Mycobacterium tuberculosis embBCodon 306 Mutations Confer Moderately Increased Resistance to Ethambutol In Vitro and In Vivo

Abstract

ABSTRACTEthambutol (EMB) is a major component of the first-line therapy of tuberculosis. Mutations in codon 306 ofembB(embB306) were suggested as a major resistance mechanism in clinical isolates. To directly analyze the impact of individualembB306 mutations on EMB resistance, we used allelic exchange experiments to generateembB306 mutants ofM. tuberculosisH37Rv. The level of EMB resistance conferred by particular mutations was measuredin vitroandin vivoafter EMB therapy by daily gavage in a mouse model of aerogenic tuberculosis. The wild-typeembB306 ATG codon was replaced byembB306 ATC, ATA, or GTG, respectively. All of the obtainedembB306 mutants exhibited a 2- to 4-fold increase in EMB MIC compared to the wild-type H37Rv.In vivo, the one selectedembB306 GTG mutant required a higher dose of ethambutol to restrict its growth in the lung compared to wild-type H37Rv. These experiments demonstrate thatembB306 point mutations enhance the EMB MICin vitroto a moderate, but significant extent, and reduce the efficacy of EMB treatment in the animal model. We propose that conventional EMB susceptibility testing, in combination withembB306 genotyping, may guide dose adjustment to avoid clinical treatment failure in these low-level resistant strains.