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ObjectivesTo determine the association between embB mutations and drug resistance, and to further investigate the mechanism of embB mutations involved in the development of ethambutol and multidrug resistance in Mycobacterium tuberculosis.MethodsOne hundred and thirty-eight multidrug-resistant clinical M. tuberculosis isolates, including 86 ethambutol-resistant and 52 ethambutol-susceptible strains, were analysed to characterize mutations within the entire coding region of the embB gene. Moreover, a two-step genotyping was performed to identify the genetic lineage.ResultsIn total, 27 embB mutation types were detected in 19 distinct codons. Though a strong association was observed between embB mutations and ethambutol resistance, 19.2% of embB306 mutants and 11.5% of embB406 or embB497 mutants were ethambutol susceptible. Among 39 ethambutol-resistant strains without embB306 mutations, 51.3% harboured mutations at codons 406 or 497. Particularly, three pairs of isolates with identical embB mutations and genotyping features were identified with variant ethambutol susceptibility. Among 77 isoniazid, rifampicin, streptomycin and ethambutol quadruple drug-resistant isolates, 89.6% carried embB mutations and 83.1% could be identified by detecting 10 embB mutations.ConclusionsOur results suggest embB mutations alone are not sufficient for the development of full resistance to ethambutol in M. tuberculosis and mutations other than embB are also needed. Our study confirms the importance of mutations at embB406 and embB497 as hotspots, in addition to embB306, for detecting ethambutol resistance. Ten selected mutations of embB, covered by a short PCR product, can be used as candidate markers for the prediction of quadruple resistance to isoniazid, rifampicin, streptomycin and ethambutol.

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BackgroundMonitoring drug resistance in Mycobacterium tuberculosis is essential to curb the spread of tuberculosis (TB). Unfortunately, drug susceptibility testing is currently not available in Papua New Guinea (PNG) and that impairs TB control in this country. We report for the first time M. tuberculosis mutations associated with resistance to first and second-line anti-TB drugs in Madang, PNG. A molecular cluster analysis was performed to identify M. tuberculosis transmission in that region.ResultsPhenotypic drug susceptibility tests showed 15.7% resistance to at least one drug and 5.2% multidrug resistant (MDR) TB. Rifampicin resistant strains had the rpoB mutations D516F, D516Y or S531L; Isoniazid resistant strains had the mutations katG S315T or inhA promoter C15T; Streptomycin resistant strains had the mutations rpsL K43R, K88Q, K88R), rrs A514C or gidB V77G. The molecular cluster analysis indicated evidence for transmission of resistant strain.ConclusionsWe observed a substantial rate of MDR-TB in the Madang area of PNG associated with mutations in specific genes. A close monitoring of drug resistance is therefore urgently required, particularly in the presence of drug-resistant M. tuberculosis transmission. In the absence of phenotypic drug susceptibility testing in PNG, molecular assays for drug resistance monitoring would be of advantage.