Lantibiotic immunity: inhibition of nisin mediated pore formation by NisI

Nisin-relevant antimicrobial peptides: synthesis strategies and applications

Small pentacyclic peptides, represented by nisin, have been successfully utilized as preservatives in the food industry and have evolved into a paradigm for understanding the genetic structure, expression, and control of genes created by lantibiotics. Due to the ever-increasing antibiotic resistance, nisin-relevant antimicrobial peptides have received much attention, which calls for a summarization of their synthesis, modification and applications. In this review, we first provided a timeline of select highlights in nisin biosynthesis and engineering. Then, we outlined the current developments in nisin synthesis. We also provided an overview of the engineering, screening, and production of nisin-relevant antimicrobial peptides based on enzyme alteration, substrate modification, and sequence mining. Furthermore, an updated summary of applications of nisin-relevant antimicrobial peptides has been developed for food applications. Finally, this study offers insights into emerging technologies, limitations and the future development of nisin-relevant antimicrobial peptides for pathogen inhibition, food preservatives, and improved health.

A Function of Amyloid-β in Mediating Activity-Dependent Axon/Synapse Competition May Unify Its Roles in Brain Physiology and Pathology

Amyloid-β protein precursor (AβPP) gives rise to amyloid-β (Aβ), a peptide at the center of Alzheimer's disease (AD). AβPP, however, is also an ancient molecule dating back in evolution to some of the earliest forms of metazoans. This suggests a possible ancestral function that may have been obscured by those that evolve later. Based on literature from the functions of Aβ/AβPP in nervous system development, plasticity, and disease, to those of anti-microbial peptides (AMPs) in bacterial competition as well as mechanisms of cell competition uncovered first by Drosophila genetics, I propose that Aβ/AβPP may be part of an ancient mechanism employed in cell competition, which is subsequently co-opted during evolution for the regulation of activity-dependent neural circuit development and plasticity. This hypothesis is supported by foremost the high similarities of Aβ to AMPs, both of which possess unique, opposite (i.e., trophic versus toxic) activities as monomers and oligomers. A large body of data further suggests that the different Aβ oligomeric isoforms may serve as the protective and punishment signals long predicted to mediate activity-dependent axonal/synaptic competition in the developing nervous system and that the imbalance in their opposite regulation of innate immune and glial cells in the brain may ultimately underpin AD pathogenesis. This hypothesis can not only explain the diverse roles observed of Aβ and AβPP family molecules, but also provide a conceptual framework that can unify current hypotheses on AD. Furthermore, it may explain major clinical observations not accounted for and identify approaches for overcoming shortfalls in AD animal modeling.

New insights into the resistance mechanism for the BceAB-type transporter SaNsrFP

Treatment of bacterial infections is one of the major challenges of our time due to the evolved resistance mechanisms of pathogens against antibiotics. To circumvent this problem, it is necessary to understand the mode of action of the drug and the mechanism of resistance of the pathogen. One of the most potent antibiotic targets is peptidoglycan (PGN) biosynthesis, as this is an exclusively occurring and critical feature of bacteria. Lipid II is an essential PGN precursor synthesized in the cytosol and flipped into the outer leaflet of the membrane prior to its incorporation into nascent PGN. Antimicrobial peptides (AMPs), such as nisin and colistin, targeting PGN synthesis are considered promising weapons against multidrug-resistant bacteria. However, human pathogenic bacteria that were also resistant to these compounds evolved by the expression of an ATP-binding cassette transporter of the bacitracin efflux (BceAB) type localized in the membrane. In the human pathogen Streptococcus agalactiae, the BceAB transporter SaNsrFP is known to confer resistance to the antimicrobial peptide nisin. The exact mechanism of action for SaNsrFP is poorly understood. For a detailed characterization of the resistance mechanism, we heterologously expressed SaNsrFP in Lactococcus lactis. We demonstrated that SaNsrFP conferred resistance not only to nisin but also to a structurally diverse group of antimicrobial PGN-targeting compounds such as ramoplanin, lysobactin, or bacitracin/(Zn)-bacitracin. Growth experiments revealed that SaNsrFP-producing cells exhibited normal behavior when treated with nisin and/or bacitracin, in contrast to the nonproducing cells, for which growth was significantly reduced. We further detected the accumulation of PGN precursors in the cytoplasm after treating the cells with bacitracin. This did not appear when SaNsrFP was produced. Whole-cell proteomic protein experiments verified that the presence of SaNsrFP in L. lactis resulted in higher production of several proteins associated with cell wall modification. These included, for example, the N-acetylmuramic acid-6-phosphate etherase MurQ and UDP-glucose 4-epimerase. Analysis of components of the cell wall of SaNsrFP-producing cells implied that the transporter is involved in cell wall modification. Since we used an ATP-deficient mutant of the transporter as a comparison, we can show that SaNsrFP and its inactive mutant do not show the same phenotype, albeit expressed at similar levels, which demonstrates the ATP dependency of the mediated resistance processes. Taken together, our data agree to a target protection mechanism and imply a direct involvement of SaNsrFP in resistance by shielding the membrane-localized target of these antimicrobial peptides, resulting in modification of the cell wall.

