The efficiency of pulsed-field gel electrophoresis (PFGE), ribotyping and restriction enzyme analysis of the virulence plasmid (REAP) for typing and subtyping strains of was compared. All three techniques gave concordant results, and the strains studied could be separated into three distinct clusters: (1) heterogeneous strains of biotype 1A and serotype O5 (1A/O5); (2) one 3/O3 strain and all 2/O9 strains; and (3) all 4/O3, 2/O5 and two 3/O3 strains. Within cluster 3, the 2/O5 and 3/O3 strains were related more closely to each other than to the 4/O3 isolates. With ribotyping, PFGE and phage-typing, the 4/O3 isolates were subdivided into two homogeneous groups, corresponding to strains of phage type IX and strains of other phage types, respectively. Similarly, ribotyping and PFGE subdivided the 2/O9 strains into two conserved groups (I and II), but REAP gave a different subdivision and identified a new REAP pattern (P3). The three techniques confirmed the clear distinction between the heterogeneous group of non-pathogenic 1A/O5 strains and the well conserved group of pathogenic 2/O5 strains. Additional plasmids were identified in some 3/O3 strains. Combined, the results indicated that REAP (with RI) and ribotyping (with RV) are valuable alternatives to bioserotype determination, and PFGE is the most suitable technique for epidemiological tracing.


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