Erythromycin-loaded nanodroplets as adjuvant therapeutics for infected chronic wounds caused by Streptococcus pyogenes

Introduction: Chronic wounds are typically characterized by persistent tissue hypoxia, exacerbated inflammation, and impaired matrix remodeling. Moreover, chronic wounds are often worsened by microbial infections, with antimicrobial therapies being hindered by emerging resistant strains. Streptococcus pyogenes infections vary from superficial infection of the pharynx to serious skin and soft tissue ones that can lead to lethal invasive disease, despite antibiotic treatment. The use of nanoparticles has advantages in treating many dermatological conditions. Nanoparticle applications facilitate the body’s response to foreign pathogens by improving innate and adaptive immune responses, increasing the effectiveness and reducing the adverse effects of antimicrobials and other therapeutic agents. The aim of this project is to assess the potential antimicrobial effect of oxygen free nanodroplets (OFNDs) and oxygen loaded nanodroplets (OLNDs) alone or coupled with erythromycin (Ery) in skin and soft tissue infections sustained by S. pyogenes, along with toxicity on human keratinocytes. Materials and methods: Low molecular weight chitosan-shelled and 2H,3H-decafluoropentane-cored NDs, both OFNDs and OLNDs, and Ery-loaded NDs (Ery-LNDs) were prepared and characterized for physico-chemical parameters and drug release kinetics. Subsequently all NDs were tested for biocompatibility with human skin cells. Complementary analysis by confocal microscopy was also performed to study the physical interaction between NDs and S. pyogenes cell wall/membranes. Finally, bacterial growth was monitored upon incubation with or without NDs for increasing times (4, 6, 24 hours). Results: All NDs were not toxic to human keratinocytes. A prolonged release kinetics of Ery from the NDs was in vitro demonstrated without initial burst effect. Analysis by confocal microscopy showed OFND and OLND internalization by S. pyogenes already after 3 h of incubation. According to cell counts, OFNDs and OLNDs displayed long term antimicrobial efficacy against S. pyogenes significantly inhibiting bacterial growth up to 24 h, thank to known chitosan antimicrobial properties. Intriguingly, Ery-LNDs were generally more effective, than erythromycin alone, in counteract streptococcal growth, probably due to Ery-LND internalization by bacteria. Discussion and Conclusions: Taken together, these results support the hypothesis that NDs are capable antibacterial/skin-friendly devices and proper Ery-NDs might be a promising strategy for the topical treatment of streptococcal skin infections.