Primitive ancestors of Homo sapiens and their colonizing bacteria have coevolved for approximately 500,000 years; some experts estimate that the total number of human cells is 1013 and the total number of colonizing microbes is 1014. Despite this 10-to-1 inequity, the balance of power is influenced by an intact human immune system and the integrity of the skin and mucous membranes. Operative procedures disrupt this balance, resulting in a risk of surgical-site infections from endogenous flora, including colonizing strains of Staphylococcus aureus. Each year in the United States, more than 30 million operative procedures are performed. The risk . . .