Minimum inhibitory concentration of essential oils against Staphylococcus aureus isolated from dogs with external otitis

Canine otitis externa is a disease that affects the external acoustic meatus of about 20% of dogs at some point in life, without predilection for race, age, or sex. It is a multifactorial disease whose etiology involves several microorganisms, detaching Staphylococcus aureus. Antimicrobials are the basis for treating this illness; however, due to the increase in antimicrobial resistance, conventional drugs have become ineffective, requiring the search for alternative therapies. In this context, essential oils (EOs) have great therapeutic potential due to their broad antimicrobial action. This study aimed to evaluate the minimum inhibitory concentration (MIC) in order to measure the MIC50 and MIC90 of gentamicin and EOs of Eugenia caryophyllata, Thymus vulgaris, Cymbopogon winterianus, Cymbopogon citratus, and Cinnamomum cassia against 62 Staphylococcus aureus strains isolated from the external acoustic meatus of dogs. All EOs showed antibacterial action against the studied microorganisms, and their MIC50 and MIC90 were as follows: Eugenia caryophyllata, 2.42 mg mL(-1) and 7.25 mg mL(-1); Thymus vulgaris, 9.51 mg mL(-1) and 22.94 mg mL(-1); Cymbopogon winterianus, 26.78 mg mL(-1) and 157.79 mg mL(-1); Cinnamomum cassia and Cymbopogon citratus, lower than 16.48 and 27.81 mg mL(-1), with the same MIC for all isolates. The MIC50 and MIC90 found for gentamicin were 1 mu g mL(-1) and 8 mu g mL(-1). The MIC range found to antibiotic in this assay was 0.5 mu g mL(-1) to 128 mu g mL(-1), and the isolates were classified as susceptible [48 strains (77.41%) - MIC range of 0.5-4.0 mu g mL(-1)], intermediate [eight strains (12.90%) - (MIC = 8.0 mu g mL(-1)], or resistant [six strains (9.68%) - MIC = 16 mu g mL(-1)]. The results, according to the in vitro assays, showed that resistance to gentamicin, one of the antimicrobials most commonly used to treat canine otitis, is present in the Staphylococcus aureus population evaluated. Additionally, the tested EOs have great potential for therapeutic use, however future studies should be carried out to evaluate their in vivo efficacy.