Single-Cell, Time-Resolved Antimicrobial Effects of a Highly Cationic, Random Nylon-3 Copolymer on Live Escherichia coli

Trap & kill: a neutrophil-extracellular-trap mimic nanoparticle for anti-bacterial therapy

Fenton chemistry-mediated antimicrobials have demonstrated great promise in antibacterial therapy. However, the short life span and diffusion distance of hydroxyl radicals dampen the therapeutic efficiency of these antimicrobials. Herein, inspired by the neutrophil extracellular trap (NET), in which bacteria are trapped and agglutinated via electronic interactions and killed by reactive oxygen species, we fabricated a NET-mimic nanoparticle to suppress bacterial infection in a "trap & kill" manner. Specifically, this NET-mimic nanoparticle was synthesized via polymerization of ferrocene monomers followed by quaternization with a mannose derivative. Similar to the NET, the NET-mimic nanoparticles trap bacteria through electronic and sugar-lectin interactions between their mannose moieties and the lectins of bacteria, forming bacterial agglutinations. Therefore, they confine the spread of the bacteria and restrict the bacterial cells to the destruction range of hydroxyl radicals. Meanwhile, the ferrocene component of the nanoparticle catalyzes the production of highly toxic hydroxyl radicals at the H2O2 rich infection foci and effectively eradicates the agglutinated bacteria. In a mouse model of an antimicrobial-resistant bacteria-infected wound, the NET-mimic nanoparticles displayed potent antibacterial activity and accelerated wound healing.

Activation of multiple stress responses in Staphylococcus aureus substantially lowers the minimal inhibitory concentration when combining two novel antibiotic drug candidates

The past few decades have been plagued by an increasing number of infections caused by antibiotic resistant bacteria. To mitigate the rise in untreatable infections, we need new antibiotics with novel targets and drug combinations that reduce resistance development. The novel β-clamp targeting antimicrobial peptide BTP-001 was recently shown to have a strong additive effect in combination with the halogenated pyrrolopyrimidine JK-274. In this study, the molecular basis for this effect was examined by a comprehensive proteomic and metabolomic study of the individual and combined effects on Staphylococcus aureus. We found that JK-274 reduced activation of several TCA cycle enzymes, likely via increasing the cellular nitric oxide stress, and BTP-001 induced oxidative stress in addition to inhibiting replication, translation, and DNA repair processes. Analysis indicated that several proteins linked to stress were only activated in the combination and not in the single treatments. These results suggest that the strong additive effect is due to the activation of multiple stress responses that can only be triggered by the combined effect of the individual mechanisms. Importantly, the combination dose required to eradicate S. aureus was well tolerated and did not affect cell viability of immortalized human keratinocyte cells, suggesting a species-specific response. Our findings demonstrate the potential of JK-274 and BTP-001 as antibiotic drug candidates and warrant further studies.

Flexible Modulation of Cellular Activities with Cationic Photosensitizers: Insights of Alkyl Chain Length on Reactive Oxygen Species Antimicrobial Mechanisms

Cationic photosensitizers have good binding ability with negatively charged bacteria and fungi, exhibiting broad applications potential in antimicrobial photodynamic therapy (aPDT). However, cationic photosensitizers often display unsatisfactory transkingdom selectivity between mammalian cells and pathogens, especially for eukaryotic fungi. It is unclear which biomolecular sites are more efficient for photodynamic damage, owing to the lack of systematic research with the same photosensitizer system. Herein, a series of cationic aggregation-induced emission (AIE) derivatives (CABs) (using berberine (BBR) as the photosensitizers core) with different length alkyl chains are successfully designed and synthesized for flexible modulation of cellular activities. The BBR core can efficiently produce reactive oxygen species (ROS) and achieve high-performance aPDT . Through the precise regulation of alkyl chain length, different bindings, localizations, and photodynamic killing effects of CABs are achieved and investigated systematically among bacteria, fungi, and mammalian cells. It is found that intracellular active substances, not membranes, are more efficient damage sites of aPDT. Moderate length alkyl chains enable CABs to effectively kill Gram-negative bacteria and fungi with light, while still maintaining excellent mammalian cell and blood compatibility. This study is expected to provide systematic theoretical and strategic research guidance for the construction of high-performance cationic photosensitizers with good transkingdom selectivity.

