Tuning the biological activity profile of antibacterial polymers via subunit substitution pattern

Antibacterial activity of hydrophobicity modulated cationic polymers with enzyme and pH-responsiveness

The membrane lipid compositions of prokaryotic and eukaryotic cells are inherently different in many aspects, although some similarities exist in their structure and composition. Therefore, selective targeting of membrane lipids with a compound of therapeutic value, such as an antibacterial copolymer, is often challenging. Hence, developing an ideal copolymer with antibacterial properties demands hydrophobicity/hydrophilicity balance with a high biosafety profile. To integrate hydrophobic/hydrophilic balance and cationic charge in an alternating antibacterial copolymer with enzyme and pH-responsiveness, a lysine appended styrenic monomer was copolymerized with a fatty acid (octanoic acid (OA) or myristic acid (MA)) tethered maleimide monomer via reversible addition-fragmentation chain transfer (RAFT) polymerization. A range of microscopic analyses, including dynamic light scattering (DLS), confirmed the formation of nanoaggregates (size ∼30-40 nm) by these polymers in aqueous solution with positive zeta potential (cationic surface charge). Hydrophobic Nile red (NR) dye was successfully encapsulated in the nanoaggregates, and the in vitro release kinetics of the NR dye were monitored at different pHs and in the presence or absence of esterase/lipase. The in vitro release kinetics of NR revealed ∼85% dye release in the presence of pH 5.5 and lipase, suggesting their suitability for pH/enzyme-triggered therapeutic payload delivery. The standard broth microdilution assay showed significant bactericidal activity against both Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacteria with an MIC50 value <30 μg mL-1. The effect of polymeric nanoaggregates on bacterial morphology and in vitro survival was further confirmed by field emission scanning electron microscopy (FESEM), agar gel disk diffusion assay, and bacterial live/dead cell count. The significantly low hemolytic activity against red blood cells (RBCs) (HC50 >103 μg mL-1) and nontoxic effect on human intestinal epithelial cells (INT 407) (EC50 >500 μg mL-1) ensure that the polymer nanoaggregates are safe for in vivo use and can serve as a potent antibacterial polymer.

Regulation of Hydrophobic Structures of Antibacterial Guanidinium-Based Amphiphilic Polymers for Subcutaneous Implant Applications

Antimicrobial peptide mimics have been used to kill bacteria and construct antibacterial materials. Precise design and construction of chemical structure are essential for easy access to highly effective antimicrobial peptide mimics. Herein, cationic guanidinium-based polymers (PGXs) with varying hydrophobic structures were synthesized to explore the structure and antibacterial activity relationship of antimicrobial peptide mimics and to construct antibacterial implants. The effect of the hydrophobic chemical structure, including carbon chain length and configuration, on the antimicrobial activities against both Escherichia coli and Staphylococcus aureus was investigated. The antibacterial activities of PGXs improved with increasing alkyl chain length, and PGXs with a straight-chain hydrophobic structure exhibited better bactericidal activities than those with cyclic alkane and aromatic hydrocarbon. Furthermore, PGXs grafted with poly(dimethylsiloxane) (PDMS-PGXs) showed a similar bactericidal change tendency of PGXs in solution. Additionally, the PDMS-PGXs showed potent antibiofilm performance in vitro, which can inhibit bacterial infection in vivo as subcutaneous implants. This study may propose a basis for the precise design and construction of antibacterial materials and provide a promising way of designing biomedical devices and implants with bacterial infection-combating activities.

Peptide-Mimicking Poly(2-oxazoline)s Possessing Potent Antifungal Activity and BBB Penetrating Property to Treat Invasive Infections and Meningitis

Invasive fungal infections, including meningitis, cause a high mortality rate due to few available antifungal drugs and frequently associated side effects and quick emergence of drug-resistant fungi. The restrictive permeability of the blood-brain barrier (BBB) further limits the efficacy of antifungal agents substantially in treating meningitis. Hereby, we design and synthesize guanidinium-functionalized poly(2-oxazoline)s by mimicking cell-penetrating peptides. The optimal polymer, PGMeOx10 bearing a methylene spacer arm, displays potent activities against the drug-resistant fungi and biofilm, negligible toxicity, and insusceptibility to antimicrobial resistance. Moreover, PGMeOx10 can break BBB retractions to exert promising antifungal functions in the brain. PGMeOx10 demonstrates potent in vivo antifungal therapeutic efficacy in mouse models including skin infection, systemic infections, and meningitis. PGMeOx10 effectively rescues infected mice and reduces fungal burden and inflammation in the brain. These results and the excellent biosafety of poly(2-oxazoline)s indicate the effectiveness and potential of our strategy to design promising antifungal agents in treating systemic infections and meningitis.

Self-Assembly of Poly(Amino Acid)s Mediated by Secondary Conformations

Self-assembly of block copolymers has recently drawn great attention due to its remarkable performance and wide variety of applications in biomedicine, biomaterials, microelectronics, photoelectric materials, catalysts, etc. Poly(amino acid)s (PAAs), formed by introducing synthetic amino acids into copolymer backbones, are able to fold into different secondary conformations when compared with traditional amphiphilic copolymers. Apart from changing the chemical composition and degree of polymerization of copolymers, the self-assembly behaviors of PAAs could be controlled by their secondary conformations, which are more flexible and adjustable for fine structure tailoring. In this article, we summarize the latest findings on the variables that influence secondary conformations, in particular the regulation of order-to-order conformational changes and the approaches used to manage the self-assembly behaviors of PAAs. These strategies include controlling pH, redox reactions, coordination, light, temperature, and so on. Hopefully, we can provide valuable perspectives that will be useful for the future development and use of synthetic PAAs.

Biocompatible cationic polypeptoids with antibacterial selectivity depending on hydrophobic carbon chain length

The overuse of antibiotics has triggered a new infection crisis and natural antimicrobial peptides (AMPs) have been extensively studied as an alternative to fight microorganisms. Polypeptoids, or polypeptide-biomimetics, offer similar properties to polypeptides and a highly tunable structure that has been synthesized by various methods such as ring opening polymerization (ROP) using N-carboxyanhydride monomers. Simultaneous high antibacterial activity and biocompatibility of a structure by efficient synthesis is desired in the application of those materials. Herein, a series of cationic polypeptoids (PNBs) with variable side chain lengths was obtained by introducing positive charges to the main chain in one step and preserving the backbone structure, namely polypeptoids (PNBM, PNBE, PNBB) with different end groups (methyl (M), ethyl (E), butyl (B)). To address the issue of infection in interventional biomedical implants, we report cost-effective modified polyurethane (PU) films (PU-PNBM, PU-PNBE, PU-PNBB) as physical-biological synergistic antibacterial surfaces that overcome problems such as steric hindrance and the solubility of the materials. Antibacterial selectivity was achieved by regulating the different side chain lengths. When methyl and ethyl were used as hydrophobic side chains, they can only selectively kill Gram-positive Staphylococcus aureus. PNBB, the most hydrophobic and with a butyl side chain can kill both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus and inhibit the growth of bacterial biofilms. Effective in both solution and modified substrate, its biocompatibility is not compromised while the antibacterial properties are substantially improved. Furthermore, PU-PNBB films demonstrated their potential in vivo antimicrobial efficiency in a model of S. aureus infection established on mouse skin. The synthesis route and the surface modification strategies are convenient, providing a solution to the problem of poor biocompatibility in antimicrobial surface applications and a strategy for the use of peptide polymers for targeted therapy after specific infections in the biomedical field.

pH Window for High Selectivity of Ionizable Antimicrobial Polymers toward Bacteria

Antimicrobial polymers exhibit great potential for treating drug-resistant bacteria; however, designing antimicrobial polymers that can selectively kill bacteria and cause relatively low toxicity to normal tissues/cells remains a key challenge. Here, we report a pH window for ionizable polymers that exhibit high selectivity toward bacteria. Ionizable polymer PC6A showed the greatest selectivity (131.6) at pH 7.4, exhibiting low hemolytic activity and high antimicrobial activity against bacteria, whereas a very high or low protonation degree (PD) produced relatively low selectivity (≤35.6). Bactericidal mechanism of PC6A primarily comprised membrane lysis without inducing drug resistance even after consecutive incubation for 32 passages. Furthermore, PC6A demonstrated synergistic effects in combination with antibiotics at pH 7.4. Hence, this study provides a strategy for designing selective antimicrobial polymers.

