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From 16 July through 27 September 1988, seven cases of nosocomial Serratia marcescens bacteremia occurred in a cardiac care unit. In all seven case patients, S. marcescens was isolated from blood cultures. Two of the seven had other microorganisms identified in the blood culture in which S. marcescens was recovered; one had Enterobacter cloacae, and one had Klebsiella pneumoniae. A case-control study was conducted to identify risk factors for bloodstream infection. Case patients were more likely than controls to have been exposed to an intra-aortic balloon pump pressure transducer (7 of 7 versus 6 of 21; P = 0.001) and to a pulmonary arterial pressure transducer (7 of 7 versus 8 of 21; P = 0.005). Cultures of in-use and in-storage transducers revealed bacterial contamination of the pressure-sensitive membranes of the transducers. S. marcescens blood culture isolates obtained from five of the seven case patients, as well as six S. marcescens isolates from cultured transducers, belonged to serotypes Oundetermined:H1 and Oundetermined:H18. E. cloacae isolates from one case patient and from two stored and two in-use transducers had identical antimicrobial suceptibility patterns. Review of cardiac care unit disinfection practices revealed that the transducers were not processed with high-level disinfection or sterilization between patient uses. We concluded that the transducers had served as reservoirs for this outbreak of bloodstream infection. Because intra-aortic balloon pumps with pressure transducers are being used more frequently in the management of critically ill cardiac patients, their role as infectious reservoirs should be considered in the investigation of nosocomial bacteremia.