Biocidal Potency of Polymers with Bulky Cations

The performance of antimicrobial polymers depends sensitively on the type of cationic species, charge density, and spatial arrangement of cations. Here we report antimicrobial polymers bearing unusually bulky tetraaminophosphonium groups as the source of highly delocalized cationic charge. The bulky cations drastically enhanced the biocidal activity of amphiphilic polymers, leading to remarkably potent activity in the submicromolar range. The cationic polynorbornenes with pendent tetraaminophosphonium groups killed over 98% E. coli at a concentration of 0.1 μg/mL and caused a 4-log reduction of E. coli within 2 h at a concentration of 2 μg/mL, showing very rapid and potent bactericidal activity. The polymers are also highly hemolytic at similar concentrations, indicating a biocidal activity profile. Polymers of a similar chemical structure but with more flexible backbones were made to examine the effects of the flexibility of polymer chains on their activity, which turned out to be marginal. We also explore variants with different spacer arm groups separating the cations from the backbone main chain. The antibacterial activity was comparably potent in all cases, but the polymers with shorter spacer arm groups showed more rapid bactericidal kinetics. Interestingly, pronounced counterion effects were observed. Tightly bound PF₆^- counteranions showed poor activity at high concentrations due to gross aggregate formation and precipitation from the assay media, whereas loosely bound Cl^- counterions resulted in very potent activity that monotonically increased with increasing concentration. In this paper, we reveal that bulky phosphonium cations are associated with markedly enhanced biocidal activity, which provides an innovative strategy to develop more effective self-disinfecting materials.

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.

Interplay of Fusion, Leakage, and Electrostatic Lipid Clustering: Membrane Perturbations by a Hydrophobic Antimicrobial Polycation

Membrane active compounds are able to induce various types of membrane perturbations. Natural or biomimetic candidates for antimicrobial treatment or drug delivery scenarios are mostly designed and tested for their ability to induce membrane permeabilization, also termed leakage. Furthermore, the interaction of these usually cationic amphiphiles with negatively charged vesicles often causes colloidal instability leading to vesicle aggregation or/and vesicle fusion. We show the interplay of these modes of membrane perturbation in mixed phosphatidyl glycerol (PG)/phosphatidyl ethanolamine (PE) by the statistical copolymer MM:CO comprising, both, charged and hydrophobic subunits. MM:CO is a representative of partially hydrophobic, highly active, but less selective antimicrobial polycations. Cryo-electron microscopy indicates vesicle fusion rather than vesicle aggregation upon the addition of MM:CO to negatively charged PG/PE (1:1) vesicles. In a combination of fluorescence-based leakage and fusion assays, there is support for membrane permeabilization and pronounced vesicle fusion activity as distinct effects. To this end, membrane fusion and aggregation were prevented by including lipids with polyethylene glycol attached to their head groups (PEG-lipids). The leakage activity of MM:CO is very similar in the absence and presence of PEG-lipids. Vesicle aggregation and fusion however are largely suppressed. This strongly suggests that MM:CO induces leakage by asymmetric packing stress because of hydrophobically driven interactions which could lead to leakage. As a further membrane perturbation effect, MM:CO causes lipid clustering in model vesicles. We address potential artifacts and misinterpretations of experiments characterizing leakage and fusion. Additional to the leakage activity, the pronounced fusogenic activity of the polymer and potentially of many other similar compounds likely has implications for antimicrobial activity and beyond.

Real-Time Fluorescence Microscopy on Living E. coli Sheds New Light on the Antibacterial Effects of the King Penguin β-Defensin AvBD103b

(1) Antimicrobial peptides (AMPs) are a promising alternative to conventional antibiotics. Among AMPs, the disulfide-rich β-defensin AvBD103b, whose antibacterial activities are not inhibited by salts contrary to most other β-defensins, is particularly appealing. Information about the mechanisms of action is mandatory for the development and approval of new drugs. However, data for non-membrane-disruptive AMPs such as β-defensins are scarce, thus they still remain poorly understood. (2) We used single-cell fluorescence imaging to monitor the effects of a β-defensin (namely AvBD103b) in real time, on living E. coli, and at the physiological concentration of salts. (3) We obtained key parameters to dissect the mechanism of action. The cascade of events, inferred from our precise timing of membrane permeabilization effects, associated with the timing of bacterial growth arrest, differs significantly from the other antimicrobial compounds that we previously studied in the same physiological conditions. Moreover, the AvBD103b mechanism does not involve significant stereo-selective interaction with any chiral partner, at any step of the process. (4) The results are consistent with the suggestion that after penetrating the outer membrane and the cytoplasmic membrane, AvBD103b interacts non-specifically with a variety of polyanionic targets, leading indirectly to cell death.

