Entrapment of antimicrobial compounds in a metal matrix for crop protection

Agricultural yields are often limited by damage caused by pathogenic microorganisms, including plant-pathogenic bacteria. The chemical control options to cope with bacterial diseases in agriculture are limited, predominantly relying on copper-based products. These compounds, however, possess limited efficacy. Therefore, there is an urgent need to develop novel technologies to manage bacterial plant diseases and reduce food loss. In this study, a new antimicrobial agent was developed using a doping method that entraps small bioactive organic molecules inside copper as the metal matrix. The food preservative agent lauroyl arginate ethyl ester (ethyl lauroyl arginate; LAE) was chosen as the doped organic compound. The new composites were termed LAE@[Cu]. Bactericidal assays against Acidovorax citrulli, a severe plant pathogen, revealed that LAE and copper in the composites possess a synergistic interaction as compared with each component individually. LAE@[Cu] composites were further characterised in terms of chemical properties and in planta assays demonstrated their potential for further development as crop protection agents.

Chlorhexidine-Silver Nanoparticle Conjugation Leading to Antimicrobial Synergism but Enhanced Cytotoxicity

This study explored the potential synergism within chlorhexidine-silver nanoparticle conjugates against Influenza type A, Staphylococcus aureus, Escherichia coli, and Candida albicans. Silver nanoparticles (SN) were obtained by the reduction of silver ions with green tea total phenolic extract and conjugated with chlorhexidine (Cx). The particles were characterized by UV-Vis and FTIR spectroscopies, dynamic light scattering, X-ray diffraction, and transmission electron microscopy. A stable negatively charged nano-silver colloid (ζ = -50.01) was obtained with an average hydrodynamic diameter of 92.34 nm. In the presence of chlorhexidine, the spectral data and the shift of the zeta potential to positive values (ζ = +44.59) revealed the successful sorption of the drug onto the silver surface. The conjugates (SN-Cx) demonstrated potentiation in their effects against S. aureus and C. albicans and synergism against E. coli with minimal inhibitory concentrations of SN at 5.5 µg/mL + Cx 8.8 µg/mL. The SN showed excellent virucidal properties, increasing with time, and demonstrated low toxicity. However, the coupling of the cationic chlorhexidine with nano-silver did not reduce its intrinsic cytotoxicity on various cell lines (MDCK, BJ, and A549). The newly synthesized antimicrobial agent exhibited an extended and promising therapeutic spectrum and needs to be further evaluated regarding the designated route of administration in three-dimensional cell models (e.g., nasal, bronchial, dermal, ocular, etc.).

Inhibitory Effect of Epigallocatechin Gallate-Silver Nanoparticles and Their Lysozyme Bioconjugates on Biofilm Formation and Cytotoxicity

Biofilms are multicellular communities of microbial cells that grow on natural and synthetic surfaces. They have become the major cause for hospital-acquired infections because once they form, they are very difficult to eradicate. Nanotechnology offers means to fight biofilm-associated infections. Here, we report on the synthesis of silver nanoparticles (AgNPs) with the antibacterial ligand epigallocatechin gallate (EGCG) and the formation of a lysozyme protein corona on AgNPs, as shown by UV-vis, dynamic light scattering, and circular dichroism analyses. We further tested the activity of EGCG-AgNPs and their lysozyme bioconjugates on the viability of Bacillus subtilis cells and biofilm formation. Our results showed that, although EGCG-AgNPs presented no antibacterial activity on planktonic B. subtilis cells, they inhibited B. subtilis biofilm formation at concentrations larger than 40 nM, and EGCG-AgNP-lysozyme bioconjugates inhibited biofilms at concentrations above 80 nM. Cytotoxicity assays performed with human cells showed a reverse trend, where EGCG-AgNPs barely affected human cell viability while EGCG-AgNP-lysozyme bioconjugates severely hampered viability. Our results therefore demonstrate that EGCG-AgNPs may be used as noncytotoxic antibiofilm agents.

Recent Advances on Stimuli-Responsive Combination Therapy against Multidrug-Resistant Bacteria and Biofilm

The widespread occurrence of infections from multidrug-resistant (MDR) bacteria is a global health problem. It has been amplified over the past few years due to the increase in adaptive traits in bacteria and lack of advanced treatment strategies. Because of the low bioavailability and limited penetration at infected sites, the existing antibiotics often fail to resist bacterial growth. Recently, developed stimuli-responsive drug delivery systems and combinatorial therapeutic systems based on nanoparticles, metal-organic frameworks, hydrogels, and organic chromophores offer the ability to improve the therapeutic efficacy of antibiotics by reducing drug resistance and other side effects. These therapeutic systems have been designed with the relevant chemical and physical properties that respond to specific triggers resulting in spatiotemporal controlled release and site-specific transportability. This review highlights the latest development of single and dual/multistimuli-responsive antibiotic delivery systems for combination therapies to treat MDR bacterial infections and biofilm eradication.

