Molecular differentiation of Mycobacterium bovis isolates. Review of main techniques and applications.

Until recently, none of the Mycobacterium bovis typing techniques permitted a satisfactory differentiation of isolates. During the last 10 years, the genome of pathogenic mycobacteria has been extensively studied, and phylogenetic analyses have shown that all (except Mycobacterium avium) belong to a single genetic species: the Mycobacterium tuberculosis complex. This increase in knowledge about the genome of these bacteria has lead to the discovery of molecular markers that allow us to differentiate isolates. Because of the phylogenetic proximity of the strains, even if most of these markers have been discovered in M. tuberculosis, they could be successfully adapted to the other bacteria of the M. tuberculosis complex, especially M. bovis. The most common markers in use today are the IS6110 insertion sequence, the direct repeat (DR) region, the poly(GC) rich (PGRS) sequences and the variable number tandem repeats (VNTR) sequences. The corresponding typing techniques are briefly described, and current knowledge of polymorphism and marker stability is detailed. If molecular markers are to offer wide perspectives for field studies, these two characteristics (polymorphism and stability) must be taken into account when choosing the marker(s) used in a study. In this context, examples of the application of molecular typing techniques for M. bovis are reviewed, on the one hand with epidemiological studies for which the major problem is the comparison between isolates and, on the other, with more general studies about the population genetics of M. bovis in a given country, and about its history and its phylogeny.