Gramicidin A Mutants with Antibiotic Activity against Both Gram‐Positive and Gram‐Negative Bacteria

Antimicrobial peptides (AMPs) have shown potential as alternatives to traditional antibiotics for fighting infections caused by antibiotic‐resistant bacteria. One promising example of this is gramicidin A (gA). In its wild‐type sequence, gA is active by permeating the plasma membrane of Gram‐positive bacteria. However, gA is toxic to human red blood cells at similar concentrations to those required for it to exert its antimicrobial effects. Installing cationic side chains into gA has been shown to lower its hemolytic activity while maintaining the antimicrobial potency. In this study, we present the synthesis and the antibiotic activity of a new series of gA mutants that display cationic side chains. Specifically, by synthesizing alkylated lysine derivatives through reductive amination, we were able to create a broad selection of structures with varied activities towards Staphylococcus aureus and methicillin‐resistant S. aureus (MRSA). Importantly, some of the new mutants were observed to have an unprecedented activity towards important Gram‐negative pathogens, including Escherichia coli, Klebsiella pneumoniae and Psuedomonas aeruginosa.     

Peptoid–Peptide Hybrid Backbone Architectures

Peptidomimetic oligomers and foldamers have received considerable attention for over a decade, with β‐peptides and the so‐called peptoids (N‐alkylglycine oligomers) representing prominent examples of such architectures. Lately, hybrid or mixed backbones consisting of both α‐ and β‐amino acids (α/β‐peptides) have been investigated in some detail as well. The present Minireview is a survey of the literature concerning hybrid structures of α‐amino acids and peptoids, including β‐peptoids (N‐alkyl‐β‐alanine oligomers), and is intended to give an overview of this area of research within the field of peptidomimetic science.     

Synthesis and Characterization of Cationic Surfactants Based on N‐Hexamethylenetetramine as Active Microfouling Agents

Four cationic surfactants of quaternary hexammonium silane chloride based on hexamethylenetetramine and alkyl chloride were synthesized. The chemical structures of the prepared cationic surfactants were elucidated using Fourier transform infrared (FT‐IR) spectroscopy and mass spectrometry analysis. The surface and thermodynamic properties of the prepared surfactants were also studied. The performance of these cationic surfactants as microfouling agents against two strains of Gram‐negative bacteria, namely, Pseudomonas aeruginosa and Escherichia coli, and two strains of Gram‐positive bacteria, namely, Staphylococcus aureus and Bacillus subtilis, were evaluated as antimicrobial agents. The results showed that the maximum antimicrobial activity was detected for N‐hexamethylenetetramine‐N‐ethyl silane ammonium trichloride (Ah). The maximum and minimum antimicrobial activities were 73 and 60 % against S. aureus and E. coli, respectively, at a concentration of 5 mg/l, pH 7, and 37 °C.     

Modification of narrow‐spectrum peptidomimetic polyurethanes with fatty acid chains confers broad‐spectrum antibacterial activity

Each year, thousands of patients die from antimicrobial‐resistant bacterial infections that fail to respond to conventional antibiotic treatment. Antimicrobial polymers are a promising new method of combating antibiotic‐resistant bacterial infections. We have previously reported the synthesis of a series of narrow‐spectrum peptidomimetic antimicrobial polyurethanes that are effective against Gram‐negative bacteria, such as Escherichia coli; however, these polymers are not effective against Gram‐positive bacteria, such as Staphylococcus aureus. With the aim of understanding the correlation between chemical structure and antibacterial activity, we have subsequently developed three structural variants of these antimicrobial polyurethanes using post‐polymerization modification with decanoic acid and oleic acid. Our results show that such modifications converted the narrow‐spectrum antibacterial activity of these polymers into broad‐spectrum activity against Gram‐positive species such as S. aureus, however, also increasing their toxicity to mammalian cells. Mechanistic studies of bacterial membrane disruption illustrate the differences in antibacterial action between the various polymers. The results demonstrate the challenge of balancing antimicrobial activity and mammalian cell compatibility in the design of antimicrobial polymer compositions. © 2019 Society of Chemical Industry     