Antibacterial effect of silver nanoparticles mixed with calcium hydroxide or chlorhexidine on multispecies biofilms

The purpose is to evaluate the antibacterial effects of the silver nanoparticles (AgNPs) (Nanografi, METU Teknokent, Ankara, Turkey) mixed with calcium hydroxide (Ca(OH)₂) (Ultracal XS, Ultradent, St Louis, US) or chlorhexidine gel (CHX) (Gluco-Chex, Cerkamed, Stalowa Wola, Poland) against a multispecies biofilm, by confocal laser scanning microscopy (CLSM) and culture-based analysis. Dentine blocks were inoculated with Enterococcus faecalis, Streptococcus mutans, Lactobacillus acidophilus and Actinomyces naeslundii for 1 week. Infected dentine blocks were randomly divided into groups according to medication; saline solution (SS), Ca(OH)₂, Ca(OH)₂ + AgNP, 2%CHX gel and 2%CHX gel + AgNP and time of application: 1 and 7 days (all groups, n = 5). Bacterial samples were collected before and after medication to quantify the bacterial load. Biofilm elimination was quantitatively analyzed by Live/Dead BacLight Bacterial Viability staining and CLSM. The addition of AgNPs to Ca(OH)₂ increased the effectiveness of medicament in terms of bacterial reduction in both application times (1 and 7 days) (p < 0.05: ANOVA, Tukey's test) according to culture-based analysis. The CLSM images revealed that mixture of AgNP with CHX killed significantly more bacteria when compared with all other medicaments at 1- and 7-day application times (p < 0.05 and p > 0.05, respectively: Kruskal-Wallis, Dunn post hoc tests). The efficacy of Ca(OH)₂ mixed with AgNPs was superior to Ca(OH)₂ used alone in both application times (p < 0.05) according to CLSM analysis. The present study put forth the potential use of AgNPs mixed with Ca(OH)₂ or CHX on multispecies (Enterococcus faecalis, Streptococcus mutans, Lactobacillus acidophilus and Actinomyces naeslundii) biofilm in 1 and 7day application periods.

Antimicrobial Activity of Silver-Treated Bacteria against other Multi-Drug Resistant Pathogens in Their Environment

Silver is a potent antimicrobial agent against a variety of microorganisms and once the element has entered the bacterial cell, it accumulates as silver nanoparticles with large surface area causing cell death. At the same time, the bacterial cell becomes a reservoir for silver. This study aims to test the microcidal effect of silver-killed E. coli O104: H4 and its supernatant against fresh viable cells of the same bacterium and some other species, including E. coli O157: H7, Multidrug Resistant (MDR) Pseudomonas aeruginosa and Methicillin Resistant Staphylococcus aureus (MRSA). Silver-killed bacteria were examined by Transmission Electron Microscopy (TEM). Agar well diffusion assay was used to test the antimicrobial efficacy and durability of both pellet suspension and supernatant of silver-killed E. coli O104:H4 against other bacteria. Both silver-killed bacteria and supernatant showed prolonged antimicrobial activity against the tested strains that extended to 40 days. The presence of adsorbed silver nanoparticles on the bacterial cell and inside the cells was verified by TEM. Silver-killed bacteria serve as an efficient sustained release reservoir for exporting the lethal silver cations. This promotes its use as a powerful disinfectant for polluted water and as an effective antibacterial which can be included in wound and burn dressings to overcome the problem of wound contamination.

Encoding Chiral Molecular Information in Metal Structures

The concept of encoding molecular information in bulk metals has been proposed over the past decade. The structure of various types of molecules, including enantiomers, can be imprinted in achiral substrates. Typically, to encode metals with chiral information, several approaches, based on chemical and electrochemical concepts, can be used. In this Minireview, recent achievements with respect to the development of such materials are discussed, including the entrapment of chiral biomolecules in metals, the chiral imprinting of metals, as well as the combination of imprinting with nanostructuring. The features and potential applications of these designer materials, such as chirooptical properties, enantioselective adsorption and separation, as well as their use for asymmetric synthesis will be presented. This will illustrate that the development of molecularly encoded metal structures opens up very interesting perspectives, especially in the frame of chiral technologies.

Random peptide mixtures entrapped within a copper-cuprite matrix: new antimicrobial agent against methicillin-resistant Staphylococcus aureus

The emergence of global antibiotic resistance necessitates the urgent need to develop new and effective antimicrobial agents. Combination of two antimicrobial agents can potentially improve antimicrobial potency and mitigate the development of resistance. Therefore, we have utilized metal molecular doping methodology whereby antimicrobial random peptides mixture (RPMs) are entrapped in a bactericidal copper metal matrix. The copper/RPM composite exhibits greater antimicrobial activity toward methicillin-resistant Staphylococcus aureus (MRSA) than either copper or RPMs alone. Our findings indicate that this bactericidal antimicrobial biomaterial could be utilized to efficiently eradicate antibiotic-resistant pathogenic bacteria for health, agricultural and environmental applications.

