The dawning of a ‘Golden era’ in lantibiotic bioengineering

Biocompatible strategies for peptide macrocyclisation

Peptides are increasingly important drug candidates, offering numerous advantages over conventional small molecules. However, they face significant challenges related to stability, cellular uptake and overall bioavailability. While individual modifications may not address all these challenges, macrocyclisation stands out as a single modification capable of enhancing affinity, selectivity, proteolytic stability and membrane permeability. The recent successes of in situ peptide modifications during screening in combination with genetically encoded peptide libraries have increased the demand for peptide macrocyclisation reactions that can occur under biocompatible conditions. In this perspective, we aim to distinguish biocompatible conditions from those well-known examples that are fully bioorthogonal. We introduce key strategies for biocompatible peptide macrocyclisation and contextualise them within contemporary screening methods, providing an overview of available transformations.

A Bird's-Eye View of the Pathophysiologic Role of the Human Urobiota in Health and Disease: Can We Modulate It?

For a long time, urine has been considered sterile in physiological conditions, thanks to the particular structure of the urinary tract and the production of uromodulin or Tamm-Horsfall protein (THP) by it. More recently, thanks to the development and use of new technologies, i.e., next-generation sequencing and expanded urine culture, the identification of a microbial community in the urine, the so-called urobiota, became possible. Major phyla detected in the urine are represented by Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria. Particularly, the female urobiota is largely represented by Lactobacillus spp., which are very active against urinary pathogenic Escherichia (E.) coli (UPEC) strains via the generation of lactic acid and hydrogen peroxide. Gut dysbiosis accounts for recurrent urinary tract infections (UTIs), so-called gut-bladder axis syndrome with the formation of intracellular bacterial communities in the course of acute cystitis. However, other chronic urinary tract infections are caused by bacterial strains of intestinal derivation. Monomicrobial and polymicrobial infections account for the outcome of acute and chronic UTIs, even including prostatitis and chronic pelvic pain. E. coli isolates have been shown to be more invasive and resistant to antibiotics. Probiotics, fecal microbial transplantation, phage therapy, antimicrobial peptides, and immune-mediated therapies, even including vaccines for the treatment of UTIs, will be described.

Construction and Activity Testing of a Modular Fusion Peptide against Enterococcus faecalis

The emergence of antibiotic resistance in enterococci is a great concern encountered worldwide. Almost all enterococci exhibit significant levels of resistance to penicillin, ampicillin, semi-synthetic penicillin and most cephalosporins, primarily due to the expression of low-affinity penicillin-binding proteins. The development of new and novel antibacterial agents against enterococci is a significant need of the hour. In this research, we have constructed a modular peptide against Enterococcus faecalis. The enzymatic domain of the constructed peptide BP404 is from the bacteriocin BacL1 and the cell wall binding domain from endolysin PlyV12 of phage ϕ1. The protein BP404 was found to be active against two tested strains of Enterococcus faecalis, with a reduction in cell density amounting to 85% and 65%. The cell wall binding assay confirms the binding of the protein to Enterococcus faecalis, which was not seen towards the control strain Escherichia coli, invariably pointing to the specificity of BP404. To the best of our knowledge, this is one of the first instances of the development of a chimeric peptide against Enterococcus faecalis. This study points out that novel proteins can be genetically engineered against clinically relevant enterococci.

A review on characterization, applications and structure-activity relationships of Bacillus species-produced bacteriocins

Antimicrobial peptides (AMPs) are inherently occurring proteins that are produced by microorganisms as secondary metabolites. Members of genus Bacillus produce many types of AMPs by ribosomal (bacteriocins) and non-ribosomal (polymyxins and iturins) mechanisms. Bacteriocins are ribosomally synthesized peptides that inhibit the growth of closely related bacterial strains. Moreover, bacteriocins produced by Bacillus species have been widely used in pharmaceutical, food industry, fishery, livestock as well as in agriculture sector. The objective of this review is to assess the characterization of the Bacillus-derived bacteriocins, their potential use in different sectors and structure-activity relationships.

Regulation of heterologous subtilin production in Bacillus subtilis W168

Background

Subtilin is a peptide antibiotic (lantibiotic) natively produced by Bacillus subtilis ATCC6633. It is encoded in a gene cluster spaBTCSIFEGRK (spa-locus) consisting of four transcriptional units: spaS (subtilin pre-peptide), spaBTC (modification and export), spaIFEG (immunity) and spaRK (regulation). Despite the pioneer understanding on subtilin biosynthesis, a robust platform to facilitate subtilin research and improve subtilin production is still a poorly explored spot.

Results

In this work, the intact spa-locus was successfully integrated into the chromosome of Bacillus subtilis W168, which is the by far best-characterized Gram-positive model organism with powerful genetics and many advantages in industrial use. Through systematic analysis of spa-promoter activities in B. subtilis W168 wild type and mutant strains, our work demonstrates that subtilin is basally expressed in B. subtilis W168, and the transition state regulator AbrB strongly represses subtilin biosynthesis in a growth phase-dependent manner. The deletion of AbrB remarkably enhanced subtilin gene expression, resulting in comparable yield of bioactive subtilin production as for B. subtilis ATCC6633. However, while in B. subtilis ATCC6633 AbrB regulates subtilin gene expression via SigH, which in turn activates spaRK, AbrB of B. subtilis W168 controls subtilin gene expression in SigH-independent manner, except for the regulation of spaBTC. Furthermore, the work shows that subtilin biosynthesis in B. subtilis W168 is regulated by the two-component regulatory system SpaRK and strictly relies on subtilin itself as inducer to fulfill the autoregulatory circuit. In addition, by incorporating the subtilin-producing system (spa-locus) and subtilin-reporting system (P_psdA-lux) together, we developed “online” reporter strains to efficiently monitor the dynamics of subtilin biosynthesis.

Conclusions

Within this study, the model organism B. subtilis W168 was successfully established as a novel platform for subtilin biosynthesis and the underlying regulatory mechanism was comprehensively characterized. This work will not only facilitate genetic (engineering) studies on subtilin, but also pave the way for its industrial production. More broadly, this work will shed new light on the heterologous production of other lantibiotics.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01782-9.

