Bacteriological and molecular analysis of rifampin and isoniazid resistant strains

To develop a better understanding of the epidemiology and molecular biology of rifampin-resistant Mycobacterium tuberculosis strains in Australia, 50 clinical isolates (33 rifampin-resistant and 17 rifampin-sensitive strains) cultured between 1990 and 1997 were analyzed by a number of bacteriological and molecular techniques. Examination of the drug resistance profiles of the 33 rifampin-resistant isolates revealed that 91% were resistant to rifampin in combination with resistance to isoniazid, 88% were resistant to rifampin on first isolation, and 81% showed cross-resistance with rifabutin. On the basis of the demographic data provided for the patients infected with the rifampin-resistant strains, 90% of the patients were born overseas. Of these patients, 64% developed clinical symptoms within 5 years of residence in Australia. On a molecular level, analysis of the rpoB gene revealed that 97% of the rifampin-resistant isolates had missense mutations within a conserved region of the gene, and eight types of missense mutations were detected. Of the 31 rifampin-resistant isolates that were typed by restriction fragment length polymorphism (RFLP) analysis, 28 distinct patterns were obtained by RFLP analysis with IS6110, and three clusters of genetically related isolates were identified. All isolates within the clusters were from patients who were born overseas and who had the same country of origin. The results from this study provide an overview of the current situation of rifampin resistance in Australia and can serve as a basis for continued monitoring of drug-resistant M. tuberculosis strains isolated within the country.