Minimal inhibitory and bactericidal concentrations of gentamicin, enrofloxacin, colistin and Melaleuca armillaris essential oil against Pseudomonas aeruginosa isolated from canine otitis externa

BUCHAMER, ANDREA; BROGLIA, GUILLERMO; BULDAIN, DANIEL; JULCA LOZANO, KAREN; MESTORINO, NORA; MARCHETTI, LAURA

Brujas

Congreso; 15th International Congress of the European Association for Veterinary Pharmacology and Toxicology; 2023

European Association for Veterinary Pharmacology and Toxicology

Introduction: Pseudomonas aeruginosa is an opportunistic bacterium usually isolated from chronic canine otitis. The loss of effectiveness of antimicrobials leads us to search for innovative therapeutic alternatives such as essential oil. The aim of this study was to determine the minimal inhibitory (MIC) and bactericidal concentration (MBC) of gentamicin (GEN), enrofloxacin (ENR), colistin (COL), and Melaleuca armillaris essential oil (EO) against P. aeruginosa isolated from the external auditory canal (EAC) of dogs with chronic otitis.Materials and Methods: 104 ear swab samples from 87 dogs with unilateral and bilateral chronic otitis externa were obtained, from September 2018 to December 2022. Each sampled ear was considered an individual sample. P. aeruginosa was identified by morphological, metabolic, and biochemical tests, and evaluated by the disk diffusion method in agar. 15 isolates were selected and tested by antibiogram for different antimicrobials (AB) and by COL-screening test in agar plates. Minimal inhibitory (MIC) and bactericidal (MBC) concentrations were determined by the serial microdilution method for GEN, ENR, COL and AE. Concentrations tested were GEN (1024–0.031 μg/mL), ENR (256–0.039 μg/mL), COL (8000–0.0156 μg/mL) and EO (10%–0.019%).GEN and ENR were selected because they are frequently used in otitis treatment; COL for its impact on Public Health, and EO as a new therapeutic alternative. P. aeruginosa ATCC 27853 was used as control strain.Results: For ENR there was a coincidence in the results obtained for both methods, qualitative (antibiogram) and quantitative (MIC), 42.86% of the isolates were resistant (6/14) in both. However, for GEN, 46.67% (7/15) of the isolates were resistant by antibiogram, but all the samples (15/15) were sensitive by MIC. Finally, in the COL screening test, 9/15 (60%) were COL-resistant, and only 1/15 (6.67%) was resistant by MIC. MIC90 for GEN was 4 μg/mL, 16 μg/mL for ENR, 0.5 μg/mL for COL, and for 2.5 μg/mL EO. MBC/MIC ratio was between 1 and 2 for all, antimicrobials and the EO studied behaved as bactericides against P. aeruginosa.Conclusions: MIC test results are more reliable than antibiogram ones. Great resistance was observed against fluoroquinolones, AB widely used to treat canine otitis. COL resistance emergence in dogs is a worrying situation since it is forbidden in veterinary. Fluoroquinolones and COL are critically important antimicrobials for humans. Close contact between critically human patients and dogs with otitis by P. aeruginosa could be a risk. The evaluation of new alternatives as essential oils is extremely important and urgent.