title: embB306 mutations as molecular indicators to predict ethambutol susceptibility in Mycobacterium tuberculosis.
abstract: BACKGROUND
  Discordant results in conventional susceptibility testing of ethambutol against Mycobacterium tuberculosis may lead to underreporting of drug resistance.
  
  
  METHODS
  A 240-bp region of the embB gene in 111 clinical isolates of M. tuberculosis was sequenced and examined for mutations linked to ethambutol resistance. The phenotypic susceptibility levels of the isolates were quantified by the BACTEC™ MGIT 960™ TB System and correlated with the genotypic test results. These data were analyzed to find information that could be used to clarify discordant ethambutol susceptibility test results.
  
  
  RESULTS
  Mutations M306I (n = 56), M306V (n = 18) and M306L (n = 3) in M. tuberculosis showed decreased susceptibility to ethambutol. The minimum inhibitory concentrations (MICs) in 73% (56/77) of embB306 mutants were at or just above the critical concentration (MICs, 5.0 to ≤12.5 µg/ml) of ethambutol reflecting borderline (or intermediate) resistance. Eight ethambutol-resistant isolates lacked embB mutations, probably due to mutational alterations elsewhere in the genome.
  
  
  CONCLUSIONS
  Our findings suggest that clinical isolates containing embB306 mutations with MICs overlapping the critical concentration are associated with discordant ethambutol susceptibility test results. The clinical significance of borderline resistance in combination treatment of tuberculosis remains to be determined before alternative ethambutol breakpoints are considered.
authors.affiliation: DST/NRF Centre of Excellence for Biomedical TB Research/MRC Centre for Molecular and Cellular Biology, Division of Molecular Biology and Human Genetics, Faculty of Health Science, Stellenbosch University, Cape Town, South Africa. fas@sun.ac.za
authors.forename: Frederick A
authors.initials: FA
authors.lastname: Sirgel
authors.forename: Robin M
authors.initials: RM
authors.lastname: Warren
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Streicher
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Victor
authors.forename: Paul D
authors.initials: PD
authors.lastname: van Helden
authors.forename: Erik C
authors.initials: EC
authors.lastname: Böttger
mesh_terms: D000995:Antitubercular Agents / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial / Q000187:drug effects; D004977:Ethambutol / Q000494:pharmacology*; D005838:Genotype; D006801:Humans; D008826:Microbial Sensitivity Tests; D009154:Mutation; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification; D014376:Tuberculosis / Q000382:microbiology
chemical_list: D000995:Antitubercular Agents; D004977:Ethambutol