A silver lining for nature-based macromolecular antibacterials

A soluble nanocomposite prepared by microwaving a solution of silver ions and a chitin-derived polymer shows promising antibacterial activity at low concentrations¹. The result, published in the journal Applied Biochemistry and Microbiology, shows that such macromolecular solutions could offer a nature-based alternative to conventional antibacterials.

Chitin is the second-most abundant biopolymer after cellulose, found in the exoskeletons of shrimps and crabs. When chemically modified it forms chitosan, a non-toxic, biodegradable polymer with unique biological and physico-chemical properties being studied for a range of possible applications. It is already used in pharmaceuticals as a filler, and in bandages to reduce bleeding. However, it is chitosan’s potential utility as a nano-scale matrix for the preparation of macromolecular composites that attracted the interest of Valentina Alexandrova and colleagues from the Topchiev Institute of Petrochemical Synthesis, part of the Russian Academy of Sciences.

“Due to increased antibiotic resistance, there’s an urgent need for alternative approaches for the production of antibacterial agents with a different action mechanism,” says Alexandrova. “Metal-polymer nanocomposite materials based on chitosan are of great interest for antimicrobial and antiviral activity.”

Using a water-soluble derivative of the normally insoluble chitosan, the researchers synthesized their nanocomposite chitosan-silver solution by microwaving a water mixture containing the chitosan derivative, silver ions, and glucose as a reducing agent.

“By optimising the conditions for synthesis, such as the concentration of the polymer solution and other reagents, and the duration of microwave irradiation, we were able to achieve even and rapid heating with lower thermal stress, which ensured the rapid and uniform reduction of silver ions, even nucleation and then formation of silver nanoparticles,” says Alexandrova. “This gave a stable macromolecular solution of silver nanoparticles suitable for biomedical purposes such as aerosols, ointments and liquid plasters.”

In collaboration with the Gause Institute of New Antibiotics, the researchers then tested the antibacterial activity of their silver nanoparticle solution by exposing common model bacteria to various concentrations.

“Silver nanoparticles have a high active surface to volume ratio, which gives effective bactericidal activity at low concentrations,” says Alexandrova. “Our in vitro experiments confirmed the antibacterial activity of our macromolecular silver solutions to Bacillus subtilis ATCC 6633 and Bacillus coagulans 429 at concentrations as low as 5.2 micrograms per millilitre.”

The researchers are now investigating the production of films based on their stable macromolecular nanocomposite solutions using chitosan succinamide and other chitosan derivatives.