Objectives: To 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.
Methods: One 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.
Results: In 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.
Conclusions: Our 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.