Novel Therapeutic Strategies for Staphylococcus aureus Biofilm Infections

Staphylococcus aureus biofilm infections — particularly those associated with indwelling medical devices and orthopaedic implants such as periprosthetic joint infection (PJI) — remain one of the most refractory problems in clinical infectious diseases. Eradication is constrained not only by classical antimicrobial resistance, but by three interlocking determinants: (i) limited and non-uniform effective drug exposure within the extracellular polymeric matrix, (ii) physiological heterogeneity and tolerance among biofilm-embedded subpopulations, and (iii) active immune evasion combined with foreign-body-associated persistence. This thesis adopts a constraint-driven, translational view of antibiofilm therapy and asks whether next-generation modalities can deliver sufficient, localised, and durable antibacterial action under biofilm-relevant boundary conditions while remaining within an acceptable safety window. The work integrates three complementary therapeutic paradigms.

In Chapter 2, a targeted radioimmunotherapy (RIT) approach was developed by conjugating the anti–wall teichoic acid monoclonal antibody 4497-IgG1 to actinium-225 (α-emitter) or lutetium-177 (β-emitter) via a DOTA chelator. Radiolabelled constructs achieved high radiochemical purity (~95–96%) and preserved immunoreactivity (~0.8), yielding multi-log reductions of planktonic S. aureus and measurable, dose-dependent killing in established biofilms, illustrating how radiation physics and biofilm architecture jointly determine achievable bactericidal activity.

Chapter 3 provides a structured review of aptamer-based strategies for bacterial diagnostics and therapy, framing diagnostics and delivery as a coupled system and identifying nuclease degradation, in vivo instability and the need for chemical modification as key translational bottlenecks.
In Chapter 4, a dual-SELEX workflow combining recombinant protein A (SpA) selection with whole-cell selection across multiple S. aureus strains, followed by next-generation sequencing, yielded five cross-enriched ssDNA aptamers with nanomolar affinity (Kd ≈ 20–57 nM). The lead candidate, optimised by 5′ PEGylation and a 3′ C8-alkyne modification, blocked the SpA–IgG interaction (Inhmax≈ 64%, EC50≈ 137 nM), inhibited early biofilm formation, enhanced neutrophil chemotaxis, and improved opsonophagocytic killing by ~0.5 log10 beyond antibody opsonisation alone — re-establishing antibody-dependent innate immunity as a therapeutic lever.

Chapter 5 describes a modular nanomedicine in which chiral histidine-stabilised gold nanoclusters (AuNCs) were encapsulated in mildly cationic lipid nanoparticles (AuNCs@LNP; ~100–120 nm, ~90–94% encapsulation efficiency). In established USA300 biofilms, free DL-AuNCs reduced viable burden by up to 5.6 log10 CFU/mL, while LNP encapsulation enhanced matrix-level engagement and biomass removal and reduced implant-associated bacterial burden by 2.6 log10 CFU in a murine subcutaneous implant model, defining a clear efficacy–tolerability window for locally delivered antibiofilm nanomedicines.

Collectively, this thesis reframes therapeutic success against S. aureus biofilm disease: rather than relying on “stronger drugs,” durable control will likely require mechanism-matched, locally deployable and combinable strategies that jointly address delivery barriers, biofilm architecture and immune dysfunction.