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.

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.

Spontaneous transmembrane pore formation by short-chain synthetic peptide

Amphiphilic β-peptides, which are synthetically designed short-chain helical foldamers of β-amino acids, are established potent biomimetic alternatives of natural antimicrobial peptides. An intriguing question is how the distinct molecular architecture of these short-chain and rigid synthetic peptides translates to its potent membrane-disruption ability. Here, we address this question via a combination of all-atom and coarse-grained molecular dynamics simulations of the interaction of mixed phospholipid bilayer with an antimicrobial 10-residue globally amphiphilic helical β-peptide at a wide range of concentrations. The simulation demonstrates that multiple copies of this synthetic peptide, initially placed in aqueous solution, readily self-assemble and adsorb at membrane interface. Subsequently, beyond a threshold peptide/lipid ratio, the surface-adsorbed oligomeric aggregate moves inside the membrane and spontaneously forms stable water-filled transmembrane pores via a cooperative mechanism. The defects induced by these pores lead to the dislocation of interfacial lipid headgroups, membrane thinning, and substantial water leakage inside the hydrophobic core of the membrane. A molecular analysis reveals that despite having a short architecture, these synthetic peptides, once inside the membrane, would stretch themselves toward the distal leaflet in favor of potential contact with polar headgroups and interfacial water layer. The pore formed in coarse-grained simulation was found to be resilient upon structural refinement. Interestingly, the pore-inducing ability was found to be elusive in a non-globally amphiphilic sequence isomer of the same β-peptide, indicating strong sequence dependence. Taken together, this work puts forward key perspectives of membrane activity of minimally designed synthetic biomimetic oligomers relative to the natural antimicrobial peptides.

Low-dimensional nanomaterials for antibacterial applications

The excessive use of antibiotics has led to a rise in drug-resistant bacteria. These "superbugs" are continuously emerging and becoming increasingly harder to treat. As a result, new and effective treatment protocols that have minimal risks of generating drug-resistant bacteria are urgently required. Advanced nanomaterials are particularly promising due to their drug loading/releasing capabilities combined with their potential photodynamic/photothermal therapeutic properties. In this review, 0-dimensional, 1-dimensional, 2-dimensional, and 3-dimensional nanomaterial-based systems are comprehensively discussed for bacterial-based diagnostic and treatment applications. Since the use of these platforms as antibacterials is relatively new, this review will provide appropriate insight into their construction and applications. As such, we hope this review will inspire researchers to explore antibacterial-based nanomaterials with the aim of developing systems for clinical applications.

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.

Hidden complexity in membrane permeabilization behavior of antimicrobial polycations

There are diverse membrane permeabilization behaviors of antimicrobial polycations in zwitterionic or charged vesicles; different mechanisms may occur over time.

Expression of Thanatin in HEK293 Cells and Investigation of its Antibacterial Effects on Some Human Pathogens

BACKGROUND

Thanatin is the smallest member of Beta-hairpin class of cationic peptide derived from insects with vast activities against various pathogens.

OBJECTIVE

In this study, the antimicrobial activity of this peptide against some species of human bacterial pathogens as well as its toxicity on NIH cells were evaluated.

METHODS

Thanatin DNA sequence was cloned into pcDNA3.1+ vector and transformed into a DH5α bacterial strain. Then the recombinant plasmids were transfected into HEK-293 cells by calcium phosphate co-precipitation. After applying antibiotic treatment, the supernatant medium containing thanatin was collected. The peptide quantity was estimated by SDS-PAGE and GelQuant software. The antimicrobial activity of this peptide was performed with Minimum Inhibitory Concentration (MIC) method. In addition, its toxicity on NIH cells were evaluated by MTT assay.

RESULTS

The peptide quantity was estimated approximately 164.21 µmolL-1. The antibacterial activity of thanatin was estimated between 0.99 and 31.58 µmolL-1 using MIC method. The result of cytotoxicity test on NIH cell line showed that the peptide toxicity up to the concentration of 394.10 µmolL-1 and for 48 hours, was not statistically significant from negative control cells (P>0.05). The antimicrobial assay demonstrated that thanatin had an antibacterial effect on some tested microorganisms. The results obtained in this study also showed that thanatin had no toxicity on mammalian cell lines including HEK293 and NIH.

CONCLUSION

Antimicrobial peptides such as thanatin are considered to be appropriate alternatives to conventional antibiotics in treating various human pathological diseases bacteria.

