Spikes formed on the surface of Cu-Polysaccharide complexes from red microalgae are responsible for its antibacterial activity

The antimicrobial activity of a complex of Cu₂O with the sulfated polysaccharide (PS) of the marine red microalga Porphyridium sp. was previously attributed to spikes formed on the complex surface. This hypothesis is further examined using other Cu-PS complexes, i.e., two with monovalent (Cu₂O, CuCl) and two with divalent (CuO, CuCl₂) copper. The nanostructure parameters of the monovalent complexes, namely, longer spikes (1000 nm) and greater density (2000-5000 spikes/µm²), were found to be related to superior inhibition of microbial growth and viability and of biofilm formation. Accordingly, when Escherichia coli TV1061, used as a bioluminescent test organism, was exposed to the monovalent Cu-PS complexes, enhanced bioluminescence accumulation was observed, probably due to membrane perforation by the spikes on the surface of the complexes and consequent cytoplasmic leakage. In addition, differences were found in the surface chemistry of the monovalent and divalent Cu-PS complexes, with the monovalent Cu-PS complexes exhibiting greater stability (ζ-potential, FT-IR spectra, and leaching out), which could be related to spike formation. This study thus supports our hypothesis that the spikes protruding from the monovalent Cu-PS surfaces, as characterized by their aspect ratio, are responsible for the antimicrobial (bacteria and fungus) and antibiofilm activities of the complexes.