Harnessing cross-resistance - Sustainable nisin production from low-value food side streams using a Lactococcus lactis mutant with higher nisin-resistance obtained after prolonged chlorhexidine exposure

Nisin has a tendency to associate with the cell wall of the producing strain, which inhibits growth and lowers the ceiling for nisin production. With the premise that resistance to the cationic chlorhexidine could reduce nisin binding, variants with higher tolerance to this compound were isolated. One of the resistant isolates, AT0606, had doubled its resistance to nisin, and produced three times more free nisin, when cultured in shake flasks. Characterization revealed that AT0606 had an overall less negatively charged and thicker cell wall, and these changes appeared to be linked to a defect high-affinity phosphate uptake system, and a mutation inactivating the oleate hydratase. Subsequently, the potential of using AT0606 for cost efficient production of nisin was explored, and it was possible to attain a high titer of 13181 IU/mL using a fermentation substrate based on molasses and a by-product from whey protein hydrolysate production.

In Silico Prediction and Analysis of Unusual Lantibiotic Resistance Operons in the Genus Corynebacterium

Post-translationally modified, (methyl-)lanthionine-containing peptides are produced by several Gram-positive bacteria. These so-called lantibiotics have potent activity against various bacterial pathogens including multidrug-resistant strains and are thus discussed as alternatives to antibiotics. Several naturally occurring mechanisms of resistance against lantibiotics have been described for bacteria, including cell envelope modifications, ABC-transporters, lipoproteins and peptidases. Corynebacterium species are widespread in nature and comprise important pathogens, commensals as well as environmentally and biotechnologically relevant species. Yet, little is known about lantibiotic biosynthesis and resistance in this genus. Here, we present a comprehensive in silico prediction of lantibiotic resistance traits in this important group of Gram-positive bacteria. Our analyses suggest that enzymes for cell envelope modification, peptidases as well as ABC-transporters involved in peptide resistance are widely distributed in the genus. Based on our predictions, we analyzed the susceptibility of six Corynebacterium species to nisin and found that those without dedicated resistance traits are more susceptible and unable to adapt to higher concentrations. In addition, we were able to identify lantibiotic resistance operons encoding for peptidases, ABC-transporters and two-component systems with an unusual predicted structure that are conserved in the genus Corynebacterium. Heterologous expression shows that these operons indeed confer resistance to the lantibiotic nisin.

Gene-Trait Matching and Prevalence of Nisin Tolerance Systems in Lactococus lactis

Lactococcus lactis cheese starter cultures typically contain a mix of many strains and may include variants that produce and/or tolerate the antimicrobial bacteriocin nisin. Nisin is well-established as an effective agent against several undesirable Gram-positive bacteria in cheese and various other foods. In the current study, we have examined the effect of nisin on 710 individual L. lactis strains during milk fermentations. Changes in milk acidification profiles with and without nisin exposure, ranging from unaltered acidification to loss of acidification, could be largely explained by the type(s) and variants of nisin immunity and nisin degradation genes present, but surprisingly, also by genotypic lineage (L. lactis ssp. cremoris vs. ssp. lactis). Importantly, we identify that nisin degradation by NSR is frequent among L. lactis and therefore likely the main mechanism by which dairy-associated L. lactis strains tolerate nisin. Insights from this study on the strain-specific effect of nisin tolerance and degradation during milk acidification is expected to aid in the design of nisin-compatible cheese starter cultures.

Insights in the Antimicrobial Potential of the Natural Nisin Variant Nisin H

Lantibiotics are a growing class of antimicrobial peptides, which possess antimicrobial activity against mainly Gram-positive bacteria including the highly resistant strains such as methicillin-resistant Staphylococcus aureus or vancomycin-resistant enterococci. In the last decades numerous lantibiotics were discovered in natural habitats or designed with bioengineering tools. In this study, we present an insight in the antimicrobial potential of the natural occurring lantibiotic nisin H from Streptococcus hyointestinalis as well as the variant nisin H F₁I. We determined the yield of the heterologously expressed peptide and quantified the cleavage efficiency employing the nisin protease NisP. Furthermore, we analyzed the effect on the modification via mass spectrometry analysis. With standardized growth inhibition assays we benchmarked the activity of pure nisin H and the variant nisin H F₁I, and their influence on the activity of the nisin immunity proteins NisI and NisFEG from Lactococcus lactis and the nisin resistance proteins SaNSR and SaNsrFP from Streptococcus agalactiae COH1. We further checked the antibacterial activity against clinical isolates of Staphylococcus aureus, Enterococcus faecium and Enterococcus faecalis via microdilution method. In summary, nisin H and the nisin H F₁I variant possessed better antimicrobial potency than the natural nisin A.

Self-immunity to antibacterial peptides by ABC transporters

Bacteria produce under certain stress conditions bacteriocins and microcins that display antibacterial activity against closely related species for survival. Bacteriocins and microcins exert their antibacterial activity by either disrupting the membrane or inhibiting essential intracellular processes of the bacterial target. To this end, they can lyse bacterial membranes and cause subsequent loss of their integrity or nutrients, or hijack membrane receptors for internalisation. Both bacteriocins and microcins are ribosomally synthesised and several are posttranslationally modified, whereas others are not. Such peptides are also toxic to the producer bacteria, which utilise immunity proteins or/and dedicated ATP-binding cassette (ABC) transporters to achieve self-immunity and peptide export. In this review, we discuss the structure and mechanism of self-protection that is conferred by these ABC transporters.

