Particle-Driven Synergistic Antibacterial Effect of Silver-Chitosan Nanocomposites Against Escherichia coli,...

Healthcare-associated infections are often challenging to treat as bacteria can rapidly adapt to conventional antibiotics. Therefore, alternative efficient antibacterials are needed. We hypothesized that combining nanosilver with chitosan would yield synergistically acting nanocomposites that could be used in topical treatments (e.g., wound dressings) and surface coatings, whereas the higher the share of chitosan, the higher the synergistic potency. To prove that, three silver-chitosan nanocomposites (nAgCSs) with different Ag-to-chitosan weight ratios (1:0.3, 1:1, and 1:3) were synthesized, physicochemically characterized and evaluated for their antibacterial potency toward clinically critical Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, and Gram-positive bacteria Staphylococcus aureus. Experiments were conducted in deionized water to minimize the speciation effects of silver. The nAgCSs proved highly antibacterial against E. coli and P. aeruginosa, whereas the nAgCSs with the highest chitosan share were the most potent (24-h MBC values 0.07-0.28 mg Ag/L). As chitosan is a polycationic polymer, we assumed the synergy observed was mainly driven by chitosan attaching to the negatively charged bacterial cells and provoking increased local shedding of silver ions. Our hypothesis was proven by analyzing the contacts between the nAgCSs and bacteria using confocal laser scanning microscopy and flow cytometry and quantifying the bioavailability of silver from the nAgCSs by an Ag-sensing biosensor. To our knowledge, this type of combined hypothesis-driven study on silver-chitosan nanocomposites' synergistic properties has not been conducted before.