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

## TL;DR

This study explores levamisole's potential to combat biofilm-forming bacteria in canine ear infections, showing it can disrupt biofilms even if it has limited direct antibacterial effects.

## Contribution

The study introduces levamisole as a novel candidate for preventing recurrent otitis externa in dogs by targeting bacterial biofilms.

## Key findings

- Levamisole reduced mature biofilm biomass with MBEC values ranging from 1.17 to 18.75 mg/mL.
- At MIC concentration, levamisole significantly reduced biofilm formation in all isolates over 120 hours.
- The drug disrupted biofilm structure and promoted bacterial cell death over time.

## Abstract

Otitis externa is an inflammation of the external ear canal with a complex and multifactorial etiology associated with predisposing, primary, and perpetuating factors in addition to secondary bacterial and fungal infections. However, the treatment of otitis is challenging when analyzing the emergence of biofilm-associated bacterial infections, which contribute to antimicrobial resistance and recurrence of the disease. In this study, levamisole was evaluated for its antimicrobial and antibiofilm effect against bacterial isolates obtained from dogs with otitis externa. The drug’s activity was assessed against bacteria both in planktonic form and biofilm-embedded. Despite its limited antimicrobial activity, levamisole was able to disrupt biofilm structure, reduce cell cohesion, and promote bacterial cell death over time. Therefore, levamisole may represent a promising strategy to prevent recurrent bacterial otitis in dogs.

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 (p < 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 (p < 0.05). Levamisole demonstrated potential as a preventive approach against biofilm-associated bacterial otitis.