Structural characterization of free and membrane-bound nisin by infrared spectroscopy

Adsorption of antimicrobial peptide onto chitosan-coated iron oxide nanoparticles fosters oxidative stress triggering bacterial cell death

In the prevailing environmental status quo, bacterial resistance has made antibiotics and antimicrobial peptides (AMPs) ineffective, imparting a serious threat and putting a much greater financial burden on the biomedical and food industries. For this reason, the present study investigates the potential of iron oxide nanoparticles (IONPs) coated with chitosan (CS-IONP) as a platform for augmenting the antimicrobial activity of antimicrobial peptides like nisin. Hence, the nisin is allowed to be adsorbed onto chitosan-coated IONPs to formulate nisin-loaded CS-IONP nanoconjugates. The nanoconjugates were characterized by various optical techniques, such as XRD, FTIR, SEM, zeta and DLS. Remarkably, lower concentrations of N-CS-IONP nanoconjugate exhibited significant and broad-spectrum antibacterial potency compared to bare IONPs and nisin against both Gram-positive and Gram-negative bacteria. Biofilm production was also found to be drastically reduced in the presence of nanoconjugates. Further investigation established a relationship between an increase in antibacterial activity and the enhanced generation of reactive oxygen species (ROS). Oxidative stress exhibited due to enhanced ROS generation is a conclusive reason for the rupturing of bacterial membranes and leakage of cytoplasmic contents, eventually leading to the death of the bacteria. Thus, the current study emphasizes the formulation of a novel antimicrobial agent which exploits magnetic nanoparticles modulated with chitosan for enhanced remediation of resistant bacteria due to oxidative stress imparted by the nanoconjugates upon interaction with the bacteria, leading to cell death.

Discovery of the Potentiator of the Pore-Forming Ability of Lantibiotic Nisin: Perspectives for Anticancer Therapy

This study was focused on the action of lantibiotic nisin on the phospholipid membranes. Nisin did not produce ion-permeable pores in the membranes composed of DOPC or DOPE. The introduction of DOPS into bilayer lipid composition led to a decrease in the threshold detergent concentration of nisin. An addition of nisin to DOPG- and TOCL-enriched bilayers caused the formation of well-defined ion pores of various conductances. The transmembrane macroscopic current increased with the second power of the lantibiotic aqueous concentration, suggesting that the dimer of nisin was at least involved in the formation of conductive subunit. The pore-forming ability of lantibiotic decreased in the series: DOPC/TOCL ≈ DOPE/TOCL >> DOPC/DOPG ≥ DOPE/DOPG. The preferential interaction of nisin to cardiolipin-enriched bilayers might explain its antitumor activity by pore-formation in mitochondrial membranes. Small natural molecules, phloretin and capsaicin, were found to potentiate the membrane activity of nisin in the TOCL-containing membranes. The effect was referred to as changes in the membrane boundary potential at the adsorption of small molecules. We concluded that the compounds diminishing the membrane boundary potential should be considered as the potentiator of the nisin pore-forming ability that can be used to develop innovative formulations for anticancer therapy.

A Synergistic New Approach Toward Enhanced Antibacterial Efficacy via Antimicrobial Peptide Immobilization on a Nitric Oxide-Releasing Surface

Despite technological advancement, nosocomial infections are prevalent due to the rise of antibiotic resistance. A combinatorial approach with multimechanistic antibacterial activity is desired for an effective antibacterial medical device surface strategy. In this study, an antimicrobial peptide, nisin, is immobilized onto biomimetic nitric oxide (NO)-releasing medical-grade silicone rubber (SR) via mussel-inspired polydopamine (PDA) as a bonding agent to reduce the risk of infection. Immobilization of nisin on NO-releasing SR (SR-SNAP-Nisin) and the surface characteristics were characterized by Fourier transform infrared spectroscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy and contact angle measurements. The NO release profile (7 days) and diffusion of SNAP from SR-SNAP-Nisin were quantified using chemiluminescence-based nitric oxide analyzers and UV-vis spectroscopy, respectively. Nisin quantification showed a greater affinity of nisin immobilization toward SNAP-doped SR. Matrix-assisted laser desorption/ionization mass spectrometry analysis on surface nisin leaching for 120 h under physiological conditions demonstrated the stability of nisin immobilization on PDA coatings. SR-SNAP-Nisin shows versatile in vitro anti-infection efficacy against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus in the planktonic and adhered states. Furthermore, the combination of NO and nisin has a superior ability to impair biofilm formation on polymer surfaces. SR-SNAP-Nisin leachates did not elicit cytotoxicity toward mouse fibroblast cells and human umbilical vein endothelial cells, indicating the biocompatibility of the material in vitro. The preventative and therapeutic potential of SR-SNAP-Nisin dictated by two bioactive agents may offer a promising antibacterial surface strategy.

