Determination of minimum inhibitory concentrations for silver sulfadiazine and other topical antimicrobial agents against strains of Pseudomonas aeruginosa isolated from canine otitis externa

Background Otitis externa is a common presenting complaint in practice. Ear infections by Pseudomonas aeruginosa are particularly problematic due to the organism's high level of resistance and ability to damage the tympanum. Treatment should be based on susceptibility testing although minimum inhibitory concentrations (MICs) are not available for all treatment options. Silver sulfadiazine has been used in cases of recurrent P. aeruginosa otitis, although a MIC for silver sulfadiazine as a single agent has not been established. Objectives To describe susceptibility patterns of P. aeruginosa isolated from canine otitis externa and determine the MIC for silver sulfadiazine and other topical antimicrobials. Animals Thirty-six P. aeruginosa isolates were collected from client-owned dogs, suffering from otitis externa. Methods and materials Susceptibility patterns were established using disc diffusion susceptibility testing against 17 antimicrobial agents. For determination of the MIC, selected strains were tested against increasing concentrations of marbofloxacin, enrofloxacin, gentamicin, polymyxin B and silver sulfadiazine using broth microdilution. Results For nine of 17 antimicrobial agents, complete resistance was seen in all isolates tested via disk diffusion susceptibility testing. Approximately 94% and 96% of isolates were susceptible to gentamicin and imipenem, respectively. These findings were consistent with broth dilution, where all strains were susceptible to gentamicin. Resistance was higher against polymyxin B and the fluoroquinolones. Silver sulfadiazine was effective in vitro with a MIC ranging from 1 to 64 mu g/mL. Conclusions and clinical significance As the MIC of silver sulfadiazine was lower than the concentration in a 1% preparation, such a product potentially represents a treatment option for dogs with P. aeruginosa otitis.