Photo-Enhanced Antimicrobial Activity of Polymers Containing an Embedded Photosensitiser.

This work presents the synthesis of a novel photosensitive acrylate monomer for use as both a self-catalyst in the photoinduced electron/energy transfer-reversible addition fragmentation chain transfer (PET-RAFT) polymerisation process and a photosensitiser (PS) for antibacterial applications. Hydrophilic, cationic, and antimicrobial formulations are explored to compare the antibacterial effects between charged and non-charged polymers. Covalent attachment of the catalyst to well-defined linear polymer chains has no effect on polymerisation control or singlet oxygen generation. The addition of the PS to polymers provides activity against S. aureus for all polymer formulations, resulting in up to a 99.99999 % killing efficacy in 30 min. Antimicrobial peptide mimetic polymers previously active against P. aeruginosa, but not S. aureus, gain significant bactericidal activity against S. aureus through the inclusion of PS groups, with 99.998 % killing efficiency after 30 min incubation with light. Thus, a broader spectrum of antimicrobial activity is achieved using two distinct mechanisms of bactericidal activity via the incorporation of a photosensitiser monomer into an antimicrobial polymer.

Effect of Hydrophobic Groups on Antimicrobial and Hemolytic Activity: Developing a Predictive Tool for Ternary Antimicrobial Polymers.

Antimicrobial polymers have emerged as a potential solution to the growing problem of antimicrobial resistance. Although several studies have examined the effects of various parameters on the antimicrobial and hemolytic activity of statistical copolymers, there are still numerous parameters to be explored. Therefore, in this study, we developed a library of 36 statistical amphiphilic ternary copolymers prepared via photoinduced electron transfer-reversible addition-fragmentation chain transfer polymerization to systematically evaluate the influence of hydrophobic groups [number of carbons (5, 7, and 9)] and chain type of the hydrophobic monomer (cyclic, aromatic, linear, or branched), monomer ratio, and degree of polymerization (DPn) on antimicrobial and hemolytic activity. To guide our synthetic strategy, we developed a pre-experimental screening approach using C log P values of oligomer models, which correspond to the logarithm of the partition coefficient of compounds between n-octanol and water. This method enabled correlation of polymer hydrophobicity with antimicrobial and hemolytic activity. In addition, this study revealed that minimizing hydrophobicity and hydrophobic content were key factors in controlling hemolysis, whereas optimizing antimicrobial activity was more complex. High antimicrobial activity required hydrophobicity (i.e., C log P, hydrophobicity index) that was neither too high nor too low, an appropriate cationic/hydrophobic balance, and structural compatibility between the chosen monomers. Furthermore, these findings could guide the design of future antimicrobial ternary copolymers and suggest that C log P values between 0 and 2 have the best balance of high antimicrobial activity and low hemolytic activity.

Antibiofilm Platform based on the Combination of Antimicrobial Polymers and Essential Oils.

The development of potent strategies to counter microbial biofilm is an urgent priority in healthcare. The majority of bacterial infections in humans are biofilm related, however, effective treatments are still lacking especially for combating multidrug-resistant (MDR) strains. Herein, we report an effective antibiofilm platform based on the use of synthetic antimicrobial polymers in combination with essential oils, where the antimicrobial polymers play a secondary role as delivery vehicle for essential oils. Two ternary antimicrobial polymers consisting of cationic primary amines, low-fouling oligo(ethylene glycol) and hydrophobic ethylhexyl groups were synthesized in the form of random and block copolymers, and mixed with either carvacrol or eugenol. Coadministration of these compounds improved the efficacy against Pseudomonas aeruginosa biofilms compared to the individual compounds. We observed about a 60-75% and 70-85% biofilm inhibition effect for all tested combinations against wild-type P. aeruginosa PAO1 and MDR strain PA37, respectively, upon 6.5 h of incubation time. While both random and block copolymers demonstrated similar biofilm inhibition potencies in combination with essential oils, only the block copolymer acted synergistically with essential oils in killing biofilm. Treatment of PAO1 biofilm for 20 min with the block copolymer-oil combinations resulted in the killing of >99.99% of biofilm bacteria. This synergistic bactericidal activity is attributed to the targeted delivery of essential oils to the biofilm, driven by the electrostatic interaction between positively charged delivery vehicles, in the form of polymeric micelles, and negatively charged bacteria. This study thus highlights the advantage of combining essential oils and antimicrobial polymers as an effective avenue for antibacterial applications.