Cationic Polymers with Tailored Structures for Rendering Polysaccharide-Based Materials Antimicrobial: An Overview

Antimicrobial polymers have attracted substantial interest due to high demands on improving the health of human beings via reducing the infection caused by various bacteria. The review presented herein focuses on rendering polysaccharides, mainly cellulosic-based materials and starch to some extent, antimicrobial via incorporating cationic polymers, guanidine-based types in particular. Extensive review on synthetic antimicrobial materials or plastic/textile has been given in the past. However, few review reports have been presented on antimicrobial polysaccharide, cellulosic-based materials, or paper packaging, especially. The current review fills the gap between synthetic materials and natural polysaccharides (cellulose, starch, and cyclodextrin) as substrates or functional additives for different applications. Among various antimicrobial polymers, particular attention in this review is paid to guanidine-based polymers and their derivatives, including copolymers, star polymer, and nanoparticles with core-shell structures. The review has also been extended to gemini surfactants and polymers. Cationic polymers with tailored structures can be incorporated into various products via surface grafting, wet-end addition, blending, or reactive extrusion, effectively addressing the dilemma of improving substrate properties and bacterial growth. Moreover, the pre-commercial trial conducted successfully for making antimicrobial paper packaging has also been addressed.

Melittin-Induced Permeabilization, Re-sealing, and Re-permeabilization of E. coli Membranes

The permeabilization of model lipid bilayers by cationic peptides has been studied extensively over decades, with the bee-sting toxin melittin perhaps serving as the canonical example. However, the relevance of these studies to the permeabilization of real bacterial membranes by antimicrobial peptides remains uncertain. Here, we employ single-cell fluorescence microscopy in a detailed study of the interactions of melittin with the outer membrane (OM) and the cytoplasmic membrane (CM) of live Escherichia coli. Using periplasmic green fluorescent protein (GFP) as a probe, we find that melittin at twice the minimum inhibitory concentration first induces abrupt cell shrinkage and permeabilization of the OM to GFP. Within ∼4 s of OM permeabilization, the CM invaginates to form inward facing "periplasmic bubbles." Seconds later the bubbles begin to leak periplasmic GFP into the cytoplasm. Permeabilization is localized, consistent with possible formation of toroidal pores. Within ∼20 s, first the OM and then the CM re-seals to GFP. Some 2-20 min later, both CM and OM are re-permeabilized to GFP. We invoke a mechanism based on curvature stress concepts derived from model bilayer studies. The permeabilization and re-sealing events involve sequential, time-dependent build-up of melittin density within the outer and inner leaflets of each bilayer. We also propose a mechanical explanation for the early cell shrinkage event induced by melittin and a variety of other cationic peptides. As peptides gain access to the periplasm, they bind to the anionic peptido-crosslinks of the lipopolysaccharide layer, increasing its longitudinal elastic modulus. The cell wall shrinks because it can withstand the same turgor pressure with smaller overall extension. Shrinkage in turn induces invagination of the CM, preserving its surface area. We conclude by comparing the behavior of different peptides.

Rigidification of the Escherichia coli cytoplasm by the human antimicrobial peptide LL-37 revealed by superresolution fluorescence microscopy

Natural antimicrobial peptides (AMPs) that exhibit broad-spectrum antibacterial activity are often highly positively charged. Fluorescence microscopy shows that after permeabilization of Escherichia coli membranes by the cationic AMP LL-37 a massive influx of peptide freezes the diffusive motion of the chromosomal DNA and a subset of ribosomes. Both are highly negatively charged. Cells cannot recover growth. We suggest that LL-37 forms noncovalent, electrostatic linkages between DNA strands and among polyribosomes, rigidifying the entire cytoplasm. While the preponderance of polyanionic biopolymers in the cytoplasm facilitates diffusion in normal growth, this same characteristic renders the bacterium highly susceptible to attack by polycationic AMPs. The results help explain why bacteria develop resistance to AMPs very slowly and may inform the design of new antibacterial agents.

