Insights into the mode of action of the two-peptide lantibiotic haloduracin

Thusin, a Novel Two-Component Lantibiotic with Potent Antimicrobial Activity against Several Gram-Positive Pathogens

Due to the rapidly increasing prevalence of multidrug-resistant bacterial strains, the need for new antimicrobial drugs to treat infections has become urgent. Bacteriocins, which are antimicrobial peptides of bacterial origin, are considered potential alternatives to conventional antibiotics and have attracted widespread attention in recent years. Among these bacteriocins, lantibiotics, especially two-component lantibiotics, exhibit potent antimicrobial activity against some clinically relevant Gram-positive pathogens and have potential applications in the pharmaceutical industry. In this study, we characterized a novel two-component lantibiotic termed thusin that consists of Thsα, Thsβ, and Thsβ' (mutation of Thsβ, A14G) and that was isolated from a B. thuringiensis strain BGSC 4BT1. Thsα and Thsβ (or Thsβ') exhibit optimal antimicrobial activity at a 1:1 ratio and act sequentially to affect target cells, and they are all highly thermostable (100°C for 30 min) and pH tolerant (pH 2.0 to 9.0). Thusin shows remarkable efficacy against all tested Gram-positive bacteria and greater activities than two known lantibiotics thuricin 4A-4 and ticin A4, and one antibiotic vancomycin against various bacterial pathogens (Bacillus cereus, Listeria monocytogenes, Staphylococcus aureus (MRSA), Staphylococcus sciuri, Enterococcus faecalis, and Streptococcus pneumoniae). Moreover, thusin is also able to inhibit the outgrowth of B. cereus spores. The potent antimicrobial activity of thusin against some Gram-positive pathogens indicates that it has potential for the development of new drugs.

Efficacy of Lantibiotic Treatment of Staphylococcus aureus-Induced Skin Infections, Monitored by In Vivo Bioluminescent Imaging

Staphylococcus aureus is a bacterial pathogen responsible for the majority of skin and soft tissue infections. Antibiotics are losing their efficacy as treatment for skin and soft tissue infections as a result of increased resistance in a variety of pathogens, including S. aureus It is thus imperative to explore alternative antimicrobial treatments to ensure future treatment options for skin and soft tissue infections. A select few lantibiotics, a group of natural defense peptides produced by bacteria, inhibit the growth of numerous clinical S. aureus isolates, including methicillin-resistant strains. In this study, the antimicrobial activities of nisin, clausin, and amyloliquecidin, separately administered, were compared to that of a mupirocin-based ointment, which is commonly used as treatment for S. aureus-induced skin infections. Full-thickness excisional wounds, generated on the dorsal surfaces of mice, were infected with a bioluminescent strain of S. aureus (strain Xen 36). The infections were monitored in real time using in vivo bioluminescent imaging. Lantibiotic treatments significantly reduced the bioluminescence of S. aureus Xen 36 to a level similar to that recorded with mupirocin treatment. Wound closure, however, was more pronounced during lantibiotic treatment. Lantibiotics thus have the potential to be used as an alternative treatment option for S. aureus-induced skin infections.

Discovery and Characterization of Bicereucin, an Unusual d-Amino Acid-Containing Mixed Two-Component Lantibiotic

Lantibiotics are a group of ribosomally synthesized and post-translationally modified peptides (RiPPs) exhibiting antimicrobial activity. They are characterized by the presence of the thioether-containing bisamino acids lanthionine and methyllanthionine. Here, we report a two-component lantibiotic from Bacillus cereus SJ1 with unusual structural features that we named bicereucin. Unlike all previous two-component lantibiotics, only one of the two peptides of bicereucin contains a lanthionine. The second peptide lacks any cysteines but contains several d-amino acids. These are installed by the dehydrogenase BsjJB, the activity of which was successfully reconstituted in vitro. The proteolytic removal of the leader peptide was also performed in vitro. Bicereucin displayed synergistic antimicrobial activities against Gram-positive strains including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococci as well as hemolytic activity. To illustrate the utility of the enzymes, an analog of the d-amino acid containing opioid dermorphin was successfully produced in E. coli by employing the dehydratase BsjM and the dehydrogenase NpnJA.

A gene network engineering platform for lactic acid bacteria

Recent developments in synthetic biology have positioned lactic acid bacteria (LAB) as a major class of cellular chassis for applications. To achieve the full potential of LAB, one fundamental prerequisite is the capacity for rapid engineering of complex gene networks, such as natural biosynthetic pathways and multicomponent synthetic circuits, into which cellular functions are encoded. Here, we present a synthetic biology platform for rapid construction and optimization of large-scale gene networks in LAB. The platform involves a copy-controlled shuttle for hosting target networks and two associated strategies that enable efficient genetic editing and phenotypic validation. By using a nisin biosynthesis pathway and its variants as examples, we demonstrated multiplex, continuous editing of small DNA parts, such as ribosome-binding sites, as well as efficient manipulation of large building blocks such as genes and operons. To showcase the platform, we applied it to expand the phenotypic diversity of the nisin pathway by quickly generating a library of 63 pathway variants. We further demonstrated its utility by altering the regulatory topology of the nisin pathway for constitutive bacteriocin biosynthesis. This work demonstrates the feasibility of rapid and advanced engineering of gene networks in LAB, fostering their applications in biomedicine and other areas.