Star Polymers.

Recent advances in controlled/living polymerization techniques and highly efficient coupling chemistries have enabled the facile synthesis of complex polymer architectures with controlled dimensions and functionality. As an example, star polymers consist of many linear polymers fused at a central point with a large number of chain end functionalities. Owing to this exclusive structure, star polymers exhibit some remarkable characteristics and properties unattainable by simple linear polymers. Hence, they constitute a unique class of technologically important nanomaterials that have been utilized or are currently under audition for many applications in life sciences and nanotechnologies. This article first provides a comprehensive summary of synthetic strategies towards star polymers, then reviews the latest developments in the synthesis and characterization methods of star macromolecules, and lastly outlines emerging applications and current commercial use of star-shaped polymers. The aim of this work is to promote star polymer research, generate new avenues of scientific investigation, and provide contemporary perspectives on chemical innovation that may expedite the commercialization of new star nanomaterials. We envision in the not-too-distant future star polymers will play an increasingly important role in materials science and nanotechnology in both academic and industrial settings.

Exploiting the Versatility of Polydopamine-Coated Nanoparticles to Deliver Nitric Oxide and Combat Bacterial Biofilm.

In this study, an antimicrobial platform in the form of nitric oxide (NO) gas-releasing polydopamine (PDA)-coated iron oxide nanoparticles (IONPs) is developed for combating bacterial biofilms. NO is bound to the PDA-coated IONPs via the reaction between NO and the secondary amine moieties on PDA to form N-diazeniumdiolate (NONOate) functionality. To impart colloidal stability to the nanoparticles in aqueous solutions (e.g., phosphate buffered saline (PBS) and bacteria cell culture media M9), a polymer bearing hydrophilic and amine pendant groups, P(OEGMA)-b-P(ABA), is synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization and is subsequently grafted onto the PDA-coated IONPs by employing the Schiff base/Michael addition reaction between o-quinone and a primary amine. These nanoparticles are able to effectively disperse Pseudomonas aeruginosa biofilms (up to 79% dispersal) at submicromolar NO concentrations. In addition, the nanoparticles demonstrate excellent bactericidal activity toward P. aeruginosa planktonic and biofilm cells (up to 5-log10 reduction).

Towards Sequence-Controlled Antimicrobial Polymers: Effect of Polymer Block Order on Antimicrobial Activity.

Synthetic polymers have shown promise in combating multidrug-resistant bacteria. However, the biological effects of sequence control in synthetic antimicrobial polymers are currently not well understood. As such, we investigate the antimicrobial effects of monomer distribution within linear high-order quasi-block copolymers consisting of aminoethyl, phenylethyl, and hydroxyethyl acrylamides made in a one-pot synthesis approach via photoinduced electron transfer-reversible addition-fragmentation chain transfer polymerisation (PET-RAFT). Through different combinations of monomer/polymer block order, antimicrobial and haemolytic activities are tuneable in a manner comparable to antimicrobial peptides.

Tuning the Properties of Polymer Capsules for Cellular Interactions.

Particle-cell interactions are governed by, among other factors, the composition and surface properties of the particles. Herein, we report the preparation of various polymer capsules with different compositions and properties via atom transfer radical polymerization mediated continuous assembly of polymers (CAPATRP), where the cellular interactions of these capsules, particularly fouling and specific targeting, are examined by flow cytometry and deconvolution microscopy. Acrylated eight-arm poly(ethylene glycol) (8-PEG) and poly(N-(2-hydroxypropyl)-methacrylamide) (PHPMA) as well as methacrylated hyaluronic acid (HA), poly(glutamic acid) (PGA), and poly(methacrylic acid) (PMA) are used as macro-cross-linkers to obtain a range of polymer capsules with different compositions (PEG, PHPMA, HA, PGA, and PMA). Capsules composed of low-fouling polymers, PEG and PHPMA, show negligible association with macrophage Raw 264.7, monocyte THP-1, and HeLa cells. HA capsules, although moderately low-fouling (<22%) to HeLa, BT474, Raw 264.7, and THP-1 cells, exhibit high targeting specificity to CD44-over-expressing MDA-MB-231 cells. In contrast, PGA and PMA capsules show high cellular association toward phagocytic Raw 264.7 and THP-1 cells. These findings demonstrate the capability of the CAPATRP technique in preparing polymer capsules with specific cellular interactions.

