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 r(2) = 0.42; poor model fit interpatient r(2) = 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. Copyright (C) 2013 by the Association for Professionals in Infection Control and Epidemiology, Inc. Published by Elsevier Inc. All rights reserved.