Multiple-locus variable tandem-repeat analysis (MLVA typing)

MLVA typing is a PCR based typing method. Typing is based on variation in the number of repeats in selected loci of the genome. (VNTR-loci = variable number of tandem repeat loci). The size of the PCR-products of such a locus will vary depending on the number of repeats, i.e. each additional repeat increases the size of the loci with the length of the repeat. In Salmonella Typhimurium, for example, 5 such loci are used for typing and in the human specific Salmonella Typhi, 3 loci are used for typing.

The loci used for typing are predefined based on sequence analysis. Two primers directed against the non-variable part of each of the loci are used to PCR amplify the particular VNTR-locus. One primer is labelled with a specific dye. Products from all VNTR loci used for typing are mixed and the sizes are determined using capillary electrophoresis. A standardized nomenclature has been agreed on for most bacteria that are commonly typed with this method. This enables international communication of types of strains observed in a particular country. For the most commonly typed bacteria, reference strains are available enabling an easy control of the system.