title: Influence of testing methodology on the tigecycline activity profile against presumably tigecycline-non-susceptible Acinetobacter spp.
abstract: OBJECTIVE
  To compare the tigecycline activity profile against Acinetobacter spp. by Etest versus broth microdilution in isolates with high Etest MIC.
  
  
  METHODS
  Acinetobacter spp. isolates with tigecycline MICs of >or=0.5 mg/L determined by commercially developed Etests strips (January 2006 to July 2007) in five Spanish hospitals were considered. Values were rounded to the nearest upper double-dilution. Susceptibility by broth microdilution following CLSI (formerly NCCLS) recommendations, as the reference method, was determined in a central laboratory. BSAC breakpoints were used: susceptible <or=1 mg/L; intermediate = 2 mg/L; and resistant >2 mg/L.
  
  
  RESULTS
  One hundred and forty-eight isolates were collected: 12 isolates with a tigecycline Etest MIC of 0.5 mg/L, 14 with 1 mg/L, 86 with 2 mg/L, 31 with 4 mg/L and 5 with 8 mg/L. Isolates with Etest MICs of 0.5-1 mg/L showed the same values by broth microdilution. Among isolates with Etest MICs of 2 mg/L, only 5.8% of strains showed the same value by both methods (88.4% showed values that were one or two dilutions lower by microdilution). None of the 36 isolates with Etest MICs of 4-8 mg/L showed the same value by both methods, with values at least two dilutions lower by microdilution. Weak correlation (R = 0.238; P <or= 0.001) was found between both methods. All 26 Etest susceptible isolates, 80/86 (93.0%) Etest intermediate and 32/36 (88.9%) Etest resistant strains were susceptible by microdilution.
  
  
  CONCLUSIONS
  Caution should be taken in interpreting Etest MICs of >or=2 mg/L for Acinetobacter spp. since strains with Etest MICs of 2-4 mg/L are susceptible when tested by microdilution. False non-susceptibility by Etest may exclude tigecycline as a therapeutic option in a field where multiresistance is the rule.
authors.affiliation: Microbiology Department, Hospital Universitario Reina Sofía, Cordoba, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Casal
authors.forename: F
authors.initials: F
authors.lastname: Rodríguez
authors.forename: B
authors.initials: B
authors.lastname: Johnson
authors.forename: E
authors.initials: E
authors.lastname: Garduno
authors.forename: F
authors.initials: F
authors.lastname: Tubau
authors.forename: R Ortiz
authors.initials: RO
authors.lastname: de Lejarazu
authors.forename: A
authors.initials: A
authors.lastname: Tenorio
authors.forename: M J
authors.initials: MJ
authors.lastname: Giménez
authors.forename: R
authors.initials: R
authors.lastname: Bartolomé
authors.forename: C
authors.initials: C
authors.lastname: Garcia-Rey
authors.forename: L
authors.initials: L
authors.lastname: Aguilar
authors.forename: N
authors.initials: N
authors.lastname: García-Escribano
mesh_terms: D000150:Acinetobacter / Q000187:drug effects* / Q000302:isolation & purification; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003951:Diagnostic Errors; D024881:Drug Resistance, Bacterial; D006761:Hospitals; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods*; D008911:Minocycline / Q000031:analogs & derivatives* / Q000494:pharmacology; D013030:Spain; D000078304:Tigecycline
chemical_list: D000900:Anti-Bacterial Agents; D000078304:Tigecycline; D008911:Minocycline