title: Assessment of early combination effects of colistin and meropenem against Pseudomonas aeruginosa and Acinetobacter baumannii in dynamic time-kill experiments.
abstract: BACKGROUND
  In view of the paucity of clinical evidence, in vitro studies are needed to find antibiotic combinations effective against multidrug-resistant Gram-negative bacteria. Interpretation of in vitro effects is usually based on bacterial growth after 24 h in time-kill and checkerboard experiments. However, the clinical relevance of the effects observed in vitro is not established. In this study we explored alternative output parameters to assess the activities of colistin and meropenem against Pseudomonas aeruginosa and Acinetobacter baumannii.
  
  
  METHODS
  Four strains each of P. aeruginosa and A. baumannii were exposed to colistin and meropenem, alone and in combination, in 8 h dynamic time-kill experiments. Initial (1 h), maximum and 8 h bacterial reductions and the area under the bacterial time-kill curve were evaluated. Checkerboards, interpreted based on fractional inhibitory concentration indices after 24 h, were performed for comparison.
  
  
  RESULTS
  In the time-kill experiments, the combination resulted in enhanced 1 h, maximum and 8 h bacterial reductions against 2, 3 and 5 of 8 strains, respectively, as compared to the single drugs. A statistically significant reduction in the area under the time-kill curve was observed for three strains. In contrast, the checkerboards did not identify synergy for any of the strains.
  
  
  CONCLUSIONS
  Combination effects were frequently found with colistin and meropenem against P. aeruginosa and A. baumannii in time-kill experiments but were not detected with the checkerboard method. We propose that the early dynamics of bacterial killing and growth, which may be of great clinical importance, should be considered in future in vitro combination studies.
authors.affiliation: a Department of Medical Sciences , Uppsala University , Uppsala , Sweden.
authors.forename: Thomas
authors.initials: T
authors.lastname: Tängdén
authors.affiliation: a Department of Medical Sciences , Uppsala University , Uppsala , Sweden.
authors.forename: Matti
authors.initials: M
authors.lastname: Karvanen
authors.affiliation: b Department of Pharmaceutical Biosciences , Uppsala University , Uppsala , Sweden.
authors.forename: Lena E
authors.initials: LE
authors.lastname: Friberg
authors.affiliation: c Department of Clinical Sciences , Lund University , Malmö , Sweden.
authors.forename: Inga
authors.initials: I
authors.lastname: Odenholt
authors.affiliation: a Department of Medical Sciences , Uppsala University , Uppsala , Sweden.
authors.forename: Otto
authors.initials: O
authors.lastname: Cars
mesh_terms: D040981:Acinetobacter baumannii / Q000187:drug effects* / Q000502:physiology; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003091:Colistin / Q000494:pharmacology*; D004347:Drug Interactions; D006801:Humans; D000077731:Meropenem; D050296:Microbial Viability / Q000187:drug effects*; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000502:physiology; D013845:Thienamycins / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D000900:Anti-Bacterial Agents; D013845:Thienamycins; D000077731:Meropenem; D003091:Colistin
keywords: Gram-negative bacteria; checkerboards; synergy; time-kill experiments