Lectin-Coated Silver/Silver Chloride Nanoparticles in Combination with Gentamicin: A Strategy to Preserve Antibiotic Efficacy at Lower Doses Against Pathogenic Planktonic Bacteria

The rampant overuse of antibiotics in recent decades has significantly contributed to the emergence of antibiotic resistance. This highlights the need for new antibacterial strategies to reduce dependence on traditional antibiotics and slow the development of resistance. Concanavalin A-coated silver/silver chloride nanoparticles (ConA/Ag/AgCl-NPs) offer a promising alternative, combining the antimicrobial activity of metallic silver with the ability of concanavalin A to interact with both membrane carbohydrates and antibiotic gentamicin. The aim of this study is to investigate the synergistic antibacterial activity of gentamicin combined with ConA/Ag/AgCl-NPs against pathogenic bacteria. Our results confirmed that gentamicin interacts with ConA/Ag/AgCl-NPs, altering their plasmonic resonance properties. The combination of gentamicin with ConA/Ag/AgCl-NPs significantly reduced the minimum inhibitory concentration (MIC) values of both, from 0.19 μg/mL and 0.04 μg/mL to 0.02 μg/mL and 0.01 μg/mL, respectively, required to inhibit planktonic <i>Staphylococcus aureus</i>. Similarly, the MIC values required to inhibit planktonic <i>Pseudomonas aeruginosa</i> reduced from 0.39 μg/mL and 0.02 μg/mL to 0.02 μg/mL and 0.0012 μg/mL, respectively. In both cases, a reduction of more than 90% in the amount of gentamicin used was observed. These data demonstrate synergistic antibacterial activities, as revealed by the fractional inhibitory concentration indexes below 0.5. Additionally, a killing kinetic assay confirmed bactericidal effects of the combination against both strains tested. Conversely, for both strains, ConA/Ag/AgCl-NPs exhibited substantial reduction in biofilm formation only at a concentration at least 500-fold greater than the MIC values observed for planktonic cells. Therefore, the strategy presented here allows for significantly lower doses of gentamicin to achieve the same level of effectiveness against pathogenic planktonic bacteria.