Nisin-Loaded Ulvan Particles: Preparation and Characterization

Nisin is an attractive alternative to chemical preservatives in the food industry. It is a cationic peptide of 34 amino acid residues that exhibits antimicrobial activity against Gram-positive bacteria. To ensure nisin stability in food matrices, new nisin-loaded ulvan particles were developed by the complexation method. The interaction of nisin with ulvan was demonstrated by FT-IR spectroscopy and differential scanning calorimetry. The encapsulation efficiency was calculated at different pH values within the range of 4.0–7.0 and was found to have the highest value at pH 7.0. The size and surface charge of particles fabricated at different concentrations of nisin and pH values were determined. Nisin-loaded ulvan particles exhibited antimicrobial activity against Gram-positive bacteria comparable to that of free nisin. Therefore, the developed complexes have the potential for application as biopreservatives in the food industry. For the first time, the potential of ulvan as a carrier of antimicrobial agent nisin was demonstrated.

Chemical and biological protection of food grade nisin through their partial intercalation in laminar hydroxide salts

The use of antimicrobial agents within a matrix, specifically layered compounds, is of growing interest for reducing contamination due to food borne pathogens and deteriorative microorganisms, one of the main health problems worldwide. In this study, zinc layered hydroxide nanoparticles were synthesized as a matrix for nisin immobilization. Layered materials were characterized by X-ray diffraction, Fourier-Transform Infrared and Ultra Violet-Visible spectra, Scanning Electron Microscopy, and by Thermogravimetric Analysis. Thermal, chemical, enzymatic, and biological stabilities were assessed against Lactobacillus brevis as control strain. Free and immobilized nisin in solution were previously subjected to 25 and 121 °C, pH (7, 9) and inactivation with protease before antimicrobial tests that lasted 21 days. Immobilized nisin was found to maintain the activity levels after the protease action while the pure nisin solution lost its activity gradually. Furthermore, immobilized nisin treated at 121 °C and pH 7 showed higher activity than pure nisin after 21 days. These results may support that immobilizing nisin in zinc layered hydroxide salts promoted extended nisin inhibitory activity in solution after thermal, chemical or enzymatic treatments. This research provides an alternative to nisin application that could be used in processes where such operating conditions take place, as in dairy products.

Interaction of the antibiotic peptide nisin with anionic membranes in different phase-states: a vibrational study

Interactions between the antibiotic peptide nisin and multilamellar vesicles of phosphoglycerol lipids in different phase-states were studied using vibrational spectroscopy. The infrared amide I' band of nisin, both in solution and in the membrane-bound state, was analyzed in the temperature range comprised between 20 and 60 °C in order to study its conformational behavior. Nisin presented mainly unordered and β-turns conformations. Their relative populations varied according to the environment and as the temperature increased: β turns were more favored in the membrane-bound state than in solution, but at higher temperatures the disordered conformation was dominant in both states. Spectral changes of specific infrared bands belonging to the hydrocarbon and polar moieties of lipids were also analyzed to evaluate the perturbation of the lipid membrane order. Nisin interactions with the membrane polar region induced a high restriction to water incorporation, promoting a small increase in the temperature of the lipid phase transition. Raman spectra of nisin/phosphoglycerol systems at ambient temperature were also analyzed. They revealed that the peptide incorporation to a membrane in the fluid phase caused drastic structural modifications in the hydrophobic region of the bilayer. Although nisin may be able to disrupt the hydrophobic portion of the bilayer in the gel phase, the most of the peptide molecule remained at the membrane surface interacting with the polar headgroups. This work provides evidence of a differential effect of nisin on anionic membranes, depending on the phase-state of the lipid.

Violacein Targets the Cytoplasmic Membrane of Bacteria

Violacein is a tryptophan-derived purple pigment produced by environmental bacteria, which displays multiple biological activities, including strong inhibition of Gram-positive pathogens. Here, we applied a combination of experimental approaches to identify the mechanism by which violacein kills Gram-positive bacteria. Fluorescence microscopy showed that violacein quickly and dramatically permeabilizes B. subtilis and S. aureus cells. Cell permeabilization was accompanied by the appearance of visible discontinuities or rips in the cytoplasmic membrane, but it did not affect the cell wall. Using in vitro experiments, we showed that violacein binds directly to liposomes made with commercial and bacterial phospholipids and perturbs their structure and permeability. Furthermore, molecular dynamics simulations were employed to reveal how violacein inserts itself into lipid bilayers. Thus, our combined results demonstrate that the cytoplasmic membrane is the primary target of violacein in bacteria. The implications of this finding for the development of violacein as a therapeutic agent are discussed.

