Biomaterial-induced inhibition of Staphylococcus aureus biofilm formation

Abstract

Biomaterial-associated infection (BAI) is a severe complication linked to medical device implantation, causing significant patient suffering and high healthcare costs. The pathogenesis of BAI is complex and not fully understood. Infections often occur during the peri-implantation period due to microbial contamination of the implant and surrounding tissues. Staphylococcus aureus is one of the primary causative agents of BAI, due to its array of toxic factors, biofilm formation, and immune evasion mechanisms, which exploit implant-associated local immunocompromise. The ongoing threat of antimicrobial resistance necessitates new strategies to treat and prevent BAI. This thesis aimed to evaluate anti-virulence strategies and infection-resistant biomaterials that intend to control BAI through the modulation of S. aureus pathogenicity, incorporation of bactericidal metals, prevention of biofilm formation, improvement of the local immune response, and enhancing tissue integration. Quorum sensing (QS) inhibition of S. aureus by sodium salicylate (NaSa) reduced the overall virulence and toxicity of S. aureus (Paper I). While NaSa did not fully prevent biofilm formation on titanium (Ti), it increased the susceptibility of S. aureus biofilms to antimicrobial agents like rifampicin and silver. The effect of NaSa on biofilm formation depended on the QS type (agr I-IV) and the material surface (polystyrene, Ti, or 3D collagen wound model). Incorporating metallic ions such as zinc and copper (Paper II and III) into an iron-manganese alloy and Ti, respectively, reduced S. aureus biofilm formation. Zinc ion release significantly reduced the biovolume and growth rate of S. aureus biofilms, while copper ion release was bactericidal towards S. aureus lab and clinical strains, achieving complete eradication in some conditions. Copper also increased phagocytosis of heat-killed S. aureus by THP-1 macrophages and contributed to their polarisation towards both M1 and M2 phenotypes. In Paper IV, PEGylated-PDLLA coatings on Ti proved cytocompatible and significantly reduced biofilm formation of multi-resistant S. aureus. This disruption increased the susceptibility of S. aureus to several antibiotics, and adding silver provided complementary bactericidal effects without impacting cytocompatibility. In Paper V, “race for the surface” experiments with macrophages and S. aureus on nano/microscale laser-modified Ti surfaces highlighted the importance of host tissue integration as a preventive strategy against BAI. When S. aureus formed a biofilm before the arrival of macrophages, S. aureus won the race, secreting virulence factors, eliciting strong cytotoxicity, and promoting biofilm persistence and intracellular survival. When macrophages reached the surface first or simultaneously with S. aureus, phagocytic efficacy improved, resulting in fewer viable intracellular S. aureus and increased macrophage colonisation. In conclusion, the investigated strategies show potential to enhance biomaterial infection resistance for the clinical management of BAI. Balancing antimicrobial properties with host integration is complex due to the multifaceted pathogenesis of BAI, making it unlikely that any single strategy will resolve the issue. This thesis provides new insights into macrophage-S. aureus interactions at the biomaterial surface and represents an in vitro proof-of-concept for the potential clinical translation of these antimicrobial strategies.