The attract–kill inhibition mechanism in Ag/chitosan hydrogel for long-acting control of Ralstonia solanacearum

In this study, we developed an innovative eco-friendly antiseptic nanocomposite, specifically a silver nanoparticle (Ag NP)-immobilized chitosan hydrogel (Ag@Cs/Cs^h gel), synthesized through a cross-linking reaction involving polyvinylpyrrolidone (PVP) and tetraethyl orthosilicate (TEOS). This nanocomposite was designed to combat Ralstonia solanacearum (R. solanacearum) infections, particularly in Solanaceae plants. Chitosan of varying molecular weights (50-190 kDa for Cs and 300-375 kDa for Cs^h) was used to control the swelling efficacy and regulate the release of Ag NPs. The gels were loaded with either 0.1 or 0.25 mg of Ag NPs and assessed for their antibacterial efficacy against R. solanacearum at various concentrations using the turbidity method. At 5 × 10⁴ CFU mL^-1, all gel samples, except for controls, exhibited notable antibacterial activity lasting up to 18.5 days. At higher concentrations of 5 × 10⁶ and 5 × 10⁸ CFU mL^-1, the 0.25-Ag@Cs/Cs^h gel achieved nearly 100% eradication over 18.5 days. Notably, parallel experiments with various concentrations of Ag NPs alone showed limited antibacterial effectiveness against R. solanacearum. Therefore, we propose an "attract and kill" antibacterial mechanism to elucidate the enhanced disinfection capability of the Ag@Cs/Cs^h gel. Initially, the positively charged chitosan hydrogel attracts negatively charged bacteria, followed by steady Ag⁺ ion release to eradicate them, providing potent and prolonged antibacterial efficacy.