Search

ABSTRACTElucidation of the molecular basis of isoniazid (INH) resistance in Mycobacterium tuberculosishas led to the development of different genotypic approaches for the rapid detection of INH resistance in clinical isolates. Mutations in katG, in particular the S315T substitution, are responsible for INH resistance in a large proportion of tuberculosis cases. However, the frequency of the katGS315T substitution varies with population samples. In this study, 52 epidemiologically unrelated clinical INH-resistant M. tuberculosisisolates collected in Australia were screened for mutations at katGcodon 315 and the fabG1-inhAregulatory region. Importantly, 52 INH-sensitive isolates, selected to reflect the geographic and genotypic diversity of the isolates, were also included for comparison. The katGS315T substitution and fabG1-inhA−15 C-to-T mutation were identified in 34 and 13 of the 52 INH-resistant isolates, respectively, and none of the INH-sensitive isolates. Three novel katGmutations, D117A, M257I, and G491C, were identified in three INH-resistant strains with a wild-type katGcodon 315, fabG1-inhAregulatory region, and inhAstructural gene. When analyzed for possible associations between resistance mechanisms, resistance phenotype, and genotypic groups, it was found that neither the katGS315T nor fabG1-inhA−15 C-to-T mutation clustered with any one genotypic group, but that the −15 C-to-T substitution was associated with isolates with intermediate INH resistance and isolates coresistant to ethionamide. In total, 90.4% of unrelated INH-resistant isolates could be identified by analysis of just two loci: katG315and the fabG1-inhA regulatory region.

ABSTRACTMutations at position 306 of embB(embB306) have been proposed as a marker for ethambutol resistance in Mycobacterium tuberculosis; however, recent reports of embB306 mutations in ethambutol-susceptible isolates caused us to question the biological role of this mutation. We tested 1,020 clinical M. tuberculosisisolates with different drug susceptibility patterns and of different geographical origins for associations between embB306 mutations, drug resistance patterns, and major genetic group. One hundred isolates (10%) contained a mutation in embB306; however, only 55 of these mutants were ethambutol resistant. Mutations in embB306 could not be uniquely associated with any particular type of drug resistance and were found in all three major genetic groups. A striking association was observed between these mutations and resistance to any drug (P< 0.001), and the association between embB306 mutations and resistance to increasing numbers of drugs was highly significant (P< 0.001 for trend). We examined the association between embB306 mutations and IS6110clustering (as a proxy for transmission) among all drug-resistant isolates. Mutations in embB306 were significantly associated with clustering by univariate analysis (odds ratio, 2.44; P= 0.004). In a multivariate model that also included mutations in katG315, katG463, gyrA95, and kasA269, only mutations in embB306 (odds ratio, 2.14; P= 0.008) and katG315 (odds ratio, 1.99; P= 0.015) were found to be independently associated with clustering. In conclusion, embB306 mutations do not cause classical ethambutol resistance but may predispose M. tuberculosisisolates to the development of resistance to increasing numbers of antibiotics and may increase the ability of drug-resistant isolates to be transmitted between subjects.

Mutations at position 306 of embB (embB306) have been proposed as a marker for ethambutol resistance in Mycobacterium tuberculosis; however, recent reports of embB306 mutations in ethambutol-susceptible isolates caused us to question the biological role of this mutation. We tested 1,020 clinical M. tuberculosis isolates with different drug susceptibility patterns and of different geographical origins for associations between embB306 mutations, drug resistance patterns, and major genetic group. One hundred isolates (10%) contained a mutation in embB306; however, only 55 of these mutants were ethambutol resistant. Mutations in embB306 could not be uniquely associated with any particular type of drug resistance and were found in all three major genetic groups. A striking association was observed between these mutations and resistance to any drug (P < 0.001), and the association between embB306 mutations and resistance to increasing numbers of drugs was highly significant (P < 0.001 for trend). We examined the association between embB306 mutations and IS6110 clustering (as a proxy for transmission) among all drug-resistant isolates. Mutations in embB306 were significantly associated with clustering by univariate analysis (odds ratio, 2.44; P = 0.004). In a multivariate model that also included mutations in katG315, katG463, gyrA95, and kasA269, only mutations in embB306 (odds ratio, 2.14; P = 0.008) and katG315 (odds ratio, 1.99; P = 0.015) were found to be independently associated with clustering. In conclusion, embB306 mutations do not cause classical ethambutol resistance but may predispose M. tuberculosis isolates to the development of resistance to increasing numbers of antibiotics and may increase the ability of drug-resistant isolates to be transmitted between subjects.