Editorials Typing of mycobacteria using spoligotyping

The potential use of spoligotyping—a polymerase chain reaction based typing system for diVerentiation of strains of Mycobacterium tuberculosis—for studying the epidemiology of tuberculosis in developing countries which do not routinely perform mycobacterial culture is described in this issue of Thorax by Heyderman et al in a study from Zimbabwe. 1 How does this newer method compare with the now more established molecular typing methods for tuberculosis? The most commonly used typing system for tuberculosis is IS6110 based restriction fragment length polymorphism (RFLP). 2 Extensive experience with IS6110 based RFLP typing has shown that there is a high degree of heterogeneity in typing patterns. This means that the technique is highly discriminatory so that it is unlikely that two organisms will share indistinguishable typing patterns by chance. 3–6 This property has allowed RFLP typing to be used in a variety of ways. For example, typing can be used to confirm outbreaks of tuberculosis 7 8 or incidents of suspected laboratory cross contamination, 9 10 and the technique has been used to investigate the relative importance of reactivation versus reinfec-tion in patients with a second episode of tuberculosis. Studies which type all available isolates from a defined geographical area over a specified time period can help to estimate the importance of recent transmission and to identify risk factors for the spread of tuberculosis. 4–6 There are a number of diYculties with standard IS6110 typing—for example, the technique is less discriminatory in isolates with low copy numbers of IS6110 (secondary typing using techniques based on diVerent genetic markers such as direct repeat (DR) typing or polymorphic GC rich repetitive sequence typing (PGCRS) typing may be needed to diVerentiate such isolates) 13 ; it requires large quantities of DNA so that prolonged culture may be required before typing causing considerable delay before results are available 2 ; it is also relatively labour intensive and thus costly; and the typing patterns produced have varying numbers of bands which may also vary in position so sophisticated software is needed to analyse large numbers of typing patterns and to compare results between laboratories. 14 Spoligotyping has some major advantages over standard IS6110 typing. It is a PCR based technique so minimal quantities of DNA are required. 15 This means that it has the potential to be used directly on clinical specimens without the need for prior culture but, as the authors report, it may be necessary to …