Bacteriocins: An Overview of Antimicrobial, Toxicity, and Biosafety Assessment by in vivo Models

The world is facing a significant increase in infections caused by drug-resistant infectious agents. In response, various strategies have been recently explored to treat them, including the development of bacteriocins. Bacteriocins are a group of antimicrobial peptides produced by bacteria, capable of controlling clinically relevant susceptible and drug-resistant bacteria. Bacteriocins have been studied to be able to modify and improve their physicochemical properties, pharmacological effects, and biosafety. This manuscript focuses on the research being developed on the biosafety of bacteriocins, which is a topic that has not been addressed extensively in previous reviews. This work discusses the studies that have tested the effect of bacteriocins against pathogens and assess their toxicity using in vivo models, including murine and other alternative animal models. Thus, this work concludes the urgency to increase and advance the in vivo models that both assess the efficacy of bacteriocins as antimicrobial agents and evaluate possible toxicity and side effects, which are key factors to determine their success as potential therapeutic agents in the fight against infections caused by multidrug-resistant microorganisms.

Inactivation of GalU Leads to a Cell Wall-Associated Polysaccharide Defect That Reduces the Susceptibility of Enterococcus faecalis to Bacteriolytic Agents

Enterococcal plasmid-encoded bacteriolysin Bac41 is a selective antimicrobial system that is considered to provide a competitive advantage to Enterococcus faecalis cells that carry the Bac41-coding plasmid. The Bac41 effector consists of the secreted proteins BacL₁ and BacA, which attack the cell wall of the target E. faecalis cell to induce bacteriolysis. Here, we demonstrated that galU, which encodes UTP-glucose-1-phosphate uridylyltransferase, is involved in susceptibility to the Bac41 system in E. faecalis Spontaneous mutants that developed resistance to the antimicrobial effects of BacL₁ and BacA were revealed to carry a truncation deletion of the C-terminal amino acid (aa) region 288 to 298 of the translated GalU protein. This truncation resulted in the depletion of UDP-glucose, leading to a failure to utilize galactose and produce the enterococcal polysaccharide antigen (EPA), which is expressed abundantly on the cell surface of E. faecalis This cell surface composition defect that resulted from galU or EPA-specific genes caused an abnormal cell morphology, with impaired polarity during cell division and alterations of the limited localization of BacL₁ Interestingly, these mutants had reduced susceptibility to beta-lactams besides Bac41, despite their increased susceptibility to other bacteriostatic antimicrobial agents and chemical detergents. These data suggest that a complex mechanism of action underlies lytic killing, as exogenous bacteriolysis induced by lytic bacteriocins or beta-lactams requires an intact cell physiology in E. faecalis IMPORTANCE Cell wall-associated polysaccharides of bacteria are involved in various physiological characteristics. Recent studies demonstrated that the cell wall-associated polysaccharide of Enterococcus faecalis is required for susceptibility to bactericidal antibiotic agents. Here, we demonstrated that a galU mutation resulted in resistance to the enterococcal lytic bacteriocin Bac41. The galU homologue is reported to be essential for the biosynthesis of species-specific cell wall-associated polysaccharides in other Firmicutes In E. faecalis, the galU mutant lost the E. faecalis-specific cell wall-associated polysaccharide EPA (enterococcal polysaccharide antigen). The mutant also displayed reduced susceptibility to antibacterial agents and an abnormal cell morphology. We demonstrated here that galU was essential for EPA biosynthesis in E. faecalis, and EPA production might underlie susceptibility to lytic bacteriocin and antibiotic agents by undefined mechanisms.

Lichenicidin rational site-directed mutagenesis library: A tool to generate bioengineered lantibiotics

Lantibiotics are ribosomally synthesized and posttranslationally modified antimicrobial peptides that arise as an alternative to the traditional antibiotics. Lichenicidin is active against clinically relevant bacteria and it was the first lantibiotic to be fully produced in vivo in the Gram-negative host Escherichia coli. Here, we present the results of a library of lichenicidin mutants, in which the mutations were generated based on the extensive bibliographical search available for other lantibiotics. The antibacterial activity of two-peptide lantibiotics, as is lichenicidin, requires the synergistic activity of two peptides. We established a method that allows screening for bioactivity which does not require the purification of the complementary peptide. It is an inexpensive, fast and user-friendly method that can be scaled up to screen large libraries of bioengineered two-peptide lantibiotics. The applied system is reliable and robust because, in general, the results obtained corroborate structure-activity relationship studies carried out for other lantibiotics.

Lactic Acid Bacteria (LAB) and Their Bacteriocins as Alternative Biotechnological Tools to Control Listeria monocytogenes Biofilms in Food Processing Facilities

Bacteriocins are antimicrobial peptides produced by bacteria Gram-negative and Gram-positive, including lactic acid bacteria (LAB), organisms that are traditionally used in food preservation practices. Bacteriocins have been shown to have an aptitude as biofilm controlling agents in Listeria monocytogenes biofilms, a major risk for consumers and the food industry. Biofilms protect pathogens from sanitization procedures, allowing them to survive and persist in processing facilities, resulting in the cross-contamination of the end products. Studies have been undertaken on bacteriocinogenic LAB, their bacteriocins, and bioengineered bacteriocin derivatives for controlling L. monocytogenes biofilms on different surfaces through inhibition, competition, exclusion, and displacement. These alternative strategies can be considered promising in preventing the development of resistance to conventional sanitizers and disinfectants. Bacteriocins are "friendly" antimicrobial agents, and with high prevalence in nature, they do not have any known associated public health risk. Most trials have been carried out in vitro, on food contact materials such as polystyrene and stainless steel, while there have been few studies performed in situ to consolidate the results observed in vitro. There are strategies that can be employed for prevention and eradication of L. monocytogenes biofilms (such as the establishment of standard cleaning procedures using the available agents at proper concentrations). However, commercial cocktails using alternatives compounds recognized as safe and environmental friendly can be an alternative approach to be applied by the industries in the future.

Genome mining, isolation, chemical synthesis and biological evaluation of a novel lanthipeptide, tikitericin, from the extremophilic microorganism Thermogemmatispora strain T81

Tikitericin, a novel lanthipeptide was isolated and characterised together with its first total synthesis.

Genome mining of the New Zealand extremophilic microorganism Thermogemmatispora strain T81 indicated the presence of biosynthetic machinery to produce several different peptidic natural products. Solid-phase culture of T81 led to the isolation of tikitericin 1, a new lanthipeptide characterised by four (methyl)lanthionine bridges. The mass-guided isolation and structural elucidation of tikitericin 1 is described together with its total synthesis via Fmoc-solid-phase peptide synthesis (SPPS). The key non-canonical (methyl)lanthionine residues were synthesised in solution phase via an improved synthetic route and subsequently assembled to construct the peptide backbone using Fmoc-SPPS. N-Terminal truncated analogues of tikitericin (2–5) were also prepared in order to evaluate the contribution of each sequential ring of the polycyclic lanthipeptide to the antibacterial activity.