Designer co-beta-peptide copolymer selectively targets resistant and biofilm Gram-negative bacteria

New antimicrobials are urgently needed to combat Gram-negative bacteria, particularly multi-drug resistant (MDR) and phenotypically resistant biofilm species. At present, only sequence-defined alpha-peptides (e.g. polymyxin B) can selectively target Gram-negative bacterial lipopolysaccharides. We show that a copolymer, without a defined sequence, shows good potency against MDR Gram-negative bacteria including its biofilm form. The tapered blocky co-beta-peptide with controlled N-terminal hydrophobicity (#4) has strong interaction with the Gram-negative bacterial lipopolysaccharides via its backbone through electrostatic and hydrogen bonding interactions but not the Gram-positive bacterial and mammalian cell membranes so that this copolymer is non-toxic to these two latter cell types. The new #4 co-beta-peptide selectively kills Gram-negative bacteria with low cytotoxicity both in vitro and in a mouse biofilm wound infection model. This strategy provides a new concept for the design of Gram-negative selective antimicrobial peptidomimetics against MDR and biofilm species.

Facial amphiphilicity index correlating chemical structures with antimicrobial efficacy

Facial amphiphilicity is an extraordinary chemical structure feature of a variety of antimicrobial peptides and polymers. Vast efforts have been dedicated to small molecular, macromolecular and dendrimer-like systems to mimic this highly preferred structure or conformation, including local facial amphiphilicity and global amphiphilicity. This work conceptualizes Facial Amphiphilicity Index (FAI) as a numerical value to quantitatively characterize the measure of chemical compositions and structural features in dictating antimicrobial efficacy. FAI is a ratio of numbers of charges to rings, representing both compositions of hydrophilicity and hydrophobicity. Cationic derivatives of multicyclic compounds were evaluated as model systems for testing antimicrobial selectivity against Gram-negative and Gram-positive bacteria. Both monocyclic and bicyclic compounds are non-antimicrobial regardless of FAIs. Antimicrobial efficacy was observed with systems having larger cross-sectional areas including tricyclic abietic acid and tetracyclic bile acid. While low and high FAIs respectively lead to higher and lower antimicrobial efficacy, in consideration of cytotoxicity, the sweet spot is typically suited with intermediate FAIs for each specific system. This can be well explained by the synergistic hydrophobic-hydrophobic and electrostatic interactions with bacterial cell membranes and the difference between bacterial and mammalian cell membranes. The adoption of FAI would pave a new avenue toward the design of next-generation antimicrobial macromolecules and peptides.

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Established a numerical index to quantify the effect of facial amphiphilicity on antimicrobial efficacy.

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Evaluated the facial amphiphilicity index of multicyclic compounds possessing various rings and cationic charges.

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Provided this index a new tool toward more quantitative designs of AMP mimics.

Switching from membrane disrupting to membrane crossing, an effective strategy in designing antibacterial polypeptide

Drug-resistant bacterial infections have caused serious threats to human health and call for effective antibacterial agents that have low propensity to induce antimicrobial resistance. Host defense peptide-mimicking peptides are actively explored, among which poly-β-l-lysine displays potent antibacterial activity but high cytotoxicity due to the helical structure and strong membrane disruption effect. Here, we report an effective strategy to optimize antimicrobial peptides by switching membrane disrupting to membrane penetrating and intracellular targeting by breaking the helical structure using racemic residues. Introducing β-homo-glycine into poly-β-lysine effectively reduces the toxicity of resulting poly-β-peptides and affords the optimal poly-β-peptide, βLys₅₀HG₅₀, which shows potent antibacterial activity against clinically isolated methicillin-resistant Staphylococcus aureus (MRSA) and MRSA persister cells, excellent biosafety, no antimicrobial resistance, and strong therapeutic potential in both local and systemic MRSA infections. The optimal poly-β-peptide demonstrates strong therapeutic potential and implies the success of our approach as a generalizable strategy in designing promising antibacterial polypeptides.

Glycopolymers for Antibacterial and Antiviral Applications

Diseases induced by bacterial and viral infections are common occurrences in our daily life, and the main prevention and treatment strategies are vaccination and taking antibacterial/antiviral drugs. However, vaccines can only be used for specific viral infections, and the abuse of antibacterial/antiviral drugs will create multi-drug-resistant bacteria and viruses. Therefore, it is necessary to develop more targeted prevention and treatment methods against bacteria and viruses. Proteins on the surface of bacteria and viruses can specifically bind to sugar, so glycopolymers can be used as potential antibacterial and antiviral drugs. In this review, the research of glycopolymers for bacterial/viral detection/inhibition and antibacterial/antiviral applications in recent years are summarized.

Antimicrobial Nanostructured Assemblies with Extremely Low Toxicity and Potent Activity to Eradicate Staphylococcus Aureus Biofilms

Self-assembled cationic polymeric nanostructures have been receiving increasing attention for efficient antibacterial agents. In this work, a new type of antibacterial agents is developed by preparing pH-dependent nanostructured assemblies from cationic copolypeptoid poly(N-allylglycine)-b-poly(N-octylglycine) (PNAG-b-PNOG) modified with cysteamine hydrochloride ((PNAG-g-NH₂ )-b-PNOG) driven by crystallization and hydrophobicity of the PNOG blocks. Due to the presence of confined domains arising from crystalline PNOG, persistent spheres and fiber-like assemblies are obtained from the same polymer upon a heating-cooling cycle. This allows for direct comparison of antimicrobial efficiency of nanostructured assemblies with various morphologies that are otherwise similar. Both nanostructured assemblies exhibit extremely low toxicity to human red blood cells, irrespective of the presence of the hydrophobic block. Enhanced antimicrobial performance of the fiber-like micelles compared to the spheres, which result in high selectivity of the fibers, is shown. Notably, the fiber-like micelles show great efficacy in inhibition of the Staphylococcus aureus (S. aureus) biofilm formations and eradication of the mature biofilms, superior to vancomycin. The micelles also show potent in vivo antimicrobial efficacy in a S. aureus infection mouse skin model. With a systematic study, it is demonstrated that both micelles kill the bacteria through a membrane disruption mechanism. These results imply great potential of polypeptoid assemblies as promising excellent candidates for antibacterial treatment and open up new possibilities for the preparation of a new generation of nanostructured antimicrobials.

