the amount of oxygen or lack thereof (redox state) and thepH and salt concentration, as well as to scavenge essentialminerals and harvest and metabolize the available nutrients,determines the numbers and nature of the species that popu-late a site of the body. Anaerobic or facultative anaerobicbacteria colonize most of the sites of the body because of thelack of oxygen in sites such as the mouth, intestine, andgenitourinary tract.