Synthesis and biocidal efficacy of self‐spreading polydimethylsiloxane oligomers possessing oxyethylene‐functionalized quaternary ammoniums

In an effort to develop a more versatile creeping biocide that is capable of self‐spreading and self‐decontaminating of pathogenic bacteria, we report the development of two new homologous series of hybrid PDMS molecules. These oligomers were synthesized with terminal quaternary ammonium functionalities bearing variable length oxyethylene moieties. It is shown that the ionic interaction of the ammonium groups with the surface onto which it spreads can be tempered by the oxyethylene segments through close association of the polar chains with the cationic centers within the hydrophobic PDMS environment, thereby promoting self‐spreading of the molecule. Once the compounds spread to a humid environment, the oxyethylene chains “blossom” and subsequently expose the biocidal centers, at which point, function as broad spectrum versatile antimicrobials. While biological evaluation showed antimicrobial activity against both Gram‐positive and Gram‐negative bacteria for all samples, one series was found to be much more effective due to lower steric hindrance surrounding the biocidal cationic termini. However, this increased exposure of the cationic center also altered the physical properties of the compounds except those isolated as a waxy solid. Self‐spreading abilities with increasing oxyethylene chain length correlating to a decreasing spread rate. The decontaminating ability of the two most active compounds was demonstrated by allowing samples to spread to pools of water contaminated with S. aureus, yielding log reductions as high as 5.7 in less than two hours without external influences. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Peptoid Efficacy against Polymicrobial Biofilms Determined by Using Propidium Monoazide‐Modified Quantitative PCR

Biofilms containing Candida albicans are responsible for a wide variety of clinical infections. The protective effects of the biofilm matrix, the low metabolic activity of microorganisms within a biofilm and their high mutation rate, significantly enhance the resistance of biofilms to conventional antimicrobial treatments. Peptoids are peptide‐mimics that share many features of host defence antimicrobial peptides but have increased resistance to proteases and therefore have better stability in vivo. The activity of a library of peptoids was tested against monospecies and polymicrobial bacterial/fungal biofilms. Selected peptoids showed significant bactericidal and fungicidal activity against the polymicrobial biofilms. This coupled with low cytotoxicity suggests that peptoids could offer a new option for the treatment of clinically relevant polymicrobial infections.     

Expanding the Scope of Sortase‐Mediated Ligations by Using Sortase Homologues

Sortase‐catalyzed transacylation reactions are widely used for the construction of non‐natural protein derivatives. However, the most commonly used enzyme for these strategies (sortase A from Staphylococcus aureus) is limited by its narrow substrate scope. To expand the range of substrates compatible with sortase‐mediated reactions, we characterized the in vitro substrate preferences of eight sortase A homologues. From these studies, we identified sortase A enzymes that recognize multiple substrates that are unreactive toward sortase A from S. aureus. We further exploited the ability of sortase A from Streptococcus pneumoniae to recognize an LPATS substrate to perform a site‐specific modification of the N‐terminal serine residue in the naturally occurring antimicrobial peptide DCD‐1L. Finally, we unexpectedly observed that certain substrates (LPATXG, X=Nle, Leu, Phe, Tyr) were susceptible to transacylation at alternative sites within the substrate motif, and sortase A from S. pneumoniae was capable of forming oligomers. Overall, this work provides a foundation for the further development of sortase enzymes for use in protein modification.     

