In vitro effects of N-acetylcysteine in combination with antifungal agents against Malassezia pachydermatis isolated from canine otitis externa

Background: Many clinicians prescribe antifungal agents to treat canine otitis externa (OE). However, studies evaluating the antifungal effects of N-acetylcysteine (NAC) and its combinations are limited. Hypothesis/objectives: The aim of this study was to evaluate the antifungal effects of NAC alone and in combination with other antifungal agents against Malassezia pachydermatis isolated from canine OE. Materials and methods: M. pachydermatis samples were collected from 13 dogs with OE. The final concentration of the inoculum suspensions of M. pachydermatis was 1–5 × 10⁶ colony forming units/mL. The concentrations of the test compounds ketoconazole (KTZ), terbinafine (TER), nystatin (NYS) and NAC were 0.02–300 µg/mL, 0.04–80 µg/mL, 0.16–40 µg/mL and 1.25–20 mg/mL, respectively. The minimum inhibitory concentration (MIC) was measured to evaluate the susceptibility of the M. pachydermatis to KTZ, TER, NYS and NAC. The checkerboard testing method and fractional inhibitory concentration index were used to evaluate the effect of NAC in combination with KTZ, TER and NYS against M. pachydermatis. Results: The MIC90 values of M. pachydermatis were 4.6875–9.375 µg/mL, 1.25 µg/mL, 5–10 µg/mL and 10 mg/mL for KTZ, TER, NYS and NAC, respectively. The synergistic effects of KTZ, TER and NYS with NAC were identified in 0/13, 2/13 and 0/13 isolates, respectively. Conclusions and clinical relevance: NAC had an antifungal effect against M. pachydermatis but did not exert synergistic effects when used with KTZ, TER and NYS. Thus, the use of NAC alone as a topical solution could be considered an effective treatment option for canine OE involving M. pachydermatis.