Multiple-locus variable-number tandem-repeat analysis of Salmonella enterica serovar Typhi.

Multilocus variable-number tandem repeats (VNTRs) are widely used as molecular markers to differentiate isolates of homogenous pathogenic clones. We explored the genomes of Salmonella enterica serovar Typhi strains CT18 and Ty2 for potential VNTRs. Among the 43 potential VNTRs screened, 2 were found to be polymorphic. Together with seven polymorphic VNTRs from previous studies, they were used to type 73 global serovar Typhi isolates. A total of 70 multilocus VNTR analysis (MLVA) profiles were found, distinguishing all except three pairs of isolates into individual profiles. The discriminatory power was 0.999. Phylogenetic analysis showed that the MLVA profiles can be divided into seven clusters. However, except for the closely related isolates, the relationships derived were in conflict with those inferred from single nucleotide polymorphism (SNP) typing using 38 SNPs done previously. We concluded that MLVA can resolve the relationships only among closely related isolates. A combination of SNP typing and MLVA typing offers the best approach for local and global epidemiology and the evolutionary analysis of serovar Typhi. We suggest that seven of the nine most polymorphic VNTRs be used as a standardized typing scheme for epidemiological typing.