Relative expression of selected genes in mammary tissue challenged with S. aureus. The relative gene expression levels of selected genes were measured by qRT-PCR. Samples were obtained from mammary tissues of three Holstein Friesian cows at peak lactation 16 hours following intramammary infusion with increasing doses of S. aureus into the separate udder quarters. Data for each gene are expressed as relative fold changes compared to expression in the intra-animal control tissue for each animal. The genes tested included: (a) α-S1-casein (CSN1S1) (turquoise diamond) and β-casein (CSN2) (red triangle); (b) Interleukin 8 (IL-8a) (blue square), Interleukin 1β (IL-1βb) (red diamond), Tumor necrosis factor α (TNFαc) (yellow diamond), Interleukin 6 (IL-6d) (green circle) and CD14 antigen (CD14e) (black triangle); (c) S100 calcium-binding protein A12 (S100A12f) (blue diamond) and Pentraxin-3 (PTX3g) (pink square). Expression data for each gene were analysed by ANOVA. The superscript letters are specific for each gene tested and signify significant differences (P < 0.01) between a specific infection dose and the corresponding intramammary control for each cow.