Structural Characterization and Bioactivity Analysis of the Two-Component Lantibiotic Flv System from a Ruminant Bacterium

The discovery of new ribosomally synthesized and post-translationally modified peptide natural products (RiPPs) has greatly benefitted from the influx of genomic information. The lanthipeptides are a subset of this class of compounds. Adopting the genome-mining approach revealed a novel lanthipeptide gene cluster encoded in the genome of Ruminococcus flavefaciens FD-1, an anaerobic bacterium that is an important member of the rumen microbiota of livestock. The post-translationally modified peptides were produced via heterologous expression in Escherichia coli. Subsequent structural characterization and assessment of their bioactivity revealed features reminiscent of and distinct from previously reported lanthipeptides. The lanthipeptides of R. flavefaciens FD-1 represent a unique example within two-component lanthipeptides, consisting of a highly conserved α-peptide and a diverse set of eight β-peptides.

Antibacterial Peptides: Opportunities for the Prevention and Treatment of Dental Caries

Dental caries is a multifactorial disease that is a growing and costly global health concern. The onset of disease is a consequence of an ecological imbalance within the dental plaque biofilm that favors specific acidogenic and aciduric caries pathogens, namely Streptococcus mutans and Streptococcus sobrinus. It is now recognized by the scientific and medical community that it is neither possible nor desirable to totally eliminate dental plaque. Conversely, the chemical biocides most commonly used for caries prevention and treatment indiscriminately attack all plaque microorganisms. These treatments also suffer from other drawbacks such as bad taste, irritability, and staining. Furthermore, the public demand for safe and natural personal hygiene products continues to rise. Therefore, there are opportunities that exist to develop new strategies for the treatment of this disease. As an alternative to conventional antibiotics, antibacterial peptides have been explored greatly over the last three decades for many different therapeutic uses. There are currently tens of hundreds of antibacterial peptides characterized across the evolutionary spectrum, and among these, many demonstrate physical and/or biological properties that may be suitable for a more targeted approach to the selective control or elimination of putative caries pathogens. Additionally, many peptides, such as nisin, are odorless, colorless, and tasteless and do not cause irritation or staining. This review focuses on antibacterial peptides for their potential role in the treatment and prevention of dental caries and suggests candidates that need to be explored further. Practical considerations for the development of antibacterial peptides as oral treatments are also discussed.

Cell Wall-active Bacteriocins and Their Applications Beyond Antibiotic Activity

Microorganisms synthesize several compounds with antimicrobial activity in order to compete or defend themselves against others and ensure their survival. In this line, the cell wall is a major protective barrier whose integrity is essential for many vital bacterial processes. Probably for this reason, it represents a 'hot spot' as a target for many antibiotics and antimicrobial peptides such as bacteriocins. Bacteriocins have largely been recognized by their pore-forming ability that collapses the selective permeability of the cytoplasmic membrane. However, in the last few years, many bacteriocins have been shown to inhibit cell wall biosyntheis alone, or in a concerted action with pore formation like nisin. Examples of cell wall-active bacteriocins are found in both Gram-negative and Gram-positive bacteria and include a wide diversity of structures such as nisin-like and mersacidin-like lipid II-binding bacteriocins, two-peptide lantibiotics, and non-modified bacteriocins. In this review, we summarize the current knowledge on these antimicrobial peptides as well as the role, composition, and biosynthesis of the bacterial cell wall as their target. Moreover, even though bacteriocins have been a matter of interest as natural food antimicrobials, we propose them as suitable tools to provide new means to improve biotechnologically relevant microorganisms.

Class I and Class II Lanthipeptides Produced by Bacillus spp

The increasing number of multidrug-resistant pathogens, along with the small number of new antimicrobials under development, leads to an increased need for novel alternatives. Class I and class II lanthipeptides (also known as lantibiotics) have been considered promising alternatives to classical antibiotics. In addition to their relevant medical applications, they are used as probiotics, prophylactics, preservatives, and additives in cosmetics and personal-care products. The genus Bacillus is a prolific source of bioactive compounds including ribosomally and nonribosomally synthesized antibacterial peptides. Accordingly, there is significant interest in the biotechnological potential of members of the genus Bacillus as producers of antimicrobial lanthipeptides. The present review focuses on aspects of the biosynthesis, gene cluster organization, structure, antibacterial spectrum, and bioengineering approaches of lanthipeptides produced by Bacillus strains. Their efficacy and potency against some clinically relevant strains, including MRSA and VRE, are also discussed. Although no lanthipeptides are currently in clinical use, the information herein highlights the potential of these compounds.

Multipronged approach for engineering novel peptide analogues of existing lantibiotics

INTRODUCTION

Lantibiotics are a class of ribosomally and post-translationally modified peptide antibiotics that are active against a broad spectrum of Gram-positive bacteria. Great efforts have been made to promote the production of these antibiotics, so that they can one day be used in our antimicrobial arsenal to combat multidrug-resistant bacterial infections.

