J. N. Jean-Giles, G. L. George, J. E. LeClerc, T. A. Cebula, Mod-1
Background: In order to evaluate tools for the differentiation of strains of foodborne pathogens, we have applied the multilocus variable-number tandem repeat analysis (MLVA) to a reference collection of diverse isolates of Escherichia coli O157:H7. The method differentiates strains by analysis of the expansion and contraction of short tandem repeats (TRs) in the genome. Since these elements vary in number at several loci, the method discriminates among strains of the same species.
Methods: MLVA analysis using a multiplex of seven targeted loci of TRs (TR1-TR7) was used to compare isolates in a collection of 120 strains of E. coli O157:H7. The MLVA pattern of each strain was determined based on the size (bp) of TRs of each locus to determine the number of repeats. The patterns were categorized using a letter coding system.
Results: From 120 strains of E. coli O157:H7 examined, 89 different TR patterns were determined. The most variable locus analyzed, a tandem repeat of six bp (TR2), showed 30 alleles and as many as 69 repeat elements.
Conclusion: Application of the MLVA analysis to a large set of diverse isolates of E. coli O157:H7 showed the duplication of MLVA patterns in independent strains. While the variability in MLVA patterns showed the genetic diversity present among many of the strains examined, the results demonstrated that additional molecular markers would be required for conclusive strain differentiation. Further evaluation of rapidly evolving loci, such as TR2, is also needed for proper interpretation of MLVA results.