Evidence for the Phospholipid Sponge Effect as the Biocidal Mechanism in Surface-Bound Polyquaternary Ammonium Coatings with Variable Cross-Linking Density

Poly quaternary -oniums derived from polyethylenimine (PEI), poly(vinyl-N-alkylpyriclinium), or chitosan belong to a class of cationic polymers that are efficient antimicrobial agents. When dissolved in solution, the positively charged polycations are able to displace the divalent cations of the cellular phospholipid bilayer and disrupt the ionic cross-links and structural integrity of the membrane. However, when immobilized to a surface where confinement limits diffusion, poly-oniums still show excellent antimicrobial activity, which implies a different biocidal mode of action. Recently, a proposed mechanism, named phospholipid sponge effect, suggested that surface-bound polycationic networks are capable of recruiting negatively charged phospholipids out of the bacterial cell membrane and sequestering them within the polymer matrix.' However, there has been insufficient evidence to support this hypothesis. In this study, a surface-bound N,N-dodecyl methyl-co-N,Nmethylbenzophenone methyl quaternary PEI (DMBQPEI) was prepared to verify the phospholipid sponge effect. By' tuning the irradiation time, the cross-linking densities of surface-bound DMBQPEI films were mediated. The modulus of films was measured by PeakForce Quantitative Nanomechanical Mapping (QNM) to indicate the cross-linking density variation with increasing irradiation time. A negative correlation between the film cross-linking density and the absorption of a negatively charged phospholipid (DPhPG) was observed, but no such correlations were observed with a neutral phospholipid (DPhPC), which strongly supported the action of anionic phospholipid suction proposed, in the lipid sponge effect. Moreover, the killing efficiency toward S. aureus and E. coli was inversely affected by the cross-linking density of the films, providing evidence for the phospholipid sponge effect. The relationship between killing efficiency and film cross-linking density is discussed.