AREAS COVERED

This review provides a synopsis of lantibiotic research aimed at furthering our understanding of the structural limitation of lantibiotics as well as identifying structural regions that can be modified to improve the bioactivity. In vivo, in vitro and chemical synthesis of lantibiotics has been useful for engineering novel variants with enhanced activities. These approaches have provided novel ways to further our understanding of lantibiotic function and have advanced the objective to develop lantibiotics for the treatment of infectious diseases.

EXPERT OPINION

Synthesis of lantibiotics with enhanced activities will lead to the discovery of new promising drug candidates that will have a long lasting impact on the treatment of Gram-positive infections. The current body of literature for producing structural variants of lantibiotics has been more of a 'proof-of-principle' approach and the application of these methods has not yet been fully utilized.

Michael-type cyclizations in lantibiotic biosynthesis are reversible

Lanthipeptides are members of the ribosomally synthesized and post-translationally modified peptides (RiPPs). They are generated in two biosynthetic steps: dehydration of Ser and Thr residues to the corresponding dehydroamino acids and subsequent conjugate addition by the thiol of Cys residues to generate the characteristic lanthionine and methyllanthionine thioether-bridged structures. Typically, a lanthipeptide contains multiple thioether cross-links. Recent studies have proposed that the final ring topology may be under thermodynamic control. If so, then the Michael-type cyclization reaction would need to be reversible, but such reversibility has never been demonstrated. We show here for the class I lanthipeptide cyclase NisC and class II lanthipeptide synthetase HalM2 that, indeed, the conjugate addition reactions are reversible and that the enzymes can open up all thioether rings in their products. We also propose that a His residue that is conserved among the lanthipeptide cyclases acts as the acid or base that protonates or generates the enolate intermediate during thioether ring formation and opening, respectively.

Engineering the heterologous expression of lanthipeptides in Escherichia coli by multigene assembly

Lantibiotics are an important class of ribosomally synthesised peptide antibiotics with a remarkable pharmacological potential. Structural variants of lantibiotics generated by peptide engineering in vivo are an important aspect for improving the peptide's efficacy, stability and bioavailability as well as production titre, which severely impacts the potential exploitation in pharmaceutical applications. Therefore, expression systems are needed which allow for a robust genetic access for ample mutagenesis experiments. Based on previous heterologous expression of the two-component lanthipeptide lichenicidin (Bliα and Bliβ) in Escherichia coli BLic5, we now employ a multigene assembly strategy for recombinant lantibiotic peptide production in the Gram-negative host. Two E. coli high copy plasmids for separate and increased expression of a two-component lantibiotic were cloned and tested for expression. From these E. coli HP expression strains, an up to 100 times increased expression was found compared with Bacillus licheniformis I89 and E. coli BLic5. Total expression yields reach 4 mg L(-1) for Bliα and 6 mg L(-1) for Bliβ. The expression system developed in this study constitutes an important cornerstone for future in vivo peptide engineering studies and is of significance for potential applications aiming at higher production titres of ribosomally synthesised, post translationally modified peptides.

Cloning and optimization of a nisin biosynthesis pathway for bacteriocin harvest

Nisin is an important antimicrobial peptide that has enormous applications in biotechnology. Despite many encouraging efforts, its overproduction has been a long-standing challenge due to the complexity of the underlying pathway and the difficulty in genetic modification of lactic acid bacteria. Here, we cloned an entire nisin biosynthesis pathway from a nisin-producing strain (Lactococcus lactis K29) into a plasmid and transplanted the plasmid into a nisin deficient strain Lactococcus lactis MG1363, resulting in successful heterologous expression of bioactive recombinant nisin. To increase nisin harvest, we also overexpressed nisA, a gene responsible for nisin precursor production, with a set of constitutive promoters. To further optimize nisin yield, we minimized the metabolic cost of the engineered strains by integrating nisA overexpression cassettes and the recombinant pathway into a single circuit. With our rational construction and optimization, our engineered optimized strain is able to produce bioactive nisin with a yield of 1098 IU/mL, which is more than six times higher than that of the original strain.

Recent advances in synthetic analogues of lantibiotics: What can we learn from these?

The lantibiotics are a family of antibacterial cyclic peptides distinguished by one or more thioether linkages between amino acid side chains, and by unique modes of action. Recent developments in the chemical synthesis, mutagenesis and mutasynthesis of these peptides are providing insights into the structural requirements for antibacterial activity and into the mode of action, as well as having the potential to produce analogues with greater stability, potency and bioavailability. This Review provides a survey of these recent advances.

Cerecidins, novel lantibiotics from Bacillus cereus with potent antimicrobial activity

Lantibiotics are ribosomally synthesized and posttranslationally modified antimicrobial peptides that are widely produced by Gram-positive bacteria, including many species of the Bacillus group. In the present study, one novel gene cluster coding lantibiotic cerecidins was unveiled in Bacillus cereus strain As 1.1846 through genomic mining and PCR screening. The designated cer locus is different from that of conventional class II lantibiotics in that it included seven tandem precursor cerA genes, one modification gene (cerM), two processing genes (cerT and cerP), one orphan regulator gene (cerR), and two immunity genes (cerF and cerE). In addition, one unprecedented quorum sensing component, comQXPA, was inserted between cerM and cerR. The expression of cerecidins was not detected in this strain of B. cereus, which might be due to repressed transcription of cerM. We constitutively coexpressed cerA genes and cerM in Escherichia coli, and purified precerecidins were proteolytically processed with the endoproteinase GluC and a truncated version of putative serine protease CerP. Thus, two natural variants of cerecidins A1 and A7 were obtained which contained two terminal nonoverlapping thioether rings rarely found in lantibiotics. Both cerecidins A1 and A7 were active against a broad spectrum of Gram-positive bacteria. Cerecidin A7, especially its mutant Dhb13A, showed remarkable efficacy against multidrug-resistant Staphylococcus aureus (MDRSA), vancomycin-resistant Enterococcus faecalis (VRE), and even Streptomyces.