A Working Module for the Neurovascular Unit in the Sexually Dimorphic Nucleus of the Preoptic Area

The neurovascular unit (NVU) can be conceptualized as a functional entity consisting of neurons, astrocytes, pericytes, and endothelial and smooth muscle cells that operate in concert to affect blood flow to a very circumscribed area. Although we are currently in a "golden era" of bioengineering, there are, as yet, no living NVUs-on-a-chip modules available and the development of a neural chip that would mimic NVUs is a seemingly lofty goal. The sexually dimorphic nucleus of the preoptic area (SDN-POA) is a tiny brain structure (between 0.001~0.007 mm³ in rats) with an assessable biological function (i.e., male sexual behavior). The present effort was undertaken to determine whether there are identifiable NVUs in the SDN-POA by assessing its vasculature relative to its known neural components. First, a thorough and systematic review of thousands of histologic and immunofluorescent images from 201 weanling and adult rats was undertaken to define the characteristics of the vessels supplying the SDN-POA: its primary supply artery/arteriole and capillaries are physically inseparable from their neural elements. A subsequent immunofluorescent study targeting α-smooth muscle actin confirmed the identity of an artery/arteriole supplying the SDN-POA. In reality, the predominant components of the SDN-POA are calbindin D28k-positive neurons that are comingled with tyrosine hydroxylase-positive projections. Finally, a schematic of an SDN-POA NVU is proposed as a working model of the basic building block of the CNS. Such modules could serve the study of neurovascular mechanisms and potentially inform the development of next generation bioengineered neural transplants, i.e., the construct of an NVU neural chip.

Anaerobes as Sources of Bioactive Compounds and Health Promoting Tools

Aerobic microorganisms have been sources of medicinal agents for several decades and an impressive variety of drugs have been isolated from their cultures, studied and formulated to treat or prevent diseases. On the other hand, anaerobes, which are believed to be the oldest life forms on earth and evolved remarkably diverse physiological functions, have largely been neglected as sources of bioactive compounds. However, results obtained from the limited research done so far show that anaerobes are capable of producing a range of interesting bioactive compounds that can promote human health. In fact, some of these bioactive compounds are found to be novel in their structure and/or mode of action.Anaerobes play health-promoting roles through their bioactive products as well as application of whole cells. The bioactive compounds produced by these microorganisms include antimicrobial agents and substances such as immunomodulators and vitamins. Bacteriocins produced by anaerobes have been in use as preservatives for about 40 years. Because these substances are effective at low concentrations, encounter relatively less resistance from bacteria and are safe to use, there is a growing interest in these antimicrobial agents. Moreover, several antibiotics have been reported from the cultures of anaerobes. Closthioamide and andrimid produced by Clostridium cellulolyticum and Pantoea agglomerans, respectively, are examples of novel antibiotics of anaerobe origin. The discovery of such novel bioactive compounds is expected to encourage further studies which can potentially lead to tapping of the antibiotic production potential of this fascinating group of microorganisms.Anaerobes are widely used in preparation of fermented foods and beverages. During the fermentation processes, these organisms produce a number of bioactive compounds including anticancer, antihypertensive and antioxidant substances. The well-known health promoting effect of fermented food is mostly due to these bioactive compounds. In addition to their products, whole cell anaerobes have very interesting applications for enhancing the quality of life. Probiotic anaerobes have been on the market for many years and are receiving growing acceptance as health promoters. Gut anaerobes have been used to treat patients suffering from severe Clostridium difficile infection syndromes including diarrhoea and colitis which cannot be treated by other means. Whole cell anaerobes are also studied to detect and cure cancer. In recent years, evidence is emerging that anaerobes constituting the microbiome are linked to our overall health. A dysfunctional microbiome is believed to be the cause of many diseases including cancer, allergy, infection, obesity, diabetes and several other disorders. Maintaining normal microflora is believed to alleviate some of these serious health problems. Indeed, the use of probiotics and prebiotics which favourably change the number and composition of the gut microflora is known to render a health promoting effect. Our interaction with the microbiome anaerobes is complex. In fact, not only our lives but also our identities are more closely linked to the anaerobic microbial world than we may possibly imagine. We are just at the beginning of unravelling the secret of association between the microbiome and human body, and a clear understanding of the association may bring a paradigm shift in the way we diagnose and treat diseases and disorders. This chapter highlights some of the work done on bioactive compounds and whole cell applications of the anaerobes that foster human health and improve the quality of life.

Mechanistic Understanding of Lanthipeptide Biosynthetic Enzymes

Lanthipeptides are ribosomally synthesized and post-translationally modified peptides (RiPPs) that display a wide variety of biological activities, from antimicrobial to antiallodynic. Lanthipeptides that display antimicrobial activity are called lantibiotics. The post-translational modification reactions of lanthipeptides include dehydration of Ser and Thr residues to dehydroalanine and dehydrobutyrine, a transformation that is carried out in three unique ways in different classes of lanthipeptides. In a cyclization process, Cys residues then attack the dehydrated residues to generate the lanthionine and methyllanthionine thioether cross-linked amino acids from which lanthipeptides derive their name. The resulting polycyclic peptides have constrained conformations that confer their biological activities. After installation of the characteristic thioether cross-links, tailoring enzymes introduce additional post-translational modifications that are unique to each lanthipeptide and that fine-tune their activities and/or stability. This review focuses on studies published over the past decade that have provided much insight into the mechanisms of the enzymes that carry out the post-translational modifications.