Silver (I) N-Heterocyclic Carbenes Carbosilane Dendritic Systems and Their Imidazolium-Terminated Analogues as Antibacterial Agents: Study of Their Mode of Action

Spherical dendrimers and dendrons containing silver(I) N-heterocyclic carbenes (Ag(I)-NHC) and additionally bow-tie metal-free dendritic systems were synthesized in a simple and straightforward synthetic procedure and subsequently characterized. The antibacterial activity was evaluated, and in parallel, a comparative study with the cationic analogue precursors was performed to explore the effect of silver ions in the dendritic structure. Other parameters, such as topology, generation, and hydrophobicity, of the imidazole substituents were also studied. All these dendritic systems presented antibacterial activity against three different bacterial strains, two Gram-positive (Staphylococcus aureus and Bacillus subtilis) and one Gram-negative (Escherichia coli). Several assays were conducted to elucidate their mechanism of action against Bacillus subtilis, by using bacterial biosensors or specific probes and fluorescent proteins sensitive to changes in the cell membrane potential. These studies are specially focused on the role of the polyvalence of our systems containing silver atoms, which may provoke interesting effects in the mode of action.

NisI Maturation and Its Influence on Nisin Resistance in Lactococcus lactis

NisI confers immunity against nisin, with high substrate specificity to prevent a suicidal effect in nisin-producing Lactococcus lactis strains. However, the NisI maturation process as well as its influence on nisin resistance has not been characterized. Here, we report the roles of lipoprotein signal peptidase II (Lsp) and prolipoprotein diacylglyceryl transferase (Lgt) in NisI maturation and nisin resistance of L. lactis F44. We found that the resistance of nisin of an Lsp-deficient mutant remarkably decreased, while no significant differences in growth were observed. We demonstrated that Lsp could cleave signal peptide of NisI precursor in vitro Moreover, diacylglyceryl modification of NisI catalyzed by Lgt played a decisive role in attachment of NisI on the cell envelope, while it exhibited no effects on cleavage of the signal peptides of NisI precursor. The dissociation constant (K_D ) for the interaction between nisin and NisI exhibited a 2.8-fold increase compared with that between nisin and pre-NisI with signal peptide by surface plasmon resonance (SPR) analysis, providing evidence that Lsp-catalyzed signal peptide cleavage was critical for the immune activity of NisI. Our study revealed the process of NisI maturation in L. lactis and presented a potential strategy to enhance industrial nisin production.IMPORTANCE Nisin, a safe and natural antimicrobial peptide, has a long and impressive history as a food preservative and is also considered a novel candidate to alleviate the increasingly serious threat of antibiotic resistance. Nisin is produced by certain L. lactis strains. The nisin immunity protein NisI, a membrane-bound lipoprotein, is expressed by nisin producers to avoid suicidal action. Here, we report the roles of Lsp and Lgt in NisI maturation and nisin resistance of L. lactis F44. The results verified the importance of Lsp to NisI-conferred immunity and Lgt to localization. Our study revealed the process of NisI maturation in L. lactis and presented a potential strategy to enhance industrial nisin production.

Kunkecin A, a New Nisin Variant Bacteriocin Produced by the Fructophilic Lactic Acid Bacterium, Apilactobacillus kunkeei FF30-6 Isolated From Honey Bees

Apilactobacillus kunkeei FF30-6 isolated from healthy honey bees synthesizes the bacteriocin, which exhibits antimicrobial activity against Melissococcus plutonius. The bacteriocin, kunkecin A, was purified through three-step chromatography, and mass spectrometry revealed that its relative molecular mass was 4218.3. Edman degradation of purified kunkecin A showed only the N-terminal residue, isoleucine. Hence, alkaline alkylation made the subsequent amino acid residues accessible to Edman degradation, and 30 cycles were sequenced with 11 unidentified residues. Whole genome sequencing of A. kunkeei FF30-6, followed by Sanger sequencing, revealed that the genes encoding the proteins involved in lantibiotic biosynthesis were within the plasmid, pKUNFF30-6. Most of the identified proteins exhibited significant sequence similarities to the biosynthetic proteins of nisin A and its variants, such as subtilin. However, the kunkecin A gene cluster lacked the genes corresponding to nisI, nisR, and nisK of the nisin A biosynthetic gene cluster. A comparison of the gene products of kukA and nisA (kunkecin A and nisin A structural genes, respectively) suggested that they had similar post-translational modifications. Furthermore, the structure of kunkecin A was proposed based on a comparison of the observed and calculated relative molecular masses of kunkecin A. The structural analysis revealed that kunkecin A and nisin A had a similar mono-sulfide linkage pattern. Purified kunkecin A exhibited a narrow antibacterial spectrum, but high antibacterial activity against M. plutonius. Kunkecin A is the first bacteriocin to be characterized in fructophilic lactic acid bacteria and is the first nisin-type lantibiotic found in the family Lactobacillaceae.