Effect of nanovesicle-encapsulated nisin on growth of Listeria monocytogenes in milk

Commercial nisin was encapsulated in nanovesicles (mean diameter 140 nm) prepared from partially purified soy lecithin. Nisin-loaded liposomes and unencapsulated (free) nisin were initially tested in BHI medium and skim milk inoculated with Listeria monocytogenes and incubated for 48 h at 30 degrees C. At such abuse temperature conditions, free nisin showed better inhibitory than the liposomal counterparts. Subsequently, the effect of encapsulated or free nisin was evaluated in combination with refrigeration (7 +/- 1 degrees C) in both whole (3.25% fat) and skim (0% fat) milk for up to 14 day. A decrease of 3-4 log cycles in L. monocytogenes counts was observed for free and encapsulated nisin at 0.5 mg/ml concentration. Liposome encapsulation of antimicrobial peptides may be important to overcome stability issues and interaction with food components. The utilization of nanovesicle-encapsulated nisin in combination with low temperatures appeared to be effective to control L. monocytogenes in milk, emphasizing the importance of hurdle technology to assure food safety.

Binding of pediocin PA-1 with anionic lipid induces model membrane destabilization

To obtain molecular insights into the action mode of antimicrobial activity of pediocin PA-1, the interactions between this bacteriocin and dimyristoylphosphatidylcholine (DMPC) or dimyristoylphosphatidylglycerol (DMPG) model membranes have been investigated in D(2)O at pD 6 by Fourier transform infrared spectroscopy. The interactions were monitored with respect to alteration of the secondary structure of pediocin, as registered by the amide I' band, and phospholipid conformation, as revealed by the methylene nu(s)(CH(2)) and carbonyl nu(C;O) stretching vibrations. The results show that no interaction between pediocin and DMPC occurs. By contrast, pediocin undergoes a structural reorganization in the presence of DMPG. Upon heating, pediocin self-aggregates, which is not observed for this pD in aqueous solution. The gel-to-crystalline phase transition of DMPG shifts to higher temperatures with a concomitant dehydration of the interfacial region. Our results indicate that pediocin is an extrinsic peptide and that its action mechanism may lie in a destabilization of the cell membrane.

Conformational changes of pediocin in an aqueous medium monitored by fourier transform infrared spectroscopy: a biological implication

Fourier transform infrared (FTIR) spectroscopy was used to investigate the secondary structure of pediocin PA-1 in different aqueous media in relation to its antimicrobial activity. The experiments were performed at pD (pH meter corrected for deuterium isotope effect) 6, 7, and 8 and during a heating-cooling cycle of 20-80 degrees C. At pD 6, (i.e. pediocin's most active form), the FTIR results show that pediocin adopts an unordered structure with a small contribution of beta-turn. After a heating-cooling cycle, thermally-induced changes in pediocin are reversed and its activity is maintained. Increasing the pD to 7 and 8 leads to a more ordered secondary structure. For these two pD values, an increase in temperature induces an irreversible aggregation of protein as revealed by the amide I' band. The analysis of the Tyr region provides more insight into the aggregation process. In fact, it appears to be a two-step process, involving first the C (carboxy)-terminus of pediocin and then the N (amino)-terminus. This study reveals two major points: (1) the preservation of pediocin flexibility is essential for maintaining its activity; and (2) the aggregation of its C-terminus is sufficient to induce a loss of activity, suggesting that this region plays an important role in the activity of pediocin.

Cinnamycin (Ro 09-0198) promotes cell binding and toxicity by inducing transbilayer lipid movement

Cinnamycin is a unique toxin in that its receptor, phosphatidylethanolamine (PE), resides in the inner layer of the plasma membrane. Little is known about how the toxin recognizes PE and causes cytotoxicity. We showed that cinnamycin induced transbilayer phospholipid movement in target cells that leads to the exposure of inner leaflet PE to the toxin. Model membrane studies revealed that cinnamycin induced transbilayer lipid movement in a PE concentration-dependent manner. Re-orientation of phospholipids was accompanied by an increase in the incidence of beta-sheet structure in cinnamycin. When the surface concentration of PE was high, cinnamycin induced membrane re-organization such as membrane fusion and the alteration of membrane gross morphology. These results suggest that cinnamycin promotes its own binding to the cell and causes toxicity by inducing transbilayer lipid movement.

