Variable-number tandem repeats typing of Mycobacterium tuberculosis isolates with low copy numbers of IS6110 in Thailand.

Spoligotyping and variable-number tandem repeats (VNTR) typing have been increasingly used for differentiating Mycobacterium tuberculosis strains with low copy numbers of IS6110. However, there are few studies comparing their potential to type the strains originating from South and Southeast Asia where many of the isolates have only a few copies, or even single copy, of IS6110. Here, we evaluated the genotyping of 187M. tuberculosis isolates harboring 1-6 copies of IS6110, available from a population-based study in Chiangrai, northern Thailand during 1998-2000, using spoligotyping and VNTR typing. The low-copy-number isolates constituted about 34% of all M. tuberculosis isolated in the province. Discriminating capacities and cluster identification by the two methods were compared with each other and to those obtained by the standard IS6110-restriction fragment length polymorphism (RFLP) method. We found that VNTR typing based on the studied 10-loci set generated more distinct patterns (151 patterns) than spoligotyping (54 patterns) and IS6110-RFLP (65 patterns). Most of the RFLP- or spoligotyping-defined clusters were subdivided by VNTR typing. Combining IS6110-RFLP with VNTR typing produced 164 distinct patterns and 21.9% of clustered isolates whereas the combination of IS6110-RFLP and spoligotyping gave 103 different patterns and 59.4% of clustered isolates. Our results confirm the utility of VNTR typing as the secondary method of choice for investigating the epidemiology of M. tuberculosis with low copy numbers of IS6110.