Degradable Selenium-Containing Polymers for Low Cytotoxic Antibacterial Materials

Developing biodegradable cationic polymers with high antibacterial efficiency and low cytotoxicity is of great significance in biological applications. Selenium is an essential trace element for the human body, and selenium-containing compounds are promising in various health-related applications. To combine selenium with biodegradability, selenide-functionalized polycaprolactones (PCL) with different hydrophobic substituents were synthesized followed by selenoniumization. The optimal polyselenonium salt showed excellent antibacterial activity with an MBC of 2 μg mL^-1 and an MIC of 1 μg mL^-1 and exhibited good biocompatibility before and after degradation. In addition, the obtained selenium polymer can be well blended with commercial PCL by electrospinning, and films with good antibacterial activity were prepared. This work enriches the knowledge of selenium derivatives and lays a foundation for follow-up research on selenium cationic polymers in the antimicrobial field.

Shape Matters: Highly Selective Antimicrobial Bottle Brush Copolymers via a One-Pot RAFT Polymerization Approach

The one-pot synthesis of antimicrobial bottle brush copolymers is presented. Reversible addition-fragmentation chain-transfer (RAFT) polymerization is used for the production of the polymeric backbone, as well as for the grafts, which were installed using a grafting-from approach. A combination of N-isopropyl acrylamide and a Boc-protected primary amine-containing acrylamide was used in different compositions. After deprotection, polymers featuring different charge densities were obtained in both linear and bottle brush topologies. Antimicrobial activity was tested against three clinically relevant bacterial strains, and growth inhibition was significantly increased for bottle brush copolymers. Blood compatibility investigations revealed strong hemagglutination for linear copolymers and pronounced hemolysis for bottle brush copolymers. However, one bottle brush copolymer with a 50% charge density revealed strong antibacterial activity and negligible in vitro blood toxicity (regarding hemolysis and hemagglutination tests) resulting in selectivity values as high as 320. Membrane models were used to probe the mechanism of shown polymers that was found to be based on membrane disruption. The trends from bioassays are accurately reflected in model systems indicating that differences in lipid composition might be responsible for selectivity. However, bottle brush copolymers were found to possess increased cytotoxicity against human embryonic kidney (HEK) cells compared with linear analogues. The introduced synthetic platform enables screening of further, previously inaccessible parameters associated with the bottle brush topology, paving the way to further improve their activity profiles.

Design, synthesis, biological evaluation and molecular docking studies of novel pleuromutilin derivatives containing nitrogen heterocycle and alkylamine groups

A series of pleuromutilin derivatives containing alkylamine and nitrogen heterocycle groups were designed and synthesised under mild conditions. The in vitro antibacterial activity of these semisynthetic derivatives against four strains of Staphylococcus aureus (MRSA ATCC 43300, S.aureus ATCC 29213, S.aureus AD3, and S.aureus 144) were evaluated by the broth dilution method. Compound 13 was found to have excellent antibacterial activity against MRSA (MIC = 0.0625 μg/mL). Furthermore, compound 13 was further studied by the time-killing kinetics and the post-antibiotic effect approach. In the mouse thigh infection model, compound 13 exhibited superior antibacterial efficacy than that of tiamulin. Meanwhile, compound 13 showed a lower inhibitory effect than that of tiamulin on RAW264.7 and 16HBE cells at the concentration of 10 μg/mL. Molecular docking study revealed that compound 13 can effectively bind to the active site of the 50S ribosome (the binding free energy = −9.66 kcal/mol).

Changes in Ion Concentrations upon the Binding of Short Polyelectrolytes on Phospholipid Bilayers: Computer Study Addressing Interesting Physiological Consequences

This computer study was inspired by the experimental observation of Y. Qian et al. published in ACS Applied Materials and Interfaces, 2018 that the short positively charged β-peptide chains and their oligomeric analogues efficiently suppress severe medical problems caused by antimicrobial drug-resistant bacteria despite them not penetrating the bacterial membrane. Our coarse-grained molecular dynamics (dissipative particle dynamics) simulations confirm the tentative explanation of the authors of the experimental study that the potent antimicrobial activity is a result of the entropically driven release of divalent ions (mainly magnesium ions essential for the proper biological function of bacteria) into bulk solution upon the electrostatic binding of β-peptides to the bacterial membrane. The study shows that in solutions containing cations Na⁺, Ca²⁺ and Mg²⁺, and anions Cl⁻, the divalent cations preferentially concentrate close to the membrane and neutralize the negative charge. Upon the addition of positively charged oligomer chains (models of β-peptides and their analogues), the oligomers electrostatically bind to the membrane replacing divalent ions, which are released into bulk solvent. Our simulations indicate that the entropy of small ions (which controls the behavior of synthetic polyelectrolyte solutions) plays an important role in this and also in other similar biologically important systems.

Artificial neural network for cytocompatibility and antibacterial enhancement induced by femtosecond laser micro/nano structures

The failure of orthopedic and dental implants is mainly caused by biomaterial-associated infections and poor osseointegration. Surface modification of biomedical materials plays a significant role in enhancing osseointegration and anti-bacterial infection. In this work, a non-linear relationship between the micro/nano surface structures and the femtosecond laser processing parameters was successfully established based on an artificial neural network. Then a controllable functional surface with silver nanoparticles (AgNPs) to was produced to improve the cytocompatibility and antibacterial properties of biomedical titanium alloy. The surface topography, wettability, and Ag⁺ release were carefully investigated. The effects of these characteristics on antibacterial activity and cytocompatibilty were also evaluated. Results show that the prepared surface is hydrophobic, which can prevent the burst release of Ag⁺ in the initial stage. The prepared surface also shows both good cytocompatibility toward the murine calvarial preosteoblasts MC3T3-E1 cells (derived from Mus musculus (mouse) calvaria) and good antibacterial effects against Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria, which is caused by the combined effect of appropriate micro/nano-structured feature and reasonable Ag⁺ release rate. We do not only clarify the antibacterial mechanism but also demonstrate the possibility of balancing the antibacterial and osteointegration-promoting properties by micro/nano-structures. The reported method offers an effective strategy for the patterned surface modification of implants.

Ni Nanocrystals Supported on Graphene Oxide: Antibacterial Agents for Synergistic Treatment of Bacterial Infections

The effects of antibiotics on bacterial infections are gradually weakened, leading to the wide development of nanoparticle-based antibacterial agents with unique physical and chemical properties and antibacterial mechanisms different from antibiotics. In this study, we fabricated the uniform and stable graphene oxide (GO)/Ni colloidal nanocrystal cluster (NCNC) nanocomposite by electrostatic self-assembly and investigated its synergistic antibacterial activity against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) in vitro. The GO/NCNC nanocomposite was shown to possess higher inhibition efficiency than a pure NCNC or GO suspension, with 99.5 and 100% inhibition against S. aureus and E. coli at a 125 μg/mL concentration, respectively. Antibacterial mechanism analysis revealed that (i) NCNCs decorated on GO can further enhance the antibacterial properties of GO by binding and capturing bacteria, (ii) the leaching of Ni²⁺ was detected during the interaction of GO/NCNCs and bacteria, resulting in a decrease in the number of bacteria, and (iii) the GO/NCNC nanocomposite can synergistically destroy the bacterial membrane through physical action and induce the reactive oxygen species generation, so as to further damage the cell membrane and affect ATPase, leakage of intercellular contents, and ultimately bacterial growth inhibition. Meanwhile, cell culture experiments demonstrated no adverse effect of GO/NCNCs on cell growth. These preliminary results indicate the high antibacterial efficiency of the GO/NCNC nanocomposite, suggesting the possibility to develop it into an effective antibacterial agent in the future against bacterial infections.

