title: In vitro synergism of combinations of colistin with selected antibiotics against colistin-resistant Acinetobacter baumannii.
abstract: OBJECTIVE
  The in vitro activity of colistin in combination with sulbactam, netilmicin, and vancomycin against colistin-resistant A. baumannii strains was investigated. Furthermore, the clonal relationship of the strains was analyzed.
  
  
  METHODS
  Clonal relationship was investigated using rep-PCR. To screen for synergysm, the fractional inhibitory concentration index (FICI) was calculated using checkerboard assay. The killing kinetics of the combination of colistin with vancomycin was assessed using time-kill assay.
  
  
  RESULTS
  Three different clones were found among 10 clinical isolates of colistin-resistant A. baumannii strains. Thereof, 8 strains were susceptible to netilmicin. Synergistic interaction was detected in 1 strain with the combination of colistin-netilmicin, in 5 strains with colistin-sulbactam, and in 9 strains with colistin-vancomycin. None of combinations had antagonistic activity. Colistin-vancomycin combination resulted in rapid bactericidal activity.
  
  
  CONCLUSIONS
  These results show a distinct in vitro synergism between colistin and vancomycin, which might be useful to treat infection with multiple-resistant strains, prevent emergence of resistant strains, and to lower doses for both antibiotics to be used.
authors.affiliation: Department of Microbiology, Faculty of Medicine, Erciyes University, Kayseri, Turkey.
authors.forename: Duygu
authors.initials: D
authors.lastname: Percin
authors.affiliation: Department of Microbiology, Istanbul University Cerrahpasa Medical Faculty, Istanbul, Turkey.
authors.forename: Selcan
authors.initials: S
authors.lastname: Akyol
authors.affiliation: Department of Infectious Diseases and Clinical Microbiology, Yozgat State Hospital, Yozgat, Turkey.
authors.forename: Gamze
authors.initials: G
authors.lastname: Kalin
keywords: Acinetobacter baumannii; colistin resistance; fractional inhibitory concentration index (FICI); netilmicin; sulbactam; synergism; time-kill assay; vancomycin