocyclic monoterpene compound isolated from Thymus vulgaris, has been reported to have antibacterial properties. However, the effect of thymol on S. aureus internalization into bovine mammary epithelial cells (bMEC) has not been investigated. In this study, we evaluated the effect of thymol on S. aureus internalization into bMEC, the expression of tracheal antimicrobial peptide (TAP) and β-defensin (BNBD5), and the inhibition of NF-κB activation in bMEC infected with S. aureus. Our results showed that thymol (16–64 μg/ml) could reduce the internalization of S. aureus into bMEC and down-regulate the mRNA expression of TAP and BNBD5 in bMEC infected with S. aureus. In addition, thymol was found to inhibit S. aureus-induced nitric oxide (NO) production in bMEC and suppress S. aureus-induced NF-κB activation in a dose-dependent manner. In conclusion, these results indicated that thymol inhibits S. aureus internalization into bMEC by inhibiting NF-κB activation.