Indoloazepinone‐Constrained Oligomers as Cell‐Penetrating and Blood–Brain‐Barrier‐Permeating Compounds

Non‐cationic and amphipathic indoloazepinone‐constrained (Aia) oligomers have been synthesized as new vectors for intracellular delivery. The conformational preferences of the [l ‐Aia‐Xxx]_n oligomers were investigated by circular dichroism (CD) and NMR spectroscopy. Whereas Boc‐[l ‐Aia‐Gly]_2,4‐OBn oligomers 12 and 13 and Boc‐[l ‐Aia‐β³‐h‐l ‐Ala]_2,4‐OBn oligomers 16 and 17 were totally or partially disordered, Boc‐[l ‐Aia‐l ‐Ala]₂‐OBn ( 14 ) induced a typical turn stabilized by C₅‐ and C₇‐membered H‐bond pseudo‐cycles and aromatic interactions. Boc‐[l ‐Aia‐l ‐Ala]₄‐OBn ( 15 ) exhibited a unique structure with remarkable T‐shaped π‐stacking interactions involving the indole rings of the four l ‐Aia residues forming a dense hydrophobic cluster. All of the proposed FITC‐6‐Ahx‐[l ‐Aia‐Xxx]₄‐NH₂ oligomers 19 – 23 , with the exception of FITC‐6‐Ahx‐[l ‐Aia‐Gly]₄‐NH₂ ( 18 ), were internalized by MDA‐MB‐231 cells with higher efficiency than the positive references penetratin and Arg₈. In parallel, the compounds of this series were successfully explored in an in vitro blood–brain barrier (BBB) permeation assay. Although no passive diffusion permeability was observed for any of the tested Ac‐[l ‐Aia‐Xxx]₄‐NH₂ oligomers in the PAMPA model, Ac‐[l ‐Aia‐l ‐Arg]₄‐NH₂ ( 26 ) showed significant permeation in the in vitro cell‐based human model of the BBB, suggesting an active mechanism of cell penetration.     

Preparation of antimicrobial polycarboxybetaine‐based hydrogels for studies of drug loading and release

A series of cationic poly(N‐isopropyl acrylamide) (PNIPAM)‐g‐poly(carboxybetaine ester) (PCBMAE) hydrogels were prepared by reversible addition–fragmentation chain‐transfer polymerization with PCBMAE precursors reacting with N‐isopropyl acrylamide in the presence of N,N′‐methylene bisacrylamide. These hydrogels exhibited excellent antimicrobial activities against Staphylococcus aureus and could switch to nontoxic zwitterionic hydrogels after hydrolysis. Nonionic tetracycline hydrochloride (TCHC) and anionic sodium salicylate (SA) were selected to evaluate the loading capacities and release kinetics of the cationic hydrogels. We found that the loading efficiencies of TCHC in the PNIPAM‐g‐PCBMAE hydrogels were approximately twice as high as those of SA. However, the cumulative release amount of TCHC was lower than that of SA from the corresponding cationic hydrogel at 37°C. In addition, the PNIPAM‐g‐PCBMAE hydrogels exhibited accelerated release rates of both TCHC and SA with increasing content of (2‐carboxymethyl)?3‐acryloxyethyldimethylammonium chloride methyl ester. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39839.     

Antimicrobial poly(N‐vinyl‐2‐pyrrolidone‐alt‐maleic anhydride)/poly(ethylene imine) macrocomplexes

The antimicrobial polymer/polymer macrocomplexes were synthesized by radical alternating copolymerization of N‐vinyl‐2‐pyrrolidone with maleic anhydride [poly(VP‐alt‐MA)] with 2,2′‐azobis‐isobutyronitrile as an initiator at 65°C in dioxane solutions under nitrogen atmosphere, and interaction of prepared copolymer with poly(ethylene imine) (PEI) in aqueous solutions. The susceptibility of some Gram‐negative (Salmonella enteritidis and Escherichia coli) and Gram‐positive (Staphylococcus aureus and Listeria monocytogenes) bacteria to the alternating copolymer and its PEI macrocomplexes with different compositions in microbiological medium was studied using pour‐plate technique. All the studied polymers, containing biologically active moieties in the form of ionized cyclic amide, and macrobranched aliphatic amine groups and acid/amine complexed fragments, were more effective against L. monocytogenes than those for Gram‐positive S. aureus bacterium. This fact was explained by different surface layer structural architectures of biomacromolecules of tested bacteria. The resulting polymeric antimicrobial materials are expected to be used in various areas of medicine and food industry. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:5841–5847, 2006     