The high resolution structure of tyrocidine A reveals an amphipathic dimer

Tyrocidine A, one of the first antibiotics ever to be discovered, is a cyclic decapeptide that binds to membranes of target bacteria, disrupting their integrity. It is active against a broad spectrum of Gram-positive organisms, and has recently engendered interest as a potential scaffold for the development of new drugs to combat antibiotic-resistant pathogens. We present here the X-ray crystal structure of tyrocidine A at a resolution of 0.95Å. The structure reveals that tyrocidine forms an intimate and highly amphipathic homodimer made up of four beta strands that associate into a single, highly curved antiparallel beta sheet. We used surface plasmon resonance and potassium efflux assays to demonstrate that tyrocidine binds tightly to mimetics of bacterial membranes with an apparent dissociation constant (K(D)) of 10 μM, and efficiently permeabilizes bacterial cells at concentrations equal to and below the K(D). Using variant forms of tyrocidine in which the fluorescent probe p-cyano-phenylalanine had been inserted on either the polar or apolar face of the molecule, we performed fluorescence quenching experiments, using both water-soluble and membrane-embedded quenchers. The quenching results, together with the structure, strongly support a membrane association model in which the convex, apolar face of tyrocidine's beta sheet is oriented toward the membrane interior, while the concave, polar face is presented to the aqueous phase.

Fermentation stage-dependent adaptations of Bacillus licheniformis during enzyme production

Background

Industrial fermentations can generally be described as dynamic biotransformation processes in which microorganisms convert energy rich substrates into a desired product. The knowledge of active physiological pathways, reflected by corresponding gene activities, allows the identification of beneficial or disadvantageous performances of the microbial host. Whole transcriptome RNA-Seq is a powerful tool to accomplish in-depth quantification of these gene activities, since the low background noise and the absence of an upper limit of quantification allow the detection of transcripts with high dynamic ranges. Such data enable the identification of potential bottlenecks and futile energetic cycles, which in turn can lead to targets for rational approaches to productivity improvement. Here we present an overview of the dynamics of gene activity during an industrial-oriented fermentation process with Bacillus licheniformis, an important industrial enzyme producer. Thereby, valuable insights which help to understand the complex interactions during such processes are provided.

Results

Whole transcriptome RNA-Seq has been performed to study the gene expression at five selected growth stages of an industrial-oriented protease production process employing a germination deficient derivative of B. licheniformis DSM13. Since a significant amount of genes in Bacillus strains are regulated posttranscriptionally, the generated data have been confirmed by 2D gel-based proteomics. Regulatory events affecting the coordinated activity of hundreds of genes have been analyzed. The data enabled the identification of genes involved in the adaptations to changing environmental conditions during the fermentation process. A special focus of the analyses was on genes contributing to central carbon metabolism, amino acid transport and metabolism, starvation and stress responses and protein secretion. Genes contributing to lantibiotics production and Tat-dependent protein secretion have been pointed out as potential optimization targets.

Conclusions

The presented data give unprecedented insights into the complex adaptations of bacterial production strains to the changing physiological demands during an industrial-oriented fermentation. These are, to our knowledge, the first publicly available data that document quantifiable transcriptional responses of the commonly employed production strain B. licheniformis to changing conditions over the course of a typical fermentation process in such extensive depth.

High-resolution crystal structure reveals molecular details of target recognition by bacitracin

Bacitracin is a metalloantibiotic agent that is widely used as a medicine and feed additive. It interferes with bacterial cell-wall biosynthesis by binding undecaprenyl-pyrophosphate, a lipid carrier that serves as a critical intermediate in cell wall production. Despite bacitracin's broad use, the molecular details of its target recognition have not been elucidated. Here we report a crystal structure for the ternary complex of bacitracin A, zinc, and a geranyl-pyrophosphate ligand at a resolution of 1.1 Å. The antibiotic forms a compact structure that completely envelopes the ligand's pyrophosphate group, together with flanking zinc and sodium ions. The complex adopts a highly amphipathic conformation that offers clues to antibiotic function in the context of bacterial membranes. Bacitracin's efficient sequestration of its target represents a previously unseen mode for the recognition of lipid pyrophosphates, and suggests new directions for the design of next-generation antimicrobial agents.

