In vitro evaluation of equine bone-marrow derived mesenchymal stromal cells to combat orthopedic biofilm infections

Infections of fracture fixation implants and synovial structures are a primary cause of complications, increased treatment costs, and mortality in people and horses. Treatment failure is often due to biofilms that are communities of bacteria that are adhered to a surface or to each other and are surrounded in a self-secreted extracellular matrix. The biofilm matrix protects the indwelling bacteria from being killed by antibiotics and the immune system. Biofilms also stimulate chronic inflammation and tissue destruction, including peri-implant osteolysis and subsequent implant failure and chondromalacia with subsequent osteoarthritis. In horses, the resulting lameness, reduced athletic potential, and poor quality of life may necessitate euthanasia. Equine bone marrow-derived mesenchymal stromal cells (MSC) reduce inflammation and promote healing in musculoskeletal injuries and have recently been discovered to have antimicrobial properties. Equine MSC kill planktonic (free-floating) bacteria and prevent biofilm establishment in laboratory models. MSC from mice and people also promote the transition from acute inflammation to tissue regeneration (resolution of inflammation) by secretion of specialized pro-resolving lipid mediators (SPM). Whether equine MSC can disrupt established biofilms of orthopedic pathogens and modulate the inflammatory response to orthopedic biofilms is unknown. Using a novel biofilm-MSC co-culture model, our objectives were two-fold. We investigated whether MSC alone or with amikacin sulfate, an antibiotic used to treat equine orthopedic infections, could reduce biomass, pellicle size, and live bacteria of biofilms of orthopedic infectious agents S. aureus and E. coli. Next, we investigated whether MSC could modulate immune response to S. aureus biofilms by reducing secretion of pro-inflammatory cytokines by peripheral blood mononuclear cells (PBMC) and by secreting SPM. MSC demonstrated partial ability to reduce biofilms but performed different