Impact of the nisin modification machinery on the transport kinetics of NisT

Lanthipeptides are ribosomally synthesized and post-translationally modified peptides containing dehydrated amino acids and (methyl-)lanthionine rings. One of the best-studied examples is nisin produced by Lactococcus lactis. Nisin is synthesized as a precursor peptide comprising of an N-terminal leader peptide and a C-terminal core peptide. Amongst others, the leader peptide is crucial for enzyme recognition and acts as a secretion signal for the ABC transporter NisT that secretes nisin in a proposed channeling mechanism. Here, we present an in vivo secretion analysis of this process in the presence and absence of the nisin maturation machinery, consisting of the dehydratase NisB and the cyclase NisC. Our determined apparent secretion rates of NisT show how NisB and NisC modulate the transport kinetics of NisA. Additional in vitro studies of the detergent-solubilized NisT revealed how these enzymes and the substrates again influence the activity of transporter. In summary, this study highlights the pivotal role of NisB for NisT in the secretion process.

Nisin Production by Enterococcus hirae DF105Mi Isolated from Brazilian Goat Milk

The purpose of this study was to select the promising biopreservation bacteriocin producer strain from goat milk and characterize the expressed bacteriocin, related to its physiological and biochemical properties and specificity of operon encoding production and expression of antimicrobial peptide. Brazilian goat milk was used as the source for the selection of bacteriocin-producing lactic acid bacteria. One strain (DF105Mi) stood out for its strong activity against several Listeria monocytogenes strains. Selected strain was identified based on the biochemical and physiological characteristics and 16s rRNA analysis. The bacteriocin production and inhibitory spectrum of strain DF105Mi were studied, together with the evaluation of the effect of temperature, pH, and chemicals on bacteriocin stability and production, activity, and adsorption to target cells as well as to the cell surface of bacteriocin producers. Physiological and bio-molecular analyses based on targeting of different genes, parts of nisin operon were performed in order to investigate the hypothesis that the studied strain can produce and express nisin. Based on biochemical, physiological, and 16s rRNA analysis, the strain DF105Mi was classified as Enterococcus hirae. The selected strain produces a bacteriocin which is stable in a wide range of pH (2.0-12.0), temperature (up to 120 °C), presence of selected chemicals and presents adsorption affinity to different test organisms, process influenced by environmental conditions. Higher bacteriocin production by Ent. hirae DF105Mi was recorded during stationary growth phase, but only when the strain was cultured at 37 °C. The strain's genetic analysis indicated presence of the genes coding for the production of the bacteriocin nisin. This result was confirmed by cross-checking the sensitivity of the produced strain to commercial nisin A. The strong anti-Listeria activity, bacteriocin adsorption, and stability of produced bacteriocin indicate that Ent. hirae DF105Mi presents a differentiated potential application for biopreservation of fermented dairy products.

LanI-Mediated Lantibiotic Immunity in Bacillus subtilis: Functional Analysis

Lantibiotics subtilin and nisin are produced by Bacillus subtilis and Lactococcus lactis, respectively. To prevent toxicity of their own lantibiotic, both bacteria express specific immunity proteins, called SpaI and NisI. In addition, ABC transporters SpaFEG and NisFEG prevent lantibiotic toxicity by transporting the respective peptides to the extracellular space. Although the three-dimensional structures of SpaI and NisI have been solved, very little is known about the molecular function of either lipoprotein. Using laser-induced liquid bead ion desorption (LILBID)-mass spectrometry, we show here that subtilin interacts with SpaI monomers. The expression of either SpaI or NisI in a subtilin-nonproducing B. subtilis strain resulted in the respective strain being more resistant against either subtilin or nisin. Furthermore, pore formation provided by subtilin and nisin was prevented specifically upon the expression of either SpaI or NisI. As shown with a nisin-subtilin hybrid molecule, the C-terminal part of subtilin but not any particular lanthionine ring was needed for SpaI-mediated immunity. With respect to growth, SpaI provided less immunity against subtilin than is provided by the ABC transporter SpaFEG. However, SpaI prevented pore formation much more efficiently than SpaFEG. Taken together, our data show the physiological function of SpaI as a fast immune response to protect the cellular membrane.IMPORTANCE The two lantibiotics nisin and subtilin are produced by Lactococcus lactis and Bacillus subtilis, respectively. Both peptides have strong antimicrobial activity against Gram-positive bacteria, and therefore, appropriate protection mechanisms are required for the producing strains. To prevent toxicity of their own lantibiotic, both bacteria express immunity proteins, called SpaI and NisI, and in addition, ABC transporters SpaFEG and NisFEG. Whereas it has been shown that the ABC transporters protect the producing strains by transporting the toxic peptides to the extracellular space, the exact mode of action and the physiological function of the lipoproteins during immunity are still unknown. Understanding the exact role of lantibiotic immunity proteins is of major importance for improving production rates and for the design of newly engineered peptide antibiotics. Here, we show (i) the specificity of each lipoprotein for its own lantibiotic, (ii) the specific physical interaction of subtilin with its lipoprotein SpaI, (iii) the physiological function of SpaI in protecting the cellular membrane, and (iv) the importance of the C-terminal part of subtilin for its interaction with SpaI.

