Temporal changes in pulsed-field gel electrophoresis banding in vancomycin-resistant Enterococcus faecium and implications for outbreak investigations

Background: Patients are often screened with surveillance cultures to discern transmissions vs transformation of an isolate to vancomycin-resistant Enterococcus faecium. To determine the amount of time between which isolates could be considered genetically similar by pulsed-field gel electrophoresis, isolate change over time within single patients was studied. Methods: A minimum of 4 isolates per patient, separated by at least 2 months, were collected from previously frozen stores. Visual comparison of banding patterns was conducted, and percent relatedness was calculated. Results: Twenty-eight isolates from 6 patients were studied. No isolate differed by more than 3 bands before 150 days, and the average percent difference per band was 3.7%. The isolates diverged genetically as a linear function of number of bands over time (good model fit intrapatient r² = 0.42; poor model fit interpatient r² = 0.0062). Conclusion: Trajectory of genetic variation appears to be isolate/patient specific; however, commonalities exist and tested isolates were relatively stable out to 150 days.