Humans exist in an environment replete with microorganisms, but only a few become resident on the skin surface. The skin possesses protective mechanisms to limit colonization, and the survival of organisms on the surface lies in part in the ability of the organisms to resist these mechanisms. Microbial colonization on the skin adds to the skin's defense against potentially pathogenic organisms. Although microbes normally live in synergy with their hosts, occasionally colonization can result in clinical infection. Common infections consist of superficial infections of the stratum corneum or appendageal structures that can respond dramatically to therapy but commonly relapse. In rare circumstances, these infections can be quite severe, particularly in immunocompromised patients or in hospitalized patients with indwelling foreign devices. These infections are often resistant to conventional antibiotics and can result in infection with other opportunistic pathogens.