Antibacterial activity of closantel against methicillin-resistant and itsbiofilm.

Objectives: The antimicrobial resistance of () has become a challenge in the treatment of infectious diseases. It is of great clinical value to discovery effective antimicrobial agents against multi-drug resistant and its biofilms. This study aims to explore the antibacterial activity of the antiparasitic drug closantel against methicillin-resistant and its biofilms through drug repurposing.

Methods: The sensitivity of to closantel was assessed using microbroth dilution and disk diffusion methods. The bacteriostatic and bactericidal activities of closantel were determined by time-kill curves and colony count. Scanning electron microscopy combined with SYTOX Green and DiSC3(5) fluorescence probes were used to study the bactericidal mechanism of closantel. The influence of resistance was assessed by continuous exposure to sub-inhibitory concentrations of closantel. The anti-biofilm activity was evaluated using 96-well plates and crystal violet staining, and cytotoxicity was measured using the CCK-8 assay.

Results: The minimal inhibitory concentration (MIC) of closantel for both methicillin-sensitive and methicillin-resistant ranged from 0.125 to 1.000 μg/mL. Disk diffusion tests showed that 80 μg of closantel created an inhibition zone, which increased in diameter with higher drug amounts. Sub-inhibitory concentrations (0.031 μg/mL) of closantel significantly inhibited proliferation, reducing bacterial turbidity from 0.26±0.00 to 0.11±0.01 (=16.06, <0.001), with stronger inhibition at higher concentrations. Closantel at 0.25×MIC inhibited proliferation for 12 hours, while 1×MIC inhibited it for over 24 hours, with the number of viable bacteria decreasing as the drug concentration increased. Mechanistic studies indicated that closantel effectively disrupted the integrity of cell membranes, significantly increasing SYTOX Green and DiSC3(5) fluorescence intensity. Even after 25 days of continuous exposure to sub-inhibitory concentrations of closantel, no resistance developed. Closantel at 0.0625 μg/mL significantly inhibited biofilm formation, reducing it from 1.29±0.16 to 0.62±0.04 (=11.62, <0.001), showing a clear dose-dependent effect. Closantel at 2 μg/mL also significantly eradicated established biofilms, reducing biofilm mass from 1.62±0.34 to 0.51±0.39 (=4.84, <0.01). Additionally, closantel exhibited extremely low cytotoxicity, with half-maximal lethal concentrations for HepG2 liver cancer cells and normal LO2 liver cells both exceeding 64 μg/mL.

Conclusions: Closantel exhibits strong antibacterial activity against and its biofilm with low cytotoxicity against human cells, making it a promising candidate for new therapeutic strategies against related infections.