A Bioengineered Nisin Derivative, M21A, in Combination with Food Grade Additives Eradicates Biofilms of Listeria monocytogenes

The burden of foodborne disease has large economic and social consequences worldwide. Despite strict regulations, a number of pathogens persist within the food environment, which is greatly contributed to by a build-up of resistance mechanisms and also through the formation of biofilms. Biofilms have been shown to be highly resistant to a number of antimicrobials and can be extremely difficult to remove once they are established. In parallel, the growing concern of consumers regarding the use of chemically derived antimicrobials within food has led to a drive toward more natural products. As a consequence, the use of naturally derived antimicrobials has become of particular interest. In this study we investigated the efficacy of nisin A and its bioengineered derivative M21A in combination with food grade additives to treat biofilms of a representative foodborne disease isolate of Listeria monocytogenes. Investigations revealed the enhanced antimicrobial effects, in liquid culture, of M21A in combination with citric acid or cinnamaldehyde over its wild type nisin A counterpart. Subsequently, an investigation was conducted into the effects of these combinations on an established biofilm of the same strain. Nisin M21A (0.1 μg/ml) alone or in combination with cinnamaldehyde (35 μg/ml) or citric acid (175 μg/ml) performed significantly better than combinations involving nisin A. All combinations of M21A with either citric acid or cinnamaldehyde eradicated the L. monocytogenes biofilm (in relation to a non-biofilm control). We conclude that M21A in combination with available food additives could further enhance the antimicrobial treatment of biofilms within the food industry, simply by substituting nisin A with M21A in current commercial products such as Nisaplin® (Danisco, DuPont).

Towards Biocontained Cell Factories: An Evolutionarily Adapted Escherichia coli Strain Produces a New-to-nature Bioactive Lantibiotic Containing Thienopyrrole-Alanine

Genetic code engineering that enables reassignment of genetic codons to non-canonical amino acids (ncAAs) is a powerful strategy for enhancing ribosomally synthesized peptides and proteins with functions not commonly found in Nature. Here we report the expression of a ribosomally synthesized and post-translationally modified peptide (RiPP), the 32-mer lantibiotic lichenicidin with a canonical tryptophan (Trp) residue replaced by the ncAA L-β-(thieno[3,2-b]pyrrolyl)alanine ([3,2]Tpa) which does not sustain cell growth in the culture. We have demonstrated that cellular toxicity of [3,2]Tpa for the production of the new-to-nature bioactive congener of lichenicidin in the host Escherichia coli can be alleviated by using an evolutionarily adapted host strain MT21 which not only tolerates [3,2]Tpa but also uses it as a proteome-wide synthetic building block. This work underscores the feasibility of the biocontainment concept and establishes a general framework for design and large scale production of RiPPs with evolutionarily adapted host strains.

Cyclisation mechanisms in the biosynthesis of ribosomally synthesised and post-translationally modified peptides

Ribosomally synthesised and post-translationally modified peptides (RiPPs) are a large class of natural products that are remarkably chemically diverse given an intrinsic requirement to be assembled from proteinogenic amino acids. The vast chemical space occupied by RiPPs means that they possess a wide variety of biological activities, and the class includes antibiotics, co-factors, signalling molecules, anticancer and anti-HIV compounds, and toxins. A considerable amount of RiPP chemical diversity is generated from cyclisation reactions, and the current mechanistic understanding of these reactions will be discussed here. These cyclisations involve a diverse array of chemical reactions, including 1,4-nucleophilic additions, [4 + 2] cycloadditions, ATP-dependent heterocyclisation to form thiazolines or oxazolines, and radical-mediated reactions between unactivated carbons. Future prospects for RiPP pathway discovery and characterisation will also be highlighted.

Amides in Nature and Biocatalysis

Amides are widespread in biologically active compounds with a broad range of applications in biotechnology, agriculture and medicine. Therefore, as alternative to chemical synthesis the biocatalytic amide synthesis is a very interesting field of research. As usual, Nature can serve as guide in the quest for novel biocatalysts. Several mechanisms for carboxylate activation involving mainly acyl-adenylate, acyl-phosphate or acyl-enzyme intermediates have been discovered, but also completely different pathways to amides are found. In addition to ribosomes, selected enzymes of almost all main enzyme classes are able to synthesize amides. In this review we give an overview about amide synthesis in Nature, as well as biotechnological applications of these enzymes. Moreover, several examples of biocatalytic amide synthesis are given.

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.

Nisin H Is a New Nisin Variant Produced by the Gut-Derived Strain Streptococcus hyointestinalis DPC6484

Accumulating evidence suggests that bacteriocin production represents a probiotic trait for intestinal strains to promote dominance, fight infection, and even signal the immune system. In this respect, in a previous study, we isolated from the porcine intestine a strain of Streptococcus hyointestinalis DPC6484 that displays antimicrobial activity against a wide range of Gram-positive bacteria and produces a bacteriocin with a mass of 3,453 Da. Interestingly, the strain was also found to be immune to a nisin-producing strain. Genome sequencing revealed the genetic determinants responsible for a novel version of nisin, designated nisin H, consisting of the nshABTCPRKGEF genes, with transposases encoded between nshP and nshR and between nshK and nshG. A similar gene cluster is also found in S. hyointestinalis LMG14581. Notably, the cluster lacks an equivalent of the nisin immunity gene, nisI. Nisin H is proposed to have the same structure as the prototypical nisin A but differs at 5 amino acid positions-Ile1Phe (i.e., at position 1, nisin A has Ile while nisin H has Phe), Leu6Met, Gly18Dhb (threonine dehydrated to dehydrobutyrine), Met21Tyr, and His31Lys--and appears to represent an intermediate between the lactococcal nisin A and the streptococcal nisin U variant of nisin. Purified nisin H inhibits a wide range of Gram-positive bacteria, including staphylococci, streptococci, Listeria spp., bacilli, and enterococci. It represents the first example of a natural nisin variant produced by an intestinal isolate of streptococcal origin.

Efficacies of nisin A and nisin V semipurified preparations alone and in combination with plant essential oils for controlling Listeria monocytogenes

The food-borne pathogenic bacterium Listeria is known for relatively low morbidity and high mortality rates, reaching up to 25 to 30%. Listeria is a hardy organism, and its control in foods represents a significant challenge. Many naturally occurring compounds, including the bacteriocin nisin and a number of plant essential oils, have been widely studied and are reported to be effective as antimicrobial agents against spoilage and pathogenic microorganisms. The aim of this study was to investigate the ability of semipurified preparations (SPP) containing either nisin A or an enhanced bioengineered derivative, nisin V, alone and in combination with low concentrations of the essential oils thymol, carvacrol, and trans-cinnamaldehyde, to control Listeria monocytogenes in both laboratory media and model food systems. Combinations of nisin V-containing SPP (25 μg/ml) with thymol (0.02%), carvacrol (0.02%), or cinnamaldehyde (0.02%) produced a significantly longer lag phase than any of the essential oil-nisin A combinations. In addition, the log reduction in cell counts achieved by the nisin V-carvacrol or nisin V-cinnamaldehyde combinations was twice that of the equivalent nisin A-essential oil treatment. Significantly, this enhanced activity was validated in model food systems against L. monocytogenes strains of food origin. We conclude that the fermentate form of nisin V in combination with carvacrol and cinnamaldehyde offers significant advantages as a novel, natural, and effective means to enhance food safety by inhibiting food-borne pathogens such as L. monocytogenes.

