Using pulsed-field gel electrophoresis in the molecular investigation of an outbreak of Serratia marcescens infection in an intensive care unit

H Alfizah, AJ Nordiah, WS Rozaidi
Correspondence: Alfizah Hanafiah, alfizah@mail.hukm.ukm.my

ABSTRACT
Introduction Serratia marcescens is a well-known cause of nosocomial infections and outbreaks, particularly in immunocompromised patients with severe underlying disease. An outbreak due to S. marcescens infection was detected from 13 to 22 February 2001 at the intensive care unit (ICU) of our institution. We used pulsed-field gel electrophoresis (PFGE) typing to analyse the outbreak strains involved.
Methods A total of 25 isolates were included in this study: 12 isolates from infected patients, nine isolates from insulin solution, one isolate from sedative solution (midazolam and morphine infusion) and one isolate from frusemide solution. Two isolates from other wards which were epidemiologically-unrelated were also included.
Results The S. marcescens from patients, insulin solution and sedative solution showed an identical PFGE fingerprint pattern. The isolate from the frusemide solution had a closely-related PFGE pattern to the outbreak strain with one band difference. Attempts were made in the present study to identify the environmental reservoir of S. marcescens during the outbreak. We found that the insulin and sedative solutions used by the patients were contaminated with S. marcescens which was proven to be the source of the outbreak.
Conclusion Using PFGE, we showed that the outbreak in the ICU of our hospital was due to the clonal spread of a single strain of S. marcescens.

Keywords: intensive care unit, nosocomial infection, pulsed-field gel electrophoresis, Serratia marcescens
Singapore Med J 2004; 45(5): 214-218