Systematic characterization of position one variants within the lantibiotic nisin

Lantibiotics are a growing class of natural compounds, which possess antimicrobial activity against a broad range of Gram-positive bacteria. Their high potency against human pathogenic strains such as MRSA and VRE makes them excellent candidates as substitutes for classic antibiotics in times of increasing multidrug resistance of bacterial strains. New lantibiotics are detected in genomes and can be heterologously expressed. The functionality of these novel lantibiotics requires a systematic purification and characterization to benchmark them against for example the well-known lantibiotic nisin. Here, we used a standardized workflow to characterize lantibiotics consisting of six individual steps. The expression and secretion of the lantibiotic was performed employing the promiscuous nisin modification machinery. We mutated the first amino acid of nisin into all proteinaceous amino acids and compared their bactericidal potency against sensitive strains as well as strains expressing nisin resistance proteins. Interestingly, we can highlight four distinct groups based on the residual activity of nisin against sensitive as well as resistant L. lactis strains.

New bacitracin-resistant nisin-producing strain of Lactococcus lactis and its physiological characterization

Nisin A belonging to the class I bacteriocins and produced by Lactococcus lactis subsp. lactis is widely used in many countries as highly efficient and safe preservative preventing growth of undesirable bacteria in food products. Though this compound is efficient at very low concentrations, reduction of its manufacturing cost is still relevant problem. An increased nisin A production requires improved resistance of its producer to nisin. According to some studies, mechanisms of microbial resistance to nisin A and bacitracin have a similar basis, and the same transporters are used to export these antibiotics from cells. To obtain strains with improved growth rate and nisin A productivity, selection of spontaneous bacitracin-resistant L. lactis mutants followed by examination of their stability as well as physiological and fermentation characteristics was carried out. Spontaneous mutants were obtained by culturing of L. lactis VKPM B-2092 strain on selective bacitracin-containing agar medium. The obtained bacitracin-resistant strain FL-75 was characterized by accelerated growth rate, doubled biomass accumulation, and improved nisin A resistance. The nisin A productivity of FL-75 exceeded that of the parental strain by 25% reaching 8902 U/mL after 14-h cultivation. In addition, FL-75 was characterized by the improved resistance to oxidative stress that has never been reported earlier for bacitracin-resistant microorganisms. Based on the performed characterization of FL-75, we can consider it as a new independent strain promising for the industrial production of food and feed biopreservatives. Comparison of published data and the obtained results allowed us to suppose that the bacitracin resistance mutation in FL-75 is determined rather by an increased expression of a gene homologous to the bcrC gene of Bacillus sp. than by the activation of multidrug resistance mechanisms. The revealed resistance of FL-75 to bacitracin and oxidative stress can be regulated by a common transcription factor activating in response to various environmental stresses.

Contribution of YthA, a PspC Family Transcriptional Regulator of Lactococcus lactis F44 Acid Tolerance and Nisin Yield: a Transcriptomic Approach

To overcome the adverse impacts of environmental stresses during growth, different adaptive regulation mechanisms can be activated in Lactococcus lactis In this study, the transcription levels of eight transcriptional regulators of L. lactis subsp. lactis F44 under acid stress were analyzed using quantitative reverse transcription-PCR. Eight gene-overexpressing strains were then constructed to examine their influences on acid-resistant capability. Overexpressing ythA, a PspC family transcriptional regulator, increased the survival rate by 3.2-fold compared to the control at the lethal pH 3.0 acid shock. Moreover, the nisin yield was increased by 45.50%. The ythA-overexpressing strain FythA appeared to have higher intracellular pH stability and nisin-resistant ability. Subsequently, transcriptome analysis revealed that the vast majority of genes associated with amino acid biosynthesis, including arginine, serine, phenylalanine, and tyrosine, were predominantly upregulated in FythA. Arginine biosynthesis (argG and argH), arginine deiminase pathway, and polar amino acid transport (ysfE and ysfF) were proposed to be the main regulation mechanisms of YthA. Furthermore, the transcription of genes associated with pyrimidine and exopolysaccharide biosynthesis were upregulated. The transcriptional levels of nisIPRKFEG genes were substantially higher in FythA, which directly contributed to the yield and resistance of nisin. Three potential DNA-binding sequences were predicted by computer analysis using the upstream regions of genes with prominent changes. This study showed that YthA could increase acid resistance and nisin yield and revealed a putative regulation mechanism of YthA.IMPORTANCE Nisin, produced by Lactococcus lactis subsp. lactis, is widely used as a safe food preservative. Acid stress becomes the primary restrictive factor of cell growth and nisin yield. In this research, we found that the transcriptional regulator YthA was conducive to enhancing the acid resistance of L. lactis F44. Overexpressing ythA could significantly improve the survival rate and nisin yield. The stability of intracellular pH and nisin resistance were also increased. Transcriptome analysis showed that nisin immunity and the biosynthesis of some amino acids, pyrimidine, and exopolysaccharides were enhanced in the engineered strain. This study elucidates the regulation mechanism of YthA and provides a novel strategy for constructing robust industrial L. lactis strains.

