Niacin is a precursor of coenzymes NAD and NADP and plays a critical role in electron transfer during the metabolic process.In addition to its nutrimental function,niacin has long been used for the treatment of lipid disorders and cardiovascular disease.However, the effect of niacin on Staphylococcus aureus (S.aureus) internalization into bovine mammary epithelial cells (bMEC) remains unclear.Here we sought to examine the effect of niacin on S.aureus internalization into bovine mammary epithelial cells (bMEC) and to investigate the potential mechanism.In this study, the growth of S.aureus supplemented with niacin (0.5-2 mmol/L) was monitored turbidimetrically at 600 nm for 24 h and cell viability was measured by MTT assay.Gentamicin protection assay was carried out to determine the effect of niacin on S.aureus internalization into bMEC.To determine the potential mechanism, tracheal antimicrobial peptide (TAP) and β-defensin (BNBD5) expressions were detected by quantitative Real-time polymerase chain reaction (qRT-PCR).The activation of nuclear factor-kappa B (NF-κB) was determined by Western blotting.The results showed that niacin (0.5-2 mmol/L) did not affect S.aureus growth and bMEC viability, whereas it inhibits S.aureus internalization ranging from 13% to 42% and down-regulated the mRNA expression of TAP and BNBD5 compared to the control group.No exactly relationship was discovered between S.aureus internalization into bMEC and antimicrobial peptide expression, while niacin inhibited S.aureus-induced NF-κB activation in a dose manner.These dates suggest that inhibiting NF-κB activation may be the potential mechanism of niacin on modulating S.aureus internalization into bMEC.