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 avail able technique that can establish clonal relationships among many common ho spital-derived organisms with a high degree of accuracy and can yield resul ts in a sufficiently short time to guide interventions during an outbreak i nvestigation. Methods. The CHEF Genomic Bacterial DNA Plug Kit (Bio-Rad) was applied to a n unfolding nursery outbreak of Serratia marcescens infections according to the manufacturer's guidelines. Bacterial genomic DNA was digested with Xba I or SpeI and separated on 1% agarose gels, and the isolates were grouped b y 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 ide ntification of the first eight patients, when closure of the nursery was im minent, revealed that the epidemic was caused by two groups of four isolate s each. In both instances the group was geographically contained, and the n ursery remained open. A second PFGE analysis indicated that a ninth S, marc escens 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 infectio n 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 ap propriate infection control measures had been imposed.