Contribution of YthA, a PspC Family Transcriptional Regulator of Lactococcus lactis F44 Acid Tolerance and Nisin Yield: a Transcriptomic Approach

To overcome the adverse impacts of environmental stresses during growth, different adaptive regulation mechanisms can be activated in Lactococcus lactis In this study, the transcription levels of eight transcriptional regulators of L. lactis subsp. lactis F44 under acid stress were analyzed using quantitative reverse transcription-PCR. Eight gene-overexpressing strains were then constructed to examine their influences on acid-resistant capability. Overexpressing ythA, a PspC family transcriptional regulator, increased the survival rate by 3.2-fold compared to the control at the lethal pH 3.0 acid shock. Moreover, the nisin yield was increased by 45.50%. The ythA-overexpressing strain FythA appeared to have higher intracellular pH stability and nisin-resistant ability. Subsequently, transcriptome analysis revealed that the vast majority of genes associated with amino acid biosynthesis, including arginine, serine, phenylalanine, and tyrosine, were predominantly upregulated in FythA. Arginine biosynthesis (argG and argH), arginine deiminase pathway, and polar amino acid transport (ysfE and ysfF) were proposed to be the main regulation mechanisms of YthA. Furthermore, the transcription of genes associated with pyrimidine and exopolysaccharide biosynthesis were upregulated. The transcriptional levels of nisIPRKFEG genes were substantially higher in FythA, which directly contributed to the yield and resistance of nisin. Three potential DNA-binding sequences were predicted by computer analysis using the upstream regions of genes with prominent changes. This study showed that YthA could increase acid resistance and nisin yield and revealed a putative regulation mechanism of YthA.IMPORTANCE Nisin, produced by Lactococcus lactis subsp. lactis, is widely used as a safe food preservative. Acid stress becomes the primary restrictive factor of cell growth and nisin yield. In this research, we found that the transcriptional regulator YthA was conducive to enhancing the acid resistance of L. lactis F44. Overexpressing ythA could significantly improve the survival rate and nisin yield. The stability of intracellular pH and nisin resistance were also increased. Transcriptome analysis showed that nisin immunity and the biosynthesis of some amino acids, pyrimidine, and exopolysaccharides were enhanced in the engineered strain. This study elucidates the regulation mechanism of YthA and provides a novel strategy for constructing robust industrial L. lactis strains.

Crystal Structure of NisI in a Lipid-Free Form, the Nisin Immunity Protein, from Lactococcus lactis

Nisin is a lantibiotic, a member of a family of polypeptides containing lanthionine with antimicrobial activity. Nisin-producing microorganisms require immunity proteins for self-protection from nisin itself. Lactococcus lactis, a microorganism that synthesizes nisin, has an integral NisFEG ABC transporter and an NisI lipoprotein that function in nisin immunity. Here, we present the crystal structure of the full length of NisI_22-C, a lipid-free form of NisI, determined at 1.9-Å resolution. As with the nuclear magnetic resonance (NMR) structures of the N- and C-terminal domains of NisI, NisI_22-C is composed of N- and C-terminal domains, both of which display a fold similar to that found in SpaI, a lipoprotein with immunity against subtilin in Bacillus subtilis The full-length structure of NisI_22-c reveals a large, deep cleft by the interdomain association, one side of which is occupied by the residues important for immunity. Opposite the cleft, a shallow groove is found where nisin-interacting residues are distributed in the periphery composed of the C-terminal negative patch. Based on a sulfate ion found in the large and deep cleft, a model of NisI in complex with a farnesyl diphosphate backbone of lipid II is proposed, suggesting a mechanism for increasing the chances of encountering nisin.

Insight into Two ABC Transporter Families Involved in Lantibiotic Resistance

Antimicrobial peptides, which contain (methyl)-lanthionine-rings are called lantibiotics. They are produced by several Gram-positive bacteria and are mainly active against these bacteria. Although these are highly potent antimicrobials, some human pathogenic bacteria express specific ABC transporters that confer resistance and counteract their antimicrobial activity. Two distinct ABC transporter families are known to be involved in this process. These are the Cpr- and Bce-type ABC transporter families, named after their involvement in cationic peptide resistance in Clostridium difficile, and bacitracin efflux in Bacillus subtilis, respectively. Both resistance systems differentiate to each other in terms of the proteins involved. Here, we summarize the current knowledge and describe the divergence as well as the common features present in both the systems to confer lantibiotic resistance.

The N-terminal Region of Nisin Is Important for the BceAB-Type ABC Transporter NsrFP from Streptococcus agalactiae COH1

Lantibiotics are (methyl)-lanthionine-containing antimicrobial peptides produced by several Gram-positive bacteria. Some human pathogenic bacteria express specific resistance proteins that counteract this antimicrobial activity of lantibiotics. In Streptococcus agalactiae COH1 resistance against the well-known lantibiotic nisin is conferred by, the nisin resistance protein (NSR), a two-component system (NsrRK) and a BceAB-type ATP-binding cassette (ABC) transporter (NsrFP). The present study focuses on elucidating the function of NsrFP via its heterologous expression in Lactococcus lactis. NsrFP is able to confer a 16-fold resistance against wild type nisin as determined by growth inhibition experiments and functions as a lantibiotic exporter. Several C-terminal nisin mutants indicated that NsrFP recognizes the N-terminal region of nisin. The N-terminus harbors three (methyl)-lanthionine rings, which are conserved in other lantibiotics.