Antibacterial activities of nisin Z encapsulated in liposomes or produced in situ by mixed culture during cheddar cheese ripening

This study investigated both the activity of nisin Z, either encapsulated in liposomes or produced in situ by a mixed starter, against Listeria innocua, Lactococcus spp., and Lactobacillus casei subsp. casei and the distribution of nisin Z in a Cheddar cheese matrix. Nisin Z molecules were visualized using gold-labeled anti-nisin Z monoclonal antibodies and transmission electron microscopy (immune-TEM). Experimental Cheddar cheeses were made using a nisinogenic mixed starter culture, containing Lactococcus lactis subsp. lactis biovar diacetylactis UL 719 as the nisin producer and two nisin-tolerant lactococcal strains and L. casei subsp. casei as secondary flora, and ripened at 7 degrees C for 6 months. In some trials, L. innocua was added to cheese milk at 10(5) to 10(6) CFU/ml. In 6-month-old cheeses, 90% of the initial activity of encapsulated nisin (280 +/- 14 IU/g) was recovered, in contrast to only 12% for initial nisin activity produced in situ by the nisinogenic starter (300 +/- 15 IU/g). During ripening, immune-TEM observations showed that encapsulated nisin was located mainly at the fat/casein interface and/or embedded in whey pockets while nisin produced by biovar diacetylactis UL 719 was uniformly distributed in the fresh cheese matrix but concentrated in the fat area as the cheeses aged. Cell membrane in lactococci appeared to be the main nisin target, while in L. casei subsp. casei and L. innocua, nisin was more commonly observed in the cytoplasm. Cell wall disruption and digestion and lysis vesicle formation were common observations among strains exposed to nisin. Immune-TEM observations suggest several modes of action for nisin Z, which may be genus and/or species specific and may include intracellular target-specific activity. It was concluded that nisin-containing liposomes can provide a powerful tool to improve nisin stability and availability in the cheese matrix.

Combination of antibiotic mechanisms in lantibiotics

Recent studies on the mode of action have revealed exciting features of multiple activities of nisin and related lantibiotics making these peptides interesting model systems for the design of new antibiotics (Molec. Microbiol. 30 (1998) 317; Science 286 (1999) 2361; J. Biol. Chem. 276 (2001) 1772.). In contrast to other groups of antibiotic peptides, the lantibiotics display a substantial degree of specificity for particular components of bacterial membranes. Mersacidin and actagardine were shown to bind with high affinity to the lipid coupled peptidoglycan precursor, the so-called lipid II, which prevents the polymerisation of the cell wall monomers into a functional murein sacculus. The lantibiotics nisin and epidermin also bind tightly to this cell wall precursor; however, for these lantibiotics the binding of lipid II has two consequences. Like with mersacidin blocking of lipid II inhibits peptidoglycan biosynthesis; in addition, lipid II is used as a specific docking molecule for the formation of pores. This combination of lethal effects explains the potency of these peptides, which are active in nanomolar concentration. Other type-A lantibiotics are believed to also use docking molecules for pore formation, although identification of such membrane components has not yet been achieved.

The lantibiotic nisin, a special case or not?

Nisin is a 34-residue-long peptide belonging to the group A lantibiotics with antimicrobial activity against Gram-positive bacteria. The presence of dehydrated residues and lanthionine rings (thioether bonds) in nisin, imposing structural restrains on the peptide, make it an interesting case for studying the mode of action. In addition, the relatively high activity (nM range) of nisin against Gram-positive bacteria indicates that nisin may be a special case in the large family of pore-forming peptides antibiotics. In this review, we attempted to dissect the mode of action of nisin concentrating on studies that used model membranes or biological membranes. The picture that emerges suggests that in model membrane systems, composed of only phospholipids, nisin behaves similar to the antimicrobial peptide magainin, albeit with an activity that is much lower as compared to its activity towards biological membranes. This difference can be contributed to a missing factor which nisin needs for its high activity. Novel results have identified the factor as Lipid II, a precursor in the bacterial cell wall synthesis. The special high affinity interaction of nisin with Lipid II resulting in high activity and the active role of Lipid II in the pore-formation process make nisin a special case.

