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. 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.&#xa0;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&#xa0;min with each antibiotic alone and in combination with Tris-EDTA at concentrations of 0.075&#xa0;&#x3bc;g/mL to 5&#xa0;mg/mL for marbofloxacin, 0.001&#xa0;&#x3bc;g/mL to 10&#xa0;mg/mL for gentamicin and 17.8:4.7 to 0.14:0.04&#xa0;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&#xa0;&#x3bc;g/mL; P&#xa0;<&#xa0;0.001) and MIC (from 29.3 to 2.4&#xa0;&#x3bc;g/mL; P&#xa0;=&#xa0;0.008) of marbofloxacin, and the median MBC (from 625 to 39.1&#xa0;&#x3bc;g/mL) and MIC (from 19.5 to 1.2&#xa0;&#x3bc;g/mL) of gentamicin (both P&#xa0;<&#xa0;0.001). CONCLUSIONS AND CLINICAL IMPORTANCE: Tris-EDTA significantly reduced the MBCs and MICs of marbofloxacin and gentamicin for multidrug-resistant P.&#xa0;aeruginosa in vitro. This may be of use to clinicians managing these infections in dogs.