Phage-antibiotic synergy dismantles multidrug-resistant Enterococcus faecalis infection in SD rat apical periodontitis model.

Apical periodontitis poses a significant challenge, as 80% of cases are linked to Enterococcus faecalis, which is often protected within a biofilm. Phage-antibiotic synergy (PAS) presents a promising approach to overcoming resistance, though evidence from disease-relevant models, particularly in vivo, remains limited. In this study, we demonstrate that the lytic phage PEf771, when combined with either amoxicillin or ampicillin trihydrate, effectively eliminates multidrug-resistant E. faecalis YN771 across various contexts, including planktonic cultures, mature biofilms, ex vivo infected teeth, and in rat models of peritoneal and apical periodontitis infection. Co-treatment reduced planktonic viable counts by up to 5.4 log10 CFU/mL within 12 h and removed 62-68% of preformed biofilm within 4 h. The combination also converted the transient transcriptional repression achieved by phage alone into durable silencing of the biofilm determinants ebpC, gelE, and esp, which remained 68-81% below control levels. In vivo, the regimens preserved animal survival, limited organ damage, and enriched commensal taxa including Akkermansia muciniphila and Ruminococcaceae UCG-014, while histological analysis revealed reduced periapical inflammation and early periodontal-ligament repair. These findings establish PAS as an immediately translatable, resistance-sparing adjunct for refractory endodontic infections and provide a framework for pairing phages with front-line β-lactams against recalcitrant biofilm diseases.