Proposal for standardization of optimized mycobacterial interspersed repetitive unit-variable-number tandem repeat typing of Mycobacterium tuberculosis

Molecular typing based on 12 loci containing variable numbers of tandem repeats of mycobacterial interspersed repetitive units (MIRU-VNTRs) has been adopted in combination with spoligotyping as the basis for large-scale, high-throughput genotyping of Mycobacterium tuberculosis. However, even the combination of these two methods is still less discriminatory than IS6110 fingerprinting. Here, we define an optimized set of MIRU-VNTR loci with a significantly higher discriminatory power. The resolution and the stability/robustness of 29 loci were analyzed, using a total of 824 tubercle bacillus isolates, including representatives of the main lineages identified worldwide so far. Five loci were excluded for lack of robustness and/or stability in serial isolates or isolates from epidemiologically linked patients. The use of the 24 remaining loci increased the number of types by 40%-and by 23% in combination with spoligotyping-among isolates from cosmopolitan origins, compared to those obtained with the original set of 12 loci. Consequently, the clustering rate was decreased by fourfold-by threefold in combination with spoligotyping-under the same conditions. A discriminatory subset of 15 loci with the highest evolutionary rates was then defined that concentrated 96% of the total resolution obtained with the full 24-locus set. Its predictive value for evaluating M. tuberculosis transmission was found to be equal to that of IS6110 restriction fragment length polymorphism typing, as shown in a companion population-based study. This 15-locus system is therefore proposed as the new standard for routine epidemiological discrimination of M. tuberculosis isolates and the 24-locus system as a high-resolution tool for phylogenetic studies.