Improving the Hemocompatibility of Antimicrobial Peptidomimetics through Amphiphilicity Masking Using a Secondary Amphiphilic Polymer

Antimicrobial peptidomimetics (AMPMs) have received widespread attention as potentially powerful weapons against antibiotic resistance. However, AMPMs' membrane disruption mechanism not only brings resistance-resistant nature, but also nonspecific binding and disruption toward eukaryotic cell membranes, and consequently, their hemolytic activity is the primary concern on clinical applications. Here, the preparation and screening of an AMPM library is reported, through which a best-performing hit, PT-b1, can be obtained. To further improve PT-b1's hemocompatibility, a strategy is devised to mask the amphiphilicity of the AMPM using a charge-free, FDA-approved amphiphilic polymer, Pluronic F-127 (PF127). A PF127 solution containing PT-b1 can form a temperature-sensitive, absorbable hydrogel at higher concentration, but dissolve and complex with PT-b1 through hydrophobic interactions at lower concentration or lower temperature. The complexation from PF127 can mask the amphiphilicity of PT-b1 and render it extremely hemocompatible, yet the reversibility in such nanocomplexation and the existence of a secondary mechanism of action ensure that the AMPM's potency remains unchanged. The in vivo effectiveness of this antimicrobial hydrogel system is demonstrated using a mice wound infection model established with Methicillin-resistant Staphylococcus aureus, and observations indicate the hydrogel can promote wound healing and suppress bacteria-caused inflammation even when resistant pathogens are involved.

An Effective Strategy to Develop Potent and Selective Antifungal Agents from Cell Penetrating Peptides in Tackling Drug-Resistant Invasive Fungal Infections

The high mortality rate of invasive fungal infections and quick emergence of drug-resistant fungal pathogens urgently call for potent antifungal agents. Inspired by the cell penetrating peptide (CPP) octaarginine (R8), we elongated to 28 residues poly(d,l-homoarginine) to obtain potent toxicity against both fungi and mammalian cells. Further incorporation of glutamic acid residues shields positive charge density and introduces partial zwitterions in the obtained optimal peptide polymer that displays potent antifungal activity against drug-resistant fungi superior to antifungal drugs, excellent stability upon heating and UV exposure, negligible in vitro and in vivo toxicity, and strong therapeutic effects in treating invasive fungal infections. Moreover, the peptide polymer is insusceptible to antifungal resistance owing to the unique CPP-related antifungal mechanism of fungal membrane penetration followed by disruption of organelles within fungal cells. All these merits imply the effectiveness of our strategy to develop promising antifungal agents.

Optimizing the Bacteriostatic and Cytocompatibility Properties of Poly(hexamethylene guanidine) Hydrochloride (PHMG) via the Guanidine/Alkane Ratio

The emergence of "superbugs" is not only problematic and potentially lethal for infected subjects but also poses serious challenges for the healthcare system. Although existing antibacterial agents have been effective in some cases, the side effects and biocompatibility generally present difficulties. The development of new antibacterial agents is therefore urgently required. In this work, we have adapted a strategy for the improvement of poly(hexamethylene guanidine) hydrochloride (PHMG), a common antibacterial agent. This involves copolymerization of separate monomer units in varying ratios to find the optimum ratio of the hydrocarbon to guanidine units for antibacterial activity. A series of these copolymers, designated as PGB, was synthesized. By varying the guanidine/hydrophobic ratio and the copolymer molecular weight, a structure-optimized PGB was identified that showed broad-spectrum antibacterial activity and excellent biocompatibility in solution. In an antibacterial assay, the copolymer with the optimum composition (hydrophobic unit content 25%) inhibited >99% Staphylococcus aureus and was compatible with mammalian cells. A polyurethane emulsion containing this PGB component formed transparent, flexible films (PGB-PU films) on a wide range of substrate surfaces, including soft polymers and metals. The PGB-PU films showed excellent bacteriostatic efficiency against nosocomial drug-resistant bacteria, such as Pseudomonas aeruginosa and methicillin-resistant S. aureus (MRSA). It is concluded that our PGB polymers can be used as bacteriostatic agents generally and in particular for the design of antibacterial surfaces in medical devices.

Influence of PEG Subunit on the Biological Activity of Ionenes: Synthesis of Novel Polycations, Antimicrobial and Toxicity Studies

An alarming increase of antibiotic resistance among pathogens creates an urgent need to develop new antimicrobial agents. Many reported polycations show high antimicrobial activity along with low hemolytic activity. Unfortunately, most of those molecules remain highly cytotoxic against various mammalian cells. In this work, we present a systematic study on the impact of triethylene glycol monomethyl ether side groups (short PEG analog) on antimicrobial, hemolytic, and cytotoxic properties of novel amphiphilic ionenes. A detailed description of synthesis, leading to well-defined alternating polymers, which differ in structural elements responsible for hydrophilicity (PEG) and hydrophobicity (alkyl chain), is presented. Obtained results show that the PEG moiety and fine-tuned hydrophilic-lipophilic balance of ionenes synergistically lead to low-cytotoxic, low-hemolytic molecules with high activity against S. aureus, including methicillin-resistant strains (MRSA). Additionally, the results of mechanistic studies on bacterial cells and fluorescently labeled liposomes are also discussed. This article is protected by copyright. All rights reserved.

Short Guanidinium-Functionalized Poly(2-oxazoline)s Displaying Potent Therapeutic Efficacy on Drug-Resistant Fungal Infections

New antifungals are urgently needed to combat invasive fungal infections, due to limited types of available antifungal drugs and frequently encountered side effects, as well as the quick emergence of drug-resistance. We previously developed amine-pendent poly(2-oxazoline)s (POXs) as synthetic mimics of host defense peptides (HDPs) to have antibacterial properties, but with poor antifungal activity. Hereby, we report the finding of short guanidinium-pendent POXs, inspired by cell-penetrating peptides, as synthetic mimics of HDPs to display potent antifungal activity, superior mammalian cells versus fungi selectivity, and strong therapeutic efficacy in treating local and systemic fungal infections. Moreover, the unique antifungal mechanism of fungal cell membrane penetration and organelle disruption explains the insusceptibility of POXs to antifungal resistance. The easy synthesis and structural diversity of POXs imply their potential as a class of promising antifungal agents.

Dendritic Hydrogels with Robust Inherent Antibacterial Properties for Promoting Bacteria-Infected Wound Healing

Bacterial infections are a common problem associated with wound treatment that imposes a significant burden on healthcare systems and patients. As a result, healthcare providers urgently need new treatment strategies to protect people. Hydrogel biomaterials with inherent antimicrobial properties offer an attractive and viable solution to this issue. Here, for the first time, we have developed a new efficient synthetic strategy to prepare cationic hydrogels (PHCI) with intrinsically efficient antimicrobial properties by chemically cross-linking trans-1,4-cyclohexanediamine with 1,3-dibromo-2-propanol using a condensation reaction without the use of toxic cross-linking agents. As expected, the prepared PHCI hydrogel possessed an inherent antibacterial ability that can adsorb and kill Staphylococcus aureus and Escherichia coli electrostatically. Notably, in vivo experiments on normal and diabetic rat models confirmed that the PHCI hydrogel can quickly stop bleeding, efficiently kill bacteria, promote the conversion of macrophages from the proinflammatory M1 phenotype to the repaired M2 phenotype, and accelerate collagen deposition and blood vessel formation, thereby achieving rapid wound healing. Overall, this work presents an effective antibacterial dressing that might provide a facile but effective approach for clinical wound management.

