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Unformatted text preview: The epidemiology of antibiotic resistance in hospitals: Paradoxes and prescriptions Marc Lipsitch* , Carl T. Bergstrom*, and Bruce R. Levin* *Department of Biology, Emory University, 1510 Clifton Road, Atlanta, GA 30322; and Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115 Edited by Robert May, University of Oxford, Oxford, United Kingdom, and approved December 17, 1999 (received for review August 23, 1999) A simple mathematical model of bacterial transmission within a hospital was used to study the effects of measures to control nosocomial transmission of bacteria and reduce antimicrobial re- sistance in nosocomial pathogens. The model predicts that: ( i ) Use of an antibiotic for which resistance is not yet present in a hospital will be positively associated at the individual level (odds ratio) with carriage of bacteria resistant to other antibiotics, but negatively associated at the population level (prevalence). Thus inferences from individual risk factors can yield misleading conclusions about the effect of antibiotic use on resistance to another antibiotic. ( ii ) Nonspecific interventions that reduce transmission of all bacteria within a hospital will disproportionately reduce the prevalence of colonization with resistant bacteria. ( iii ) Changes in the prevalence of resistance after a successful intervention will occur on a time scale of weeks to months, considerably faster than in community- acquired infections. Moreover, resistance can decline rapidly in a hospital even if it does not carry a fitness cost. The predictions of the model are compared with those of other models and published data. The implications for resistance control and study design are discussed, along with the limitations and assumptions of the model. A ntimicrobial resistance is a growing problem in many bacterial pathogens and is of particular concern for hospital-acquired (nosocomial) infections (1, 2). Frequently, the bacteria responsible for nosocomial infections are members of the normal, usually commensal bacterial flora that become pathogenic when they multiply in normally sterile sites, such as the lower respiratory tract or the blood. Thus, hospital-acquired infections often involve transmission from patients who carry the bacteria asymptomatically (or symptomatically) to other patients, some of whom then may become infected with the colonizing strain (3). A number of interventions have been proposed and tried to limit nosocomial infections and particularly to stem the spread of antibiotic-resistant bacteria. Some of these measures, such as hand washing and barrier precautions, are designed to reduce overall transmission of bacteria within the hospital (4, 5). In some cases, hospitals have increased the prophylactic use of drugs other than those for which resistance is currently a problem, to prevent or terminate colonization with the resistant bacteria (6, 7). Another strategy has been cyclingresistant bacteria (6, 7)....
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