Effect of organoclay and silver‐substituted zeolite on the mechanical and antibacterial properties of a silicone rubber filled with 2‐hydroxypropyl‐3‐piperazinyl‐quinoline carboxylic acid methacrylate

Silicone rubber compounds filled with different loadings of organoclay (OC) and silver substituted zeolite (SSZ) solid fillers were prepared and cured with 2,5‐dimethyl‐2,5‐di(tert‐butylperoxy) hexane. The rubber vulcanizates contained an antimicrobial agent to protect them against Escherichia coli (E. coli ATCC 25922) and Staphylococcus aureus (S. aureus ATCC 25923) bacteria. The tensile strength, elongation at break, stored energy density at break, Young's modulus, modulus at 100% elongation, cyclic fatigue life, and glass transition temperature of the rubber vulcanizates were subsequently measured. The antimicrobial performance of the rubber surfaces were determined by disk diffusion testing and plate count agar method. The antimicrobial agent had an adverse effect on the mechanical properties, but the cyclic fatigue life of the rubber vulcanizate improved. The addition of OC and SSZ could improve the tensile strength, elongation at break and stored energy density at break, but deteriorated the tear energy, Young's modulus and modulus at 100% elongation. The inclusion of the fillers was not beneficial to the antimicrobial activity of the rubber against bacteria. The HPQM in the rubber was effective more against E. coli. than against S. aureus. Furthermore, the antimicrobial activity increased when the contact time in the test solution was increased. POLYM. ENG. SCI., 54:932–941, 2014. © 2013 Society of Plastics Engineers     

Synthesis and ultraviolet‐curing behaviors of vinyl ether functionalized polyurethane oligomers

The vinyl ether functionalized oligomer is one of the most basic components of vinyl ether functionalized materials for cationic UV‐curable coatings. In this study, three types of vinyl ether functionalized polyurethane oligomers (i.e., polyether, polyester, and polydimethylsiloxane) were synthesized with diisocyanate, diol, and hydroxyethyl vinyl ether. These oligomers were characterized by IR, ¹H‐NMR, and ¹³C‐NMR spectroscopy. The effect of the raw material ratio on the oligomer, UV‐curing behaviors, and thermal properties of these oligomers were investigated. The UV‐curing behavior was analyzed by real‐time Fourier transform infrared spectroscopy. The vinyl ether terminated polyester urethane oligomer exhibited better UV curing, with a higher final conversion and maximum UV‐curing rates. In addition, the light intensity was enhanced for oligomers with better UV‐curing properties. Research on these vinyl ether functionalized oligomers is essential to the development and applications of cationic vinyl ethers systems. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40501.     

Synthesis and antimicrobial properties of a guanidine‐based oligomer grafted with a reactive cationic surfactant through Michael addition

A guanidine‐based oligomer grafted with a reactive cationic surfactant was designed and synthesized through Michael addition in an attempt to combine its antibacterial and emulsification properties. This was also an excellent and efficient strategy for preparing more kinds of guanidine derivatives. Fourier transform infrared spectroscopy, ¹H‐NMR, and ¹³C‐NMR showed that the guanidine‐based oligomer grafted with reactive cationic surfactant was synthesized successfully. The antimicrobial activity and antimicrobial mechanism were investigated with several approaches. The antimicrobial activity results indicated that the introduction of a cationic surfactant into the guanidine‐based oligomer effectively raised the antimicrobial activity and showed a synergistic effect. The UV absorption at 260 nm was used to detect the dynamic antimicrobial process of the modified guanidine oligomer. Further, the results of scanning electron microscopy and atomic force microscopy implied that the antimicrobial mechanism of the modified guanidine oligomer changed the permeability of the cell membrane of the bacteria and caused the leakage of intracellular components of the Escherichia coli cells. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3489–3497, 2013     

