In Vitro and In Vivo Antibacterial Efficacy of a Ciprofloxacin Delivery System Based on Streptococcus suis Extracellular Vesicles

Conventional antibiotics exhibit limited ability to penetrate host cell membranes, making intracellular bacterial infections difficult to eradicate completely. As naturally derived nanoscale membrane structures, bacterial extracellular vesicles (EVs) possess excellent biocompatibility and intrinsic transmembrane transport capability, thereby demonstrating unique advantages for in vivo drug delivery. The present study investigated the feasibility of using EVs derived from the avirulent Streptococcus suis T15 as novel carriers for ciprofloxacin delivery. We also comprehensively evaluated the biosafety and anti-infective efficacy of this nanodrug delivery system in vitro and in vivo. Cytotoxicity assays, live/dead cell staining, and hemolysis analyses demonstrated that T15-derived EVs at concentrations below 50 μg/mL did not cause significant cellular damage or hemolysis. Serum biochemical analyses in mice further confirmed the absence of obvious organ toxicity, indicating favorable biosafety within the tested concentration range. Ciprofloxacin was successfully loaded into EVs using a combination of ultrasonication and electroporation, achieving a drug concentration of 438.6 μg/mL and a loading efficiency of 10.96%. The ciprofloxacin-loaded EVs (EV-CIP) exhibited significantly greater antibacterial activity than free ciprofloxacin against both intracellular bacteria and fluoroquinolone-resistant strains exhibiting efflux pump activity. Evaluation in animal infection models showed that EV-CIP markedly reduced mortality in infected Galleria mellonella larvae. It also decreased bacterial burdens in multiple mouse organs and significantly alleviated histopathological damage. These results collectively suggest that EVs derived from the avirulent S. suis T15 were safe and effective within the tested concentration range and experimental conditions. The EV-based ciprofloxacin delivery system substantially enhanced the clearance of intracellular pathogens and fluoroquinolone efflux pump-positive bacteria, suggesting its potential application in the treatment of difficult-to-treat bacterial infections. This study provides a theoretical and experimental basis for the further development of novel EV-based anti-infective drug delivery strategies for livestock and poultry.