Superresolution, single-particle tracking reveals effects of the cationic antimicrobial peptide LL-37 on the Escherichia coli cytoplasm. Seconds after LL-37 penetrates the cytoplasmic membrane, the chromosomal DNA becomes rigidified on a length scale of ∼30 nm, evidenced by the loss of jiggling motion of specific DNA markers. The diffusive motion of a subset of ribosomes is also frozen. The mean diffusion coefficients of the DNA-binding protein HU and the nonendogenous protein Kaede decrease twofold. Roughly 10⁸ LL-37 copies flood the cell (mean concentration ∼90 mM). Much of the LL-37 remains bound within the cell after extensive rinsing with fresh growth medium. Growth never recovers. The results suggest that the high concentration of adsorbed polycationic peptides forms a dense network of noncovalent, electrostatic linkages within the chromosomal DNA and among 70S-polysomes. The bacterial cytoplasm comprises a concentrated collection of biopolymers that are predominantly polyanionic (e.g., DNA, ribosomes, RNA, and most globular proteins). In normal cells, this provides a kind of electrostatic lubrication, enabling facile diffusion despite high biopolymer volume fraction. However, this same polyanionic nature renders the cytoplasm susceptible to massive adsorption of polycationic agents once penetration of the membranes occurs. If this phenomenon proves widespread across cationic agents and bacterial species, it will help explain why resistance to antimicrobial peptides develops only slowly. The results suggest two design criteria for polycationic peptides that efficiently kill gram-negative bacteria: facile penetration of the outer membrane and the ability to alter the cytoplasm by electrostatically linking double-stranded DNA and 70S-polysomes.

HaloTag Assay Suggests Common Mechanism of E. coli Membrane Permeabilization Induced by Cationic Peptides

Permeabilization of the Gram-negative bacterial outer membrane (OM) by antimicrobial peptides (AMPs) is the initial step enabling access of the AMP to the cytoplasmic membrane. We present a new single-cell, time-resolved fluorescence microscopy assay that reports on the permeabilization of the E. coli OM to small molecules with a time resolution of 3 s or better. When profluorophore JF₆₄₆ (702 Da) crosses the outer membrane (OM) and gains access to the periplasm, it binds to the localized HaloTag protein (34 kDa) and fluoresces in a characteristic hollow spatial pattern. Previous work used the much larger periplasmic GFP (27 kDa) probe, which reports on OM permeabilization to globular proteins. We test the assay on three cationic agents: Gellman random β-peptide copolymer MM₆₃:CHx₃₇, human AMP LL-37, and synthetic hybrid AMP CM15. These results combined with the previous work suggest a unifying sequence of OM and cytoplasmic membrane (CM) events that may prove commonplace in the attack of cationic peptides on Gram-negative bacteria. The peptide initially induces gradual OM permeabilization to small molecules, likely including the peptide itself. After a lag time, abrupt permeabilization of the OM, abrupt resealing of the OM, and abrupt permeabilization of the CM (all to globular proteins) occur in rapid sequence. We propose a mechanism based on membrane curvature stress induced by the time-dependent differential binding of peptide to the outer leaflet of the OM and CM. The results provide fresh insight into the critical OM-permeabilization step leading to a variety of damaging downstream events.

Peptide-Like Nylon-3 Polymers with Activity against Phylogenetically Diverse, Intrinsically Drug-Resistant Pathogenic Fungi

Understanding the dimensions of fungal diversity has major implications for the control of diseases in humans, plants, and animals and in the overall health of ecosystems on the planet. One ancient evolutionary strategy organisms use to manage interactions with microbes, including fungi, is to produce host defense peptides (HDPs). HDPs and their synthetic analogs have been subjects of interest as potential therapeutic agents. Due to increases in fungal disease worldwide, there is great interest in developing novel antifungal agents. Here we describe activity of polymeric HDP analogs against fungi from 18 pathogenic genera composed of 41 species and 72 isolates. The synthetic polymers are members of the nylon-3 family (poly-β-amino acid materials). Three different nylon-3 polymers show high efficacy against surprisingly diverse fungi. Across the phylogenetic spectrum (with the exception of Aspergillus species), yeasts, dermatophytes, dimorphic fungi, and molds were all sensitive to the effects of these polymers. Even fungi intrinsically resistant to current antifungal drugs, such as the causative agents of mucormycosis (Rhizopus spp.) and those with acquired resistance to azole drugs, showed nylon-3 polymer sensitivity. In addition, the emerging pathogens Pseudogymnoascus destructans (cause of white nose syndrome in bats) and Candida auris (cause of nosocomial infections of humans) were also sensitive. The three nylon-3 polymers exhibited relatively low toxicity toward mammalian cells. These findings raise the possibility that nylon-3 polymers could be useful against fungi for which there are only limited and/or no antifungal agents available at present.IMPORTANCE Fungi reside in all ecosystems on earth and impart both positive and negative effects on human, plant, and animal health. Fungal disease is on the rise worldwide, and there is a critical need for more effective and less toxic antifungal agents. Nylon-3 polymers are short, sequence random, poly-β-amino acid materials that can be designed to manifest antimicrobial properties. Here, we describe three nylon-3 polymers with potent activity against the most phylogenetically diverse set of fungi evaluated thus far in a single study. In contrast to traditional peptides, nylon-3 polymers are highly stable to proteolytic degradation and can be produced efficiently in large quantities at low cost. The ability to modify nylon-3 polymer composition easily creates an opportunity to tailor efficacy and toxicity, which makes these materials attractive as potential broad-spectrum antifungal therapeutics.

