<i>Pseudomonas aeruginosa</i> Adhesion and Biofilm Formation on Poly(<scp>l</scp>-lysine)-Tethered Hydrogels: Synergistic Effect of Substrate Stiffness and Positive Charge Density

Infections associated with antibacterial-resistant <i>Pseudomonas aeruginosa</i> (<i>P. aeruginosa</i>) are the major cause of morbidity and mortality of patients, presenting one of the greatest therapeutic challenges for treatment of community-acquired and nosocomial infections. To develop antimicrobial hydrogel coatings to control the adhesion and subsequent biofilm formation of <i>P. aeruginosa</i>, we have used photo-cross-linked poly(ethylene glycol) diacrylate (PEGDA) hydrogels with varied cross-linking densities and covalently grafted poly(l-lysine) (PLL) at different weight compositions (ϕ_PLL). Both surface stiffness and positive charge density of the hydrogels were efficiently tuned over a broad range to investigate their effects on two main strains of <i>P. aeruginosa</i>, PA01 and PA14. We found that both number and viability of attached cells were positively correlated with the hydrogel stiffness, leading to thicker and larger coverage of cell colonies at 72 h postseeding on the stiffer substrates. The dependence of both PA01 and PA14 strains on ϕ_PLL, however, was nonmonotonic. Positive charges from dissociated amine groups in the grafted PLL chains significantly promoted initial adhesion and proliferation of both strains at low ϕ_PLL and developed into the thickest biofilms on the stiffest hydrogels grafted with ϕ_PLL of 1-2%. Nevertheless, on the softest hydrogels grafted with PLL at high ϕ_PLL of 7-10%, the bacteria no longer attached or survived. These results not only improved our fundamental understanding of bacteria-material interactions but also provided a series of PLL-grafted PEGDA hydrogels with controlled stiffness and positive charge density as ideal surface coating materials to prevent bacterial infections.