Antibacterial Copolypeptoids with Potent Activity against Drug Resistant Bacteria and Biofilms, Excellent Stability, and Recycling Property

The preparation of a type of innovative cationic copolypeptoid antimicrobials containing various hydrophobic moieties that resemble both structure and membrane-lytic antibacterial mechanism of natural antimicrobial peptides (AMPs) is reported. By finely tuning the hydrophilic/hydrophobic balance, the polypeptoids exhibit a wide spectrum of antibacterial activity against both Gram-positive bacteria and Gram-negative bacteria with the lowest minimum inhibitory concentration (MIC) at only 2 µg mL^-1 , whereas they also show low haemolytic properties. In particular, high selectivity (>128) is achieved from the polymers with butyl moieties. Moreover, the polypeptoids can readily inhibit the formation of biofilms and effectively eradicate the bacteria embedded in the mature biofilms, which is superior to many natural AMPs and vancomycin. Unlike conventional antibiotics, the polypeptoids possess potent activity against drug-resistant bacteria without visible resistance development after repeated usage. Notably, the polypeptoid antimicrobials not only have inherently fast bactericidal properties and excellent stability against incubation with human plasma, but also show excellent in vivo antibacterial effect. The prepared antimicrobials, coated onto magnetic nanospheres show recycling properties and enhanced antibacterial activity as combined with near-infrared (NIR)-induced photothermal antibacterial therapy.

Quaternized Polysaccharide-Based Cationic Micelles as a Macromolecular Approach to Eradicate Multidrug-Resistant Bacterial Infections while Mitigating Antimicrobial Resistance

Microbial infections and microbial resistance lead to a high demand for new antimicrobial agents. Quaternized polysaccharides are cationic antimicrobial candidates; however, the limitation of homogeneous synthesis solvents that affect the molecular structure and biological activities, as well as their drug resistance remains unclear. Therefore, the authors homogeneously synthesize a series of quaternized chitin (QC) and quaternized chitosan (QCS) derivatives via a green and effective KOH/urea system and investigate their structure-activity relationship and biological activity in vivo and in vitro. Their study reveals that a proper match of degree of quaternization (DQ) and degree of deacetylation (DD') of QC or QCS is key to balance antimicrobial property and cytotoxicity. They identify QCS-2 as the optimized antimicrobial agent with a DQ of 0.46 and DD' of 82%, which exhibits effective broad-spectrum antimicrobial properties, good hemocompatibility, excellent cytocompatibility, and effective inhibition of bacterial biofilm formation and eradication of mature bacterial biofilms. Moreover, QCS-2 exhibits a low propensity for development of drug resistance and significant anti-infective effects on MRSA in vivo comparable to that of vancomycin, avoiding excessive inflammation and promoting the formation of new blood vessels, hair follicles, and collagen deposition to thus expedite wound healing.

Secondary Amine Pendant β-Peptide Polymers Displaying Potent Antibacterial Activity and Promising Therapeutic Potential in Treating MRSA-Induced Wound Infections and Keratitis

Interest in developing antibacterial polymers as synthetic mimics of host defense peptides (HPDs) has accelerated in recent years to combat antibiotic-resistant bacterial infections. Positively charged moieties are critical in defining the antibacterial activity and eukaryotic toxicity of HDP mimics. Most examples have utilized primary amines or guanidines as the source of positively charged moieties, inspired by the lysine and arginine residues in HDPs. Here, we explore the impact of amine group variation (primary, secondary, or tertiary amine) on the antibacterial performance of HDP-mimicking β-peptide polymers. Our studies show that a secondary ammonium is superior to either a primary ammonium or a tertiary ammonium as the cationic moiety in antibacterial β-peptide polymers. The optimal polymer, a homopolymer bearing secondary amino groups, displays potent antibacterial activity and the highest selectivity (low hemolysis and cytotoxicity). The optimal polymer displays potent activity against antibiotic-resistant bacteria and high therapeutic efficacy in treating MRSA-induced wound infections and keratitis as well as low acute dermal toxicity and low corneal epithelial cytotoxicity. This work suggests that secondary amines may be broadly useful in the design of antibacterial polymers.

Antimicrobial Peptides and Macromolecules for Combating Microbial Infections: From Agents to Interfaces

Bacterial resistance caused by the overuse of antibiotics and the shelter of biofilms has evolved into a global health crisis, which drives researchers to continuously explore antimicrobial molecules and strategies to fight against drug-resistant bacteria and biofilm-associated infections. Cationic antimicrobial peptides (AMPs) are considered to be a category of potential alternative for antibiotics owing to their excellent bactericidal potency and lesser likelihood of inducing drug resistance through their distinctive antimicrobial mechanisms. In this review, the hitherto reported plentiful action modes of AMPs are systematically classified into 15 types and three categories (membrane destructive, nondestructive membrane disturbance, and intracellular targeting mechanisms). Besides natural AMPs, cationic polypeptides, synthetic polymers, and biopolymers enable to achieve tunable antimicrobial properties by optimizing their structures. Subsequently, the applications of these cationic antimicrobial agents at the biointerface as contact-active surface coatings and multifunctional wound dressings are also emphasized here. At last, we provide our perspectives on the development of clinically significant cationic antimicrobials and related challenges in the translation of these materials.

Polymers as advanced antibacterial and antibiofilm agents for direct and combination therapies

The growing prevalence of antimicrobial drug resistance in pathogenic bacteria is a critical threat to global health. Conventional antibiotics still play a crucial role in treating bacterial infections, but the emergence and spread of antibiotic-resistant micro-organisms are rapidly eroding their usefulness. Cationic polymers, which target bacterial membranes, are thought to be the last frontier in antibacterial development. This class of molecules possesses several advantages including a low propensity for emergence of resistance and rapid bactericidal effect. This review surveys the structure-activity of advanced antimicrobial cationic polymers, including poly(α-amino acids), β-peptides, polycarbonates, star polymers and main-chain cationic polymers, with low toxicity and high selectivity to potentially become useful for real applications. Their uses as potentiating adjuvants to overcome bacterial membrane-related resistance mechanisms and as antibiofilm agents are also covered. The review is intended to provide valuable information for design and development of cationic polymers as antimicrobial and antibiofilm agents for translational applications.

Main-chain flexibility and hydrophobicity of ionenes strongly impact their antimicrobial activity: an extended study on drug resistance strains and Mycobacterium

The spread of antibiotic-resistant pathogens and the resurgence of tuberculosis disease are major motivations to search for novel antimicrobial agents. Some promising candidates in this respect are cationic polymers, also known as synthetic mimics of antimicrobial peptides (SMAMPs), which act through the membrane-lytic mechanism. Development of resistance toward SMAMPs is less likely than toward currently employed antibiotics; however, further studies are needed to better understand their structure–activity relationship. The main objective of this work is to understand the cross-influence of hydrophobicity, main-chain flexibility, and the topology of ionenes (polycations containing a cationic moiety within the main-chain) on activity. To fulfill this goal, a library of ionenes was developed and compared with previously investigated molecules. The obtained compounds display promising activity against the model microorganisms and drug-resistance clinical isolates, including Mycobacterium tuberculosis. The killing efficiency was also investigated, and results confirm a strong effect of hydrophobicity, revealing higher activity for molecules possessing the flexible linker within the polymer main-chain.

