Levamisole as a Strategy Against Bacteria from Canine Otitis Externa: An In Vitro Evaluation

Canine otitis externa is a prevalent condition, and inadequate treatment may favor the emergence of multidrug-resistant microorganisms. Drug repurposing provides an alternative approach for the management of this disease. Thus, this study aimed at assessing the antimicrobial and antibiofilm properties of levamisole against bacterial strains recovered from dogs with otitis externa as well as its influence on biofilm growth dynamics during 120 h. A total of 50 clinical bacterial isolates were subjected to analysis. Planktonic bacterial susceptibility to levamisole was assessed by broth microdilution to determine the minimum inhibitory concentration (MIC), the lowest concentration that completely inhibits bacterial growth. The activity against mature biofilms was assessed by determining the minimum biofilm eradication concentration (MBEC). The effect of levamisole on biofilm formation was evaluated at the MIC and at two subinhibitory concentrations, with daily readings recorded at 48, 72, 96, and 120 h. MICs of levamisole ranged from 0.58 to 2.34 mg/mL. Levamisole reduced the biomass of mature biofilms (<i>p</i> < 0.05), with MBEC values ranging from 1.17 to 18.75 mg/mL, and biofilm formation was significantly reduced at the MIC concentration (1.17 mg/mL) for all isolates for 120 h (<i>p</i> < 0.05). Levamisole demonstrated potential as a preventive approach against biofilm-associated bacterial otitis.