title: Allelic exchange and mutant selection demonstrate that common clinical embCAB gene mutations only modestly increase resistance to ethambutol in Mycobacterium tuberculosis.
abstract: Mutations within codon 306 of the Mycobacterium tuberculosis embB gene modestly increase ethambutol (EMB) MICs. To identify other causes of EMB resistance and to identify causes of high-level resistance, we generated EMB-resistant M. tuberculosis isolates in vitro and performed allelic exchange studies of embB codon 406 (embB406) and embB497 mutations. In vitro selection produced mutations already identified clinically in embB306, embB397, embB497, embB1024, and embC13, which result in EMB MICs of 8 or 14 microg/ml, 5 microg/ml, 12 microg/ml, 3 microg/ml, and 4 microg/ml, respectively, and mutations at embB320, embB324, and embB445, which have not been identified in clinical M. tuberculosis isolates and which result in EMB MICs of 8 microg/ml, 8 microg/ml, and 2 to 8 microg/ml, respectively. To definitively identify the effect of the common clinical embB497 and embB406 mutations on EMB susceptibility, we created a series of isogenic mutants, exchanging the wild-type embB497 CAG codon in EMB-susceptible M. tuberculosis strain 210 for the embB497 CGG codon and the wild-type embB406 GGC codon for either the embB406 GCC, embB406 TGC, embB406 TCC, or embB406 GAC codon. These new mutants showed 6-fold and 3- to 3.5-fold increases in the EMB MICs, respectively. In contrast to the embB306 mutants, the isogenic embB497 and embB406 mutants did not have preferential growth in the presence of isoniazid or rifampin (rifampicin) at their MICs. These results demonstrate that individual embCAB mutations confer low to moderate increases in EMB MICs. Discrepancies between the EMB MICs of laboratory mutants and clinical M. tuberculosis strains with identical mutations suggest that clinical EMB resistance is multigenic and that high-level EMB resistance requires mutations in currently unknown loci.
authors.affiliation: Division of Infectious Disease and the Center for Emerging Pathogens, Department of Medicine, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07103, USA.
authors.forename: Hassan
authors.initials: H
authors.lastname: Safi
authors.forename: Robert D
authors.initials: RD
authors.lastname: Fleischmann
authors.forename: Scott N
authors.initials: SN
authors.lastname: Peterson
authors.forename: Marcus B
authors.initials: MB
authors.lastname: Jones
authors.forename: Behnam
authors.initials: B
authors.lastname: Jarrahi
authors.forename: David
authors.initials: D
authors.lastname: Alland
mesh_terms: D000483:Alleles; D000995:Antitubercular Agents / Q000494:pharmacology*; D003062:Codon; D017931:DNA Primers; D004269:DNA, Bacterial / Q000235:genetics; D024881:Drug Resistance, Bacterial / Q000235:genetics; D004977:Ethambutol / Q000494:pharmacology*; D005798:Genes, Bacterial / Q000235:genetics*; D008826:Microbial Sensitivity Tests; D009154:Mutation / Q000235:genetics*; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000235:genetics*; D009876:Operon; D010430:Pentosyltransferases / Q000235:genetics*; D010957:Plasmids / Q000235:genetics; D012293:Rifampin / Q000494:pharmacology
chemical_list: D000995:Antitubercular Agents; D003062:Codon; D017931:DNA Primers; D004269:DNA, Bacterial; D004977:Ethambutol; C540269:EmbB protein, Mycobacterium tuberculosis; D010430:Pentosyltransferases; D012293:Rifampin