Inflammation is Induced by Intracellular MRSA Colonization of Diabetic Foot Ulcers

Background: Persistent wound infection frequently complicates diabetic foot ulcers (DFU) treatment and is a leading cause of amputations. Dramatic increase in community-acquired Methicillin resistant Staphylococcus aureus (MRSA) infections have resulted in difficult-to-treat colonization and biofilm formation in DFUs.

Objectives: We hypothesized that intracellular MRSA colonization contributes to recurrent infection, inflammation, and delayed wound healing in DFUs. We assessed intracellular MRSA in DFUs, host innate immune response (perforin-2) and levels of pyroptosis, an inflammatory form of cell death triggered by intracellular bacterial infection.

Materials and Methods: In a 4-week longitudinal clinical study, patients with DFUs (n=23) were treated using standard of care with tissue sample collections at initial visit and 4 weeks later. DFUs were categorized as non-healers or healers based on wound size reduction. Confocal microscopy of immunostained MRSA was used to identify bacterial internalization, Western Blot analysis quantified pyroptosis, and ELISA was used to quantify levels of IL-1ß.

Outcomes: Intracellular MRSA was detected by immunofluorescence in DFU tissue samples that clinically showed no evidence of infection. Using confocal imaging we confirmed that MRSA grown in a biofilm is capable for intracellularly infecting human keratinocytes to the same extent as planktonic MRSA. Perforin-2 was found to be suppressed in DFU tissue, allowing for MRSA accumulation. Increased levels of pyroptosis markers AIM2 and gasdermin-D coupled with pro-inflammatory IL-1ß were found in tissue of non-healing when compared to healing DFU.

Benefits: Inflammation and DFU chronicity is modulated in part by perforin-2 suppression resulting in intracellular MRSA accumulation and induction of pyroptosis. Our data indicate the need to address intracellular MRSA in DFU even in patients with no clinical signs of infection and reveal perforin-2 as novel therapeutic target for treatment of persistent DFU infections.