Guide to the Application of Genotyping to Tuberculosis Prevention and Control
CDC Tuberculosis Genotyping Laboratory Procedures
Science Behind TB Genotyping
Genotyping is based on an analysis of DNA. Mycobacteria reproduce by binary fission, which means that in almost all cases each new bacillus has identical DNA, just as human identical twins are genetically identical to each other. However, changes in the DNA occur spontaneously at low frequency. Over time, these changes, known as DNA mutations, have accumulated to produce the diversity of M. tuberculosis strains currently circulating in the world.
The diversity of strains provides a means to identify instances of recent transmission of TB as well as the chains of transmission that occur among persons with TB. This diversity also helps to elucidate the patterns and dynamics of TB transmission. When a person with TB improves but then becomes ill again, this diversity can differentiate reactivation with the same strain of M. tuberculosis from reinfection with a different strain. Genotyping can also be used to identify false-positive cultures.
Advances in DNA analytic methods have made it possible for TB programs to obtain rapid and reliable genotyping results. These advances include
- the determination of the complete DNA sequence of M. tuberculosis in 1998;
- the development of IS6110-based RFLP genotyping, which provided a discriminatory typing method and led to a standardized system for genotyping M. tuberculosis isolates; and
- the development of two new methods, spoligotyping and MIRU analysis, which are based on PCR and provide much more rapid results than RFLP analysis.