Frequency and antimicrobial susceptibility of Staphylococcus pseudintermedius in dogs with otitis externa

Infections by Staphyloccocus pseudintermedius in the skin of dogs have been object of studies, since such microorganisms often present multiple resistance to antibiotics. This study aimed to identify and evaluate the antimicrobial susceptibility of Staphylococcus pseudintermedius (SP) strains isolated from dogs with otitis. Swabs from both ears of 52 dogs with otitis (n = 104) were included. Bacteria were cultured using Muller-Hinton agar (supplemented with 5% equine blood and incubated at 37 degrees C for 24 to 48 hours. All colonies underwent biochemical evaluation for identification of staphylococci. The identity of colonies as SP was confirmed by polymerase chain reaction. The antimicrobial susceptibility of SP strains was evaluated by disk diffusion. The presence of the gene mecA was evaluated in all SP isolates by PCR. Forty-four SP strains were isolated from swabs of 31 dogs (31/52, 59.6%). Seventy-five percent of the strains were susceptible to cephalexin and 93.2% to amoxicillin plus clavulanic acid. Less than 23% of the strains were susceptible to penicillin. For non-beta-lactam antimicrobials, 63.6% of the strains showed resistance to sulfamethoxazole-trimethoprim, 61.4% to tetracycline, and 38, 64% to enrofloxacin. Aminoglycoside resistance rate was 27.3% for gentamicin. Resistance to oxacillin in vitro was detected in 13 of the 44 strains (29, 55%). A total of 12 strains (27.3%) were positive for mecA gene and five of these 12 strains were susceptible to in vitro oxacillin. Twenty-six (59, 1%) strains were resistant to three or more classes of antimicrobials, and classified as multi resistant. Our results showed high frequency of SP and multi resistant isolates to antimicrobials commonly used in veterinary.