Bacteriocin protein BacL1 of Enterococcus faecalis targets cell division loci and specifically recognizes L-Ala2-cross-bridged peptidoglycan

Bacteriocin 41 (Bac41) is produced from clinical isolates of Enterococcus faecalis and consists of two extracellular proteins, BacL1 and BacA. We previously reported that BacL1 protein (595 amino acids, 64.5 kDa) is a bacteriolytic peptidoglycan D-isoglutamyl-L-lysine endopeptidase that induces cell lysis of E. faecalis when an accessory factor, BacA, is copresent. However, the target of BacL1 remains unknown. In this study, we investigated the targeting specificity of BacL1. Fluorescence microscopy analysis using fluorescent dye-conjugated recombinant protein demonstrated that BacL1 specifically localized at the cell division-associated site, including the equatorial ring, division septum, and nascent cell wall, on the cell surface of target E. faecalis cells. This specific targeting was dependent on the triple repeat of the SH3 domain located in the region from amino acid 329 to 590 of BacL1. Repression of cell growth due to the stationary state of the growth phase or to treatment with bacteriostatic antibiotics rescued bacteria from the bacteriolytic activity of BacL1 and BacA. The static growth state also abolished the binding and targeting of BacL1 to the cell division-associated site. Furthermore, the targeting of BacL1 was detectable among Gram-positive bacteria with an L-Ala-L-Ala-cross-bridging peptidoglycan, including E. faecalis, Streptococcus pyogenes, or Streptococcus pneumoniae, but not among bacteria with alternate peptidoglycan structures, such as Enterococcus faecium, Enterococcus hirae, Staphylococcus aureus, or Listeria monocytogenes. These data suggest that BacL1 specifically targets the L-Ala-L-Ala-cross-bridged peptidoglycan and potentially lyses the E. faecalis cells during cell division.

Mechanistic studies on the substrate-tolerant lanthipeptide synthetase ProcM

Lanthipeptides are a class of post-translationally modified peptide natural products. They contain lanthionine (Lan) and methyllanthionine (MeLan) residues, which generate cross-links and endow the peptides with various biological activities. The mechanism of a highly substrate-tolerant lanthipeptide synthetase, ProcM, was investigated herein. We report a hybrid ligation strategy to prepare a series of substrate analogues designed to address a number of mechanistic questions regarding catalysis by ProcM. The method utilizes expressed protein ligation to generate a C-terminal thioester of the leader peptide of ProcA, the substrate of ProcM. This thioester was ligated with a cysteine derivative that resulted in an alkyne at the C-terminus of the leader peptide. This alkyne in turn was used to conjugate the leader peptides to a variety of synthetic peptides by copper-catalyzed azide–alkyne cycloaddition. Using deuterium-labeled Ser and Thr in the substrate analogues thus prepared, dehydration by ProcM was established to occur from C-to-N-terminus for two different substrates. Cyclization also occurred with a specific order, which depended on the sequence of the substrate peptides. Furthermore, using orthogonal cysteine side-chain protection in the two semisynthetic peptide substrates, we were able to rule out spontaneous non-enzymatic cyclization events to explain the very high substrate tolerance of ProcM. Finally, the enzyme was capable of exchanging protons at the α-carbon of MeLan, suggesting that ring formation could be reversible. These findings are discussed in the context of the mechanism of the substrate-tolerant ProcM, which may aid future efforts in lanthipeptide engineering.

Antibiotic alternatives: the substitution of antibiotics in animal husbandry?

It is a common practice for decades to use of sub-therapeutic dose of antibiotics in food-animal feeds to prevent animals from diseases and to improve production performance in modern animal husbandry. In the meantime, concerns over the increasing emergence of antibiotic-resistant bacteria due to the unreasonable use of antibiotics and an appearance of less novelty antibiotics have prompted efforts to develop so-called alternatives to antibiotics. Whether or not the alternatives could really replace antibiotics remains a controversial issue. This review summarizes recent development and perspectives of alternatives to antibiotics. The mechanism of actions, applications, and prospectives of the alternatives such as immunity modulating agents, bacteriophages and their lysins, antimicrobial peptides, pro-, pre-, and synbiotics, plant extracts, inhibitors targeting pathogenicity (bacterial quorum sensing, biofilm, and virulence), and feeding enzymes are thoroughly discussed. Lastly, the feasibility of alternatives to antibiotics is deeply analyzed. It is hard to conclude that the alternatives might substitute antibiotics in veterinary medicine in the foreseeable future. At the present time, prudent use of antibiotics and the establishment of scientific monitoring systems are the best and fastest way to limit the adverse effects of the abuse of antibiotics and to ensure the safety of animal-derived food and environment.

Advances in the arsenal of tools available enabling the discovery of novel lantibiotics with therapeutic potential

INTRODUCTION

Lantibiotics are ribosomally synthesised peptides, which undergo extensive post-translational modification. Their mode of action and effectiveness against multi-drug-resistant pathogens, and relatively low toxicity, makes them attractive therapeutic options.

AREAS COVERED

This article provides background information on the four classes of lanthipeptides that have been described to date. Due to the clinical potential of these agents, specifically those from Class I and II, it is essential to identify organisms that harbour potentially interesting clusters encoding novel lantibiotics. Multiple emerging technologies have been applied to address this issue, including genome mining and specific bioinformatics programs designed to identify lantibiotic clusters present within the genome sequences. These clusters can then be effectively expressed using optimised heterologous expression systems, which are ideally amenable to large-scale production.

EXPERT OPINION

The continuing expansion of publicly available genomes, particularly genomes from microorganisms isolated from under-explored environments, combined with powerful bioinformatics tools able to accurately identify clusters of interest are of paramount importance in the discovery of novel lantibiotics. Detailed analysis of clusters drastically reduces dereplication time, which was often problematic when using the traditional method of isolation, purification and then identification. Allowing a more focused direction of 'wet lab' work, targeting the most promising agents, greatly increases the chance of novel lantibiotic discovery and development. High-throughput screening strategies are also required to enable the efficient analysis of these potentially clinically relevant agents.

