Agents, compositions, methods and kits useful for the treatment and diagnosis of Staphylococcal intramammary infection are disclosed. The agents, compositions, methods and kits are derived from genes expressed during Staphylococcal intramammary infection, and more particularly genes SACOL0029, SACOL0264, SACOL0442, SACOL0718, SACOL0720, SACOL1353, SACOL1416, SACOL1611, SACOL1944, SACOL2144, SACOL2365 or SACOL2599, based on the gene nomenclature from the Staphylococcus aureus COL (SACOL) genome.