Despite advanced sterilization and aseptic techniques, infections associated with medical implants have not been eradicated. Most present coatings cannot simultaneously fulfil the requirements of antibacterial and antifungal activity as well as biocompatibility and reusability. Here, we report an antimicrobial hydrogel based on dimethyldecylammonium chitosan (with high quaternization)-graft-poly(ethylene glycol) methacrylate (DMDC-Q-g-EM) and poly(ethylene glycol) diacrylate, which has excellent antimicrobial efficacy against Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus and Fusarium solani. The proposed mechanism of the antimicrobial activity of the polycationic hydrogel is by attraction of sections of anionic microbial membrane into the internal nanopores of the hydrogel, like an 'anion sponge', leading to microbial membrane disruption and then microbe death. We have also demonstrated a thin uniform adherent coating of the hydrogel by simple ultraviolet immobilization. An animal study shows that DMDC-Q-g-EM hydrogel coating is biocompatible with rabbit conjunctiva and has no toxicity to the epithelial cells or the underlying stroma.

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Antimicrobial peptides (AMPs), particularly those effective against methicillin-resistant Staphylococcus aureus ( S. aureus ) and antibiotic-resistant Pseudomonas aeruginosa ( P. aeruginosa ), are important alternatives to antibiotics. Typical peptide synthesis methods involving solid-phase sequential synthesis are slow and costly, which are obstacles to their more widespread application. In this paper, we synthesize peptides via ring-opening polymerization of alpha-amino acid N-carboxyanhydrides (NCA) using a transition metal initiator. This method offers high potential for inexpensive synthesis of substantial quantities of AMPs. Lysine (K) was chosen as the hydrophilic amino acid and alanine (A), phenylalanine (F), and leucine (L) as the hydrophobic amino acids. We synthesized five series of AMPs (i.e., P(KA), P(KL), P(KF), P(KAL), and P(KFL)), varied the hydrophobic amino acid content from 0 to 100%, and determined minimal inhibitory concentrations (MICs) against clinically important Gram-negative and Gram-positive bacteria and fungi (i.e., Escherichia coli ( E. coli ), P. aeruginosa , Serratia marcescens ( S. marcescens ), and Candida albicans ( C. albicans ). We found that P(K(10)F(7.5)L(7.5)) and P(K(10)F(15)) show the broadest activity against all five pathogens and have the lowest MICs against these pathogens. For P(K(10)F(7.5)L(7.5)), the MICs against E. coli , P. aeruginosa , S. marcescens , S. aureus , and C. albicans are 31 microg/mL, 31 microg/mL, 250 microg/mL, 31 microg/mL, and 62.5 microg/mL, while for P(K(10)F(15)) the respective MICs are 31 microg/mL, 31 microg/mL, 250 microg/mL, 31 microg/mL, and 125 microg/mL. These are lower than the MICs of many naturally occurring AMPs. The membrane depolarization and SEM assays confirm that the mechanism of microbe killing by P(K(10)F(7.5)L(7.5)) copeptide includes membrane disruption, which is likely to inhibit rapid induction of AMP-resistance in pathogens.

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A nanohybrid comprising silver nanoparticles within third-generation dendritic poly(amidoamine) (PAMAM) grafted onto multiwalled carbon nanotubes (MWNTs) was applied as an antimicrobial agent in solution. The high abundance of amine groups on the dendrimer-modified MWNTs (d-MWNTs) provided sites for reduction and precipitation of silver nanoparticles from silver acetate aqueous solution, resulting in carbon nanotubes/ Ag nanohybrids (d-MWNTs/Ag). The content of PAMAM grafted on d-MWNTs determined by using a thermal gravimetric analyzer (TGA) was about 45%. The silver nanoparticles produced were determined to be face-centered cubic silver nanocrystals by X-ray powder diffraction (XRD). The nanohybrids were investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, and X-ray energy dispersive spectroscopy (EDS). The antimicrobial properties of acid-treated MWNTs (MWNTs-COOH), d-MWNTs, and d-MWNTs/ Ag were investigated against Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), and Pseudomonas aeruginosa (P. aeruginosa). Against E. coli and P. aeruginosa which are Gram-negative, d-MWNTs and d-MWNTs/Ag which are equally effective were found to have a stronger antimicrobial effect than MWNTs-COOH. Against S. aureus (Gram-positive), d-MWNTs/Ag showed a stronger antimicrobial effect than d-MWNTs (92.3% kill versus 71.6% kill), while MWNTs-COOH only killed 15.4% of this bacteria. Possible mechanisms are proposed to explain the higher antimicrobial activity by d-MWNTs/Ag nanohybrids. These findings suggest that PAMAM/Ag grafted onto insoluble MWNTs may be used as effective antimicrobial materials.

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Despite unique and useful properties of multi-walled carbon nanotubes (MWNTs) such as high strength and a low synthesis cost, their weak antimicrobial property hampers their use as an antimicrobial material. Herein, we demonstrate that the immobilization of nisin, a natural and inexpensive antimicrobial peptide, with poly(ethylene glycol) (PEG(1000)) as a linker significantly enhanced the antimicrobial and anti-biofilm properties of MWNTs. The MWNT-nisin composite showed up to 7-fold higher antimicrobial property than pristine MWNTs against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Bacillus subtilis. Moreover, the MWNT-nisin composite had a dramatically improved capability to prevent biofilm formation both on a deposited film and in suspension. In particular, the MWNT-nisin deposit film exhibited a 100-fold higher anti-biofilm property than the MWNT deposit film. Further, it has been shown that PEG and nisin are covalently attached to MWNTs with excellent stability against leaching. We envision that our novel MWNT-nisin composite can serve as an effective and economical antimicrobial material.

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