THE EFFECT OF TN916 INSERTIONS ON CONTOUR-CLAMPED HOMOGENEOUS ELECTROPHORESIS PATTERNS OF ENTEROCOCCUS-FAECALIS

Contour-clamped homogeneous gel electrophoresis has increasingly been used and has generally been considered the method of choice for the de lineation of enterococcial strains. It has been suggested that the con tour-clamped homogeneous electric field (CHEF) electrophoresis digesti on patterns of genomic DNA indicate the relatedness of strains when Sm aI patterns differ by six or fewer bands. To evaluate the potential re asons for the diversity of bands among clonally related isolates, we s tudied plasmid-free Enterococcus faecalis FA2-2 and derivatives with t ransposon Tn916 (encoding for tetracycline resistance) insertions on t he chromosome. We obtained derivatives with seven different Tn916 inse rtion sites and up to two copies of Tn916 on the chromosome, resulting in differences of one to seven bands by CHEF electrophoresis; eight d ifferent patterns were observed. With Tn916 insertions, there was eith er (i) loss of a band(s) with the generation of a new band(s), (ii) th e generation of a new band(s) only, or (iii) the loss of a band(s) wit h no new visible band(s). These results indicate that strains can have up to seven different SmaI bands by CHEF electrophoresis and still be closely related.