In vitro antibacterial activity of the manuka essential oil from Leptospermum scoparium combined with Tris-EDTA against Gram-negative bacterial isolates from dogs with otitis externa.

Abstract

BACKGROUND: The increasing prevalence of antimicrobial resistance among bacteria in dogs with otitis externa has led to a need for novel therapeutic agents. HYPOTHESIS/OBJECTIVE: To examine the antibacterial effects of manuka oil combined with ethylenediaminetetraacetic acid-tromethamine (Tris-EDTA) against Gram-negative bacteria isolates from dogs with otitis externa. METHODS AND MATERIALS: A total of 53 clinical isolates including Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae ssp. pneumoniae and Proteus mirabilis. Antimicrobial susceptibility was determined using disk diffusion; the minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of manuka essential oil, with or without Tris-EDTA, were investigated. RESULTS: A total of 44 isolates were resistant to at least one antibiotic and 19 strains were multidrug-resistant, with resistance to at least one agent in three or more antimicrobial classes. The MICs and MBCs of manuka oil alone were &#x2265;1% (v/v) and &#x2265;2% (v/v), respectively. There was no antimicrobial effect of Tris-EDTA (1.125:0.3&#xa0;mg/mL) without manuka oil. However, the combination of manuka oil with Tris-EDTA significantly decreased the MICs (ranging from 0.06% to 0.5%, v/v; P&#xa0;<&#xa0;0.001) and MBCs (ranging from 0.06% to 1%, v/v; P&#xa0;<&#xa0;0.001). There also was no significant difference between multidrug-resistant and nonresistant bacterial isolates in terms of the antimicrobial activity of manuka oil with Tris-EDTA. CONCLUSIONS AND CLINICAL IMPORTANCE: The study findings suggest that manuka oil, especially when combined with Tris-EDTA, may be a promising alternative therapeutic option for Gram-negative otic pathogens. Clinical studies are needed to assess potential for in vivo ototoxic effects and efficacy.