A high significance of the main chain flexibility and an unexpected effect of hydrophobicity on the biological activity in series of ionenes was observed. The most potent among the tested polycations showed high activity toward clinical bacterial isolates.

Local rigidification and possible coacervation of the Escherichia coli DNA by cationic nylon-3 polymers

Synthetic, cationic random nylon-3 polymers (β-peptides) show promise as inexpensive antimicrobial agents less susceptible to proteolysis than normal peptides. We have used superresolution, single-cell, time-lapse fluorescence microscopy to compare the effects on live Escherichia coli cells of four such polymers and the natural antimicrobial peptides LL-37 and cecropin A. The longer, densely charged monomethyl-cyclohexyl (MM-CH) copolymer and MM homopolymer rapidly traverse the outer membrane and the cytoplasmic membrane. Over the next ∼5 min, they locally rigidify the chromosomal DNA and slow the diffusive motion of ribosomal species to a degree comparable to LL-37. The shorter dimethyl-dimethylcyclopentyl (DM-DMCP) and dimethyl-dimethylcyclohexyl (DM-DMCH) copolymers, and cecropin A are significantly less effective at rigidifying DNA. Diffusion of the DNA-binding protein HU and of ribosomal species is hindered as well. The results suggest that charge density and contour length are important parameters governing these antimicrobial effects. The data corroborate a model in which agents having sufficient cationic charge distributed across molecular contour lengths comparable to local DNA-DNA interstrand spacings (∼6 nm) form a dense network of multivalent, electrostatic "pseudo-cross-links" that cause the local rigidification. In addition, at times longer than ∼30 min, we observe that the MM-CH copolymer and the MM homopolymer (but not the other four agents) cause gradual coalescence of the two nucleoid lobes into a single dense lobe localized at one end of the cell. We speculate that this process involves coacervation of the DNA by the cationic polymer, and may be related to the liquid droplet coacervates observed in eukaryotic cells.

Host Defense Peptide-Mimicking Polymers and Polymeric-Brush-Tethered Host Defense Peptides: Recent Developments, Limitations, and Potential Success

Amphiphilic antimicrobial polymers have attracted considerable interest as structural mimics of host defense peptides (HDPs) that provide a broad spectrum of activity and do not induce bacterial-drug resistance. Likewise, surface engineered polymeric-brush-tethered HDP is considered a promising coating strategy that prevents infections and endows implantable materials and medical devices with antifouling and antibacterial properties. While each strategy takes a different approach, both aim to circumvent limitations of HDPs, enhance physicochemical properties, therapeutic performance, and enable solutions to unmet therapeutic needs. In this review, we discuss the recent advances in each approach, spotlight the fundamental principles, describe current developments with examples, discuss benefits and limitations, and highlight potential success. The review intends to summarize our knowledge in this research area and stimulate further work on antimicrobial polymers and functionalized polymeric biomaterials as strategies to fight infectious diseases.

Addressing MRSA infection and antibacterial resistance with peptoid polymers

Methicillin-Resistant Staphylococcus aureus (MRSA) induced infection calls for antibacterial agents that are not prone to antimicrobial resistance. We prepare protease-resistant peptoid polymers with variable C-terminal functional groups using a ring-opening polymerization of N-substituted N-carboxyanhydrides (NNCA), which can provide peptoid polymers easily from the one-pot synthesis. We study the optimal polymer that displays effective activity against MRSA planktonic and persister cells, effective eradication of highly antibiotic-resistant MRSA biofilms, and potent anti-infectious performance in vivo using the wound infection model, the mouse keratitis model, and the mouse peritonitis model. Peptoid polymers show insusceptibility to antimicrobial resistance, which is a prominent merit of these antimicrobial agents. The low cost, convenient synthesis and structure diversity of peptoid polymers, the superior antimicrobial performance and therapeutic potential in treating MRSA infection altogether imply great potential of peptoid polymers as promising antibacterial agents in treating MRSA infection and alleviating antibiotic resistance.

Antibiotic resistance is a major issue in medicine and new antimicrobials for treating resistant infection are needed. Here, the authors report on antibacterial peptoid polymers, prepared via NNCA ring-opening polymerization, demonstrating antibacterial function against MRSA in vitro and in in vivo infection models.

High Antibacterial Activity and Selectivity of the Versatile Polysulfoniums that Combat Drug Resistance

Sulfonium-ion-containing polymers exhibit significant potential benefits for various applications. An efficient strategy to synthesize a type of antibacterial sulfonium-ion-bearing polypeptoids via a combination of ring-opening polymerization and a post-polymerization functionalization with various functional epoxides is presented. A systematic investigation is further performed in order to explore the influence of the overall hydrophobic/hydrophilic balance on the antimicrobial activity and selectivity of the prepared polysulfoniums. Notably, those chlorepoxypropane-modified polysulfoniums with an optimized amphiphilic balance show higher selectivity toward both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus, than to red blood cells. The polymers also show great efficiency in inhibiting S. aureus biofilm formations, as well as in further eradicating the mature biofilms. Remarkably, negligible antibacterial resistance and cross-resistance to commercial antibiotics is shown in these polymers. The polysulfoniums further show their potent in vivo antimicrobial efficacy in a multidrug-resistant S. aureus infection model that is developed on mouse skin. Similar to the antimicrobial peptides, the polysulfoniums are demonstrated to kill bacteria through membrane disruption. The obtained polypeptoid sulfoniums, with high selectivity and potent antibacterial property, are excellent candidates for antibacterial treatment and open up new possibilities for the preparation of a class of innovative antimicrobials.

Broad-Spectrum Bactericidal Activity and Remarkable Selectivity of Main-Chain Sulfonium-Containing Polymers with Alternating Sequences

Incorporation of cationic groups into polymers represents one of the most widely used strategies to prepare antibacterial materials. Sulfonium, as a typical cationic moiety, displays potent antibacterial efficacy in the form of small molecules, however, has long underperformed in polymeric systems. Herein, we developed a series of alternating polysulfoniums, where the hydrophobicity of each alternating unit can be accurately tuned by altering the monomer precursors. Excellent antibacterial activity against a broad spectrum of clinically relevant bacteria, including Methicillin-resistant Staphylococcus aureus, can be obtained in the optimal compositions with minimum bactericidal concentrations in the range of 1.25-10 μg/mL, as well as negligible hemolytic effect at polymer concentrations even up to 10000 μg/mL. Bacteria do not readily develop resistance to polysulfoniums due to the antibacterial action is possibly the membrane disrupting mechanism. This work demonstrates sulfonium-based polymers with well-defined sequences can function as a promising candidate to combat drug-resistant bacterial infection.

Advancements in release-active antimicrobial biomaterials: A journey from release to relief

Escalating medical expenses due to infectious diseases are causing huge socioeconomic pressure on mankind globally. The emergence of antibiotic resistance has further aggravated this problem. Drug-resistant pathogens are also capable of forming thick biofilms on biotic and abiotic surfaces to thrive in a harsh environment. To address these clinical problems, various strategies including antibacterial agent delivering matrices and bactericidal coatings strategies have been developed. In this review, we have discussed various types of polymeric vehicles such as hydrogels, sponges/cryogels, microgels, nanogels, and meshes, which are commonly used to deliver antibiotics, metal nanoparticles, and biocides. Compositions of these polymeric matrices have been elaborately depicted by elucidating their chemical interactions and potential activity have been discussed. On the other hand, various implant/device-surface coating strategies which exploit the release-active mechanism of bacterial killing are discussed in elaboration. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Cardiovascular Disease Implantable Materials and Surgical Technologies > Nanomaterials and Implants Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease.

