Bacterial sunscreen: layer-by-layer deposition of UV-absorbing polymers on whole-cell biosensors.

UV-protective coatings on live bacterial cells were created from the assembly of cationic and UV-absorbing anionic polyelectrolytes using layer-by-layer (LbL) methodology. A cationic polymer (polyallylamine) and three different anionic polymers with varying absorbance in the UV range (poly(vinyl sulfate), poly(4-styrenesulfonic acid), and humic acid) were used to encapsulate Escherichia coli cells with two different green fluorescent protein (GFP) expression systems: constitutive expression of a UV-excitable GFP (GFPuv) and regulated expression of the intensely fluorescent GFP from amphioxus (GFPa1) through a theophylline-inducible riboswitch. Riboswitches activate protein expression after specific ligand-RNA binding events. Hence, they operate as a cellular biosensor that will activate reporter protein synthesis after exposure to a ligand target. E. coli cells coated with UV-absorbing polymers demonstrated enhanced protection of GFP stability, metabolic activity, and viability after prolonged exposure to radiation from a germicidal lamp. The results show the effectiveness of LbL coatings to provide UV protection to living cells for biotechnological applications.

Supramolecular assembly of a biomineralizing antimicrobial peptide in coarse-grained Monte Carlo simulations.

Monte Carlo simulations are used to model the self-organizing behavior of the biomineralizing peptide KSL (KKVVFKVKFK) in the presence of phosphate. Originally identified as an antimicrobial peptide, KSL also directs the formation of biosilica through a hypothetical supramolecular template that requires phosphate for assembly. Specificity of each residue and the interactions between the peptide and phosphate are considered in a coarse-grained model. Both local and global physical quantities are calculated as the constituents execute their stochastic motion in the presence and absence of phosphate. Ordered peptide aggregates develop after simulations reach thermodynamic equilibrium, wherein phosphates form bridging ligands with lysines and are found interdigitated between peptide molecules. Results demonstrate that interactions between the lysines and phosphate drive self-organization into lower energy conformations of interconnected peptide scaffolds that resemble the supramolecular structures of polypeptide- and polyamine-mediated silica condensation systems. Furthermore, the specific phosphate-peptide organization appears to mimic the zwitterionic structure of native silaffins (scaffold proteins of diatom shells), suggesting a similar template organization for silica deposition between the in vitro KSL and silaffin systems.

Encapsulation of bacterial spores in nanoorganized polyelectrolyte shells.

Layer-by-layer assembly uses alternating charged layers of polyionic polymers to coat materials sequentially in a sheath of functionalized nanofilms. Bacterial spores were encapsulated in organized ultrathin shells using layer-by-layer assembly in order to assess the biomaterial as a suitable core and determine the physiological effects of the coating. The shells were constructed on Bacillus subtilis spores using biocompatible polymers polyglutamic acid, polylysine, albumin, lysozyme, gelatin A, protamine sulfate, and chondroitin sulfate. The assembly process was monitored by measuring the electrical surface potential (zeta-potential) of the particles at each stage of assembly. Fluorescent laser confocal microscopy and scanning electron microscopy confirmed the formation of uniform coatings on the spores. The coating surface charge and thickness (20-100 nm) could be selectively tuned by using appropriate polymers and the number of bilayers assembled. The effect of each coating type on germination was assessed and compared to native spores. The coated spores were viable, but the kinetics and extent of germination were changed from control spores in all instances. The results and insight gained from the experiments may be used to design various bioinspired systems. The spores can be made dormant for a desired amount of time using the LbL encapsulation technique and can be made active when appropriate.

Lysozyme catalyzes the formation of antimicrobial silver nanoparticles.

Hen egg white lysozyme acted as the sole reducing agent and catalyzed the formation of silver nanoparticles in the presence of light. Stable silver colloids formed after mixing lysozyme and silver acetate in methanol and the resulting nanoparticles were concentrated and transferred to aqueous solution without any significant changes in physical properties. Activity and antimicrobial assays demonstrated lysozyme-silver nanoparticles retained the hydrolase function of the enzyme and were effective in inhibiting growth of Escherichia coli, Staphylococcus aureus, Bacillus anthracis, and Candida albicans. Remarkably, lysozyme-silver nanoparticles demonstrated a strong antimicrobial effect against silver-resistant Proteus mirabilis strains and a recombinant E. coli strain containing the multiple antibiotic- and silver-resistant plasmid, pMG101. Results of toxicological studies using human epidermal keratinocytes revealed that lysozyme-silver nanoparticles are nontoxic at concentrations sufficient to inhibit microbial growth. Overall, the ability of lysozyme to assemble silver nanoparticles in a one-step reaction offers a simple and environmentally friendly approach to form stable colloids of nontoxic silver nanoparticles that combine the antimicrobial properties of lysozyme and silver. The results expand the functionality of nanomaterials for biological systems and represent a novel antimicrobial composite for potential aseptics and therapeutic use in the future.

