Antibacterial effect of N-acetylcysteine in combination with antimicrobials on common canine otitis externa bacterial isolates.

Abstract

Approved treatments for canine otitis externa are limited in variety and may contain ototoxic ingredients. With bacterial resistance an ongoing concern, it would be ideal if non-ototoxic agents combined with antibiotics resulted in a synergistic effect, requiring lower antibiotic concentrations to treat infections. Evidence of synergism and antagonism between N-acetylcysteine (NAC) and various antibiotic classes has been reported; the present research group was interested in examining these interactions. To determine if NAC, an otoprotective and antimicrobial compound, has synergistic activity when combined with enrofloxacin or gentamicin in vitro against bacterial isolates causing canine otitis externa. Twenty-two isolates from canine clinical cases of otitis externa were identified and tested, including seven Staphylococcus pseudintermedius, 12 Pseudomonas aeruginosa and three Corynebacterium spp. isolates. Each isolate was grown on blood agar for 24h and transferred to Mueller-Hinton broth (MHB), with a final concentration of 5×105 cfu/mL. Each well was inoculated with 50μL of bacterial suspension. N-acetylcysteine was diluted in MHB to a starting concentration of 160mg/mL. Enrofloxacin and gentamicin were diluted to 64μg/mL. Individual and checkerboard serial microdilution assays were performed in triplicate with negative controls for all isolates tested. Interactions observed for NAC and enrofloxacin were synergistic (4.5%), indifferent (77.3%) or antagonistic (18.2%). Interactions observed for NAC and gentamicin were synergistic (4.5%), indifferent (45.5%) or antagonistic (50%). Most interactions between NAC and enrofloxacin or gentamicin were indifferent or antagonistic at the concentrations tested in vitro.