Low-fouling, biospecific films prepared by the continuous assembly of polymers.

We report that the continuous assembly of polymers (CAP) approach, mediated by ring-opening metathesis polymerization (ROMP), is a facile and versatile technology to prepare engineered nanocoatings for various biomedical applications. Low-fouling coatings on particles were obtained by the formation of multicompositional, layered films via simple and efficient tandem CAP(ROMP) processes that are analogous to chain extension reactions. In addition, the CAP(ROMP) approach allows for the efficient postfunctionalization of the CAP films with bioactive moieties via cross-metathesis reactions between the surface-immobilized catalysts and symmetrical alkene derivatives. The combined features of the CAP(ROMP) approach (i.e., versatile polymer selection and facile functionalization) allow for the fabrication and surface modification of various types of polymer films, including those with intrinsic protein-repellent properties and selective protein recognition capabilities. This study highlights the various types of advanced coatings and materials that the CAP approach can be used to generate, which may be useful for biomedical applications.

Bioactive Synthetic Polymers.

Synthetic polymers are omnipresent in society as textiles and packaging materials, in construction and medicine, among many other important applications. Alternatively, natural polymers play a crucial role in sustaining life and allowing organisms to adapt to their environments by performing key biological functions such as molecular recognition and transmission of genetic information. In general, the synthetic and natural polymer worlds are completely separated due to the inability for synthetic polymers to perform specific biological functions; in some cases, synthetic polymers cause uncontrolled and unwanted biological responses. However, owing to the advancement of synthetic polymerization techniques in recent years, new synthetic polymers have emerged that provide specific biological functions such as targeted molecular recognition of peptides, or present antiviral, anticancer, and antimicrobial activities. In this review, the emergence of this generation of bioactive synthetic polymers and their bioapplications are summarized. Finally, the future opportunities in this area are discussed.

Two plus One: Combination Therapy Tri-systems Involving Two Membrane-Disrupting Antimicrobial Macromolecules and Antibiotics.

The escalating issue of multidrug-resistant (MDR) bacteria indicates the urgent need for new and effective strategies to combat this global health challenge. Here, we describe a new combinatorial approach that can be put forward for experimental therapy application against MDR bacteria. Specifically, we have developed a tri-system that includes the coadministration of two different membrane-disrupting-type antimicrobial agents─a synthetic antimicrobial polymer P and an antimicrobial peptide (AMP) colistin methanesulfonate (Col)─in conjunction with an antibiotic [doxycycline (Dox), rifampicin (Rif), or azithromycin (Azi)]. Traditionally, the administration of membrane-disrupting antimicrobial agents causes toxicity, but, in comparison, we demonstrated synergy and biocompatibility using this combinatorial approach. Checkerboard assays showed the occurrence of synergistic interactions in Col-Dox-P, Col-Rif-P, and Col-Azi-P tri-systems against wild-type and MDR Pseudomonas aeruginosa, with the Col-Dox-P system being the most effective. The ability to synergize thus enables the use of a lower dosage in combinations compared to the standalone agents. The tri-systems not only demonstrated bacteriostatic activity but were also bactericidal. For example, the Col-Dox-P system (at 8, 4, and 8 µg mL-1, respectively) and the Col-Rif-P system (at 4, 8, and 16 µg mL-1, respectively) were able to kill >99.999% of planktonic P. aeruginosa cells within 3 h of treatment. More importantly, an improvement of the therapeutic/selectivity index was achieved via combination therapy. Taking the Col-Dox-P system as an example, its biocompatibility with murine embryonic fibroblast cells was found to be comparable to that of polymer P alone despite the synergistic enhancement in antimicrobial activity of the combination. This resulted in a significant increase in selectivity by 16-fold for the Col-Dox-P combination system compared to P alone. Furthermore, the broad applicability of this tri-system strategy was demonstrated via the successful application of the AMP melittin in place of Col or P. Overall, this study sheds new insights on the application of membrane-disrupting antimicrobial agents in combination therapy and their potential for safer clinical use. Additionally, the information gathered in this study could inform the development of future combination therapy systems involving the simultaneous employment of multiple AMPs with antibiotics.

