A comparison of three typing methods for Serratia marcescens during an outbreak across four neonatal intensive care units

The aim of this study was to compare pulsed-field gel electrophoresis (PFGE), random amplified polymorphic DNA (RAPD) - polymerase chain reaction (PCR) and automated ribotyping for the typing of Serratia marcescens across four geographically separate neonatal intensive care units. A selection of clinical and screening isolates of S. marcescens were obtained from patients during an outbreak period and tested along with archived specimens from the previous ten years. Sixty three selected isolates of S. marcescens were tested, 34 by PFGE, RAPD-PCR and automated ribotyping (RiboPrinter®) and 29 by PFGE and RAPD only. Of the 34 isolates tested by all three methods, 29 had concordant results. Four of the remaining five isolates had concordant results by RAPD and PFGE, but not by automated ribotyping. Of the 29 isolates tested by RAPD and PFGE only, 27/29 test results were concordant. Typing and discrimination of isolates was achieved with all three molecular methods. The decision as to which method to use will be guided by the level of requirement for timely epidemiological and standardized results balanced against the laboratory costs and availability of labor resources.