Epidemiological analysis of imipenem-resistant Serratia marcescens in hospitalized patients

Our objective was to examine epidemiological characteristics of hospitalize d patients with imipenem-resistant Serratia marcescens. We performed a case -control study using data collected from computerized databases and chart r eview. Molecular typing by pulsed field gel electrophoresis of available is olates was performed. One hundred and ten patients had Serratia spp isolate d during the 23-month study period. Twelve were infected or colonized with S. marcescens resistant or of intermediate susceptibility to imipenem. Elev en of the 12 patients were detected during a seven-month period between Aug ust 1994 and February 1995, suggesting the possible occurrence of an outbre ak. However, the patients were admitted to different wards and services and , in eight patients, imipenem- resistant S. marcescens were isolated within 48 h of admission. None of the patients had epidemiological links within o ther institutions. The 12 cases were not more likely to have been exposed t o p-lactam antibiotics, including imipenem, than patients with imipenem-sus ceptible isolates. Six isolates were available for typing by PFGE; three we re indistinguishable or closely related whereas each of the other three iso lates were unique. In conclusion both the prevalence of imipenem-resistant S. marcescens and its unusual epidemiologic characteristics warrant further study.