Macromolecular Nanotherapeutics and Antibiotic Adjuvants to Tackle Bacterial and Fungal Infections

The escalating rise in the population of multidrug-resistant (MDR) pathogens coupled with their biofilm forming ability has struck the global health as nightmare. Alongwith the threat of aforementioned menace, the sluggish development of new antibiotics and the continuous deterioration of the antibiotic pipeline has stimulated the scientific community toward the search of smart and innovative alternatives. In near future, membrane targeting antimicrobial polymers, inspired from antimicrobial peptides, can stand out significantly to combat against the MDR superbugs. Many of these amphiphilic polymers can form nanoaggregates through self-assembly with superior and selective antimicrobial efficacy. Additionally, these macromolecular nanoaggregrates can be utilized to engineer smart antibiotic-delivery system for on-demand drug-release, exploiting the infection site's micoenvironment. This strategy substantially increases the local concentration of antibiotics and reduces the associated off-target toxicity. Furthermore, amphiphilc macromolecules can be utilized to rejuvinate obsolete antibiotics to tackle the drug-resistant infections. This review article highlights the recent developments in macromolecular architecture to design numerous nanostructures with broad-spectrum antimicrobial activity, their application in fabricating smart drug delivery systems and their efficacy as antibiotic adjuvants to circumvent antimicrobial resistance. Finally, the current challenges and future prospects are briefly discussed for further exploration and their practical application in clinical settings.

Influence of lipid bilayer composition on the activity of antimicrobial quaternary ammonium ionenes, the interplay of intrinsic lipid curvature and polymer hydrophobicity, the role of cardiolipin

Incorporation of hydrophobic component into amphiphilic polycations structure is frequently accompanied by an increase of antimicrobial activity. There is, however, a group of relatively hydrophilic polycations containing quaternary ammonium moieties along mainchain, ionenes, which also display strong antimicrobial and limited hemolytic properties. In this work, an influence of a hydrophobic side group length on antimicrobial mechanism of action is investigated in a series of novel amphiphilic ionenes. High antimicrobial activity was found by determination of minimum inhibitory concentration (MIC) and minimum bactericidal, and fungicidal concentration (MBC and MFC) in both growth media and a buffer. Biocompatibility was estimated by hemolytic and mammalian cells viability assays. Mechanistic studies were performed using large unilamellar vesicles (LUVs) with different lipid composition, as simplified models of cell membranes. The investigated ionenes are potent and selective antimicrobial molecules displaying a decrease of antimicrobial activity correlated with increase of hydrophobicity. Studies using LUVs revealed that the cardiolipin is an essential component responsible for the lipid bilayer permeabilization by investigated ionens. In contrast to relatively hydrophilic ionenes, more hydrophobic polymers showed an ability to stabilize membranes composed of lipids with negative spontaneous curvature in a certain range of polymer to lipid ratio. The results substantially contribute to the understanding of antimicrobial activity of the investigated class of polymers.

Cationic Homopolymers Inhibit Spore and Vegetative Cell Growth of Clostridioides difficile

A wide range of synthetic polymers have been explored for antimicrobial activity. These materials usually contain both cationic and hydrophobic subunits because these two characteristics are prominent among host-defense peptides. Here, we describe a series of nylon-3 polymers containing only cationic subunits and their evaluation against the gastrointestinal, spore-forming pathogen Clostridioides difficile. Despite their highly hydrophilic nature, these homopolymers showed efficacy against both the vegetative and spore forms of the bacterium, including an impact on C. difficile spore germination. The polymer designated P34 demonstrated the greatest efficacy against C. difficile strains, along with low propensities to lyse human red blood cells or intestinal epithelial cells. To gain insight into the mechanism of P34 action, we evaluated several cell-surface mutant strains of C. difficile to determine the impacts on growth, viability, and cell morphology. The results suggest that P34 interacts with the cell wall, resulting in severe cell bending and death in a concentration-dependent manner. The unexpected finding that nylon-3 polymers composed entirely of cationic subunits display significant activities toward C. difficile should expand the range of other polymers considered for antibacterial applications.

Effective and biocompatible antibacterial surfaces via facile synthesis and surface modification of peptide polymers

It is an urgent need to tackle drug-resistance microbial infections that are associated with implantable biomedical devices. Host defense peptide-mimicking polymers have been actively explored in recent years to fight against drug-resistant microbes. Our recent report on lithium hexamethyldisilazide-initiated superfast polymerization on amino acid N-carboxyanhydrides enables the quick synthesis of host defense peptide-mimicking peptide polymers. Here we reported a facile and cost-effective thermoplastic polyurethane (TPU) surface modification of peptide polymer (DLL: BLG = 90 : 10) using plasma surface activation and substitution reaction between thiol and bromide groups. The peptide polymer-modified TPU surfaces exhibited board-spectrum antibacterial property as well as effective contact-killing ability in vitro. Furthermore, the peptide polymer-modified TPU surfaces showed excellent biocompatibility, displaying no hemolysis and cytotoxicity. In vivo study using methicillin-resistant Staphylococcus aureus (MRSA) for subcutaneous implantation infectious model showed that peptide polymer-modified TPU surfaces revealed obvious suppression of infection and great histocompatibility, compared to bare TPU surfaces. We further explored the antimicrobial mechanism of the peptide polymer-modified TPU surfaces, which revealed a surface contact-killing mechanism by disrupting the bacterial membrane. These results demonstrated great potential of the peptide-modified TPU surfaces for practical application to combat bacterial infections that are associated with implantable materials and devices.

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A convenient surface modification of peptide polymer 90 : 10 DLL : BLG to enable material surfaces antibacterial properties.

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The modified thermoplastic polyurethane (TPU) surfaces show board-spectrum antibacterial performance and excellent biocompatibility both in vitro and in vivo.

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The contact-killing surfaces demonstrate great potential for practical application to combat bacterial infections associated with implantable materials and devices.

A convenient approach for antibacterial polypeptoids featuring sulfonium and oligo(ethylene glycol) subunits

Bioinspired polypeptoids show great potential in many applications. Here, we report a convenient approach to synthesize a novel type of polypeptoid containing both sulfonium and oligo(ethylene glycol) (OEG) moieties by ring-opening polymerization (ROP) and a post-modification strategy. Three types of epoxides with (OEG)n (n = 1-3) groups are involved to offer various functionalities. The obtained polypeptoid sulfonium salts show positive ζ potential, irrespective of the solution pH and the degree of polymerization (DP). We demonstrate that the polypeptoids exhibit excellent antibacterial activity against Staphylococcus aureus (S. aureus) with MIC (minimal inhibitory concentration) in the range of 3.9-7.8 μg mL-1. In addition, the polypeptoids have a low hemolysis property and good in vitro biocompatibility. Remarkably, the as-prepared polypeptoids show rapid and potent antibacterial activity within 5 min. These features suggest that the obtained polypeptoids offer great potential for antimicrobial agents.

