In vitro antibacterial activity of hemp (Cannabis sativa L.) extract seed oil against multidrug resistant bacterial pathogens in small animal veterinary dermatology

There is an urgent need for alternative antimicrobial therapies in veterinary small animal dermatology due to the limited therapeutic options available for treatment of infections caused by multidrug-resistant bacteria. This study aimed to evaluate the potential of hemp (Cannabis sativa L.) seed oil for topical treatment of localized infections of the skin, such as otitis externa. Antimicrobial activity was determined by broth microdilution using a strain collection of bacterial pathogens associated with skin infections, including Staphylococcus pseudintermedius (n = 120), Staphylococcus aureus (n = 48), and Pseudomonas aeruginosa (n = 26). Checkerboard dilution tests were used to assess the interaction of hemp seed oil with two antimicrobials used for management of otitis externa, gentamicin and enrofloxacin, while in vitro cytotoxicity was evaluated by the cellular 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction assay on mouse fibroblast cell line L929. Minimum inhibitory concentrations (MICs) in staphylococci (0.025-0.2 % v/v) were markedly lower than in P. aeruginosa (>0.4 % v/v). Within S. pseudintermedius, methicillin-resistant strains displayed lower susceptibility compared to susceptible strains. Hemp seed oil showed synergy with gentamicin (Fractional Inhibitory Concentration Index < 0.5), reducing the MIC of gentamicin-resistant S. pseudintermedius strains (≥16 µg/ml) below the clinical susceptibility breakpoint (≤4 µg/ml). No changes in cell viability were observed at concentrations below 2 % v/v. These findings suggest that hemp seed oil could be an effective and safe alternative or adjuvant to conventional antimicrobials for managing otitis externa and other skin focal infections caused by staphylococci, including methicillin-resistant strains.