Cycloaddition Reactions of 1,3‐Diaza‐2‐azoniaallene Salts and Glycals

The 1,3‐diaza‐2‐azoniaallene salt 3a reacts stereoselectively with glycals ( 5a—e ) to afford pyrano[2,3‐d]‐1,2,3‐ triazolium salts 6a—e . In contrast to other 1,3‐dipolar cycloadditions of glycals reported so far, the stereoselectivity of compounds 6 is not determined by the substituent on C‐3 of the glycal. Both cis ( 6a,b ) and trans ( 6d,e ) substitutions on C‐7 and C‐7a were found for bicyclic compounds 6 (crystal structure of 6a ). Under the influence of acid 6e opens the pyran ring to give the triazolium salt 9 . Addition of antimony pentachloride to a solution of the glycal 5e and the chlorotriazene 2a results in the formation of the pyranotriazene 12 containing two triazene units. In the presence of acid the pyranotriazene 6c reacts with alcohols to afford 2‐hydrazino glycosides 13a,b, 15 , which with zinc dust in acetic acid are reduced to 2‐amino glycosides 14a,b .     

Alanine and Lysine Scans of the LL‐37‐Derived Peptide Fragment KR‐12 Reveal Key Residues for Antimicrobial Activity

The human host defence peptide LL‐37 is a broad‐spectrum antibiotic with immunomodulatory functions. Residues 18–29 in LL‐37 have previously been identified as a minimal peptide (KR‐12) that retains antibacterial activity with decreased cytotoxicity. In this study, analogues of KR‐12 were generated by Ala and Lys scans to identify key elements for activity. These were tested against a panel of human pathogens and for membrane permeabilisation on liposomes. Replacements of hydrophobic and cationic residues with Ala were detrimental for antibiotic potency. Substitutions by Lys increased activity, as long as the increase in cationic density did not disrupt the amphiphilic disposition of the helical structure. Importantly, substitutions showed differential effects against different organisms. Replacement of Gln5 with Lys and Asp9 with Ala or Lys improved the broad‐spectrum activity most, each resulting in up to an eightfold increase in potency against Staphylococcus aureus, Pseudomonas aeruginosa and Candida albicans. The improved analogues displayed no significant toxicity against human cells, and thus, KR‐12 is a tuneable template for antibiotic development.     

Electrospun nylon 6 nanofibers incorporated with 2‐substituted N‐alkylimidazoles and their silver(I) complexes for antibacterial applications

The article presents the incorporation of biocides [2‐substituted N‐alkylimidazoles and their silver(I) complexes] into electrospun nylon 6 nanofibers for application as antimicrobial materials. The electrospun nylon 6/biocides nanofiber composites were characterized by IR spectroscopy (ATR‐FTIR) and scanning electron microscopy (SEM‐EDX). The antimicrobial activity of the electrospun nylon 6/biocides nanofiber composites was evaluated against Escherichia coli, Staphylococcus aureus, and Bacillus subtilis subsp. spizizenii using the disk diffusion method, the American Association for Textile Chemists and Colorists test method 100‐2004 and the dynamic shake flask method (American Society for Testing and Materials E2149‐10). The electrospun nylon 6 nanofibers incorporated with 2‐substituted N‐alkylimidazoles displayed moderate to excellent levels of growth reduction against S. aureus (73.2–99.8%). For the electrospun nylon 6 nanofibers incorporated with silver(I) complexes, the levels of growth reduction were >99.99%, for both E. coli and S. aureus, after the antimicrobial activity evaluation using the shake flask method. The study demonstrated that the electrospun nanofibers, fabricated using the incorporation strategy, have the potential to be used as attractive antimicrobial materials. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39783.     

A comparison of linear and cyclic peptoid oligomers as potent antimicrobial agents

We investigated the antimicrobial activities of N-substituted glycine "peptoid" oligomers incorporating cationic and hydrophobic side chains. Head-to-tail macrocyclization was employed to enhance antimicrobial activity. Both linear and cyclic peptoids, ranging from six to ten residues, demonstrate potent antimicrobial activity against Gram-positive and Gram-negative bacteria. These peptoids do not cause significant lysis of human erythrocytes, indicating selective antimicrobial activity. Conformational ordering established upon macrocyclization is generally associated with an enhanced capacity to inhibit bacterial cell growth. Moreover, increased hydrophobic surface area also plays a role in improving antimicrobial activity. We demonstrate the potency of a cyclic peptoid in exerting antimicrobial activity against clinical strains of S. aureus while deterring the emergence of antimicrobial resistance.     

