Veterinary Microbiology 94 (2003) 295–301 Mechanisms of fluoroquinolone resistance in Pseudomonas aeruginosa isolates from canine infections Mar´ ıa Teresa Tejedor a,∗ , Jose Luis Mart´ ın a , Margarita Navia b , Joan Freixes c , Jordi Vila b a Microbiology, Facultad de Veterinaria, Universidad de Las Palmas de Gran Canaria, P.O. Box 550, 35080 Las Palmas de Gran Canaria, Canary Islands, Spain b Servicio de Microbiolog´ ıa, Hospital Clinic, Barcelona, Spain c Cenavisa, S.A., Cam´ ı Pedra Estela, s/n, 43205 Reus, Tarragona, Spain Received 25 October 2002; received in revised form 4 March 2003; accepted 17 March 2003 Abstract Chronic otitis externa in dogs is often associated with Pseudomonas aeruginosa infection. Fluoro- quinolones are often used for treating such infections. Fluoroquinolone resistance mechanisms were characterized in 10 strains of P. aeruginosa isolated from chronic canine otitis externa. Nine out of ten strains harbored a mutation in the gyrA gene and presented an overexpression of efflux pump(s). There was a good correlation between the lipophilicity of the fluoroquinolone being tested and the effect of the efflux pump inhibitor in the final MIC. Therefore, both mechanisms, mutation in the gyrA gene and increased efflux pump(s), seem to play an important role in the acquisition of fluoroquinolone resistance in veterinary clinical isolates of P. aeruginosa. Levels of resistance to fluoroquinolones suggest that they could not be a good choice for systemic therapy of Pseudomonas otitis. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Dog; Fluoroquinolones; Mechanisms of resistance; Pseudomonas aeruginosa; Otitis externa 1. Introduction The fluoroquinolones are a group of synthetic antibacterial agents active against a wide variety of clinically important microorganisms. Their targets are two essential bacterial enzymes, DNA gyrase and topoisomerase IV, responsible for the negative supercoiling of ∗ Corresponding author. Tel.: +34-928-454358; fax: +34-928-451142. E-mail address: mtejedor@dcc.ulpgc.es (Mar´ ıa Teresa Tejedor). 0378-1135/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-1135(03)00129-9