Lipid-induced conformation and lipid-binding properties of cytolytic and antimicrobial peptides: determination and biological specificity

While antimicrobial and cytolytic peptides exert their effects on cells largely by interacting with the lipid bilayers of their membranes, the influence of the cell membrane lipid composition on the specificity of these peptides towards a given organism is not yet understood. The lack of experimental model systems that mimic the complexity of natural cell membranes has hampered efforts to establish a direct correlation between the induced conformation of these peptides upon binding to cell membranes and their biological specificities. Nevertheless, studies using model membranes reconstituted from lipids and a few membrane-associated proteins, combined with spectroscopic techniques (i.e. circular dichroism, fluorescence spectroscopy, Fourier transform infra red spectroscopy, etc.), have provided information on specific structure-function relationships of peptide-membrane interactions at the molecular level. Reversed phase-high performance chromatography (RP-HPLC) and surface plasmon resonance (SPR) are emerging techniques for the study of the dynamics of the interactions between cytolytic and antimicrobial peptides and lipid surfaces. Thus, the immobilization of lipid moieties onto RP-HPLC sorbent now allows the investigation of peptide conformational transition upon interaction with membrane surfaces, while SPR allows the observation of the time course of peptide binding to membrane surfaces. Such studies have clearly demonstrated the complexity of peptide-membrane interactions in terms of the mutual changes in peptide binding, conformation, orientation, and lipid organization, and have, to a certain extent, allowed correlations to be drawn between peptide conformational properties and lytic activity.

Characterization of permeability and morphological perturbations induced by nisin on phosphatidylcholine membranes

Nisin is an antimicrobial peptide used as food preservative. To gain some insights into the hypothesis that its bactericidal activity is due to the perturbation of the lipid fraction of the bacterial plasmic membrane, we have investigated the effect of nisin on model phosphatidylcholine (PC) membranes. We show that nisin affects the PC membrane permeability, and this perturbation is modulated by the lipid composition. Nisin-induced leakage from PC vesicles is inhibited by the presence of cholesterol. This inhibition is associated with the formation of a liquid ordered phase in the presence of cholesterol, which most likely reduces nisin affinity for the membrane. Conversely, phosphatidylglycerol (PG), an anionic lipid, promotes nisin-induced leakage, and this promotion is associated with an increased affinity of the peptide for the bilayer because nisin is a cationic peptide. When the electrostatic interactions are encouraged by the presence of 70 mol% PG in PC, the inhibitory effect of cholesterol is not observed anymore. Nisin drastically modifies the morphology of the dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) multilamellar dispersion without causing a significant change in the gel-to-liquid crystalline phase transition of the lipid. The morphological changes are observed from (31)P and (2)H NMR and cryo-electron microscopy. From the NMR point of view, the interactions giving rise to a broad signal (quadrupolar interactions and chemical shift anisotropy for (2)H NMR and (31)P NMR, respectively) are partly averaged out in the presence of nisin. This phenomenon is interpreted by the formation of curved lipid planes that lead to the lipid lateral diffusion occurring in the intermediate motional regime. By cryo-electron microscopy, large amorphous aggregates containing small dense globular particles are observed for samples quenched from 25 and 50 degrees C. Long thread-like structures are also observed in the fluid phase. A structural description of DPPC/nisin complex, consistent with the experimental observation, is proposed. The presence of 30 mol% cholesterol in DPPC completely inhibits the morphological changes induced by nisin. Therefore, it is concluded that nisin can significantly perturb PC bilayers from both the permeability and the structural points of view, and these perturbations are modulated by the lipidic species in the bilayer.

Nisin promotes the formation of non-lamellar inverted phases in unsaturated phosphatidylethanolamines

Nisin, a peptide used as a food preservative, is shown, by 31P-nuclear magnetic resonance and infrared spectroscopy, to perturb the structure of membranes formed of unsaturated phosphatidylethanolamine (PE) and to induce the formation of inverted non-lamellar phases. In the case of dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE), nisin promotes the formation of inverted hexagonal phase. Similarly, the peptide induces the formation of an isotropic phase, most likely a cubic phase, with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE). It is proposed that the insertion of the peptide in the bilayer shifts the amphiphilic balance by increasing the hydrophobic contribution and is at the origin of the changes in the polymorphic propensities of PE. This is supported by the fact that the presence of cholesterol in the PE bilayer inhibits the power of nisin to perturb the membrane structure, most likely because the peptide insertion is difficult in the fluid ordered phase. This finding provides insight into possible antibacterial mechanisms of nisin.

Influence of lipid composition on pediocin PA-1 binding to phospholipid vesicles

Pediocin PA-1 bound to anionic lipid vesicles with saturated or unsaturated fatty acid chains in a lipid concentration-dependent fashion. Little change in binding parameters was observed for zwitterionic lipid vesicles. Decreasing the anionic lipid content of the vesicles gave a higher relative dissociation constant for the peptide-lipid interactions and further supports the electrostatic interaction model of binding.