Anti-infective properties of bacteriocins: an update

Bacteriocin production is a widespread phenomenon among bacteria. Bacteriocins hold great promise for the treatment of diseases caused by pathogenic bacteria and could be used in the future as alternatives to existing antibiotics. The anti-infective potential of bacteriocins for inhibiting pathogens has been shown in various food matrices including cheese, meat, and vegetables. However, their inhibition of pathogens in vivo remains unclear and needs more investigation, due mainly to difficulties associated with demonstrating their health benefits. Many bacteriocins produced by established or potential probiotic organisms have been evaluated as potential therapeutic agents and interesting findings have been documented in vitro as well as in a few in vivo studies. Some recent in vivo studies point to the efficacy of bacteriocin-based treatments of human and animal infections. While further investigation remains necessary before the possibilities for bacteriocins in clinical practice can be described more fully, this review provides an overview of their potential applications to human and veterinary health.

A general method for fluorescent labeling of the N-termini of lanthipeptides and its application to visualize their cellular localization

Labeling of natural products with biophysical probes has greatly contributed to investigations of their modes of action and has provided tools for visualization of their targets. A general challenge is the availability of a suitable functional group for chemoselective modification. We demonstrate here that an N-terminal ketone is readily introduced into various lanthipeptides by the generation of a cryptic N-terminal dehydro amino acid by the cognate biosynthetic enzymes. Spontaneous hydrolysis of the N-terminal enamines results in α-ketoamides that site-specifically react with an aminooxy-derivatized alkyne or fluorophore. The methodology was successfully applied to prochlorosins 1.7 and 2.8, as well as the lantibiotics lacticin 481, haloduracin α, and haloduracin β. The fluorescently modified lantibiotics were added to bacteria, and their cellular localization was visualized by confocal fluorescence microscopy. Lacticin 481 and haloduracin α localized predominantly at sites of new and old cell division as well as in punctate patterns along the long axis of rod-shaped bacilli, similar to the localization of lipid II. On the other hand, haloduracin β was localized nonspecifically in the absence of haloduracin α, but formed specific patterns when coadministered with haloduracin α. Using two-color labeling, colocalization of both components of the two-component lantibiotic haloduracin was demonstrated. These data with living cells supports a model in which the α component recognizes lipid II and then recruits the β-component.

Antimicrobial mechanism of lantibiotics

Lantibiotics are ribosomally synthesized antimicrobial peptides that commonly target the cell wall precursor lipid II during their antimicrobial mechanism and exert their inhibitory activity by (i) inhibition of cell wall biosynthesis, and (ii) stable pore formation in the target membrane. Type-A(I) (i.e. nisin) and two-component (i.e. lacticin 3147) lantibiotics initially interact with lipid II to stabilize the complex, which then proceeds to inhibit cell wall biosynthesis and pore formation. Type-A(II) (i.e. nukacin ISK-1) and type-B (i.e. mersacidin) lantibiotics also use lipid II as a docking molecule, but can only inhibit cell wall biosynthesis without forming pores. In the present paper, we review the antimicrobial mechanism of different types of lantibiotics, their current progress and future prospect.

The sequence of the enterococcal cytolysin imparts unusual lanthionine stereochemistry

The enterococcal cytolysin is a two-component lantibiotic of unknown structure with hemolytic activity that is important for virulence. We prepared cytolysin by coexpression of each precursor peptide with the synthetase CylM in Escherichia coli and characterized its structure. Unexpectedly, cytolysin is to our knowledge the first example of a lantibiotic containing lanthionine and methyllanthionine structures with different stereochemistries in the same peptide. The stereochemistry is determined by the sequence of the substrate peptide.

Peptide antimicrobials: cell wall as a bacterial target

Endogenous host defense peptides (HDPs) are among the most ancient immune mediators, constituting a first line of defense against invading pathogens across the evolutionary continuum. Generally, HDPs are small (<10 kDa), cationic, and amphipathic polypeptides, often broadly classified based on structure. In eukaryotes, major HDP classes include disulfide-stabilized (e.g., defensins), and α-helical or extended (e.g., cathelicidins) peptides. Prokaryote HDPs are generally referred to as bacteriocins, colicins, or lantibiotics, many of which undergo extensive posttranslational modifications. One target for prokaryotic and eukaryotic HDPs is the bacterial cell wall, an essential structural feature conserved among broad classes of bacteria. A primary building block of the cell wall is peptidoglycan, a macromolecular complex that arises through a series of reactions including membrane translocation, extracellular anchoring, and side chain cross-linking. Each of these steps represents a potential target for HDP inhibition, leading to bacteriostatic or bactericidal outcomes. Thus, understanding the relationships between HDPs and cell wall targets may shed light on new peptide antimicrobial agents and strategies to meet the daunting challenge of antibiotic resistance.

Non-proteinogenic amino acids in lacticin 481 analogues result in more potent inhibition of peptidoglycan transglycosylation

Lantibiotics are ribosomally synthesized and post-translationally modified peptide natural products that contain the thioether structures lanthionine and methyllanthionine and exert potent antimicrobial activity against Gram-positive bacteria. At present, detailed modes-of-action are only known for a small subset of family members. Lacticin 481, a tricyclic lantibiotic, contains a lipid II binding motif present in related compounds such as mersacidin and nukacin ISK-1. Here, we show that lacticin 481 inhibits PBP1b-catalyzed peptidoglycan formation. Furthermore, we show that changes in potency of analogues of lacticin 481 containing non-proteinogenic amino acids correlate positively with the potency of inhibition of the transglycosylase activity of PBP1b. Thus, lipid II is the likely target of lacticin 481, and use of non-proteinogenic amino acids resulted in stronger inhibition of the target. Additionally, we demonstrate that lacticin 481 does not form pores in the membranes of susceptible bacteria, a common mode-of-action of other lantibiotics.

