Use of real time pulsed field gel electrophoresis to guide interventions during a nursery outbreak of Serratia marcescens infection

Background. Pulsed field gel electrophoresis (PFGE) is a commercially available technique that can establish clonal relationships among many common hospital-derived organisms with a high degree of accuracy and can yield results in a sufficiently short time to guide interventions during an outbreak investigation. Methods. The CHEF Genomic Bacterial DNA Plug Kit (Bio-Rad) was applied to an unfolding nursery outbreak of Serratia marcescens infections according to the manufacturer's guidelines. Bacterial genomic DNA was digested with XbaI or SpeI and separated on 1% agarose gels, and the isolates were grouped by restriction endonuclease patterns according to established standards. Results. S. marcescens was isolated from nine patients in an intensive care nursery during an 8-week period, Initial PFGE analysis performed after identification of the first eight patients, when closure of the nursery was imminent, revealed that the epidemic was caused by two groups of four isolates each. In both instances the group was geographically contained, and the nursery remained open. A second PFGE analysis indicated that a ninth S, marcescens isolate, recovered in Week 8, was genetically unrelated to the other two. Surveillance during an additional 6 weeks revealed no new cases, and the epidemic was declared over. No cases of invasive S. marcescens infection were identified during the subsequent 10 months. Conclusion. Real time PFGE determined that an apparent nursery outbreak of S, marcescens infection was, in fact, caused by three genetically distinct strains. This information allowed the nursery to remain open after other appropriate infection control measures had been imposed.