Molecular epidemiologic evaluation of transmissibility and virulence of Mycobacterium tuberculosis

Discovery of genotypic markers associated with increased transmissibility i n Mycobacterium tuberculosis would represent an important step in advancing mycobacterial virulence studies. M. tuberculosis strains may be classified into one of three genotypes on the basis of the presence of specific nucle otide substitutions in codon 463 of the katG gene (katG-463) and codon 95 o f the gyrA gene (gyrA-95). It has previously been reported that two of thes e three genotypes are associated with increased IS110-based clustering, a p otential proxy of virulence. We designed a case-control analysis of U.S.-bo rn patients with tuberculosis in San Francisco, Calif., between 1991 and 19 97 to investigate associations between katG-463 and gyrA-95 genotypes and e pidemiologically determined measures of strain-specific infectivity and pat hogenicity and IS6110-based clustering status. We used a new class of molec ular probes called molecular beacons to genotype the isolates rapidly. Infe ctivity was defined as the propensity of isolates to cause tuberculin skin test conversions among named contacts, and pathogenicity was defined as the ir propensity to cause active disease among named contacts. The molecular b eacon assay was a simple and reproducible method for the detection of known single nucleotide polymorphisms in large numbers of clinical M. tuberculos is isolates. The results showed that no genotype of the katG-463- and gyrA- 95-based classification system was associated with increased infectivity an d pathogenicity or with increased IS6110-based clustering in San Francisco during the study period. We speculate that molecular epidemiologic studies investigating clinically relevant outcomes may contribute to the knowledge of the significance of laboratory-derived virulence factors in the propagat ion of tuberculosis in human communities.