Contribution of the Actinobacteria to the growing diversity of lantibiotics

Currently, 76 lantibiotics have been described; the vast majority being produced by members of the Firmicute phylum of bacteria. There is a growing number being identified from the Actinobacteria phylum and some of these exhibit novel modifications leading to an increased functional diversity among lantibiotics. In this review, we discuss the currently characterized lantibiotics highlighting the expanding diversity provided by those from the Actinobacteria. This increased diversity has the potential to expand lantibiotic applications as antimicrobials in foods and pharmaceuticals. In addition, a phylogenetic classification system based on the full prepropeptide sequences showed remarkable consistency with current classification systems and may provide a more rapid and convenient means for classifying lantibiotics.

Chemical synthesis and biological activity of analogues of the lantibiotic epilancin 15X

Lantibiotics are a large family of antibacterial peptide natural products containing multiple post-translational modifications, including the thioether structures lanthionine and methyllanthionine. Efforts to probe structure-activity relationships and engineer improved pharmacological properties have driven the development of new methods to produce non-natural analogues of these compounds. In this study, solid-supported chemical synthesis was used to produce analogues of the potent lantibiotic epilancin 15X, in order to assess the importance of several N-terminal post-translational modifications for biological activity. Surprisingly, substitution of these moieties, including the unusual N-terminal D-lactyl moiety, resulted in relatively small changes in the antimicrobial activity and pore-forming ability of the peptides.

Discovery, biosynthesis, and engineering of lantipeptides

Aided by genome-mining strategies, knowledge of the prevalence and diversity of ribosomally synthesized natural products (RNPs) is rapidly increasing. Among these are the lantipeptides, posttranslationally modified peptides containing characteristic thioether cross-links imperative for bioactivity and stability. Though this family was once thought to be a limited class of antimicrobial compounds produced by gram-positive bacteria, new insights have revealed a much larger diversity of activity, structure, biosynthetic machinery, and producing organisms than previously appreciated. Detailed investigation of the enzymes responsible for installing the posttranslational modifications has resulted in improved in vivo and in vitro engineering systems focusing on enhancement of the therapeutic potential of these compounds. Although dozens of new lantipeptides have been isolated in recent years, bioinformatic analyses indicate that many hundreds more await discovery owing to the widespread frequency of lantipeptide biosynthetic machinery in bacterial genomes.

Ring A of nukacin ISK-1: a lipid II-binding motif for type-A(II) lantibiotic

Ring A of nukacin ISK-1, which is also present in different type-A(II) lantibiotics, resembles a lipid II-binding motif (TxS/TxD/EC, x denotes undefined residues) similar to that present in mersacidin (type-B lantibiotics), which suggests that nukacin ISK-1 binds to lipid II as a docking molecule. Results from our experiments on peptidoglycan precursor (UDP-MurNAc-pp) accumulation and peptide antagonism assays clearly indicated that nukacin ISK-1 inhibits cell-wall biosynthesis, accumulating lipid II precursor inside the cell, and the peptide activity can be repressed by lipid I and lipid II. Interaction analysis of nukacin ISK-1 and different ring A variants with lipid II revealed that nukacin ISK-1 and nukacin D13E (a more active variant) have a high affinity (K(D) = 0.17 and 0.19 μM, respectively) for lipid II, whereas nukacin D13A (a less active variant) showed a lower affinity, and nukacin C14S (a negative variant lacking the ring A structure) exhibited no interaction. Therefore, on the basis of the structural similarity and positional significance of the amino acids in this region, we concluded that nukacin ISK-1 binds lipid II via its ring A region and may lead to the inhibition of cell-wall biosynthesis.

Lantibiotics: how do producers become self-protected?

Lantibiotics are small peptides produced by Gram-positive bacteria, which are ribosomally synthesized as a prepeptide. Their genes are highly organized in operons containing all the genes required for maturation, transport, immunity and synthesis. The best-characterized lantibiotic is nisin from Lactococcus lactis. Nisin is active against other Gram-positive bacteria via various modes of actions. To prevent activity against its producer strain, an autoimmunity system has developed consisting of different proteins, the ABC transporter NisFEG and a membrane anchored protein NisI. Together, they circumvent the ability of nisin to fulfill its action and cause cell death of L. lactis. Within this review, the mechanism of regulation, biosynthesis and activity of the immunity machinery will be discussed. Furthermore a short description about the application of these immunity proteins in both medical and industrial fields is highlighted.

Investigating the importance of charged residues in lantibiotics

Lantibiotics are antimicrobial peptides which can have a broad spectrum activity against many Gram positive pathogens. Many of these peptides contain charged amino acids which may be of critical importance with respect to antimicrobial activity. We have recently carried out an in-depth bioengineering based investigation of the importance of charged residues in a representative two peptide lantibiotic, lacticin 3147, and here we discuss the significance of these findings in the context of other lantibiotics and cationic antimicrobial peptides.

