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 differently on S. aureus versus E. coli biofilms. Co-culture of biofilms with MSC significantly reduced pellicle area of biofilms of both bacteria, reduced biomass of S. aureus biofilms, and killed live S. aureus bacteria. MSC combined with amikacin also significantly reduced S. aureus biomass to a greater extent compared to amikacin alone. The resolution in detecting differences between groups for E. coli was diminished because of high variation between biofilms treated with MSC between different donors and between control biofilms between experiments.

Using the same experimental system, culture of S. aureus biofilms with MSC in the transwell inserts and PBMC in the bottom wells significantly reduced biofilm size compared to untreated biofilms. Co-culture of MSC and PBMC with S. aureus biofilms also significantly increased detection of multiple SPM on lipid chromatography-mass spectrometry compared to MSC or PBMC cultures alone. Using a commercial equine multiplex bead ELISA, multiple inflammatory cytokines and chemokines were increased when S. aureus biofilms were cultured with MSC and PBMC; however, these were not different from untreated biofilms. Our results indicate that the utility of MSC in combating orthopedic biofilm infections lies in their ability to disrupt the biofilm matrix and promote inflammation resolution. These findings support continued investigation into and optimization of the anti-biofilm mechanisms of MSC.