Novel lytic phage vB-Vp-ZX1018 potentiates ofloxacin activity against multidrug-resistant Vibrio parahaemolyticus infections.

The increasing prevalence of multidrug-resistant (MDR) Vibrio parahaemolyticus poses a growing threat to public health and aquaculture, underscoring the need for alternative therapeutic strategies. In this study, a clinically derived MDR Vibrio parahaemolyticus strain (Vp1018) was characterized; its genome encodes multiple antimicrobial resistance determinants and exhibits clear pathogenicity despite lacking the canonical virulence genes tdh and trh. Using Vp1018 as the host, a lytic phage, vB-Vp-ZX1018, was isolated and identified as a Siphoviridae (genus Mardecavirus) double-stranded DNA phage with a high burst size, strong physicochemical stability, and no detectable virulence or resistance genes, supporting its potential as a therapeutic antibacterial agent. In vitro bactericidal assays demonstrated marked phage-antibiotic synergy (PAS) between vB-Vp-ZX1018 and ofloxacin (OFL), in which low phage inputs combined with sub-inhibitory antibiotic concentrations achieved effective bacterial suppression while substantially reducing the required antibiotic dose. In vivo, PAS markedly improved therapeutic outcomes. In the Galleria mellonella model, the combination increased survival from 0% to 80% and reduced the EC50 of OFL by ∼15.85-fold compared with antibiotic monotherapy. These benefits were further validated in the mouse oral infection model, where combination treatment produced the highest survival rates, substantially lowered intestinal and fecal bacterial burdens, and markedly alleviated histopathological injury relative to either agent alone. Taken together, our data indicate that vB-Vp-ZX1018 represents a safe therapeutic candidate and that rationally optimized PAS may offer an effective strategy for the treatment of MDR Vibrio parahaemolyticus infections.