Inactivation of Clostridium difficile spore outgrowth by synergistic effects of nisin and lysozyme

Inactivating Clostridium difficile spores is difficult, as they are resistant to heat, chemicals, and antimicrobials. However, this note describes inactivation of C. difficile spore outgrowth by incubation in a solution containing a germinant (1% (m/v) sodium taurocholate), co-germinants (1% (m/v) tryptose and 1% (m/v) NaCl), and natural antimicrobials (20 nmol·L^-1 nisin and 0.2 mmol·L^-1 lysozyme). Clostridium difficile spores were resistant to nisin and lysozyme but became susceptible during germination and outgrowth triggered and promoted by sodium taurocholate, tryptose, and NaCl. The degree of inactivation of germinated and outgrowing C. difficile spores by both nisin and lysozyme was greater than the sum of that by nisin and lysozyme individually, suggesting synergistic inactivation of C. difficile spores. The germinant, co-germinants, and natural antimicrobials used in this study are safe for human contact and consumption. Therefore, these findings will facilitate the development of a safe and effective method to inactivate C. difficile spore.

Stoichiometry and structure of a lantibiotic maturation complex

Lantibiotics are ribosomally synthesized antimicrobial peptides secreted by mainly Gram-positive bacteria. Class 1 lantibiotics mature via two modification steps introduced by a modification LanBC complex. For the lantibiotic nisin, the dehydratase NisB catalyzes the dehydration of serine and threonine residues in the so-called core peptide. Second, five (methyl)-lanthionine rings are introduced in a regio- and stereospecific manner by the cyclase NisC. Here, we characterized the assembly of the NisBC complex in vitro, which is only formed in the presence of the substrate. The complex is composed of a NisB dimer, a monomer of NisC and one prenisin molecule. Interestingly, the presence of the last lanthionine ring prevented complex formation. This stoichiometry was verified by small-angle X-ray scattering measurements, which revealed the first structural glimpse of a LanBC complex in solution.

Interfacial assembly at silver nanoparticle enhances the antibacterial efficacy of nisin

Nisin is a well-recognised antimicrobial peptide (AMP) used in food industry. However, efficacy of the peptide has been compromised due to development of resistance in different bacterial strains. Here, efficacy of the peptide upon assembly at a silver nanoparticle (AgNP) interface has been characterized. To this end, experimental and simulation studies are done to characterize the interfacial assembly of nisin and underlie antibacterial mechanism. Being an AMP, efficacy of an intact nisin is explored against Gram-positive and Gram-negative bacteria, and compared with antibacterial propensity of the interfacially assembled nisin. Antibacterial propensity, upon the assembly, increases against both kinds of bacteria. Interestingly, the growth inhibition studies of the interfacially assembled nisin indicate that the originally nisin resistant Gram-negative bacteria become sensitive to the nanomolar nisin concentrations. Furthermore, reactive oxygen species (ROS) measurements together with confocal microscopy imaging indicate that the increase in interfacial and intracellular ROS production upon the treatment is underling mechanism of enhanced antibacterial propensity of the assembled nisin. Thus, the study observed that the interfacial assembly of nisin at AgNP interface enhances the efficacy of nisin against different spectrum of bacteria, where the intact nisin is largely ineffective for the studied concentrations.

Development and Characterization of Nisin Nanoparticles as Potential Alternative for the Recurrent Vaginal Candidiasis Treatment

The aim of this work was the development and characterization of nisin-loaded nanoparticles and the evaluation of its potential antifungal activity. Candidiasis is a fungal infection caused by Candida sp. considered as one of the major public health problem currently. The discovery of antifungal agents that present a reduced or null resistance of Candida sp. and the development of more efficient drug release mechanisms are necessary for the improvement of candidiasis treatment. Nisin, a bacteriocin commercially available for more than 50 years, exhibits antibacterial action in food products with potential antifungal activity. Among several alternatives used to modulate antifungal activity of bacteriocins, polymeric nanoparticles have received great attention due to an effective drug release control and reduction of therapeutic dose, besides the minimization of adverse effects by the preferential accumulation in specific tissues. The nisin nanoparticles were prepared by double emulsification and solvent evaporation methods. Nanoparticles were characterized by dynamic light scattering, zeta potential, Fourier transform infrared, X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy. Antifungal activity was accessed by pour plate method and cell counting using Candida albicans strains. The in vitro release profile and in vitro permeation studies were performed using dialysis bag method and pig vaginal mucosa in Franz diffusion cell, respectively. The results revealed nisin nanoparticles (300 nm) with spherical shape and high loading efficiency (93.88 ± 3.26%). In vitro test results suggest a promising application of these nanosystems as a prophylactic agent in recurrent vulvovaginal candidiasis and other gynecological diseases.

