Frequency of rpoB mutations inside and outside the cluster I region in rifampin-resistant clinical Mycobacterium tuberculosis isolates

The prevalence of recently described mutation V176F, located in the beginni ng of the rpoB gene and associated with rifampin resistance and the wild-ty pe cluster I sequence, was determined by analyzing the distribution of rpoB mutations among 80 rifampin (RIF)-resistant Mycobacterium tuberculosis str ains isolated in Germany during 1997. The most frequent rpoB mutations were changes in codon 456 (52 isolates, 65%), followed by changes in codon 441 (13 isolates, 16%) and codon 451 (11 isolates, 14%). The V176F mutation was detected in one isolate of the study population and in 5 of 18 RIF-resista nt strains with no cluster I mutation from six previously published studies . In three isolates, a mixture of resistant and susceptible subpopulations (heteroresistance) prohibited the detection of rpoB mutations in the initia l analysis; however, in these isolates, cluster I mutations could be verifi ed after a passage on RIF-containing medium, IS6110 DNA fingerprinting of 7 6 strains revealed eight clusters comprising 27 strains with identical rest riction fragment length polymorphism patterns that mainly also show identic al rpoB mutations and identical or similar drug resistance patterns. In con clusion, our results indicate that the V176F mutation should be included in molecular tests for prediction of RIF resistance in M. tuberculosis. We fu rther demonstrated that heteroresistance caused by a mixture of mycobacteri al subpopulations with different susceptibilities to RIF may influence the sensitivity of molecular tests for detection of resistance.