Structure–Activity-Relationship Studies to Elucidate Sources of Antibacterial Activity of Modular Polyacrylate Microgels

Emerging infections of unknown origin and increasing bacterial resistance against available antibiotics necessitate the development of different antimicrobial agents with unconventional mechanisms of action. A promising strategy to meet this need may be found by combining polymeric scaffolds with transition metals, e.g., by decorating polyacrylate-based microgels with Cu­(II) complexes. A series of structure–activity relationship studies using broth microdilution assays with such materials and Staphylococcus aureus concluded that the antimicrobial activity of microgels can be tailored during their synthesis by choice of co-monomers, by design of the binding strength between Cu­(II) ions and backbone ligands, and by selection of the counter ions for coordination to the metal complexes. A microgel ^Cu₂LP­(EG) (L = VBbsdpo) with an optimized minimal inhibitory concentration of 0.39 ± 0.03 μg/mL is thereby derived and synthesized from 60 mol % of cross-linking ethylene glycol dimethacrylate, 40 mol % butyl acrylate, 0.5 mol % VBbsdpo ligand with 1 mol % Cu­(II) ions, and 5 mol % ethylene glycol as counter ions. The antimicrobial activity of the microgel has a lifetime of over 18 months at ambient temperature. Bactericidal activity of the same microgel is observed by replating assays in less than 15 min when exposing S. aureus to microgel concentrations of 1.5-fold of its minimum inhibitory concentration (MIC) value or higher. Furthermore, spectrophotometric evaluations at 260 nm revealed time- and concentration-dependent release of intracellular bacterial components after interactions with the microgel indicating irreversible damage to the bacterial cell membrane as a possible mechanism of activity. Preliminary results indicate that the selected microgels are not cytotoxic toward human dermal fibroblasts at MIC value concentrations for over 20 h.