Self-Defensive Antimicrobial Surfaces Using Polymyxin-Loaded Poly(styrene sulfonate) Microgels

Self-defensive antimicrobial surfaces are of interest because they can inhibit bacterial colonization while minimizing unnecessary antimicrobial release in the absence of a bacterial challenge. One self-defensive approach uses self-assembly to first deposit a submonolayer coating of polyelectrolyte microgels and subsequently load those microgels by complexation with small-molecule antimicrobials. The microgel/antimicrobial complexation strength is a key parameter that controls the ability of the antimicrobial both to remain sequestered within the microgels when exposed to medium and to release in response to a bacterial challenge. Here we study the relative complexation strengths of two FDA-approved cationic antibiotics─colistin (polymyxin E) and polymyxin B─with microgels of poly(styrene sulfonate) (PSS). These polymyxins are similar cyclic polypeptides with +5 charge at pH 7.4. However, polymyxin B substitutes an aromatic ring for a dimethyl moiety in colistin, and this aromaticity can influence complexation via π and hydrophobic interactions. Coarse-grained molecular dynamics shows that the free-energy change associated with polymyxin B/PSS complexation is more negative than that of colistin/PSS complexation. Experimentally, in situ optical microscopy of microgel deswelling shows that both antibiotics load quickly from low-ionic-strength phosphate buffer. The enhanced polymyxin B/PSS complexation strength is then manifested by subsequent exposure to flowing antibiotic-free buffer with varying NaCl concentration. Microgels loaded with polymyxin B remain stably deswollen to higher salt concentrations than do colistin/PSS microgels. Importantly, exposing loaded microgels to E. coli in nutrient-free-flowing phosphate buffer shows that bacteria are killed by physical contact with the loaded microgels consistent with the contact-transfer mechanism of self-defensiveness. In vitro culture experiments show that these same surfaces, nevertheless, support the adhesion, spreading and proliferation of human fetal osteoblasts. These findings suggest a pathway to create a self-defensive antimicrobial surface effective under physiological conditions based on the nonmetabolic bacteria-triggered release of FDA-approved antibiotics.