Intensive mutagenesis of the nisin hinge leads to the rational design of enhanced derivatives

Nisin A is the most extensively studied lantibiotic and has been used as a preservative by the food industry since 1953. This 34 amino acid peptide contains three dehydrated amino acids and five thioether rings. These rings, resulting from one lanthionine and four methyllanthionine bridges, confer the peptide with its unique structure. Nisin A has two mechanisms of action, with the N-terminal domain of the peptide inhibiting cell wall synthesis through lipid II binding and the C-terminal domain responsible for pore-formation. The focus of this study is the three amino acid ‘hinge’ region (N 20, M 21 and K 22) which separates these two domains and allows for conformational flexibility. As all lantibiotics are gene encoded, novel variants can be generated through manipulation of the corresponding gene. A number of derivatives in which the hinge region was altered have previously been shown to possess enhanced antimicrobial activity. Here we take this approach further by employing simultaneous, indiscriminate site-saturation mutagenesis of all three hinge residues to create a novel bank of nisin derivative producers. Screening of this bank revealed that producers of peptides with hinge regions consisting of AAK, NAI and SLS displayed enhanced bioactivity against a variety of targets. These and other results suggested a preference for small, chiral amino acids within the hinge region, leading to the design and creation of producers of peptides with hinges consisting of AAA and SAA. These producers, and the corresponding peptides, exhibited enhanced bioactivity against Lactococcus lactis HP, Streptococcus agalactiae ATCC 13813, Mycobacterium smegmatis MC2155 and Staphylococcus aureus RF122 and thus represent the first example of nisin derivatives that possess enhanced activity as a consequence of rational design.

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.

Codon randomization for rapid exploration of chemical space in thiopeptide antibiotic variants

Thiopeptide antibiotics exhibit a profound level of chemical diversity that is installed through cascades of posttranslational modifications on ribosomal peptides. Here, we present a technique to rapidly explore the chemical space of the thiopeptide GE37468 through codon randomization, yielding insights into thiopeptide maturation as well as structure and activity relationships. In this incarnation of the methodology, we randomized seven residues of the prepeptide-coding region, enabling the generation of 133 potential thiopeptide variants. Variant libraries were subsequently queried in two ways. First, high-throughput MALDI-TOF mass spectrometry was applied to colony-level expressions to sample mutants that permitted full maturation of the antibiotic. Second, the activity of producing mutants was detected in an antibiotic overlay assay. In total, 29 of the 133 variants produced mature compound, 12 of which retained antibiotic activity and 1 that had improved activity.

Subtilomycin: a new lantibiotic from Bacillus subtilis strain MMA7 isolated from the marine sponge Haliclona simulans

Bacteriocins are attracting increased attention as an alternative to classic antibiotics in the fight against infectious disease and multidrug resistant pathogens. Bacillus subtilis strain MMA7 isolated from the marine sponge Haliclona simulans displays a broad spectrum antimicrobial activity, which includes Gram-positive and Gram-negative pathogens, as well as several pathogenic Candida species. This activity is in part associated with a newly identified lantibiotic, herein named as subtilomycin. The proposed biosynthetic cluster is composed of six genes, including protein-coding genes for LanB-like dehydratase and LanC-like cyclase modification enzymes, characteristic of the class I lantibiotics. The subtilomycin biosynthetic cluster in B. subtilis strain MMA7 is found in place of the sporulation killing factor (skf) operon, reported in many B. subtilis isolates and involved in a bacterial cannibalistic behaviour intended to delay sporulation. The presence of the subtilomycin biosynthetic cluster appears to be widespread amongst B. subtilis strains isolated from different shallow and deep water marine sponges. Subtilomycin possesses several desirable industrial and pharmaceutical physicochemical properties, including activity over a wide pH range, thermal resistance and water solubility. Additionally, the production of the lantibiotic subtilomycin could be a desirable property should B. subtilis strain MMA7 be employed as a probiotic in aquaculture applications.

Antimicrobial peptides in oyster hemolymph: the bacterial connection

We have explored antimicrobial compounds in oyster hemolymph and purified four active peptides with molecular masses of 4464, 3158, 655 and 636 Da. While no exploitable structural elements were obtained for the former three, a partial amino acid sequence (X-P-P-X-X-I-V) was obtained for the latter, named Cg-636. Due to both its low MM and the presence of exotic amino acid residue (X), we suspected a bacterial origin and tracked cultivable hemolymph-resident bacteria of oyster for their antimicrobial abilities. Supernatants of 224 hemolymph resident bacteria coming from 60 oysters were screened against 10 target bacteria including aquaculture pathogens. Around 2% (5 strains) revealed antimicrobial activities. They belong to Pseudoalteromonas and Vibrio genera. Two closely related strains named hCg-6 and hCg-42 have been shown to produce Bacteriocin-Like Inhibitory Substances (BLIS) even in oyster hemolymph. We report herein first BLIS-producing bacteria isolated from bivalve hemolymph. These results strongly suggest that hemolymph resident bacteria may prevent pathogen establishment and pave the way for considering a role of resident bacteria into bivalve defense.

'Bac' to the future: bioengineering lantibiotics for designer purposes

Bacteriocins are bacterially produced peptides or proteins that inhibit the growth of other bacterial strains. They can have a broad (effective against multiple genera) or narrow (effective against specific species) spectrum of activity. The diversity of bacteriocins found in Nature, in terms of both spectrum of activity and physiochemical properties, offers the possibility of multiple applications in the food and pharmaceutical industries. However, traditional screening strategies may not provide a sufficient range of natural molecules with specifically desired properties. Research suggests that bioengineering of existing inhibitors has the potential to address this issue, extending the application of natural bacteriocins for use in novel settings and against different targets. In the present paper, we discuss the successful implementation of bioengineering strategies to alter and even improve the functional characteristics of a bacteriocin, using the prototypical lantibiotic nisin as an example. Additionally, we describe the recent use of the nisin-modification machinery in vivo to enhance the properties of medically significant peptides.