Surface Modified with a Host Defense Peptide-Mimicking β-Peptide Polymer Kills Bacteria on Contact with High Efficacy

Methicillin-resistant Staphylococcus aureus (MRSA) has been one of the major nosocomial pathogens to cause frequent and serious infections that are associated with various biomedical surfaces. This study demonstrated that surface modified with host defense peptide-mimicking β-peptide polymer, has surprisingly high bactericidal activities against Escherichia coli ( E. coli) and MRSA. As surface-tethered β-peptide polymers cannot move freely to adopt the collaborative interactions with bacterial membrane and are too short to penetrate the cell envelop, we proposed a mode of action by diffusing away the cell membrane-stabilizing divalent ions, Ca²⁺ and Mg²⁺. This hypothesis was supported by our study that Ca²⁺ and Mg²⁺ supplementation in the assay medium causes up to 80% loss of bacterial killing efficacy and that the addition of divalent ion chelating ethylenediaminetetraacetic acid into the above assay medium leads to significant recovery of the bacterial killing efficacy. In addition to its potent bacterial killing efficacy, the surface-tethered β-peptide polymer also demonstrated excellent biocompatibility by displaying no hemolysis and supporting mammalian cell adhesion and growth. In conclusion, this study demonstrated the potential of β-peptide polymer-modified surface in addressing nosocomial infections that are associated with various surfaces in biomedical applications.

Self-immolative polymers with potent and selective antibacterial activity by hydrophilic side chain grafting

Self-immolative polymers, which exert potent antibacterial activity with low hemolytic toxicity to red blood cells, are triggered to unzip into small molecules by a chemical stimulus.

Lipidated Peptide Dendrimers Killing Multidrug-Resistant Bacteria

New antibiotics are urgently needed to address multidrug-resistant (MDR) bacteria. Herein we report that second-generation (G2) peptide dendrimers bearing a fatty acid chain at the dendrimer core efficiently kill Gram-negative bacteria including Pseudomonas aeruginosa and Acinetobacter baumannii, two of the most problematic MDR bacteria worldwide. Our most active dendrimer TNS18 is also active against Gram-positive methicillin-resistant Staphylococcus aureus. Based on circular dichroism and molecular dynamics studies, we hypothesize that TNS18 adopts a hydrophobically collapsed conformation in water with the fatty acid chain backfolded onto the peptide dendrimer branches and that the dendrimer unfolds in contact with the membrane to expose its lipid chain and hydrophobic residues, thereby facilitating membrane disruption leading to rapid bacterial cell death. Dendrimer TNS18 shows promising in vivo activity against MDR clinical isolates of A. baumannii and Escherichia coli, suggesting that lipidated peptide dendrimers might become a new class of antibacterial agents.

Structure and Interactions of A Host Defense Antimicrobial Peptide Thanatin in Lipopolysaccharide Micelles Reveal Mechanism of Bacterial Cell Agglutination

Host defense cationic Antimicrobial Peptides (AMPs) can kill microorganisms including bacteria, viruses and fungi using various modes of action. The negatively charged bacterial membranes serve as a key target for many AMPs. Bacterial cell death by membrane permeabilization has been well perceived. A number of cationic AMPs kill bacteria by cell agglutination which is a distinctly different mode of action compared to membrane pore formation. However, mechanism of cell agglutinating AMPs is poorly understood. The outer membrane lipopolysaccharide (LPS) or the cell-wall peptidoglycans are targeted by AMPs as a key step in agglutination process. Here, we report the first atomic-resolution structure of thanatin, a cell agglutinating AMP, in complex with LPS micelle by solution NMR. The structure of thanatin in complex with LPS, revealed four stranded antiparallel β-sheet in a ‘head-tail’ dimeric topology. By contrast, thanatin in free solution assumed an antiparallel β-hairpin conformation. Dimeric structure of thanatin displayed higher hydrophobicity and cationicity with sites of LPS interactions. MD simulations and biophysical interactions analyses provided mode of LPS recognition and perturbation of LPS micelle structures. Mechanistic insights of bacterial cell agglutination obtained in this study can be utilized to develop antibiotics of alternative mode of action.

