Towards novel antibiofilm strategies

Abstract

The rise of multidrug resistant bacteria has global implications posing a threat to human health. Bacteria naturally reside in biofilms as complex communities of cells encased in a self-assembled matrix. The biofilm state renders bacteria up to 1000-fold less susceptible to antimicrobial treatments, while unarming the body’s immune response and promoting antibiotic resistance. Biofilms are recognised as the origin of devastating, antibiotic-refractory diseases and are associated with 80% of infections in the body, including chronic rhinosinusitis. The capability of bacteria in biofilms to resist current antibiotic therapies emphasises the need for novel therapeutic strategies. Whilst oral drug delivery is frequently ineffective to treat biofilm-related infections, topical treatments have the potential to deliver higher drug concentrations to the infection-site while reducing systemic side-effects. In this thesis, the development of two innovative topical strategies against antibiotic resistant bacteria and bacterial biofilms were explored, specifically: (i) colloidal silver nanoparticles (AgNPs) and (ii) a treatment combining the iron chelator deferiprone (Def) and the haem analogue gallium-protoporphyrin (GaPP). (i) Whilst the antimicrobial activity of spherical AgNPs is well described in planktonic bacteria, little is known about their antibiofilm effects and the influence of particle shape. AgNP spheres, cubes and stars were synthesised and their cytotoxicity towards human macrophages and human bronchial epithelial cells, as well as their activities against S. aureus, MRSA and P. aeruginosa biofilms were evaluated. While non-desirable toxicity and stability limited the utilisation of AgNP cubes and stars, AgNP spheres showed significant antibiofilm activity against clinically relevant biofilms in vitro and in an in vivo infection model in C. elegans. Moreover, AgNP spheres were physically stable in suspension for over 6 months with no observed loss of antibiofilm activity. This research has led to a phase I human clinical trial that commenced in October 2016 at The Queen Elizabeth Hospital, Woodville, SA, Australia. (ii) The antibiofilm activity of a novel treatment combining Def and GaPP was investigated. These compounds interfere with bacterial iron metabolism, which presents a unique alternative target vital for all human pathogens. Def-GaPP demonstrated synergistic antibiofilm effects against a series of bacteria, including reference strains and multidrug resistant clinical isolates of S. aureus, S. aureus small colony variants, MRSA, S. epidermidis, P. aeruginosa and A. johnsonii. Furthermore, Def-GaPP potentiated the activity of antibiotics. In vitro cell culture studies confirmed no toxicity of Def-GaPP in murine fibroblasts and human bronchial epithelial cells. Moreover, a clinically used chitosan-dextran hydrogel for wound healing was used as a delivery vehicle for Def-GaPP, thereby complementing wound healing effects with strong antibacterial properties. The Def-GaPP gel showed significant antibiofilm activity in an in vitro wound model and in an in vivo infection model in C. elegans. This work resulted in a patent approval. Two innovative strategies (i.e. colloidal AgNPs and Def-GaPP gel) have arisen from this thesis that hold significant promise as topical antibiofilm treatments. Both strategies have potential as alternatives to antibiotics or as adjuvants for the treatment of multidrug resistant bacteria and biofilm-associated infections and are advancing for clinical use.