Chemical synthesis of the lantibiotic lacticin 481 reveals the importance of lanthionine stereochemistry

Lantibiotics are a family of antibacterial peptide natural products characterized by the post-translational installation of the thioether-containing amino acids lanthionine and methyllanthionine. Until recently, only a single naturally occurring stereochemical configuration for each of these cross-links was known. The discovery of lantibiotics with alternative lanthionine and methyllanthionine stereochemistry has prompted an investigation of its importance to biological activity. Here, solid-supported chemical synthesis enabled the total synthesis of the lantibiotic lacticin 481 and analogues containing cross-links with non-native stereochemical configurations. Biological evaluation revealed that these alterations abolished the antibacterial activity in all of the analogues, revealing the critical importance of the enzymatically installed stereochemistry for the biological activity of lacticin 481.

Revealing nature's synthetic potential through the study of ribosomal natural product biosynthesis

Ribosomally synthesized posttranslationally modified peptides (RiPPs) are a rapidly growing class of natural products with diverse structures and activities. In recent years, a great deal of progress has been made in elucidating the biosynthesis of various RiPP family members. As with the study of nonribosomal peptide and polyketide biosynthetic enzymes, these investigations have led to the discovery of entirely new biological chemistry. With each unique enzyme investigated, a more complex picture of Nature's synthetic potential is revealed. This Review focuses on recent reports (since 2008) that have changed the way that we think about ribosomal natural product biosynthesis and the enzymology of complex bond-forming reactions.

Saturation mutagenesis of lysine 12 leads to the identification of derivatives of nisin A with enhanced antimicrobial activity

It is becoming increasingly apparent that innovations from the "golden age" of antibiotics are becoming ineffective, resulting in a pressing need for novel therapeutics. The bacteriocin family of antimicrobial peptides has attracted much attention in recent years as a source of potential alternatives. The most intensively studied bacteriocin is nisin, a broad spectrum lantibiotic that inhibits gram-positive bacteria including important food pathogens and clinically relevant antibiotic resistant bacteria. Nisin is gene-encoded and, as such, is amenable to peptide bioengineering, facilitating the generation of novel derivatives that can be screened for desirable properties. It was to this end that we used a site-saturation mutagenesis approach to create a bank of producers of nisin A derivatives that differ with respect to the identity of residue 12 (normally lysine; K12). A number of these producers exhibited enhanced bioactivity and the nisin A K12A producer was deemed of greatest interest. Subsequent investigations with the purified antimicrobial highlighted the enhanced specific activity of this modified nisin against representative target strains from the genera Streptococcus, Bacillus, Lactococcus, Enterococcus and Staphylococcus.

Saturation mutagenesis of selected residues of the α-peptide of the lantibiotic lacticin 3147 yields a derivative with enhanced antimicrobial activity

The lantibiotic lacticin 3147 consists of two ribosomally synthesized and post-translationally modified antimicrobial peptides, Ltnα and Ltnβ, which act synergistically against a wide range of Gram-positive microorganisms. We performed saturation mutagenesis of specific residues of Ltnα to determine their functional importance. The results establish that Ltnα is more tolerant to change than previously suggested by alanine scanning mutagenesis. One substitution, LtnαH23S, was identified which improved the specific activity of lacticin 3147 against one pathogenic strain, Staphylococcus aureus NCDO1499. This represents the first occasion upon which the activity of a two peptide lantibiotic has been enhanced through bioengineering.

In vivo activity of nisin A and nisin V against Listeria monocytogenes in mice

BACKGROUND

Lantibiotics are post-translationally modified antimicrobial peptides, of which nisin A is the most extensively studied example. Bioengineering of nisin A has resulted in the generation of derivatives with increased in vitro potency against Gram-positive bacteria. Of these, nisin V (containing a Met21Val change) is noteworthy by virtue of exhibiting enhanced antimicrobial efficacy against a wide range of clinical and food-borne pathogens, including Listeria monocytogenes. However, this increased potency has not been tested in vivo.

RESULTS

Here we address this issue by assessing the ability of nisin A and nisin V to control a bioluminescent strain of Listeria monocytogenes EGDe in a murine infection model.More specifically, Balb/c mice were infected via the intraperitoneal route at a dose of 1 × 10(5) cfu/animal and subsequently treated intraperitoneally with either nisin V, nisin A or a PBS control. Bioimaging of the mice was carried out on day 3 of the trial. Animals were then sacrificed and levels of infection were quantified in the liver and spleen.

CONCLUSION

This analysis revealed that nisin V was more effective than Nisin A with respect to controlling infection and therefore merits further investigation with a view to potential chemotherapeutic applications.

Bacteriocins - a viable alternative to antibiotics?

Solutions are urgently required for the growing number of infections caused by antibiotic-resistant bacteria. Bacteriocins, which are antimicrobial peptides produced by certain bacteria, might warrant serious consideration as alternatives to traditional antibiotics. These molecules exhibit significant potency against other bacteria (including antibiotic-resistant strains), are stable and can have narrow or broad activity spectra. Bacteriocins can even be produced in situ in the gut by probiotic bacteria to combat intestinal infections. Although the application of specific bacteriocins might be curtailed by the development of resistance, an understanding of the mechanisms by which such resistance could emerge will enable researchers to develop strategies to minimize this potential problem.

Bioengineered nisin A derivatives with enhanced activity against both Gram positive and Gram negative pathogens

Nisin is a bacteriocin widely utilized in more than 50 countries as a safe and natural antibacterial food preservative. It is the most extensively studied bacteriocin, having undergone decades of bioengineering with a view to improving function and physicochemical properties. The discovery of novel nisin variants with enhanced activity against clinical and foodborne pathogens has recently been described. We screened a randomized bank of nisin A producers and identified a variant with a serine to glycine change at position 29 (S29G), with enhanced efficacy against S. aureus SA113. Using a site-saturation mutagenesis approach we generated three more derivatives (S29A, S29D and S29E) with enhanced activity against a range of Gram positive drug resistant clinical, veterinary and food pathogens. In addition, a number of the nisin S29 derivatives displayed superior antimicrobial activity to nisin A when assessed against a range of Gram negative food-associated pathogens, including E. coli, Salmonella enterica serovar Typhimurium and Cronobacter sakazakii. This is the first report of derivatives of nisin, or indeed any lantibiotic, with enhanced antimicrobial activity against both Gram positive and Gram negative bacteria.