Oxidative stress induced in E. coli by the human antimicrobial peptide LL-37

Antimicrobial peptides (AMPs) are thought to kill bacterial cells by permeabilizing their membranes. However, some antimicrobial peptides inhibit E. coli growth more efficiently in aerobic than in anaerobic conditions. In the attack of the human cathelicidin LL-37 on E. coli, real-time, single-cell fluorescence imaging reveals the timing of membrane permeabilization and the onset of oxidative stress. For cells growing aerobically, a CellROX Green assay indicates that LL-37 induces rapid formation of oxidative species after entry into the periplasm, but before permeabilization of the cytoplasmic membrane (CM). A cytoplasmic Amplex Red assay signals a subsequent burst of oxidative species, most likely hydrogen peroxide, shortly after permeabilization of the CM. These signals are much stronger in the presence of oxygen, a functional electron transport chain, and a large proton motive force (PMF). They are much weaker in cells growing anaerobically, by either fermentation or anaerobic respiration. In aerobic growth, the oxidative signals are attenuated in a cytochrome oxidase–bd deletion mutant, but not in a –bo₃ deletion mutant, suggesting a specific effect of LL-37 on the electron transport chain. The AMPs melittin and LL-37 induce strong oxidative signals and exhibit O₂-sensitive MICs, while the AMPs indolicidin and cecropin A do not. These results suggest that AMP activity in different tissues may be tuned according to the local oxygen level. This may be significant for control of opportunistic pathogens while enabling growth of commensal bacteria.

Antimicrobial peptides play a significant role in the innate immune response of plants and animals, including humans. While it is well known that AMPs can permeabilize bacterial cell membranes, a growing body of evidence indicates that they cause a variety of additional deleterious effects. Here we use single-cell imaging methods to study the induction of oxidative stress in live E. coli by several natural cationic AMPs, including the human cathelicidin LL-37. Strong fluorescence signals indicative of oxidative stress correlate with smaller minimum inhibitory concentrations (MICs) in aerobic vs anaerobic growth conditions. A detailed mechanistic study suggests that LL-37 disrupts the proper flow of electrons through the electron transport chain, releasing oxidative species into the periplasm. Based on these results, we suggest that the degree of aeration in different tissue types may be used by the host to modulate AMP efficacy.

Synthetic Random Copolymers as a Molecular Platform To Mimic Host-Defense Antimicrobial Peptides

Synthetic polymers have been used as a molecular platform to develop host-defense antimicrobial peptide (AMP) mimetics which are effective in killing drug-resistant bacteria. In this topical review, we will discuss the AMP-mimetic design and chemical optimization strategies as well as the biological and biophysical implications of AMP mimicry by synthetic polymers. Traditionally, synthetic polymers have been used as a chemical means to replicate the chemical functionalities and physicochemical properties of AMPs (e.g., cationic charge, hydrophobicity) to recapitulate their mode of action. However, we propose a new perception that AMP-mimetic polymers are an inherently bioactive platform as whole molecules, which mimic more than the side chain functionalities of AMPs. The tunable nature and chemical simplicity of synthetic random polymers facilitate the development of potent, cost-effective, broad-spectrum antimicrobials. The polymer-based approach offers the potential for many antimicrobial applications to be used directly in solution or attached to surfaces to fight against drug-resistant bacteria.

Pyruvate Oxidase as a Critical Link between Metabolism and Capsule Biosynthesis in Streptococcus pneumoniae

The pneumococcus is one of the most prodigious producers of hydrogen peroxide amongst bacterial pathogens. Hydrogen peroxide production by the pneumococcus has been implicated in antibiotic synergism, competition between other bacterial colonizers of the nasopharynx, and damage to epithelial cells. However, the role during invasive disease has been less clear with mutants defective in hydrogen peroxide production demonstrating both attenuation and heightened invasive disease capacity depending upon strain and serotype background. This work resolves these conflicting observations by demonstrating that the main hydrogen peroxide producing enzyme of the pneumococcus, SpxB, is required for capsule formation in a strain dependent manner. Capsule production by strains harboring capsules with acetylated sugars was dependent upon the presence of spxB while capsule production in serotypes lacking such linkages were not. The spxB mutant had significantly lower steady-state cellular levels of acetyl-CoA, suggesting that loss of capsule arises from dysregulation of this intermediary metabolite. This conclusion is corroborated by deletion of pdhC, which also resulted in lower steady-state acetyl-CoA levels and phenocopied the capsule expression profile of the spxB mutant. Capsule and acetyl-CoA levels were restored in the spxB and lctO (lactate oxidase) double mutant, supporting the connection between central metabolism and capsule formation. Taken together, these data show that the defect in pathogenesis in the spxB mutant is due to a metabolic imbalance that attenuates capsule formation and not to reduced hydrogen peroxide formation.

