Tris-EDTA significantly enhances antibiotic efficacy against multidrug-resistant Pseudomonas aeruginosa in vitro

Vet Dermatol. 2013 Oct;24(5):519-e122. doi: 10.1111/vde.12071.


Background: Multidrug-resistant Pseudomonas aeruginosa commonly complicates chronic bacterial otitis in dogs.

Hypothesis/objectives: 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 and clinical importance: 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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anti-Bacterial Agents / pharmacology*
  • Drug Resistance, Multiple, Bacterial*
  • Drug Synergism
  • Edetic Acid / pharmacology*
  • Fluoroquinolones / administration & dosage
  • Fluoroquinolones / pharmacology*
  • Gentamicins / administration & dosage
  • Gentamicins / pharmacology
  • Microbial Sensitivity Tests
  • Pseudomonas aeruginosa / drug effects*
  • Tromethamine / pharmacology*


  • Anti-Bacterial Agents
  • Fluoroquinolones
  • Gentamicins
  • Tromethamine
  • marbofloxacin
  • Edetic Acid