Temporal changes in pulsed-field gel electrophoresis banding in vancomycin-resistantEnterococcus faeciumand implications for outbreak investigations

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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.


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.


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).


Trajectory of genetic variation appears to be isolate/patient specific; however, commonalities exist and tested isolates were relatively stable out to 150 days.

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