Optimization of variable number tandem repeat typing set for differentiating Mycobacterium tuberculosis strains in the Beijing family

Variable number tandem repeat (VNTR) typing of Mycobacterium tuberculosis, as presently used, has often failed to differentiate clonal strains especially in areas where the Beijing family genotype is predominant. We have evaluated mycobacterial interspersed repetitive units (MIRUs), Queen's University of Belfast (QUB) loci and exact tandem repeat (ETR) loci individually for their abilities to differentiate the Beijing and non-Beijing genotype families of M. tuberculosis. The best locus was QUB 3232 followed by QUB 11b. Available MIRU, QUB and ETR loci were then consecutively arranged in declining order of discriminatory power, and combined to form an optimal set of 12-loci VNTR typing (MIRU 10, 26, 39, 40; QUB 11a, 11b, 15, 18, 26, 3232, 3336; ETR A) that possessed comparable discriminatory ability to IS6110 restriction fragment length polymorphism. This optimized 12-loci VNTR typing set can become an important tool for tracking M. tuberculosis of the Beijing family.