A combination of two genetic markers is sufficient for restriction fragment length polymorphism typing of Mycobacterium tuberculosis complex in areas with a high incidence of tuberculosis.

The incidence of tuberculosis (TB) in Madagascar is 150 cases per 100,000 people. Because of this endemicity, we studied the genetic diversity of Mycobacterium tuberculosis strains isolated in four big cities in 1994 to 1995 with the aim of monitoring TB transmission. Isolates from 316 cases of pulmonary TB (PTM(+)) were typed by Southern hybridization with genetic markers IS6110 and DR. Of the 316 PTM(+) strains, 66 (20.8%) had a single IS6110 band and were differentiated by the DR marker into 33 profiles. Using both markers, 37.7% (119) of the patients were clustered, a proportion similar to that in countries with a high prevalence of TB. There was no significant difference between clustered and nonclustered patients in age, sex, Mycobacterium bovis BCG status, and drug susceptibility of strains. Clustering was significantly greater in the capital, Antananarivo, than in the other cities, suggesting a higher rate of transmission. However, most of the patients in clusters were living in different areas, and, within a distance of 0.7 km, we did not find epidemiologically unrelated strains with the same restriction fragment length polymorphism profile. Despite an apparently low polymorphism, genetic markers such as IS6110 are potentially valuable for monitoring TB transmission. However, the high proportion of Malagasy isolates with a single IS6110 copy makes this marker alone unsuitable for typing. Additional markers such as DR are necessary for the differentiation of the isolates and for epidemiological surveys.