In Vitro Antibacterial Potentials of Cinnamomum verum J. Presl (Cinnamon), Citrus x aurantium L. (Petitgrain) Pelargonium graveolens L’Hér (Rose Geranium), and Thymus vulgaris L. (Thyme) Essential Oil-Containing Ointments Against Canine Otitis Extern

Background/Objectives: Otitis externa is a common infection in dogs. The most important aetiological agent is Pseudomonas aeruginosa, and an increasing number of cases are proving difficult to treat due to this pathogen’s antibiotic resistance. Methods: We determined the composition of cinnamon (Cinnamomum verum J. Presl), petitgrain (Citrus x aurantium L.), rose geranium (Pelargonium graveolens L’Hér), and thyme (Thymus vulgaris L.) essential oils (EOs) using gas chromatography-mass spectrometry (GC-MS). We then used the bioautographic method to identify antibacterial components. Two-and-a-half- and five percent concentrations of the essential oils were added to four different ointment bases, such as hydrogelum methylcellulose, unguentum oleosum, unguentum stearini, and vaselinum cholesterinatum, and the in vitro antibacterial effects were tested on lawns of six P. aeruginosa isolates using the antibiotic-containing product Marbogen complex as a control. The keepability of the methylcellulose-based ointments was monitored at 4 °C and 27 °C for 12 months, with efficacy losses evaluated monthly. Results: Hydrogel methylcellulose (HM) was the most effective at supporting the therapeutic efficacy of each oil, reducing the bacterial count on the exposed area by 99.999%, 100%, 100%, and 100% in the case of 5% cinnamon, petitgrain, rose geranium, and thyme, respectively. EO-based HM ointments take effect within 15 min, with only slight improvement after 30, 45, and 60 min. Cinnamon was the most effective when applied in the HM matrix. In contrast to the HM and US gel matrices, the antibacterial supportive effects of Unguentum oleosum and Vazelinum cholestrinatum were drastically lower. The antimicrobial efficacy of essential oils in hydrogel formulation varied significantly depending on the type of oil (p < 0.001), its concentration (p < 0.001), the exposure time (p < 0.001), and the P. aeruginosa isolate used (p < 0.001). Cinnamon oil demonstrated the highest efficacy, achieving near-complete bacterial elimination across all isolates within 15 min. There was no significant difference between the 2.5% and 5% concentrations (EMM: 0.016 vs. 0.004; p = 0.904). Conclusions: Our results demonstrate that essential oil-based products can be effective alternatives to antibiotics when used in conjunction with an effective gel matrix.