Post-antibiotic effect of orbifloxacin against Escherichia coli and Pseudomonas aeruginosa isolates from dogs

Orbifloxacin is a fluoroquinolone developed for use in companion animal medicine. This antimicrobial agent exhibits bactericidal activity against numerous gram-negative and gram-positive bacteria. In canine practice, orbifloxacin is indicated for the treatment of various infections, including urinary, skin, and otitis infections, and is available in many countries including Japan.Pharmacodynamic variables such as the post-antibiotic effect (PAE) and post-antibiotic sub-minimum inhibitory concentration effect (PA-SME) have increasingly become the focus of investigations designed to determine optimal dosage regimens for antimicrobial agents. The PAE is defined as the length of time that bacterial growth is suppressed following brief exposure to an antibiotic [1]. PAE has been investigated for several veterinary fluoroquinolones, such as enrofloxacin, marbofloxacin, and difloxacin [2-5], but not for orbifloxacin. On the other hand, PA-SME is defined as the time interval that includes the PAE plus the additional time during which growth is suppressed by sub-MICs, and has not been investigated for all veterinary fluoroquinolones, including orbifloxacin. In this study, we examined the in vitro PAEs and PA-SMEs of orbifloxacin against Escherichia coli and Pseudomonas aeruginosa, which are representative gram-negative pathogens responsible for urinary and skin infections, respectively, in dogs, and these values were compared with those for enrofloxacin.Two strains each of E. coli (strains 09-207 and 09-225) and P. aeruginosa (strains 33 and 72) were used in this study. These organisms were isolated from canine urine (E. coli) and skin (P. aeruginosa), and identified by gram stain, catalase, and oxidase tests and Api 20E kit (Bio Merieux, France). MICs of orbifloxacin and enrofloxacin were determined by the agar dilution method according to the guidelines of the Clinical and Laboratory Standards Institute [6]. E. coli ATCC 25922 and P. aeruginosa ATCC 27853 were used as qual