Antibacterial Activities of Aliphatic Polyester Nanocomposites with Silver Nanoparticles and/or Graphene Oxide Sheets

Aliphatic polyesters such as poly(lactic acid) (PLA), polycaprolactone (PCL) and poly(lactic-co-glycolic) acid (PLGA) copolymers have been widely used as biomaterials for tissue engineering applications including: bone fixation devices, bone scaffolds, and wound dressings in orthopedics. However, biodegradable aliphatic polyesters are prone to bacterial infections due to the lack of antibacterial moieties in their macromolecular chains. In this respect, silver nanoparticles (AgNPs), graphene oxide (GO) sheets and AgNPs-GO hybrids can be used as reinforcing nanofillers for aliphatic polyesters in forming antimicrobial nanocomposites. However, polymeric matrix materials immobilize nanofillers to a large extent so that they cannot penetrate bacterial membrane into cytoplasm as in the case of colloidal nanoparticles or nanosheets. Accordingly, loaded GO sheets of aliphatic polyester nanocomposites have lost their antibacterial functions such as nanoknife cutting, blanket wrapping and membrane phospholipid extraction. In contrast, AgNPs fillers of polyester nanocomposites can release silver ions for destroying bacterial cells. Thus, AgNPs fillers are more effective than loaded GO sheets of polyester nanocomposiites in inhibiting bacterial infections. Aliphatic polyester nanocomposites with AgNPs and AgNPs-GO fillers are effective to kill multi-drug resistant bacteria that cause medical device-related infections.

Entrapment of Drugs within Metallic Platinum and Their Delivery

Platinum has been a widely used metal for a variety of implanted medical devices, because of its inertness, low corrosion rate, high biocompatibility, high electric conductivity, and good mechanical stability. A highly desirable property still in need to be addressed is the tailoring of drug-delivery ability to that metal. This is needed in order to treat infections due to the process of implanting, to treat postoperation pain, and to prevent blood clotting. Can Pt itself serve as a delivery matrix? A review on metallic implants (Lyndon, J. A.; Boyd, B. J.; Birbilis, N. Metallic implant drug/device combinations for controlled drug release in orthopaedic applications. J. Control. Release 2014, 179, 63-75) proposes that "Metals themselves can be used for delivering pharmaceutics" but adds that "there has been no current research into [that] possibility" despite its advantages. Here we present a solution to that challenge and show a new method of using an inert metal as a 3D matrix from within which entrapped drug molecules are released. This new type of drug-delivery system is fabricated by the methodolodgy of entrapment of molecules within metals, resulting in various drugs@Pt. Specifically the following drugs have been entrapped and released: the pain-killer and platelet-inhibitor nonsteroidal anti-inflammatory drugs (NSAIDs) ibuprofen and naproxen, the antibiotic ciprofloxacin, and the antiseptic chlorhexidine. The delivery profiles of all biocomposites were studied in two forms, powders and pressed discs, showing, in general, fast followed by slow first order release profiles. It is shown that the delivery kinetics can be tailored by changing the entrapment process, by applying different pressures in the disc preparation, and by changing the delivery temperature. The latter was also used to determine the activation energy for the release. Full characterization of the metallic biomaterials is provided, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDAX), thermogravimetric analysis (TGA), and surface area/porosity analysis.

Sustained release from a metal - Analgesics entrapped within biocidal silver

Matrices for sustained release of drugs have been based on polymers, biomaterials and oxides. The use of the major family of metals as matrices for sustained release is, to the best of our knowledge, unknown. In this context we describe a new family of bio-composites for sustained release of drugs, namely analgesic drugs entrapped within metallic silver. Synthetic methodologies were developed for the preparation of ibuprofen@Ag, naproxen@Ag, tramadol@Ag and bupivacaine@Ag composites. Detailed kinetic analysis of the release of the drugs from within the metal, is provided, demonstrating that metals can indeed serve as reservoirs for drug release. The metal in our case acts not only as a drug releasing source, but also as an antibacterial agent and this property of the composites was studied. Unexpectedly, it was found that the entrapment of the analgesics within silver, dramatically enhances the growth inhibition activity of wild type Pseudomonas aeruginosa, exceeding by far the inhibition activity of the separate components. A mechanism for this interesting observation is provided. The strong antimicrobial activity combined with the analgesic activity open the road for future applications of these materials as dual-purpose components in wound treatment.

