Antimicrobial activity of thyme oil, oregano oil, thymol and carvacrol against sensitive and resistant microbial isolates from dogs with otitis externa.

BACKGROUND: Multidrug-resistant pathogens present a major global challenge in antimicrobial therapy and frequently complicate otitis externa in dogs. HYPOTHESIS/OBJECTIVES: In vitro efficacy of oregano oil, thyme oil and their main phenolic constituents against bacterial and fungal isolates associated with canine otitis externa were investigated. It was hypothesized that the main phenolic components would have greater antimicrobial activity compared to the relative essential oil. METHODS AND MATERIALS: Antimicrobial susceptibility testing was performed using broth microdilution with spot-plating technique to determine minimum inhibitory and bactericidal/fungicidal concentrations (MICs, MBCs and MFCs). A time-kill kinetics assay was performed to confirm the bactericidal and fungicidal activity of the oils and their phenolic constituents. One hundred bacterial and fungal isolates, including meticillin-susceptible Staphylococcus pseudintermedius (n = 10), meticillin-resistant S. pseudintermedius (n = 10), ß-haemolytic Streptococcus spp. (n = 20), Pseudomonas aeruginosa (n = 20; including 10 isolates resistant to one or two antimicrobials), Proteus mirabilis (n = 20) and Malassezia pachydermatis (n = 20) from dogs with otitis externa were used. RESULTS: Oregano oil, thyme oil, carvacrol and thymol exhibited antibacterial activity against all bacterial and fungal isolates tested. MIC90 values ranged from 0.015 to 0.03% (146-292 µg/mL) for the Gram-positive bacteria and P. mirabilis. For P. aeruginosa and M. pachydermatis, MIC90 values ranged from 0.09 to 0.25% (800-2,292 µg/mL). CONCLUSIONS AND CLINICAL SIGNIFICANCE: Oregano oil, thyme oil, carvacrol and thymol showed good in vitro bactericidal and fungicidal activity against 100 isolates from dogs with otitis externa, including some highly drug-resistant isolates. These essential oils and their main phenolic constituents have the potential to be further investigated in vivo for the treatment of canine otitis externa.

In vitro antimicrobial activity of seven adjuvants against common pathogens associated with canine otitis externa.

BACKGROUND: An antibiotic adjuvant is a chemical substance used to modify or augment the effectiveness of primary antimicrobial agents against drug-resistant micro-organisms. Its use provides an alternative approach to address the global issue of antimicrobial resistance and enhance antimicrobial stewardship. HYPOTHESIS/OBJECTIVES: To determine the antimicrobial activity of a panel of potential antimicrobial adjuvants against common pathogens associated with canine otitis externa (OE). ANIMALS/ISOLATES: A number of type strains and clinical isolates (n = 110) from canine OE were tested including Staphylococcus pseudintermedius, ß-haemolytic Streptococcus spp., Pseudomonas aeruginosa, Proteus mirabilis and Malassezia pachydermatis. METHODS AND MATERIALS: Antimicrobial activities of monolaurin, monocaprin, N-acetylcysteine (NAC), polymyxin B nonapeptide, Tris-EDTA, Tris-HCL and disodium EDTA were tested using microdilution methodology according to CLSI guidelines. RESULTS: N-acetylcysteine, Tris-EDTA and disodium EDTA had antimicrobial activity against both type strains and otic pathogens. The other adjuvants tested had limited to no efficacy. NAC had a minimal inhibitory concentration (MIC) range of 2,500-10,000 µg/mL for the various organisms. Pseudomonas aeruginosa isolates were eight times more susceptible to disodium EDTA in the presence of Tris-HCL in comparison to disodium EDTA alone. Malassezia pachydermatis isolates were most susceptible to Tris-EDTA (MIC90  = 190/60 µg/mL) and disodium EDTA (MIC90  = 120 µg/mL). CONCLUSIONS AND CLINICAL RELEVANCE: N-acetylcysteine, Tris-EDTA and disodium EDTA have intrinsic antimicrobial activity and represent promising adjuvants that could be used to enhance the efficacy of existing antibiotics against Gram-negative and multidrug-resistant bacterial infections. These agents could be combined with other antimicrobial agents in a multimodal approach for mixed ear infections in dogs.