Minimum Spanning Tree. Minimum spanning tree of the Multi-locus variable number of tandem repeat analysis (MLVA) of the fifty-four Escherichia coli O157 isolates. The numbering shown within each circle denotes the pen number from where each isolate was recovered. Each isolate is colour coded according to their corresponding MLVA type. The colour coding is identified by the key accompanying this figure (see also Table 1). The numbering shown between the circles represents the distance between each node.