title: In vitro Activity of Colistin in Combination with Tigecycline against Carbapenem-Resistant Acinetobacter baumannii Strains Isolated from Patients with Ventilator-Associated Pneumonia.
abstract: OBJECTIVE
  This study investigated the minimum inhibitory concentration (MIC) values and in vitro activity of colistin in combination with tigecycline against carbapenem-resistant Acinetobacter baumannii strains isolated from patients with ventilator-associated pneumonia (VAP) using the E-test method.
  
  
  METHODS
  A total of 40 A. baumannii strains, identified using the Phoenix Automated Microbiology System (Becton, Dickinson and Co., Franklin Lakes, NJ, USA) by conventional methods, were included in this study. Pulsed-field gel electrophoresis was performed to examine the clonal relationships between isolates. The carbapenem resistance of the strains to colistin and tigecycline was assessed using the E-test method (Liofilchem, Roseto Degli Abruzzi, Italy). The in vitro activity of colistin in combination with tigecycline was evaluated using the fractional inhibitor concentration (FIC) index.
  
  
  RESULTS
  While only 1 of 40 A. baumannii strains was determined to be colistin resistant, 6 were tigecycline resistant. The MIC50, MIC90, and MIC intervals of the A. baumannii strains were 0.19, 1.5, and 0.064‒4 μg/ml for colistin and 1, 8, and 0.094‒256 μg/ml for tigecycline, respectively. No synergistic effect was observed using the FIC index; 8 strains exhibited an indifferent effect and 32 exhibited an antagonist effect. Three of the six strains that were resistant to tigecycline were indifferent; the remaining three were antagonistic. The colistin-resistant strain also exhibited an antagonist effect.
  
  
  CONCLUSIONS
  In contrast to their synergistic effect against carbapenem-resistant A. baumannii isolates, colistin and tigecycline were highly antagonistic to carbapenem-resistant A. baumannii strains isolated from patients with VAP when the drugs were administered together. Therefore, alternative treatment options should be used during the treatment of VAP attributed to A. baumannii.
authors.affiliation: 1. Department of Medical Microbiology, Faculty of Medicine, Erzincan University, Erzincan, Turkey.
authors.forename: Aytekin
authors.initials: A
authors.lastname: Cikman
authors.affiliation: 1. Department of Medical Microbiology, Faculty of Medicine, Erzincan University, Erzincan, Turkey.
authors.forename: Baris
authors.initials: B
authors.lastname: Gulhan
authors.affiliation: 1. Department of Medical Microbiology, Faculty of Medicine, Erzincan University, Erzincan, Turkey.
authors.forename: Merve
authors.initials: M
authors.lastname: Aydin
authors.affiliation: 2. Viransehir State Hospital, Department of Infectious Diseases and Clinical Microbiology, Sanliurfa, Turkey.
authors.forename: Mehmet Resat
authors.initials: MR
authors.lastname: Ceylan
authors.affiliation: 3. Department of Medical Microbiology, Faculty of Medicine, Yuzuncu Yil University, Van, Turkey.
authors.forename: Mehmet
authors.initials: M
authors.lastname: Parlak
authors.affiliation: 4. Department of Infectious Diseases and Clinical Microbiology, Faculty of Medicine, Erzincan University, Erzincan, Turkey.
authors.forename: Faruk
authors.initials: F
authors.lastname: Karakecili
authors.affiliation: 5. Molecular Microbiology Research and Application Laboratory, Public Health Institution of Turkey, Ankara, Turkey.
authors.forename: Alper
authors.initials: A
authors.lastname: Karagoz
mesh_terms: D040981:Acinetobacter baumannii / Q000187:drug effects* / Q000302:isolation & purification; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D015780:Carbapenems / Q000494:pharmacology; D002648:Child; D002675:Child, Preschool; D003091:Colistin / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial / Q000187:drug effects; D004359:Drug Therapy, Combination; D005260:Female; D006801:Humans; D008297:Male; D008826:Microbial Sensitivity Tests; D008875:Middle Aged; D008911:Minocycline / Q000031:analogs & derivatives* / Q000494:pharmacology; D053717:Pneumonia, Ventilator-Associated / Q000382:microbiology*; D000078304:Tigecycline; D055815:Young Adult
chemical_list: D000900:Anti-Bacterial Agents; D015780:Carbapenems; D000078304:Tigecycline; D008911:Minocycline; D003091:Colistin
keywords: Acinetobacter baumannii; carbapenem-resistant; colistin; tigecycline; ventilator-associated pneumonia