Synthetic Antimicrobial Polymers in Combination Therapy: Tackling Antibiotic Resistance.

Antibiotic resistance is a critical global healthcare issue that urgently needs new effective solutions. While small molecule antibiotics have been safeguarding us for nearly a century since the discovery of penicillin by Alexander Fleming, the emergence of a new class of antimicrobials in the form of synthetic antimicrobial polymers, which was driven by the advances in controlled polymerization techniques and the desire to mimic naturally occurring antimicrobial peptides, could play a key role in fighting multidrug resistant bacteria in the near future. By harnessing the ability to control chemical and structural properties of polymers almost at will, synthetic antimicrobial polymers can be strategically utilized in combination therapy with various antimicrobial coagents in different formats to yield more potent (synergistic) outcomes. In this review, we present a short summary of the different combination therapies involving synthetic antimicrobial polymers, focusing on their combinations with nitric oxide, antibiotics, essential oils, and metal- and carbon-based inorganics.

Incorporation and antimicrobial activity of nisin Z within carrageenan/chitosan multilayers.

An antimicrobial peptide, nisin Z, was embedded within polyelectrolyte multilayers (PEMs) composed of natural polysaccharides in order to explore the potential of forming a multilayer with antimicrobial properties. Using attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR), the formation of carrageenan/chitosan multilayers and the inclusion of nisin Z in two different configurations was investigated. Approximately 0.89 µg cm-2 nisin Z was contained within a 4.5 bilayer film. The antimicrobial properties of these films were also investigated. The peptide containing films were able to kill over 90% and 99% of planktonic and biofilm cells, respectively, against Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA) strains compared to control films. Additionally, surface topography and wettability studies using atomic force microscopy (AFM) and the captive bubble technique revealed that surface roughness and hydrophobicity was similar for both nisin containing multilayers. This suggests that the antimicrobial efficacy of the peptide is unaffected by its location within the multilayer. Overall, these results demonstrate the potential to embed and protect natural antimicrobials within a multilayer to create functionalised coatings that may be desired by industry, such as in the food, biomaterials, and pharmaceutical industry sectors.

Synergy between Synthetic Antimicrobial Polymer and Antibiotics: A Promising Platform To Combat Multidrug-Resistant Bacteria.

The failure of many antibiotics in the treatment of chronic infections caused by multidrug-resistant (MDR) bacteria necessitates the development of effective strategies to combat this global healthcare issue. Here, we report an antimicrobial platform based on the synergistic action between commercially available antibiotics and a potent synthetic antimicrobial polymer that consists of three key functionalities: low-fouling oligoethylene glycol, hydrophobic ethylhexyl, and cationic primary amine groups. Checkerboard assays with Pseudomonas aeruginosa (P. aeruginosa) and Escherichia coli demonstrated synergy between our synthetic antimicrobial polymer and two antibiotics, doxycycline and colistin. Coadministration of these compounds significantly improved the bacteriostatic efficacy especially against MDR P. aeruginosa strains PA32 and PA37, where the minimal inhibitory concentrations (MICs) of polymer and antibiotics were reduced by at least 4-fold. A synergistic killing activity was observed when the antimicrobial polymer was used in combination with doxycycline, killing >99.999% of planktonic and biofilm P. aeruginosa PAO1 upon a 20 min treatment at a polymer concentration of 128 µg mL-1 (4.6 µM) and doxycycline concentration of 64 µg mL-1 (133.1 µM). In addition, this synergistic combination reduced the rate of resistance development in P. aeruginosa compared to individual compounds and was also capable of reviving susceptibility to treatment in the resistant strains.

