In vitro antimicrobial activity of narasin against common clinical isolates associated with canine otitis externa

Background - Antimicrobial resistance and antimicrobial stewardship are of ever-increasing importance in veterinary medicine. Re-purposing of old drugs that are not used in human medicine is one approach that addresses the emergence of multidrug resistance in canine skin and ear infections, and can reduce the use of critically important human antibiotic classes. Hypothesis/Objectives - To determine the antimicrobial activity of narasin, a polyether ionophore conventionally used as a rumen modifier and anticoccidial agent in production animals, against common clinical isolates of canine otitis externa (OE). Animals/Isolates - Clinical isolates (n = 110) from canine OE were tested, including 17 meticillin-susceptible Staphylococcus pseudintermedius (MSSP), 13 multidrug-resistant Staphylococcus pseudintermedius (MDRSP), and 20 each of -haemolytic Streptococcus spp., Pseudomonas aeruginosa, Proteus mirabilis and Malassezia pachydermatis. Methods - Bacterial and yeast isolates were subcultured, suspended in broth and inoculated into 96-well plates. Organisms were tested against concentrations of narasin ranging from 0.03 to 128 mu g/mL. Minimal inhibitory concentrations (MICs) were determined after overnight incubation. Results - Narasin MICs for staphylococcal and streptococcal isolates ranged from 0.06 to 0.25 mu g/mL; MIC50 and MIC90 values for both organisms were 0.125 mu g/mL. No MICs were achieved for Pseudomonas or Proteus isolates. There was a weak antifungal effect against M. pachydermatis isolates (MIC 32 to >128 mu g/mL). Conclusions and Clinical Relevance - Narasin was effective against Gram-positive bacteria and had antifungal activity at higher concentrations against M. pachydermatis. However, the lack of Gram-negative activity would prevent its use as a sole antimicrobial agent in cases of canine OE.