Hybrid antimicrobial enzyme and silver nanoparticle coatings for medical instruments.

We report a method for the synthesis of antimicrobial coatings on medical instruments that combines the bacteriolytic activity of lysozyme and the biocidal properties of silver nanoparticles. Colloidal suspensions of lysozyme and silver nanoparticles were electrophoretically deposited onto the surface of stainless steel surgical blades and needles. Electrodeposited films firmly adhered to stainless steel surfaces even after extensive washing and retained the hydrolytic properties of lysozyme. The antimicrobial efficacy of coatings was tested by using blades and needles in an in vitro lytic assay designed to mimic the normal application of the instruments. Coated blades and needles were used to make incisions and punctures, respectively, into agarose infused with bacterial cells. Cell lysis was seen at the contact sites, demonstrating that antimicrobial activity is transferred into the media, as well as retained on the surface of the blades and needles. Blade coatings also exhibited antimicrobial activity against a range of bacterial species. In particular, coated blades demonstrated potent bactericidal activity, reducing cell viability by at least 3 log within 1.5 h for Klebsiella pneumoniae, Bacillus anthracis Sterne, and Bacillus subtilis and within 3 h for Staphylococcus aureus and Acinetobacter baylyi. The results confirmed that complex antimicrobial coatings can be created using facile methods for silver nanoparticle synthesis and electrodeposition.

Synthesis of bioinorganic antimicrobial peptide nanoparticles with potential therapeutic properties.

Amphiphilicity and cationicity are properties shared between antimicrobial peptides and proteins that catalyze biomineralization reactions. Merging these two functionalities, we demonstrate a reaction where a cationic antimicrobial peptide catalyzes self-biomineralization within inorganic matrices. The resultant antimicrobial peptide nanoparticles retain biocidal activity, protect the peptide from proteolytic degradation, and facilitate a continuous release of the antibiotic over time. Taken together, these properties demonstrate the therapeutic potential of self-synthesizing biomaterials that retain the biocidal properties of antimicrobial peptides.

Histidine ligand protonation and redox potential in the rieske dioxygenases: role of a conserved aspartate in anthranilate 1,2-dioxygenase.

The Rieske dioxygenase, anthranilate 1,2-dioxygenase, catalyzes the 1,2-dihydroxylation of anthranilate (2-aminobenzoate). As in all characterized Rieske dioxygenases, the catalytic conversion to the diol occurs within the dioxygenase component, AntAB, at a mononuclear iron site which accepts electrons from a proximal Rieske [2Fe-2S] center. In the related naphthalene dioxygenase (NDO), a conserved aspartate residue lies between the mononuclear and Rieske iron centers, and is hydrogen-bonded to a histidine ligand of the Rieske center. Engineered substitutions of this aspartate residue led to complete inactivation, which was proposed to arise from elimination of a productive intersite electron transfer pathway [Parales, R. E., Parales, J. V., and Gibson, D. T. (1999) J. Bacteriol. 181, 1831-1837]. Substitutions of the corresponding aspartate, D218, in AntAB with alanine, asparagine, or glutamate also resulted in enzymes that were completely inactive over a wide pH range despite retention of the hexameric quaternary structure and iron center occupancy. The Rieske center reduction potential of this variant was measured to be approximately 100 mV more negative than that for the wild-type enzyme at neutral pH. The wild-type AntAB became completely inactive at pH 9 and exhibited an altered Rieske center absorption spectrum which resembled that of the D218 variants at neutral pH. These results support a role for this aspartate in maintaining the protonated state and reduction potential of the Rieske center. Both the wild-type and D218A variant AntABs exhibited substrate-dependent rapid phases of Rieske center oxidations in stopped-flow time courses. This observation does not support a role for this aspartate in a facile intersite electron transfer pathway or in productive substrate gating of the Rieske center reduction potential. However, since the single turnovers resulted in anthranilate dihydroxylation by the wild-type enzyme but not by the D218A variant, this aspartate must also play a crucial role in substrate dihydroxylation at or near the mononuclear iron site.

Redox-dependent structural changes in archaeal and bacterial Rieske-type [2Fe-2S] clusters.

Proteins containing Rieske-type [2Fe-2S] clusters play important roles in many biological electron transfer reactions. Typically, [2Fe-2S] clusters are not directly involved in the catalytic transformation of substrate, but rather supply electrons to the active site. We report herein X-ray absorption spectroscopic (XAS) data that directly demonstrate an average increase in the iron-histidine bond length of at least 0.1 A upon reduction of two distantly related Rieske-type clusters in archaeal Rieske ferredoxin from Sulfolobus solfataricus strain P-1 and bacterial anthranilate dioxygenases from Acinetobacter sp. strain ADP1. This localized redox-dependent structural change may fine tune the protein-protein interaction (in the case of ARF) or the interdomain interaction (in AntDO) to facilitate rapid electron transfer between a lower potential Rieske-type cluster and its redox partners, thereby regulating overall oxygenase reactions in the cells.