The genomic relationship between isolates representing 17 definitive phage types (DTs) of subsp. serotype were analysed using three different typing methods: IS200 typing using the restriction enzymes RI and ll, ribotyping using and RI, and PFGE using These methods were used to study four DTs in greater detail; in all 18 (DT 49), 10 (DT 110), five (DT 120) and seven (DT 135) isolates were studied. The combined data generated two large clusters, which could be divided into five groups. Within the first cluster, a close similarity was indicated between isolates of the following phage types: group A — DTs 44, 49, 135 and 204c, with DT 9 distantly related; group B — DTs 95 and 99; and group C — DTs 104a, 110 and 120. The other large cluster contained group D — DTs 10, 20 and 146, with DT 12 distantly related, and group E — DTs 69, 103 and 153. The same grouping was observed by principal component analysis, but a minimum spanning tree linked DT 12 to group E and not group D in this analysis. Among the typing methods used, IS gave the best representation of the overall similarity between the isolates. Five different IS profiles were obtained among isolates belonging to DT 49. Only one profile was observed within each of the phage types DT 110, 120 and 135. All isolates within each of these four phage types were of one ribotype. Isolates of DT 49 showed four PFGE patterns, while one pattern was present within isolates of the three other phage types. Members of these four phage types were found to be clonally related as they formed tight subclusters separated from isolates of other phage types.


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