Antibiofilm Nitric Oxide-Releasing Polydopamine Coatings.

The growing number of patient morbidity related to nosocomial infections has placed an importance on the development of new antibacterial coatings for medical devices. Here, we utilize the versatile adhesion property of polydopamine (pDA) to design an antibacterial coating that possesses low-fouling and nitric oxide (NO)-releasing capabilities. To demonstrate this, glass substrates were functionalized with pDA via immersion in alkaline aqueous solution containing dopamine, followed by grafting of low-fouling polymer (poly(ethylene glycol) (PEG)) via Michael addition and subsequent formation of N-diazeniumdiolate functionalities (NO precursors) by purging with NO gas. X-ray photoelectron spectroscopy confirmed the successful grafting of PEG and formation of N-diazeniumdiolate on polydopamine-coated substrates. NO release from the coating was observed over 2 days, and NO loading is tunable by the pDA film thickness. The antibacterial efficiency of the coatings was assessed using Gram-negative Pseudomonas aeruginosa (i.e., wild-type PAO1 and multidrug-resistant PA37) and Gram-positive Staphylococcus aureus (ATCC 29213). The NO-releasing PEGylated pDA film inhibited biofilm attachment by 96 and 70% after exposure to bacterial culture solution for 24 and 36 h, respectively. In contrast, films that do not contain NO failed to prevent biofilm formation on the surfaces at these time points. Furthermore, this coating also showed 99.9, 97, and 99% killing efficiencies against surface-attached PAO1, PA37, and S. aureus bacteria. Overall, the combination of low-fouling PEG and antibacterial activity of NO in pDA films makes this coating a potential therapeutic option to inhibit biofilm formation on medical devices.

Highly Bactericidal Macroporous Antimicrobial Polymeric Gel for Point-of-Use Water Disinfection.

Access to clean and safe water supply remains inadequate in many developing countries. One of the key challenges is to remove pathogenic bacteria from the water supply via effective water disinfection technologies to prevent the spread of diseases and to ensure the safety of consumers. Herein, a highly effective point-of-use (on-demand) water disinfection technology, in the form of a polymeric scaffold called macroporous antimicrobial polymeric gel (MAPG), is demonstrated. MAPG is easy to fabricate, completely organic and possess inherent antimicrobial property which makes it non-reliant on inorganic compounds such as silver where the long-term toxicity remains unknown. MAPG is highly bactericidal and can disinfect bacteria-contaminated water (ca. 108 CFU mL-1) at a capacity of about >50 times the mass of the organic material used, inactivating >99% of both Gram-negative and Gram-positive bacteria including Escherichia coli, Vibrio cholerae and Staphylococcus aureus within 20 minutes of treatment. When fabricated in a syringe, MAPG eliminates E. coli from contaminated water source by >8.0 log10 reduction in bacteria counts (i.e., no viable bacteria were detected after treatment), and the syringe can be reused multiple times without losing potency. The MAPG technology is not only restricted to water disinfection but may also be applicable in other bacteria inactivation applications.

Rational Design of Single-Chain Polymeric Nanoparticles That Kill Planktonic and Biofilm Bacteria.

Infections caused by multidrug-resistant bacteria are on the rise and, therefore, new antimicrobial agents are required to prevent the onset of a postantibiotic era. In this study, we develop new antimicrobial compounds in the form of single-chain polymeric nanoparticles (SCPNs) that exhibit excellent antimicrobial activity against Gram-negative bacteria (e.g., Pseudomonas aeruginosa) at micromolar concentrations (e.g., 1.4 µM) and remarkably kill ≥99.99% of both planktonic cells and biofilm within an hour. Linear random copolymers, which comprise oligoethylene glycol (OEG), hydrophobic, and amine groups, undergo self-folding in aqueous systems due to intramolecular hydrophobic interactions to yield these SCPNs. By systematically varying the hydrophobicity of the polymer, we can tune the extent of cell membrane wall disruption, which in turn governs the antimicrobial activity and rate of resistance acquisition in bacteria. We also show that the incorporation of OEG groups into the polymer design is essential in preventing complexation with proteins in biological medium, thereby maintaining the antimicrobial efficacy of the compound even in in vivo mimicking conditions. In comparison to the last-resort antibiotic colistin, our lead agents have a higher therapeutic index (by ca. 2-3 times) and hence better biocompatibility. We believe that the SCPNs developed here have potential for clinical applications and the information pertaining to their structure-activity relationship will be valuable toward the general design of synthetic antimicrobial (macro)molecules.

