title: Tris-EDTA significantly enhances antibiotic efficacy against multidrug-resistant Pseudomonas aeruginosa in vitro.
abstract: BACKGROUND
  Multidrug-resistant Pseudomonas aeruginosa commonly complicates chronic bacterial otitis in dogs.
  
  
  OBJECTIVE
  The aim of this in vitro study was to determine the effect of ethylenediaminetetraacetic acid-tromethamine (Tris-EDTA) on the minimal bactericidal concentrations (MBCs) and minimal inhibitory concentrations (MICs) of marbofloxacin and gentamicin for multidrug-resistant P. aeruginosa isolates from cases of canine otitis.
  
  
  METHODS
  Eleven isolates were identified as multidrug resistant on disc diffusion; 10 were resistant to marbofloxacin and two were resistant to gentamicin. Isolates were incubated for 90 min with each antibiotic alone and in combination with Tris-EDTA at concentrations of 0.075 μg/mL to 5 mg/mL for marbofloxacin, 0.001 μg/mL to 10 mg/mL for gentamicin and 17.8:4.7 to 0.14:0.04 mg/mL for Tris-EDTA. Positive and negative controls were included. Aliquots of each antibiotic and/or Tris-EDTA concentration were subsequently transferred to sheep blood agar to determine the MBCs, and tryptone soy broth was added to the remaining suspensions to determine the MICs.
  
  
  RESULTS
  Tris-EDTA alone was bacteriostatic but not bactericidal at any concentration. The addition of Tris-EDTA significantly reduced the median MBC (from 625 to 468.8 μg/mL; P < 0.001) and MIC (from 29.3 to 2.4 μg/mL; P = 0.008) of marbofloxacin, and the median MBC (from 625 to 39.1 μg/mL) and MIC (from 19.5 to 1.2 μg/mL) of gentamicin (both P < 0.001).
  
  
  CONCLUSIONS
  Tris-EDTA significantly reduced the MBCs and MICs of marbofloxacin and gentamicin for multidrug-resistant P. aeruginosa in vitro. This may be of use to clinicians managing these infections in dogs.
authors.affiliation: School of Veterinary Science, The University of Liverpool, Leahurst Campus, Neston, Cheshire, CH64 7TE, UK.
authors.forename: Laura M
authors.initials: LM
authors.lastname: Buckley
authors.forename: Neil A
authors.initials: NA
authors.lastname: McEwan
authors.forename: Tim
authors.initials: T
authors.lastname: Nuttall
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D024901:Drug Resistance, Multiple, Bacterial; D004357:Drug Synergism; D004492:Edetic Acid / Q000494:pharmacology*; D024841:Fluoroquinolones / Q000008:administration & dosage / Q000494:pharmacology*; D005839:Gentamicins / Q000008:administration & dosage / Q000494:pharmacology; D008826:Microbial Sensitivity Tests; D011550:Pseudomonas aeruginosa / Q000187:drug effects*; D014325:Tromethamine / Q000494:pharmacology*
chemical_list: D000900:Anti-Bacterial Agents; D024841:Fluoroquinolones; D005839:Gentamicins; D014325:Tromethamine; C080260:marbofloxacin; D004492:Edetic Acid