Highly diverse variable number tandem repeat loci in the E. coli O157:H7 and O55:H7 genomes for high-resolution molecular typing

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Abstract
Aim: Evaluation of the Escherichia coli genome for variable number tandem repeat (VNTR) loci in order to provide a subtyping tool with greater discrimination and more efficient capacity. Methods and Results: Twenty‐nine putative VNTR loci were identified from the E. coli genomic sequence. Their variability was validated by characterizing the number of repeats at each locus in a set of 56 E. coli O157:H7/HN and O55:H7 isolates. An optimized multiplex assay system was developed to facility high capacity analysis. Locus diversity values ranged from 0·23 to 0·95 while the number of alleles ranged from two to 29. This multiple‐locus VNTR analysis (MLVA) data was used to describe genetic relationships among these isolates and was compared with PFGE (pulse field gel electrophoresis) data from a subset of the same strains. Genetic similarity values were highly correlated between the two approaches, through MLVA was capable of discrimination amongst closely related isolates when PFGE similar values were equal to 1·0. Conclusions: Highly variable VNTR loci exist in the E. coli O157:H7 genome and are excellent estimators of genetic relationships, in particular for closely related isolates. Significance and Impact of the Study: Escherichia coli O157:H7 MLVA offers a complimentary analysis to the more traditional PFGE approach. Application of MLVA to an outbreak cluster could generate superior molecular epidemiology and result in a more effective public health response.