Activities and mechanisms of oregano, marjoram and rosemary essential oils against Malassezia pachydermatitis isolates from canine and feline otitis against Malassezia pachydermatitis isolates from canine and feline otitis

Malassezia pachydermatis is an opportunistic yeast found in the ear canal of small animals; however, the current azole-based therapy applied to it has failed to achieve clinical success due to the antifungal resistance. This issue has encouraged the studies in natural products, such as Origanum vulgare (oregano), Origanum majorana (marjoram) and Rosmarinus officinalis (rosemary) essential oils, although their mechanism of action remains unclear. Malassezia pachydermatis specimens deriving from otitis cases in dogs (n = 22) and cats (n = 2) were subjected to CLSI M27-A3. Sorbitol protection and ergosterol effect were analyzed to investigate their mechanism of action. Fungistatic (MIC) and fungicidal (MFC) activities were observed for oregano (MIC$_{90}$/MFC$_{90}$: 0.625 mg/mL); marjoram (MIC$_{90}$/MFC$_{90}$: 2.5 mg/mL) and rosemary MIC$_{90}$/MFC$_{90}$ > 2.5 mg/mL). Oregano showed superior antifungal effect even at lower MIC and MFC values. All three oils acted on cell wall and at complexation to fungal ergosterol. By gas chromatography (GC-FID), carvacrol was the major compound found in oregano (73.9%); 1,8-cineole was for marjoram and rosemary (20.9% and 49.4%, respectively). These findings support the potential use of these essential oils to treat canine and feline otitis caused by Malassezia pachydermatis.