Background -Moxifloxacin is a fourth-generation fluoroquinolone (FQ) that is approved for use in people to treat a variety of infections. Some veterinary microbiology laboratories report moxifloxacin in culture and sensitivity profiles for Staphylococcus pseudintermedius and S. schleiferi albeit using Clinical & Laboratory Standards Institute (CLSI) breakpoints for S. aureus. Previous studies have shown that S. aureus breakpoints can mischaracterize S. pseudintermedius susceptibility to various drugs. Pradofloxacin is a third generation veterinary FQ with a similar mechanism of action and spectrum of activity to moxifloxacin; however, the dose format (25 mg/mL solution) available in the USA may limit its practical use in large dogs.Objective -To determine the minimum inhibitory concentration (MIC), mutant prevention concentration (MPC) and mutant selection window (MSW) of moxifloxacin and pradofloxacin for isolates of S. pseudintermedius and S. schleiferi.Methods and materials -Pulsed-field gel electrophoresis was performed to establish that each bacterial isolate selected for testing represented an unique strain. The MIC, MPC and MSW for moxifloxacin and pradofloxacin were determined from 60 strains of S. pseudintermedius and seven strains of S. schleiferi.Results -The MIC and MPC ranges of moxifloxacin and pradofloxacin for meticillin-susceptible S. pseudintermedius were similar. However, MIC and MPC ranges were much wider and resistance to both drugs was more common for meticillin-resistant strains of S. pseudintermedius and S. schleiferi.Conclusions and Clinical Importance -The narrow MSW of these drugs may reduce the risk of selecting for antibiotic-resistant subpopulations. Pharmacokinetic, pharmacodynamic and safety studies are needed.

Background Repurposing existing drugs is one approach to address the growing concerns of multi‐drug resistant bacterial pathogens in veterinary medicine. Oxyclozanide is in the anthelmintic drug class salicylanilide, which has been used primarily as a treatment and preventative for Fasciola hepatica in ruminants. The antimicrobial activity of oxyclozanide has been studied in human medicine; its activity against common small animal bacterial pathogens such as Staphylococcus pseudintermedius has yet to be determined. Objective The aim of this study was to measure and establish the minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) of oxyclozanide against S. pseudintermedius and other common small animal bacterial pathogens. Methods and materials The MIC and MPC of oxyclozanide were determined from eighteen meticillin sensitive S. pseudintermedius (MSSP) isolates and eleven meticillin‐resistant S. pseudintermedius (MRSP), as well as single isolates of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and Enterococcus faecalis. Results The MIC of the eighteen meticillin‐sensitive S. pseudintermedius isolates was 0.5–1 μg/mL and the MPC ranged between 16 and 32 μg/mL. The MIC of the eleven meticillin‐resistant strains of S. pseudintermedius ranged from 0.5 to 2 μg/mL with a MPC ranging between 16 and 32 μg/mL. A single isolate of meticillin‐resistant S. aureus (MRSA) had an MIC of 1 μg/mL and MPC 16 μg/mL. No inhibition of growth was seen at the concentrations tested for bacterial isolate strains E. coli, P. aeruginosa and E. faecalis. Conclusion and clinical importance Oxyclozanide demonstrated in‐vitro antibacterial activity against meticillin‐resistant S. pseudintermedius. Further studies are needed to evaluate the potential use of oxyclozanide as a topical bactericidal agent.