Squilla-derived chitosan-silver/copper nanocomposites as sustainable biomaterials for targeting multidrug resistant pathogens

Antimicrobial resistance remains a global health challenge, necessitating innovative biomaterials with synergistic antimicrobial potential. Herein, the efficacy of squilla-chitosan (Cs) as a natural polymer was evaluated to combat antibiotic-resistant bacteria and its synergy with 14 commercial antibiotics. The combination of cefepime (FEP) with Cs showed synergistic effects against all tested strains, with fractional inhibitory concentrations (FICI) ranging from 0.14 to 0.31 μg/mL. Chitosan obtained after deacetylation (Cs1 and Cs2) also exhibited synergistic effects with FEP. The incorporation of silver nanoparticles (AgNPs) and copper nanoparticles (CuNPs) onto Cs1 and Cs2 (AgNPs-NCs1, AgNPs-NCs2, CuNPs-NCs1, CuNPs-NCs2) was confirmed by TEM. The checkerboard method revealed that combinations of FEP with AgNPs-NCs1 and AgNPs-NCs2 showed the strongest synergy, particularly for S. aureus and A. baumannii, with FICI values of 0.17 and 0.03 μg/mL, respectively. These combinations resulted in significant growth reductions (3log10 units for Gram-positive and 2log10 units for Gram-negative bacteria) compared to FEP alone. TEM revealed severe damage to P. aeruginosa cells treated with FEP/AgNPs-NCs2. Additionally, the two most effective combinations displayed antifungal activity against Rhizopus nigricans with inhibition zones of 28 mm and 20 mm, respectively. This study highlights the potential of chitosan-based nanoparticles as efficient drug carriers with antimicrobial and antifungal properties.