Depolymerase as a potent adjunct to polymyxin for targeting KL160 pandrug-resistant Acinetobacter baumannii in a murine bacteremia model.

Acinetobacter baumannii bacteremia caused by pandrug-resistant strains poses a major challenge in intensive care units, necessitating novel therapeutic approaches. Phage-derived depolymerases offer a promising adjunct to conventional antibiotics. However, studies on A. baumannii phage depolymerases have been limited to non-mammalian models. This study investigates the therapeutic efficacy, safety, and potential mechanisms of action of DPO-HL, both as a monotherapy and in combination with polymyxin B, in a murine model of A. baumannii bacteremia. DPO-HL was expressed and purified via Ni-NTA affinity chromatography. Its bactericidal activity was assessed through dynamic killing and biofilm disruption assays. Interaction with human plasma was examined to determine its impact on plasma's bactericidal activity. Synergy with polymyxin B was evaluated by MIC reduction. Safety was assessed via cytotoxicity, haemolysis, and acute toxicity tests. A mouse bacteremia model was established to evaluate therapeutic efficacy via intraperitoneal and intravenous administration. DPO-HL, targeting KL160 capsular polysaccharide, exhibited stability in plasma and enhanced plasma's bactericidal effect. It showed strong synergy with polymyxin B, reducing its MIC by 16-fold, and efficiently eradicated mature biofilms. DPO-HL alone reduced bacterial load and endotoxin levels but rescued only 30% of bacteremia mice. Combination therapy (1.45 mg/kg DPO-HL + 0.5 mg/kg polymyxin B) significantly reduced endotoxin levels and achieved 100% survival, regardless of administration route. This study identifies a KL160-targeting depolymerase and demonstrates its potent synergy with polymyxin B in treating A. baumannii bacteremia, supporting its potential for clinical application.