Haloduracin α binds the peptidoglycan precursor lipid II with 2:1 stoichiometry

The two-peptide lantibiotic haloduracin is composed of two post-translationally modified polycyclic peptides that synergistically act on gram-positive bacteria. We show here that Halα inhibits the transglycosylation reaction catalyzed by PBP1b by binding in a 2:1 stoichiometry to its substrate lipid II. Halβ and the mutant Halα-E22Q were not able to inhibit this step in peptidoglycan biosynthesis, but Halα with its leader peptide still attached was a potent inhibitor. Combined with previous findings, the data support a model in which a 1:2:2 lipid II:Halα:Halβ complex inhibits cell wall biosynthesis and mediates pore formation, resulting in loss of membrane potential and potassium efflux.

Easy and rapid purification of highly active nisin

Nisin is an antimicrobial peptide produced and secreted by several L. lactis strains and is specifically active against Gram-positive bacteria. In previous studies, nisin was purified via cation exchange chromatography at low pH employing a single-step elution using 1 M NaCl. Here, we describe an optimized purification protocol using a five-step NaCl elution to remove contaminants. The obtained nisin is devoid of impurities and shows high bactericidal activity against the nisin-sensitive L. lactis strain NZ9000. Purified nisin exhibits an IC₅₀ of ~3 nM, which is a tenfold improvement as compared to nisin obtained via the one-step elution procedure.

Mechanism of inhibition of Bacillus anthracis spore outgrowth by the lantibiotic nisin

The lantibiotic nisin inhibits growth of vegetative Gram-positive bacteria by binding to lipid II, which disrupts cell wall biosynthesis and facilitates pore formation. Nisin also inhibits the outgrowth of bacterial spores, including spores of Bacillus anthracis, whose structural and biochemical properties are fundamentally different from those of vegetative bacteria. The molecular basis of nisin inhibition of spore outgrowth had not been identified, as previous studies suggested that inhibition of spore outgrowth involved either covalent binding to a spore target or loss of membrane integrity; disruption of cell wall biosynthesis via binding to lipid II had not been investigated. To provide insights into the latter possibility, the effects of nisin were compared with those of vancomycin, another lipid II binding antibiotic that inhibits cell wall biosynthesis but does not form pores. Nisin and vancomycin both inhibited the replication of vegetative cells, but only nisin inhibited the transition from a germinated spore to a vegetative cell. Moreover, vancomycin prevented nisin’s activity in competition studies, suggesting that the nisin-lipid II interaction is important for inhibition of spore outgrowth. In experiments with fluorescently labeled nisin, no evidence was found for a covalent mechanism for inhibition of spore outgrowth. Interestingly, mutants in the hinge region (N20P/M21P and M21P/K22P) that still bind lipid II but cannot form pores had potent antimicrobial activity against vegetative B. anthracis cells but did not inhibit spore outgrowth. Therefore, pore formation is essential for the latter activity but not the former. Collectively, these studies suggest that nisin utilizes lipid II as the germinated spore target during outgrowth inhibition and that nisin-mediated membrane disruption is essential to inhibit spore development into vegetative cells.

Lichenicidin biosynthesis in Escherichia coli: licFGEHI immunity genes are not essential for lantibiotic production or self-protection

This study demonstrated, for the first time, that immunity genes licFGEHI are not essential for self-protection and production of the two-component lantibiotic lichenicidin in the Gram-negative heterologous host Escherichia coli BLic5. Additionally, it was experimentally demonstrated that lichenicidin lantibiotics are active against the E. coli imp4213 strain, a mutant strain possessing a permeable outer membrane.

Heterologous expression, biosynthesis, and mutagenesis of type II lantibiotics from Bacillus licheniformis in Escherichia coli

Lichenicidin is a class II two-component lantibiotic produced by Bacillus licheniformis. It is composed of the two peptides Bliα and Bliβ, which act synergistically against various Gram-positive bacteria. The lichenicidin gene cluster was successfully expressed in Escherichia coli, thus constituting the first report to our knowledge of a full reconstitution of a lantibiotic biosynthetic pathway in vivo by a Gram-negative host. This system was further exploited to characterize and assign the function of proteins encoded in the biosynthetic gene cluster in the maturation of lichenicidin peptides. Moreover, a trans complementation system was developed for expression of Bliα and Bliβ variants in vivo. This contribution will spur future studies in the heterologous expression and engineering of lantibiotics.

Diversity and applications of Bacillus bacteriocins

Members of the genus Bacillus are known to produce a wide arsenal of antimicrobial substances, including peptide and lipopeptide antibiotics, and bacteriocins. Many of the Bacillus bacteriocins belong to the lantibiotics, a category of post-translationally modified peptides widely disseminated among different bacterial clades. Lantibiotics are among the best-characterized antimicrobial peptides at the levels of peptide structure, genetic determinants and biosynthesis mechanisms. Members of the genus Bacillus also produce many other nonmodified bacteriocins, some of which resemble the pediocin-like bacteriocins of the lactic acid bacteria (LAB), while others show completely novel peptide sequences. Bacillus bacteriocins are increasingly becoming more important due to their sometimes broader spectra of inhibition (as compared with most LAB bacteriocins), which may include Gram-negative bacteria, yeasts or fungi, in addition to Gram-positive species, some of which are known to be pathogenic to humans and/or animals. The present review provides a general overview of Bacillus bacteriocins, including primary structure, biochemical and genetic characterization, classification and potential applications in food preservation as natural preservatives and in human and animal health as alternatives to conventional antibiotics. Furthermore, it addresses their environmental applications, such as bioprotection against the pre- and post-harvest decay of vegetables, or as plant growth promoters.