Antimicrobial N‐halamine coatings synthesized via vapor‐phase assisted polymerization

An N‐halamine monomer, 3‐allyl‐5,5‐dimethylhydantoin (ADMH), was synthesized by a Gabriel reaction of 5,5‐dimethylhydantoin and 3‐bromopropene. Antimicrobial coatings of poly[1‐(4,4‐dimethyl‐2,5‐dioxoimidazolidin‐1‐methyl)ethylene] were prepared on plasma‐treated PET fabrics via a vapor‐phase assisted polymerization (VAP) process using gasified ADMH as monomer. The coatings endow the PET fabrics with an antimicrobial efficiency greater than 80% for both Escherichia coli and Staphylococcus aureus after chlorination of the N‐halamine polymer with dilute bleach solution. The obtained antimicrobial effect has remarkable durability that can bear over 30 times of stringent laundering tests. Compared with other antimicrobial finishing methods, the VAP methodology offers great advantages in needless of organic solvents and small consumption of monomer. It has potential applications in a wide variety of fields such as hygienic clothing, underwear, socks, and medical textiles. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41824.     

Synthesis and antimicrobial activities of new water‐soluble bis‐quaternary ammonium methacrylate polymers

New methacrylate monomers containing pendant quaternary ammonium moieties based on 1,4‐diazabicyclo‐[2.2.2]‐octane (DABCO) were synthesized. The DABCO group contains either a butyl or a hexyl pendant group comprising the hydrophobic segment of the monomers and one tether group to the methacrylate moiety. The monomers were homopolymerized in water by using 2,2′‐azobis(2‐methylpropionamide) dihydrochloride (V‐50) as an initiator. The monomers and polymers were characterized by elemental analysis, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), FTIR, and ¹³C‐NMR. The antimicrobial activities of the corresponding small molecules (bis‐quaternary ammonium monocarboxylates) and polymers were investigated against Staphylococcus aureus and Escherichia coli. Although the small molecules did not show any antimicrobial activity, the polymers were moderately effective against both Gram‐positive and Gram‐negative bacteria. The minimum inhibitory concentration (MIC) values of the polymers with butyl and hexyl hydrocarbon chains against S. aureus and E. coli were found to be 250 and 62.5 μg/mL, respectively. The minimum bactericidal concentration (MBC) value for the polymer with the butyl group was higher than 1 mg/mL, whereas the MBC value for the polymer with hexyl group was found to be 62.5 μg/mL. Thus, an increase of the alkyl chain length from 4 to 6 significantly increased the antimicrobial activity of the polymer. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 635–642, 2004     

Preparation of antibacterial cellulose with s‐triazine‐based quaternarized N‐halamine

To reduce the neutral salt used in the textile finishing with s‐triazine derivatives, a novel s‐triazine‐based quaternarized N‐halamine precursor was synthesized by two simple steps and characterized by H‐NMR and FT‐IR. This compound can be effectively coated onto cellulose by nucleophilic substitution process without neutral salt. The treated cellulose was rendered with powerful biocidal efficacy after transferring to an N‐halamine structure by exposing to dilute sodium hypochlorite solution through the synergistic antimicrobial effect quaternary ammonium salt and N‐halamine. The chlorinated samples could inactivate 6‐logs of Staphylococcus aureus and Escherichia coli O157:H7 within 1 min and 5 min, respectively. In addition, about 50% of oxidant chlorine remained after 50 washing cycles and 30 days storage, and all of the lost active chlorines in the N‐halamine molecules recovered after exposing to bleach solution. With these advantages, the as‐prepared antimicrobial fabrics will have potential application, especially in the medical and healthcare textiles. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44998.