title: Mycobacterium tuberculosis strains with highly discordant rifampin susceptibility test results.
abstract: The objectives of this study were to investigate the origin of highly discordant rifampin (rifampicin) (RMP) drug susceptibility test results obtained for Mycobacterium tuberculosis strains during proficiency testing. Nine Supra-National Tuberculosis Reference Laboratories tested the RMP susceptibilities of 19 selected M. tuberculosis strains, using standard culture-based methods. The strains were classified as definitely resistant (R) (n = 6) or susceptible (S) (n = 2) or probably resistant (PR) (n = 8) or susceptible (PS) (n = 3) based on rpoB mutations and treatment outcome. All methods yielded a susceptible result for the two S and three PS strains lacking an rpoB mutation and a resistant result for one R strain with a Ser531Leu mutation and one PR strain with a double mutation. Although the remaining 12 R and PR strains had rpoB mutations (four Asp516Tyr, three Leu511Pro, two Leu533Pro, one each His526Leu/Ser, and one Ile572Phe), they were all susceptible by the radiometric Bactec 460TB or Bactec 960 MGIT methods. In contrast, only one was susceptible by the proportion method on Löwenstein-Jensen medium and two on Middlebrook 7H10 agar. Low-level but probably clinically relevant RMP resistance linked to specific rpoB mutations is easily missed by standard growth-based methods, particularly the automated broth-based systems. Further studies are required to confirm these findings, to determine the frequency of these low-level-resistant isolates, and to identify technical improvements that may identify such strains.
authors.affiliation: Mycobacteriology Unit, Institute of Tropical Medicine, Nationalestraat 155, 2000-Antwerp, Belgium. avdeun@itg.be
authors.forename: A
authors.initials: A
authors.lastname: Van Deun
authors.forename: L
authors.initials: L
authors.lastname: Barrera
authors.forename: I
authors.initials: I
authors.lastname: Bastian
authors.forename: L
authors.initials: L
authors.lastname: Fattorini
authors.forename: H
authors.initials: H
authors.lastname: Hoffmann
authors.forename: K M
authors.initials: KM
authors.lastname: Kam
authors.forename: L
authors.initials: L
authors.lastname: Rigouts
authors.forename: S
authors.initials: S
authors.lastname: Rüsch-Gerdes
authors.forename: A
authors.initials: A
authors.lastname: Wright
mesh_terms: D000995:Antitubercular Agents / Q000494:pharmacology*; D012321:DNA-Directed RNA Polymerases / Q000235:genetics; D006801:Humans; D008826:Microbial Sensitivity Tests; D020125:Mutation, Missense; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000235:genetics; D012293:Rifampin / Q000494:pharmacology*
chemical_list: D000995:Antitubercular Agents; D012321:DNA-Directed RNA Polymerases; C105078:RNA polymerase beta subunit; D012293:Rifampin