University of Pennsylvania Kennett Square, Pennsylvania
Streptococcus equi subsp equi (S. equi), significantly impacts the equine industry through chronically infected shedders. We hypothesized that persistent infection is caused by an antimicrobial-tolerant biofilm. Our objective was to determine the capability of S. equi to form a biofilm in vitro. Tryptic soy broth (TSB), equine synovial fluid (“SynF”), and equine plasma solution (“Plasma”) were inoculated with S. equi, incubated, then treated with ceftiofur and counted. To test adherence, S. equi was inoculated into collagen-coated wells containing plasma solution, incubated, and treated with penicillin. Wells were washed to remove non-adherent bacteria and counted. Aggregate architecture was visualized via scanning electron and confocal microscopy. S. equi formed aggregates by 24h post-inoculation in SynF and Plasma. Unlike treated planktonic bacteria (TSB), which showed decreased growth, aggregates displayed antimicrobial tolerance. In a collagen-rich environment analogous to the guttural pouch, S. equi formed an adhesive film resistant to washing and antimicrobials. Microscopy revealed a network of cocci chains surrounded by matrix rich in proteins and carbohydrates. To our knowledge this is the first assay demonstrating S. equi aggregates consistent with biofilm, with hallmarks like antimicrobial recalcitrance, adherence, and complex extracellular matrix structures. Biofilms in vivo differ from in vitro, so further investigations are necessary for completecharacterization. Biofilm formation provides an explanation for asymptomatic carrier horses. Biofilms are metabolically quiescent and protected by extracellular matrix, making them difficult to eradicate with conventional antimicrobial therapy. Future experiments will focus on compounds that eliminate S. equi biofilm and their safety/efficacy in equine applications.
