title: Correlation of different phenotypic drug susceptibility testing methods for four fluoroquinolones in Mycobacterium tuberculosis.
abstract: BACKGROUND
  Molecular resistance testing fails to explain all fluoroquinolone resistance, with a continued need for a suitable rapid phenotypic drug susceptibility testing method.
  
  
  OBJECTIVE
  To evaluate the optimal method for phenotypic fluoroquinolone susceptibility testing.
  
  
  METHODS
  Using Löwenstein-Jensen medium, Middlebrook 7H11 agar, BACTEC-MGIT 960 and the resazurin microtitre plate assay, we determined susceptibility to fluoroquinolones in Mycobacterium tuberculosis and investigated cross-resistance between ofloxacin, levofloxacin, moxifloxacin and gatifloxacin. We compared MICs of all four fluoroquinolones for 91 strains on Löwenstein-Jensen (as the gold standard) with their MICs in resazurin plates, and with ofloxacin susceptibility at a single concentration in MGIT and on 7H11 agar, in addition to sequencing of the gyrAB genes.
  
  
  CONCLUSIONS
  Applying a cut-off of 2 mg/L ofloxacin, 1 mg/L levofloxacin and 0.5 mg/L moxifloxacin and gatifloxacin in all methods, some discordance between solid medium and MGIT methods was observed, yet this tended to be explained by MICs around the cut-off. The high discordance between Löwenstein-Jensen (LJ) and resazurin plates suggests that the currently applied cut-offs for all fluoroquinolones in the resazurin method should decrease and minor changes in colour (from blue to purple) be considered as meaningful. High-level resistance in all assays to all drugs correlated well with the presence of gyrA mutations, in support of recent findings that fluoroquinolone resistance should be tested at different concentrations, as patients with lower levels of resistance may continue to benefit from high-dose fluoroquinolone-based therapy.
authors: Mycobacteriology Unit, Institute of Tropical Medicine, Antwerp, Belgium Department of Biomedical Sciences, Antwerp University, Antwerp, Belgium.
authors: Nele
authors: N
authors: Coeck
authors: Mycobacteriology Unit, Institute of Tropical Medicine, Antwerp, Belgium Department of Medicine, Division of Infectious Diseases, New York University, New York, USA Vaccinology Department, Medical Research Council Unit, Fajara, The Gambia.
authors: Bouke C
authors: BC
authors: de Jong
authors: Mycobacteriology Unit, Institute of Tropical Medicine, Antwerp, Belgium.
authors: Maren
authors: M
authors: Diels
authors: Mycobacteriology Unit, Institute of Tropical Medicine, Antwerp, Belgium.
authors: Pim
authors: P
authors: de Rijk
authors: Mycobacteriology Unit, Institute of Tropical Medicine, Antwerp, Belgium Médecins Sans Frontières, Paris, France.
authors: Elisa
authors: E
authors: Ardizzoni
authors: Mycobacteriology Unit, Institute of Tropical Medicine, Antwerp, Belgium.
authors: Armand
authors: A
authors: Van Deun
authors: Mycobacteriology Unit, Institute of Tropical Medicine, Antwerp, Belgium Department of Biomedical Sciences, Antwerp University, Antwerp, Belgium lrigouts@itg.be.
authors: Leen
authors: L
authors: Rigouts
mesh_terms: D000995:Antitubercular Agents / Q000494:pharmacology*; D003470:Culture Media / Q000737:chemistry; D027081:DNA Gyrase / Q000235:genetics; D024841:Fluoroquinolones / Q000494:pharmacology*; D008826:Microbial Sensitivity Tests / Q000379:methods; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics; D017422:Sequence Analysis, DNA
chemical_list: D000995:Antitubercular Agents; D003470:Culture Media; D024841:Fluoroquinolones; D027081:DNA Gyrase