Investigation of genes involved in nisin production in Enterococcus spp. strains isolated from raw goat milk

Different strains of Lactococcus lactis are capable of producing the bacteriocin nisin. However, genetic transfer mechanisms allow the natural occurrence of genes involved in nisin production in members of other bacterial genera, such as Enterococcus spp. In a previous study, nisA was identified in eight enterococci capable of producing antimicrobial substances. The aim of this study was to verify the presence of genes involved in nisin production in Enterococcus spp. strains, as well as nisin expression. The nisA genes from eight Enterococcus spp. strains were sequenced and the translated amino acid sequences were compared to nisin amino-acid sequences previously described in databases. Although containing nisin structural and maturation related genes, the enterococci strains tested in the present study did not present the immunity related genes (nisFEG and nisI). The translated sequences of nisA showed some point mutations, identical to those presented by Lactococcus strains isolated from goat milk. All enterococci were inhibited by nisin, indicating the absence of immunity and thus that nisin cannot be expressed. This study demonstrated for the first time the natural occurrence of nisin structural genes in Enterococcus strains and highlights the importance of providing evidence of a link between the presence of bacteriocin genes and their expression.

Protein Defense Systems against the Lantibiotic Nisin: Function of the Immunity Protein NisI and the Resistance Protein NSR

Lantibiotics are potential alternatives to antibiotics because of their broad-range killing spectrum. The producer strain is immune against its own synthesized lantibiotic via the expression of two proteins LanI and LanFEG. Recently, gene operons are found in mainly human pathogenic strains, which confer resistance against lantibiotics. Of all the lantibiotics discovered till date, nisin produced by some Lactococcus lactis strains is the most prominent member. Nisin has multiple mode of actions of which binding to the cell wall precursor lipid II and subsequent insertion into the bacterial membrane to form pores are the most effective. The nisin producing strains express the lipoprotein NisI to prevent a suicidal effect. NisI binds nisin, inducing a reversible cell clustering to prevent nisin from reaching the membrane. Importantly NisI does not modify nisin and releases it as soon as the concentration in the media drops below a certain level. The human pathogen Streptococcus agalactiae is naturally resistant against nisin by expressing a resistance protein called SaNSR, which is a nisin degrading enzyme. By cleaving off the last six amino acids of nisin, its effectiveness is 100-fold reduced. This cleavage reaction appears to be specific for nisin since SaNSR recognizes the C-terminal located lanthionine rings. Recently, the structures of both NisI and SaNSR were determined by NMR and X-ray crystallography, respectively. Furthermore, for both proteins the binding site for nisin was determined. Within this review, the structures of both proteins and their different defense mechanisms are described.

The Solution Structure of the Lantibiotic Immunity Protein NisI and Its Interactions with Nisin

Many Gram-positive bacteria produce lantibiotics, genetically encoded and posttranslationally modified peptide antibiotics, which inhibit the growth of other Gram-positive bacteria. To protect themselves against their own lantibiotics these bacteria express a variety of immunity proteins including the LanI lipoproteins. The structural and mechanistic basis for LanI-mediated lantibiotic immunity is not yet understood. Lactococcus lactis produces the lantibiotic nisin, which is widely used as a food preservative. Its LanI protein NisI provides immunity against nisin but not against structurally very similar lantibiotics from other species such as subtilin from Bacillus subtilis. To understand the structural basis for LanI-mediated immunity and their specificity we investigated the structure of NisI. We found that NisI is a two-domain protein. Surprisingly, each of the two NisI domains has the same structure as the LanI protein from B. subtilis, SpaI, despite the lack of significant sequence homology. The two NisI domains and SpaI differ strongly in their surface properties and function. Additionally, SpaI-mediated lantibiotic immunity depends on the presence of a basic unstructured N-terminal region that tethers SpaI to the membrane. Such a region is absent from NisI. Instead, the N-terminal domain of NisI interacts with membranes but not with nisin. In contrast, the C-terminal domain specifically binds nisin and modulates the membrane affinity of the N-terminal domain. Thus, our results reveal an unexpected structural relationship between NisI and SpaI and shed light on the structural basis for LanI mediated lantibiotic immunity.

The C-terminus of nisin is important for the ABC transporter NisFEG to confer immunity in Lactococcus lactis

The lantibiotic nisin is a small 3.4 kDa antimicrobial peptide, which acts against Gram-positive bacteria in the nmol/L range. Nisin is produced and secreted by several Lactococcus lactis strains to ensure advantages against other bacteria in their habitat. Nisin contains five specific lanthionine rings of which the first two are important for Lipid II binding and the last two are crucial for the pore formation in the membrane. To gain immunity against nisin, the producing strain is expressing an ABC transporter called NisFEG, which expels nisin from the membrane. As a result six to eightfold more nisin is needed to affect the cells. The hydrolysis of ATP by NisFEG is required for this immunity as shown by a mutant, where the ATP hydrolysis is disrupted (NisF_H181AEG). Furthermore, NisFEG recognizes the C-terminus of nisin, since deletion of the last six amino acids as well as of the last ring lowered the fold of immunity displayed by NisFEG.