In vitro antimicrobial activity of essential oils against Malassezia pachydermatis isolated from canine ear swabs

The yeast Malassezia pachydermatis (MP) can cause canine otitis. The in vitro efficacy of 22 commercially available essential oils (EO) was tested against 15 isolates of MP rom canine ear swabs derived from routine diagnostic of Laboklin using the agar disk diffusion test (aromatogram).

Briefly, an agar plate is inoculated with MP and a paper filter disk with 10 µl of the EO is placed in the middle of the plate. After incubation (4 days, 36 °C) the zone of inhibition of yeast growth is determined. The in vitro efficacy of EO was classified as follows: not sensitive (< 8 mm), slightly sensitive (8 – 13 mm), moderately sensitive (14 – 19 mm), very sensitive (≥20 mm) and extremely sensitive, if total growth was inhibited. The EOs were also tested in a dilution of 1:5 in sesame oil and in case of a growth inhibiting effect in this dilution in a 1:10 dilution as well.

Lemon grass, winter savory, oregano and rose geranium oil showed mainly extremely inhibitory effects. Palamarosa, indian melissa, Thyme thymol 19%, thyme, thyme linalool, cinnamon leaf, clove and coriander seed oil were effective against all tested isolates, but in different degrees. Manuka, fennel, tea tree, lavandin super, lavender fine, lemon, clary sage and angelica root oil showed inhibiting activity in some cases. Ravintsara and neroli oil were not effective. In a dilution of 1:5 only some EOs had inhibitory effects: Lemon grass, oregano, palmarosa, cinnamon leaf and winter savory oil were effective in all cases. Clove, indian melissa and rose geranium oil had an inhibitory effect in some cases. In a dilution of 1:10 only lemon grass oil was effective, thus being the most potent of the tested EO.

These in vitro results identify some EO as a promising candidate for therapy of MP associated canine otitis.