Bioengineering: a bacteriocin perspective

While the bacteriocin Nisin has been employed by the food industry for 60 y, it remains the only bacteriocin to be extensively employed as a food preservative. This is despite the fact that the activity of Nisin against several food spoilage and pathogenic bacteria is poor and the availability of many other bacteriocins with significant potential in this regard. An alternative route to address the deficiencies of Nisin is the application of bioengineered derivatives of the peptide which, despite differing only subtly, possess enhanced capabilities of commercial value. The career path which has taken me from learning for the first time what bacteriocins are to understanding the potential of bacteriocin bioengineering has been a hugely enjoyable experience and promises to get even more interesting in the years to come.

Bioluminescence-based identification of nisin producers - a rapid and simple screening method for nisinogenic bacteria in food samples

We present a simple and rapid method for screening nisin producers that directly identifies nisinogenic bacteria by induction of bioluminescence within the Lactococcus lactis NZ9800lux biosensor strain (Immonen and Karp, 2007, Biosensors and Bioelectronics 22, 1982-7). An overlay of putative nisinogenic colonies with the biosensor strain gives identification results within 1h. Functionality and specificity of the method were verified by screening nisin producers among 144 raw milk colonies and a panel of 91 lactococcal strains. Studies performed on strains and colonies that did not induce bioluminescence but inhibited growth of the biosensor demonstrated that only nisinogenic bacteria can cause induction. Bacteria known to produce bacteriocins other than nisin failed to induce bioluminescence, further verifying the specificity of the assay. We discovered a non-inducing but inhibitory lactococcal strain harboring a modified nisin Z gene, and demonstrated that the source of the inhibitory action is not a non-inducing variant of nisin, but a bacteriocin of lower molecular weight. The concentration of nisin producers in a raw milk sample was 1.3 × 10(2)CFU/ml. We identified from raw milk a total of seven nisin Z producing L. lactis subsp. lactis colonies, which were shown by genetic fingerprinting to belong to three different groups. Among the panel of 91 lactococci, four strains were nisin A producers, and one strain harbored the modified nisin Z gene. The method presented here is robust, cost-effective and simple to perform, and avoids the pitfalls of traditional screening methods by directly specifying the identity of the inhibitory substance.

Retention of nisin activity at elevated pH in an organic acid complex and gold nanoparticle composite

Biocompatible organic acids and citrate-stabilized gold nanoparticles were interacted with nisin to generate robust antimicrobial agents, which display archetypical nisin activity even at elevated pH.

Antibacterial peptides "bacteriocins": an overview of their diverse characteristics and applications

Bacteriocins are ribosomally synthesized antibacterial peptides produced by bacteria that inhibit the growth of similar or closely related bacterial strains. A number of bacteriocins from a wide variety of bacteria have been discovered, and their diverse structures have been reported. Growing evidence suggests that bacteriocins have diverse structures, modes of action, mechanisms of biosynthesis and self-immunity, and gene regulation. Bacteriocins are considered as an attractive compound in food and pharmaceutical industries to prevent food spoilage and pathogenic bacterial growth. Furthermore, elucidation of their biosynthesis has led to the use of bacteriocin-controlled gene-expression systems and the biosynthetic enzymes of lantibiotics, a class of bacteriocins, as tools to design novel peptides. In this review, we summarize and discuss currently known information on bacteriocins produced by Gram-positive bacteria and their applications.

Insights into Lantibiotic Immunity Provided by Bioengineering of LtnI

The lantibiotic lacticin 3147 has been the focus of much research due to its broad spectrum of activity against many microbial targets, including drug-resistant pathogens. In order to protect itself, a lacticin 3147 producer must possess a cognate immunity mechanism. Lacticin 3147 immunity is provided by an ABC transporter, LtnFE, and a dedicated immunity protein, LtnI, both of which are capable of independently providing a degree of protection. In the study described here, we carried out an in-depth investigation of LtnI structure-function relationships through the creation of a series of fusion proteins and LtnI determinants that have been the subject of random and site-directed mutagenesis. We establish that LtnI is a transmembrane protein that contains a number of individual residues and regions, such as those between amino acids 20 and 27 and amino acids 76 and 83, which are essential for LtnI function. Finally, as a consequence of the screening of a bank of 28,000 strains producing different LtnI derivatives, we identified one variant (LtnI I81V) that provides enhanced protection. To our knowledge, this is the first report of a lantibiotic immunity protein with enhanced functionality.

New horizons for host defense peptides and lantibiotics

Antimicrobial peptides from either microbial sources, or based on host defense peptides (HDPs) from higher organisms, show promising activity against human pathogens. Lantibiotics have been extensively engineered by either molecular biology approaches or chemistry and both natural and modified entities have been shown to have good efficacy in animal models of infection. Amongst HDPs either truncated peptides or non-peptide mimetic molecules show substantial promise both for their direct antibiotic action and also modulation of host functions. Members of both classes have reached clinical development for therapy of systemic infections and Clostridium difficile infection of the gastrointestinal tract.

Heterologous expression and purification of NisA, the precursor peptide of lantibiotic nisin from Lactococcus lactis

The lantibiotic nisin is a ribosomally synthesised and post-translationally modified antimicrobial peptide produced by strains of Lactococcus lactis, and used as safe and natural preservative in food industry. The nisA structural gene encodes ribosomally synthesised and biologically inactive a 57 amino acid precursor peptide (NisA) which undergoes several post-translational modifications. In this study, we report the expression of precursor nisin as a His6-tagged peptide in Escherichia coli and its purification using a nickel affinity column. The technique of spliced-overlap extension PCR was used to amplify the nisA gene and the T7 promoter region of pET-15b vector. This approach was used to introduce six histidine residues at the C-terminus of prenisin. The identity of the expressed peptide was confirmed by N-terminal sequencing. The expressed His-tagged prenisin was purified under denaturing conditions, and named as prenisin-His6. The purified prenisin-His6 was analyzed by SDS-PAGE, Western blotting and mass spectroscopy. These results showed that the nisin precursor peptide can be successfully produced using an E. coli expression system.

In vitro selection of functional lantipeptides

In this report we present a method to identify functional artificial lantipeptides. In vitro translation coupled with an enzyme-free protocol for posttranslational modification allows preparation of more than 10(11) different lanthionine containing peptides. This diversity can be searched for functional molecules using mRNA-lantipeptide display. We validated this approach by isolating binders toward Sortase A, a transamidase which is required for virulence of Staphylococcus aureus. The interaction of selected lantipeptides with Sortase A is highly dependent on the presence of a (2S,6R)-lanthionine in the peptide and an active conformation of the protein.