The impact of changing pathogens of serious infections in hospitalized patients.

New epidemiological patterns are being observed for multidrug-resistant nosocomial organisms. Current problems include the appearance of resistance determinants in organisms that are virulent enough to cause infection in patients with normal host defenses. In addition, multidrug-resistant organisms are spreading from health care to community settings, and organisms from the community are spreading to health care settings. The appearance and spread of resistance can be examined both at a molecular level and on a larger scale involving several pathways. Potential pathways within institutions include the following: introduction of new strains from outside sources (e.g., patients or health care workers from other institutions); exchange of resistance determinants via genetic mutation or transfer of genetic material; emergence or selection of resistant strains following exposure to antimicrobials; and clonal dissemination. Strategies such as multidisciplinary management of infections, appropriate infection control measures, and surveillance of resistance patterns are necessary to address the problem of resistance.