The pneumococcus polysaccharide capsule is one of the most critical virulence determinants produced by this major human pathogen. The pneumococcus also produces prodigious amounts of hydrogen peroxide via the enzymatic reaction catalyzed by pyruvate oxidase, SpxB. Deletion of spxB resulted in the loss of surface polysaccharide capsule production in a serotype dependent manner with a mirrored effect on the virulence of the mutants. We observed that deletion of spxB reduced the steady-state levels of acetyl-CoA, a key metabolic intermediate in peptidoglycan, fatty acid biosynthesis, and in capsule biosynthesis in a subset of serotypes. These data suggest that the defect in capsule production was due to altered metabolism that results in reduced acetyl-CoA availability. Corroborating these data, we found that capsule biosynthesis was impaired upon loss of PDHC, an additional metabolic enzyme that generates acetyl-CoA. These data reveal a critical link between pneumococcal metabolism and capsule biosynthesis as well as provide a striking example of how a virulence gene can have a differential contribution to pathogenesis dependent upon strain background.

Recent Advances in Antimicrobial Polymers: A Mini-Review

Human safety and well-being is threatened by microbes causing numerous infectious diseases resulting in a large number of deaths every year. Despite substantial progress in antimicrobial drugs, many infectious diseases remain difficult to treat. Antimicrobial polymers offer a promising antimicrobial strategy for fighting pathogens and have received considerable attention in both academic and industrial research. This mini-review presents the advances made in antimicrobial polymers since 2013. Antimicrobial mechanisms exhibiting either passive or active action and polymer material types containing bound or leaching antimicrobials are introduced. This article also addresses the applications of these antimicrobial polymers in the medical, food, and textile industries.

Focal Targeting of the Bacterial Envelope by Antimicrobial Peptides

Antimicrobial peptides (AMPs) are utilized by both eukaryotic and prokaryotic organisms. AMPs such as the human beta defensins, human neutrophil peptides, human cathelicidin, and many bacterial bacteriocins are cationic and capable of binding to anionic regions of the bacterial surface. Cationic AMPs (CAMPs) target anionic lipids [e.g., phosphatidylglycerol (PG) and cardiolipins (CL)] in the cell membrane and anionic components [e.g., lipopolysaccharide (LPS) and lipoteichoic acid (LTA)] of the cell envelope. Bacteria have evolved mechanisms to modify these same targets in order to resist CAMP killing, e.g., lysinylation of PG to yield cationic lysyl-PG and alanylation of LTA. Since CAMPs offer a promising therapeutic alternative to conventional antibiotics, which are becoming less effective due to rapidly emerging antibiotic resistance, there is a strong need to improve our understanding about the AMP mechanism of action. Recent literature suggests that AMPs often interact with the bacterial cell envelope at discrete foci. Here we review recent AMP literature, with an emphasis on focal interactions with bacteria, including (1) CAMP disruption mechanisms, (2) delocalization of membrane proteins and lipids by CAMPs, and (3) CAMP sensing systems and resistance mechanisms. We conclude with new approaches for studying the bacterial membrane, e.g., lipidomics, high resolution imaging, and non-detergent-based membrane domain extraction.

Lights, Camera, Action! Antimicrobial Peptide Mechanisms Imaged in Space and Time

Deeper understanding of the bacteriostatic and bactericidal mechanisms of antimicrobial peptides (AMPs) should help in the design of new antibacterial agents. Over several decades, a variety of biochemical assays have been applied to bulk bacterial cultures. While some of these bulk assays provide time resolution of the order of 1min, they do not capture faster mechanistic events. Nor can they provide subcellular spatial information or discern cell-to-cell heterogeneity within the bacterial population. Single-cell, time-resolved imaging assays bring a completely new spatiotemporal dimension to AMP mechanistic studies. We review recent work that provides new insights into the timing, sequence, and spatial distribution of AMP-induced effects on bacterial cells.