title: Minimal inhibitory concentrations of first-line drugs of multidrug-resistant tuberculosis isolates.
abstract: BACKGROUND
  The treatment of multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis (TB) is consistently difficult. Besides resistances, drug availability can be problematic and costs for therapy are high.
  
  
  OBJECTIVE
  Our aim was to evaluate alternatives in treatment of MDR and XDR TB other than using second-line drugs.
  
  
  METHODS
  We analyzed retrospectively the minimal inhibitory concentrations (MICs) of first-line drugs for 44 multidrug-resistant Mycobacterium tuberculosis isolates determined in our institute over a period of 20 years (1990 - 2010, n = 44). Drug susceptibility testing (DST) was performed using the proportion method on Lowenstein-Jensen Medium or Middlebrook 7H10 agar. MICs were defined as the lowest drug concentration after two-fold serially diluted concentration of the drugs that inhibits growth of more than 99.0% of a bacterial proportion of the tested M. tuberculosis within 14 to 21 days of incubation at 37°C.
  
  
  METHODS
  Summation.
  
  
  RESULTS
  The MICs of isoniazid and ethambutol were equal or slightly above the critical concentration in most of the strains (92% and 84%, respectively), defined as "low-level resistance". Rifampicin and streptomycin exhibited very high MICs in most of the strains (100% and 77%, respectively), indicating a "high-level resistance".
  
  
  CONCLUSIONS
  Our results indicate that isoniazid and ethambutol could still play a role in treating MDR and XDR TB patients if low-level resistance is detected. Quantitative DST seems to be promising for the recognition of residual drug activity, but has to be confirmed by clinical studies.
authors.affiliation: Department of Pneumology, Lungenklinik Heckeshorn, HELIOS Klinikum Emil von Behring, Walterhöferstr. 11, Berlin, Germany.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Schönfeld
authors.forename: Thorsten
authors.initials: T
authors.lastname: Bergmann
authors.forename: Silvan
authors.initials: S
authors.lastname: Vesenbeckh
authors.forename: Harald
authors.initials: H
authors.lastname: Mauch
authors.forename: Gudrun
authors.initials: G
authors.lastname: Bettermann
authors.forename: Torsten T
authors.initials: TT
authors.lastname: Bauer
authors.forename: Holger
authors.initials: H
authors.lastname: Rüssmann