An antibacterial copper composite more bioactive than metallic silver

Although known for its biocidal activity, copper is still not considered as a viable alternative to silver in many of its biocidal applications, mainly because it is generally considered to be a milder antibacterial metal. As copper is much cheaper than silver (1/100), it is potentially more accessible to the health and hygiene needs of third-world countries, to large volume consumer products, and to large-scale agricultural and water treatment needs. Therefore, enhancing the biocidal efficacy of copper is a sought-after goal. We report a method for achieving this goal: by entrapping molecules of the biocidal agent chlorhexidine (CH) within a metallic copper metal powder, using a new materials methodology, the antibacterial efficacy of copper towards two model nosocomial opportunistic bacteria - the Gram-negative Pseudomonas aeruginosa and the Gram-positive Staphylococcus epidermidis- is enhanced to provide a powerful antibacterial agent exceeding the activity of silver. ICP-MS elemental analysis and UV-spectroscopy indicated that the enhanced bactericidal effects of the synthesized composite, CH@Cu, are associated with the sustained release of both copper ions and CH, giving rise to synergistically enhanced activity.

A comparative antimicrobial and toxicological study of gold(iii) and silver(i) complexes with aromatic nitrogen-containing heterocycles: synergistic activity and improved selectivity index of Au(iii)/Ag(i) complexes mixture

Combination therapy may be applied in the case of gold(iii) and silver(i) complexes with aromatic nitrogen-containing heterocycles to improve their antimicrobial activity and reduce toxic-side effects.

Antibacterial activity of silver-killed bacteria: the "zombies" effect

We report a previously unrecognized mechanism for the prolonged action of biocidal agents, which we denote as the zombies effect: biocidally-killed bacteria are capable of killing living bacteria. The concept is demonstrated by first killing Pseudomonas aeruginosa PAO1 with silver nitrate and then challenging, with the dead bacteria, a viable culture of the same bacterium: Efficient antibacterial activity of the killed bacteria is observed. A mechanism is suggested in terms of the action of the dead bacteria as a reservoir of silver, which, due to Le-Chatelier's principle, is re-targeted to the living bacteria. Langmuirian behavior, as well as deviations from it, support the proposed mechanism.

Molecularly doped metals

The many millions of organic, inorganic, and bioorganic molecules represent a very rich library of chemical, biological, and physical properties that do not show up among the approximately 100 metals. The ability to imbue metals with any of these molecular properties would open up tremendous potential for the development of new materials. In addition to their traditional features and their traditional applications, metals would have new traits, which would merge their classical virtues such as conductivity and catalytic activity with the diverse properties of these molecules. In this Account, we describe a new materials methodology, which enables, for the first time, the incorporation and entrapment of small organic molecules, polymers, and biomolecules within metals. These new materials are denoted dopant@metal. The creation of dopant@metal yields new properties that are more than or different from the sum of the individual properties of the two components. So far we have developed methods for the doping of silver, copper, gold, iron, palladium, platinum, and some of their alloys, as well as Hg-Ag amalgams. We have successfully altered classical metal properties (such as conductivity), induced unorthodox properties (such as rendering a metal acidic or basic), used metals as heterogeneous matrices for homogeneous catalysts, and formed new metallic catalysts such as metals doped with organometallic complexes. In addition, we have created materials that straddle the border between polymers and metals, we have entrapped enzymes to form bioactive metals, we have induced chirality within metals, we have made corrosion-resistant iron, we formed efficient biocidal materials, and we demonstrated a new concept for batteries. We have developed a variety of methods for synthesizing dopant@metals including aqueous homogeneous and heterogeneous reductions of the metal cations, reductions in DMF, electrochemical entrapments, thermal decompositions of zerovalent metal carbonyls, and dissolution during amalgam formation. The structures of these dopant@metal materials indicate that metals entrap the organic molecules within their agglomerated nanocrystals. As a result, these materials are porous, making the dopant accessible for chemical reactions, in particular for catalysis. We have prepared these materials in a variety of forms, including powder, granules, pressed discs, thin films, thick films, sub-micrometer particles, and nanometric particles decorating ceramic nanofibers. Entrapment and adsorption are very different processes. If entrapped, water-soluble molecules cannot be extracted, but the same molecules, if adsorbed, are easily washed away. Likewise, most of the special properties that we have observed, such as major improvements or changes in catalytic activity, completely different thermal gravimetric analysis behavior, and more, are observed only in the entrapped cases.

Fabrication of bioactive glass-introduced nanofibrous membranes with multifunctions for potential wound dressing

Bioactive glass-introduced gelatin/chitosan nanofibrous dressings were developedviaelectrospinning to endow improved antibacterial activity, adjustable bioactivity and water uptake capacity for enhancing chronic wound healing.