Bionano Interaction Study on Antimicrobial Star-Shaped Peptide Polymer Nanoparticles.

'Structurally nanoengineered antimicrobial peptide polymers' (SNAPPs), in the form of star-shaped peptide polymer nanoparticles, have been recently demonstrated as a new class of antimicrobial agents with superior in vitro and in vivo efficacy against Gram-negative pathogens, including multidrug-resistant species. Herein, we present a detailed bionano interaction study on SNAPPs by assessing their antimicrobial activities against several Gram-negative bacteria in complex biological matrices. Simulated body fluid and animal serum were used as test media to reveal factors that influence the antimicrobial efficacy of SNAPPs. With the exception of Acinetobacter baumannii, the presence of divalent cations at physiological concentrations reduced the antimicrobial efficacy of SNAPPs from minimum inhibitory concentrations (MICs) within the nanomolar range (40-300 nM) against Escherichia coli, Pseudomanas aeruginosa, and Klebsiella pneumoniae to 0.6-4.7 µM. By using E. coli as a representative bacterial species, we demonstrated that the reduction in activity was due to a decrease in the ability of SNAPPs to cause outer and inner membrane disruption. This effect could be reversed through coadministration with a chelating agent. Interestingly, the potency of SNAPPs against A. baumannii was retained even under high salt concentrations. The presence of serum proteins was also found to affect the interaction of SNAPPs with bacterial membranes, possibly through intermolecular binding. Collectively, this study highlights the need to consider the possible interactions of (bio)molecules present in vivo with any new antimicrobial agent under development. We also demonstrate that outer membrane disruption/destabilization is an important but hitherto under-recognized target for the antimicrobial action of peptide-based agents, such as antimicrobial peptides (AMPs). Overall, the findings presented herein could aid in the design of more efficient peptide-based antimicrobial agents with uncompromised potency even under physiological conditions.

Photocontrolled Cargo Release from Dual Cross-Linked Polymer Particles.

Burst release of a payload from polymeric particles upon photoirradiation was engineered by altering the cross-linking density. This was achieved via a dual cross-linking concept whereby noncovalent cross-linking was provided by cyclodextrin host-guest interactions, and irreversible covalent cross-linking was mediated by continuous assembly of polymers (CAP). The dual cross-linked particles (DCPs) were efficiently infiltrated (∼80-93%) by the biomacromolecule dextran (molecular weight up to 500 kDa) to provide high loadings (70-75%). Upon short exposure (5 s) to UV light, the noncovalent cross-links were disrupted resulting in increased permeability and burst release of the cargo (50 mol % within 1 s) as visualized by time-lapse fluorescence microscopy. As sunlight contains UV light at low intensities, the particles can potentially be incorporated into systems used in agriculture, environmental control, and food packaging, whereby sunlight could control the release of nutrients and antimicrobial agents.

Polypeptide films via N-carboxyanhydride ring-opening polymerization (NCA-ROP): past, present and future.

The formation of surface-grafted polypeptide films and interfaces via N-carboxyanhydride ring-opening polymerization (NCA-ROP) holds great potential for the facile preparation of next-generation multifunctional and responsive materials with excellent biocompatibility, biodegradability, tunable conformations and chemical functionalities. Therefore, the aim of this feature article is to provide a topical review of developments in the area of polypeptide films derived through NCA-ROP. It is evident that studies reported thus far have only started to unveil the full potential of peptide-based interfaces and materials, and with continued advancements it is anticipated that the strategic combination of NCA-ROP with modern synthetic chemistries will continue to yield versatile platforms for broader applications in the fields of polymer therapeutics, tissue engineering, (bio)nanocoatings, (bio)chemosensors, catalysis and separation technologies.