A call for action to the biomaterial community to tackle antimicrobial resistance

The global surge of antimicrobial resistance (AMR) is a major concern for public health and proving to be a key challenge in modern disease treatment, requiring action plans at all levels. Microorganisms regularly and rapidly acquire resistance to antibiotic treatments and new drugs are continuously required. However, the inherent cost and risk to develop such molecules has resulted in a drying of the pipeline with very few compounds currently in development. Over the last two decades, efforts have been made to tackle the main sources of AMR. Nevertheless, these require the involvement of large governmental bodies, further increasing the complexity of the problem. As a group with a long innovation history, the biomaterials community is perfectly situated to push forward novel antimicrobial technologies to combat AMR. Although this involvement has been felt, it is necessary to ensure that the field offers a united front with special focus in areas that will facilitate the development and implementation of such systems. This paper reviews state of the art biomaterials strategies striving to limit AMR. Promising broad-spectrum antimicrobials and device modifications are showcased through two case studies for different applications, namely topical and implantables, demonstrating the potential for a highly efficacious physical and chemical approach. Finally, a critical review on barriers and limitations of these methods has been developed to provide a list of short and long-term focus areas in order to ensure the full potential of the biomaterials community is directed to helping tackle the AMR pandemic.

Regiospecific vs. non regiospecific click azide-alkyne polymerization: In vitro study of water-soluble antibacterial poly(amide aminotriazole)s

Novel linear cationic poly(amide aminotriazole)s (PATnD) with secondary amine groups in the backbone were obtained by using azide-alkyne 1,3-dipolar cycloaddition reactions: metal- and solvent-free (thermal conditions, PATTnD) or copper(I)-catalyzed (Sharpless conditions, PATCnD). PATnD were investigated in vitro against strains of E. coli, P. aeruginosa, S. aureus, and S. epidermidis. Hemolytic activity was tested using human red blood cells (hRBC), and very low or no hemolytic activity was observed. The cytotoxicity of PATnD polymers against Human Gingival Fibroblasts (HGnF) cells was concentration-dependent, and significant differences between PATT1D and PATC1D were observed. The ability of these polymers to induce resistance against both Gram-positive and Gram-negative bacteria was also assessed. Studied bacterial strains acquired resistance to catalytic polymers (PATCnD) in initial passages meanwhile resistance to thermal polymers (PATTnD) appears in later passages, being the increase of the minimum inhibitory concentration lower than in catalytic polymers. This result, together with the higher biocidal capacity of thermal polymers compared to catalytic ones, seems to suggest an influence of the regiospecificity of the polymers on their antibacterial characteristics. This study also demonstrates that PAT1D polymers, which do not appear to have strong hydrophobic residues, can exert significant antimicrobial activity against Gram-positive bacteria such as S. epidermidis. This pair of polymers, PATC1D and PATT1D, displays the greatest antimicrobial activity while not causing significant hemolysis along with the lowest susceptibility for resistance development of the polymers evaluated.

Beyond Amphiphilic Balance: Changing Subunit Stereochemistry Alters the Pore-Forming Activity of Nylon-3 Polymers

Amphiphilic nylon-3 polymers have been reported to mimic the biological activities of natural antimicrobial peptides, with high potency against bacteria and minimal toxicity toward eukaryotic cells. Amphiphilic balance, determined by the proportions of hydrophilic and lipophilic subunits, is considered one of the most important features for achieving this activity profile for nylon-3 polymers and many other antimicrobial polymers. Insufficient hydrophobicity often correlates with weak activities against bacteria, whereas excessive hydrophobicity correlates with high toxicity toward eukaryotic cells. To ask whether factors beyond amphiphilic balance influence polymer activities, we synthesized and evaluated new nylon-3 polymers with two stereoisomeric subunits, each bearing an ethyl side chain and an aminomethyl side chain. Subunits that differ only in stereochemistry are predicted to contribute equally to amphiphilic balance, but we observed that the stereochemical difference correlates with significant changes in biological activity profile. Antibacterial activities were not strongly affected by subunit stereochemistry, but the ability to disrupt eukaryotic cell membranes varied considerably. Experiments with planar lipid bilayers and synthetic liposomes suggested that eukaryotic membrane disruption results from polymer-mediated formation of large pores. Collectively, our results suggest that factors other than amphiphilic balance influence the membrane activity profile of synthetic polymers. Subunits that differ in stereochemistry are likely to have distinct conformational propensities, which could potentially lead to differences in the average shapes of polymer chains, even when the subunits are heterochiral. These findings highlight a dimension of polymer design that should be considered more broadly in efforts to improve specificity and efficacy of antimicrobial polymers.

Recent advances in design of antimicrobial peptides and polypeptides toward clinical translation

The recent outbreaks of infectious diseases caused by multidrug-resistant pathogens have sounded a piercing alarm for the need of new effective antimicrobial agents to guard public health. Among different types of candidates, antimicrobial peptides (AMPs) and the synthetic mimics of AMPs (SMAMPs) have attracted significant enthusiasm in the past thirty years, due to their unique membrane-active antimicrobial mechanism and broad-spectrum antimicrobial activity. The extensive research has brought many drug candidates into clinical and pre-clinical development. Despite tremendous progresses have been made, several major challenges inherent to current design strategies have slowed down the clinical translational development of AMPs and SMAMPs. However, these challenges also triggered many efforts to redesign and repurpose AMPs. In this review, we will first give an overview on AMPs and their synthetic mimics, and then discuss the current status of their clinical translation. Finally, the recent advances in redesign and repurposing AMPs and SMAMPs are highlighted.

Diverse Impacts on Prokaryotic and Eukaryotic Membrane Activities from Hydrophobic Subunit Variation Among Nylon-3 Copolymers

Synthetic, sequence-random polymers that feature a wide range of backbone and side chain structures have been reported to function as mimics of natural host-defense peptides, inhibiting bacterial growth while exerting little or no toxicity toward eukaryotic cells. The common themes among these materials are net positive charge, which is thought to confer preferential action toward prokaryotic vs eukaryotic cells, and the presence of hydrophobic components, which are thought to mediate membrane disruption. This study is based on a set of new binary cationic-hydrophobic nylon-3 copolymers that was designed to ask whether factors beyond net charge and net hydrophobicity influence the biological activity profile. In previous work, we found that nonpolar subunits preorganized by a ring led to copolymers with a diminished tendency to disrupt human cell membranes (as measured via lysis of red blood cells) relative to copolymers containing more flexible nonpolar subunits. An alternative mode of conformational restriction, involving geminal substitution, also minimized hemolysis. Here, we asked whether combining a cyclic constraint and geminal substitution would be synergistic; the combination was achieved by introducing backbone methyl groups to previously described cyclopentyl and cyclohexyl subunits. The new cyclic subunits containing two quaternary backbone carbons (i.e, two sites of geminal substitution) were comparable or slightly superior in terms of antibacterial potency but markedly superior in terms of low hemolytic activity, relative to cyclic subunits lacking the quaternary carbons. However, new cyclic units containing only one quaternary carbon were very hemolytic, which was unanticipated. Variations in net hydrophobicity cannot explain the trend in hemolysis, in contrast to the standard perspective in this field. The impact of each new polymer on live E. coli cells was evaluated via fluorescence microscopy. All new polymers moved rapidly across the outer membrane without large-scale disruption of barrier function. Increasing the number of quaternary carbons in the nonpolar subunit correlated with an increased propensity to permeabilize the cytoplasmic membrane of E. coli cells. Collectively, these findings show that relationships between nonpolar subunit identity and biological activity are influenced by factors in addition to hydrophobicity and charge. We propose that the variation of subunit conformational properties may be one such factor.