Large-scale DNA fingerprinting of Mycobacterium tuberculosis strains as a tool for epidemiological studies of tuberculosis

We conducted a large-scale DNA fingerprinting analysis of Mycobacterium tuberculosis strains in a country in which tuberculosis is endemic (Tunisia) in order to evaluate the importance of microepidemics in the maintenance of the disease within the population. The genetic polymorphisms of 201 strains of M. tuberculosis isolated from 196 unrelated patients living in four districts of northern Tunisia during a 3-year period were studied by restriction fragment length polymorphism (RFLP) analysis by using the insertion sequence IS6110 as a probe. Seventy-three strains isolated from 68 patients living in the districts of Tunis, Nabeul, and Jendouba generated 67 different RFLPs, indicating a high degree of polymorphism of the M. tuberculosis strains within these areas. In contrast, the 128 strains isolated from individuals in the district of Menzel Bourguiba appeared much less heterogeneous since they often generated identical or very similar fingerprints. Seventeen of 29 cases (58%) of active tuberculosis in the city of Menzel Bourguiba could be traced to as few as four M. tuberculosis strains. These results indicate the persistence of underestimated microepidemics in this region. The RFLP typing of a large number of randomly collected strains provides a general picture of the strains involved in tuberculosis. The systematic study of limited areas where tuberculosis is endemic can provide evidence for the existence of persisting epidemics. This stresses the different problems which remain to be solved in order to improve the control of tuberculosis.