Bacteriophage vB_SEEA3_AUVM_23KS1 and Its Endolysin as Biocontrol Agents Against Streptococcus equi subsp. equi, the Causative Agent of Strangles

Strangles, a major contagious disease in horses caused by Streptococcus equi subsp. equi (SEE), is difficult to control because of limitations associated with antimicrobial treatment and persistent carrier states. In this study, we characterized the bacteriophage, vB_SEEA3_AUVM_23KS1, and its multicomponent endolysin, PlyKS1, as potential biocontrol agents against SEE. vB_SEEA3_AUVM_23KS1, with no known genes of concern in its genome, showed lytic activity against 116 of 124 diverse SEE isolates in spot assays, whereas PlyKS1 lysed all 124 SEE isolates tested, including those not lysed by the phage. The broad host range of vB_SEEA3_AUVM_23KS1 could be explained by its ability to recognize cell-surface carbohydrates shared among SEE. Both agents rapidly reduced viable bacterial counts, inhibited biofilm formation, and reduced established biofilm biomass. The simultaneous administration of either agent with the bacterial inoculum significantly improved survival in a Galleria mellonella infection model. The phage remained stable for 14 days at 4 °C and 25 °C, suggesting favorable storage stability. In addition, it retained most of its infectivity after 24 h at 37 °C and remained stable over a broad pH range, supporting its potential therapeutic application. Together, these findings warrant further investigation of vB_SEEA3_AUVM_23KS1 and PlyKS1 as complementary biocontrol agents against SEE for strangles management.