The anion-exchange capacity of the cell-wall sulfated polysaccharide of the red microalga Porphyridium sp. can be exploited for the complexation of metal ions (e.g., Cu, Zn, Ag) to produce novel materials with new bioactivities. In this study, we investigated this algal polysaccharide as a platform for the incorporation of copper as Cu2O. Chemical and rheological characterization of the Cu2O–polysaccharide complex showed that the copper is covalently bound to the polysaccharide and that the complex exhibits higher viscosity and conductivity than the native polysaccharide. Examination of the complex’s inhibitory activity against the bacteria Acinetobacter baumannii, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, and Bacillus subtilis and the fungus Candida albicans revealed a relatively high antimicrobial activity, especially against C. albicans (92% growth inhibition) as compared to the polysaccharide and to Cu2O alone. The antibiofilm activity was also found against P. aeruginosa PA14 and C. albicans biofilms. An atomic force microscopy examination of the surface morphology of the complex revealed needle-like structures (spikes), approximately 10 nm thick, protruding from the complex surface to a maximum height of 1000 nm, at a density of about 5000/μm2, which were not detected in the native polysaccharide. It seems that the spikes on the surface of the Cu2O–polysaccharide complex are responsible for the antimicrobial activities of the complex, that is, for disruption of microbial membrane permeability, leading to cell death. The study thus indicates that the superior qualities of the novel material formed by complexion of Cu2O to the polysaccharide should be studied further for various biotechnological applications.

Chitosan hydrogels have widespread industrial applications due to their versatility and antimicrobial potential. However, their applicability can be limited by poor mechanical properties or because their fabrication requires the use of toxic compounds which can leach into their environment. Additionally, their poor water solubility under neutral conditions restricts their fabrication and applications to low pHs. Here, we synthesized a modified derivative [N-(2-hydroxy-3-trimethylammonium)propyl] chitosan chloride (HTCC), which is soluble and antimicrobial at neutral pH, and used it to compare the effect of three crosslinking agents on the formation of industrially relevant hydrogels. The crosslinkers sodium tripolyphosphate (TPP), glutaraldehyde (GA), and citric acid (CA) were compared in terms of their impact on the swelling potential, hydrophobicity, and mechanical properties. Swelling degrees ranging from 350 to 2350% for GA and TPP, respectively, were observed. Silver nanoparticles (Ag NPs) were synthesized in situ, leading to improved mechanical properties as evidenced by an increase in the Young modulus from 10.3 MPa for TPP-crosslinked systems to 87.4 MPa for TPP-crosslinked/Ag NP composites. Ag ion release rather than Ag NP leaching was determined to be the dominant strategy for antimicrobial action against Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, and Acinetobacter baumanii, causing significant increases (p < 0.05) in clearance ratios and biofilm shape factors, pointing to a synergism between the crosslinked HTCC and Ag NPs. The exceptional broad-spectrum antimicrobial/antifouling properties of these materials regardless of the crosslinking method allow for selection of different preparation techniques to tune desired traits for diverse industrial applications.

Metal–polysaccharides have recently raised significant interest due to their multifunctional bioactivities. 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 (roughness). This hypothesis was further examined here using other Cu–PS complexes (i.e., monovalent-Cu2O, CuCl and divalent-CuO, CuCl2). 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 the superior inhibition of microbial growth and viability and biofilm formation. 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, FTIR 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 and antibiofilm activities of the complexes.

No abstract