Discovery and development of lantibiotics; antimicrobial agents that have significant potential for medical application

INTRODUCTION

Antimicrobial drug resistance is driving the need for novel therapeutics. Amongst the most promising antibacterial agents that are being investigated as replacements for current therapeutic antibiotics are antibacterial peptides, such as the lanthionine-containing peptide antibiotics (lantibiotics).

AREAS COVERED

This review focuses on the current methods used for discovery of potentially exploitable lantibiotics for medical applications and discusses relevant recent innovations that will have a positive impact on the discovery of useful lantibiotics.

EXPERT OPINION

Recent technological advances in a number of fields mean that increased research into the identification and characterisation of new lantibiotics is feasible. We need to increase our understanding of the various mechanisms of antibacterial action exhibited by lantibiotics and apply this knowledge to peptide engineering or novel practical applications. The advent of next-generation sequencing approaches now negate the need for extensive reverse genetics and employment of bioinformatics approaches is greatly assisting the identification of potentially useful inhibitors in the genomes of a range of clinically significant bacteria. These advances in genetic analysis and engineering will facilitate increased exploitation of lantibiotics in medical therapy.

Production of lantipeptides in Escherichia coli

Lantipeptides are ribosomally synthesized and posttranslationally modified peptides containing thioether cross-links. We describe the preparation of seven different lantipeptides in Escherichia coli and demonstrate that this methodology can be used to incorporate nonproteinogenic amino acids.

Plantazolicin, a novel microcin B17/streptolysin S-like natural product from Bacillus amyloliquefaciens FZB42

Here we report on a novel thiazole/oxazole-modified microcin (TOMM) from Bacillus amyloliquefaciens FZB42, a Gram-positive soil bacterium. This organism is well known for stimulating plant growth and biosynthesizing complex small molecules that suppress the growth of bacterial and fungal plant pathogens. Like microcin B17 and streptolysin S, the TOMM from B. amyloliquefaciens FZB42 undergoes extensive posttranslational modification to become a bioactive natural product. Our data show that the modified peptide bears a molecular mass of 1,335 Da and displays antibacterial activity toward closely related Gram-positive bacteria. A cluster of 12 genes that covers ∼10 kb is essential for the production, modification, export, and self-immunity of this natural product. We have named this compound plantazolicin (PZN), based on the association of several producing organisms with plants and the incorporation of azole heterocycles, which derive from Cys, Ser, and Thr residues of the precursor peptide.

Isolation, structure elucidation, and synergistic antibacterial activity of a novel two-component lantibiotic lichenicidin from Bacillus licheniformis VK21

A novel synergetic lantibiotic pair, Lchalpha (3249.51 Da) and Lchbeta (3019.36 Da), termed lichenicidin VK21, was isolated from the producer strain Bacillus licheniformis VK21. Chemical and spatial structures of Lchalpha and Lchbeta were determined. Each peptide contains 31 amino acid residues linked by 4 intramolecular thioether bridges and the N-terminal 2-oxobutyryl group. Spatial structures of Lchalpha and Lchbeta were studied by NMR spectroscopy in methanol solution. The Lchalpha peptide displays structural homology with mersacidin-like lantibiotics and involves relatively well-structured N- and C-terminal domains connected by a flexible loop stabilized by a thioether bridge Ala11-S-Ala21. In contrast, the Lchbeta peptide represents a prolonged hydrophobic alpha-helix flanked with more flexible N- and C-terminal domains. A lantibiotic cluster of the Bacillus licheniformis VK21 genome which comprises the structural genes, lchA1 and lchA2, encoding the lantibiotics precursors, as well as the gene of a modifying enzyme lchM1, was amplified and sequenced. The mature peptides, Lchalpha and Lchbeta, interact synergistically to possess antibiotic activity against Gram-positive bacteria within a nanomolar concentration range, though the individual peptides were shown to be active at micromolar concentrations. Our results afford molecular insight into the mechanism of lichenicidin VK21 action.

Antibiotic discovery in the twenty-first century: current trends and future perspectives

New antibiotics are necessary to treat microbial pathogens that are becoming increasingly resistant to available treatment. Despite the medical need, the number of newly approved drugs continues to decline. We offer an overview of the pipeline for new antibiotics at different stages, from compounds in clinical development to newly discovered chemical classes. Consistent with historical data, the majority of antibiotics under clinical development are natural products or derivatives thereof. However, many of them also represent improved variants of marketed compounds, with the consequent risk of being only partially effective against the prevailing resistance mechanisms. In the discovery arena, instead, compounds with promising activities have been obtained from microbial sources and from chemical modification of antibiotic classes other than those in clinical use. Furthermore, new natural product scaffolds have also been discovered by ingenious screening programs. After providing selected examples, we offer our view on the future of antibiotic discovery.