The genes encoding the virulence factors alpha-hemolysin (hla), aureolysin (aur) and a glutamyl endopeptidase (sspA) as well as the virulence regulators SarR (sarR) and RNAIII were expressed to a lesser extent by S. aureus infecting the more susceptible 4-APP-treated or MyD88-deficient C57BL/6 mice than during infection of normal C57BL/6 mice. The gene discussed is APP; the disease is infection.