Biosynthesis of bacteriocins in lactic acid bacteria

Surface glycosaminoglycans protect eukaryotic cells against membrane-driven peptide bacteriocins

Enzymatic elimination of surface glycosaminoglycans or inhibition of their sulfation provokes sensitizing of HT-29 and HeLa cells toward the peptide bacteriocins nisin A, plantaricin C, and pediocin PA-1/AcH. The effect can be partially reversed by heparin, which also lowers the susceptibility of Lactococcus lactis to nisin A. These data indicate that the negative charge of the glycosaminoglycan sulfate residues binds the positively charged bacteriocins, thus protecting eukaryotic cells from plasma membrane damage.

Production, purification and characterization of bacteriocin from Lactobacillus murinus AU06 and its broad antibacterial spectrum

OBJECTIVE

To study the production, purification and characterization of bacteriocin from Lactobacillus murinus AU06 isolated from marine sediments and its broad spectrum of inhibition against fish pathogens.

METHODS

The selected strain was used in production, purification and characterized of bacteriocin. In addition, purified bacteriocin was tested for its antimicrobial activity against fish pathogens.

RESULTS

In the present study, the bacteriocin production was found to be higher at 35 °C, pH 6.0 and was purified to 4.74 fold with 55. 38 U/mg of specific activity with the yield of 28.92%. The molecular weight of the purified bacteriocin was estimated as 21 kDa. The purified bacteriocin exhibited complete inactivation of antimicrobial activity when treated with proteinase K, pronase, chymotrypsin, trypsin, pepsin and papain. The purified bacteriocin exhibited broad inhibitory spectrum against both Gram positive and negative bacteria.

CONCLUSIONS

It is concluded that the ability of bacteriocin in inhibiting a wide-range of pathogenic bacteria is of potential interest for food safety and may have future applications in food preservative.

Gene cluster responsible for secretion of and immunity to multiple bacteriocins, the NKR-5-3 enterocins

Enterococcus faecium NKR-5-3, isolated from Thai fermented fish, is characterized by the unique ability to produce five bacteriocins, namely, enterocins NKR-5-3A, -B, -C, -D, and -Z (Ent53A, Ent53B, Ent53C, Ent53D, and Ent53Z). Genetic analysis with a genome library revealed that the bacteriocin structural genes (enkA [ent53A], enkC [ent53C], enkD [ent53D], and enkZ [ent53Z]) that encode these peptides (except for Ent53B) are located in close proximity to each other. This NKR-5-3ACDZ (Ent53ACDZ) enterocin gene cluster (approximately 13 kb long) includes certain bacteriocin biosynthetic genes such as an ABC transporter gene (enkT), two immunity genes (enkIaz and enkIc), a response regulator (enkR), and a histidine protein kinase (enkK). Heterologous-expression studies of enkT and ΔenkT mutant strains showed that enkT is responsible for the secretion of Ent53A, Ent53C, Ent53D, and Ent53Z, suggesting that EnkT is a wide-range ABC transporter that contributes to the effective production of these bacteriocins. In addition, EnkIaz and EnkIc were found to confer self-immunity to the respective bacteriocins. Furthermore, bacteriocin induction assays performed with the ΔenkRK mutant strain showed that EnkR and EnkK are regulatory proteins responsible for bacteriocin production and that, together with Ent53D, they constitute a three-component regulatory system. Thus, the Ent53ACDZ gene cluster is essential for the biosynthesis and regulation of NKR-5-3 enterocins, and this is, to our knowledge, the first report that demonstrates the secretion of multiple bacteriocins by an ABC transporter.

Defining the structure and receptor binding domain of the leaderless bacteriocin LsbB

LsbB is a class II leaderless lactococcal bacteriocin of 30 amino acids. In the present work, the structure and function relationship of LsbB was assessed. Structure determination by NMR spectroscopy showed that LsbB has an N-terminal α-helix, whereas the C-terminal of the molecule remains unstructured. To define the receptor binding domain of LsbB, a competition assay was performed in which a systematic collection of truncated peptides of various lengths covering different parts of LsbB was used to inhibit the antimicrobial activity of LsbB. The results indicate that the outmost eight-amino acid sequence at the C-terminal end is likely to contain the receptor binding domain because only truncated fragments from this region could antagonize the antimicrobial activity of LsbB. Furthermore, alanine substitution revealed that the tryptophan in position 25 (Trp(25)) is crucial for the blocking activity of the truncated peptides, as well as for the antimicrobial activity of the full-length bacteriocin. LsbB shares significant sequence homology with five other leaderless bacteriocins, especially at their C-terminal halves where all contain a conserved KXXXGXXPWE motif, suggesting that they might recognize the same receptor as LsbB. This notion was supported by the fact that truncated peptides with sequences derived from the C-terminal regions of two LsbB-related bacteriocins inhibited the activity of LsbB, in the same manner as found with the truncated version of LsbB. Taken together, these structure-function studies provide strong evidence that the receptor-binding parts of LsbB and sequence-related bacteriocins are located in their C-terminal halves.

Characterization of some bacteriocins produced by lactic acid bacteria isolated from fermented foods

Lactic acid bacteria (LAB) isolated from different sources (dairy products, fruits, fresh and fermented vegetables, fermented cereals) were screened for antimicrobial activity against other bacteria, including potential pathogens and food spoiling bacteria. Six strains have been shown to produce bacteriocins: Lactococcus lactis 19.3, Lactobacillus plantarum 26.1, Enterococcus durans 41.2, isolated from dairy products and Lactobacillus amylolyticus P40 and P50, and Lactobacillus oris P49, isolated from bors. Among the six bacteriocins, there were both heat stable, low molecular mass polypeptides, with a broad inhibitory spectrum, probably belonging to class II bacteriocins, and heat labile, high molecular mass proteins, with a very narrow inhibitory spectrum, most probably belonging to class III bacteriocins. A synergistic effect of some bacteriocins mixtures was observed. We can conclude that fermented foods are still important sources of new functional LAB. Among the six characterized bacteriocins, there might be some novel compounds with interesting features. Moreover, the bacteriocin-producing strains isolated in our study may find applications as protective cultures.

Impact of genomics on the field of probiotic research: historical perspectives to modern paradigms

For thousands of years, humans have safely consumed microorganisms through fermented foods. Many of these bacteria are considered probiotics, which act through diverse mechanisms to confer a health benefit to the host. However, it was not until the availability of whole-genome sequencing and the era of genomics that mechanisms of probiotic efficacy could be discovered. In this review, we explore the history of the probiotic concept and the current standard of integrated genomic techniques to discern the complex, beneficial relationships between probiotic microbes and their hosts.

Draft Genome Sequence of a New Homofermentative, Lactic Acid-Producing Enterococcus faecalis Isolate, CBRD01

We report here the draft genome sequence of the novel homofermentative Enterococcus faecalis isolate CBRD01, which is capable of high lactic acid productivity and yields, with minimal nutritional requirements. The genome is 2.8 Mbp, with 37% G+C, and contains genes for two lactate dehydrogenase (LDH) enzymes found in related organisms.

Qualitative detection of class IIa bacteriocinogenic lactic acid bacteria from traditional Chinese fermented food using a YGNGV-motif-based assay

In the present study, a YGNGV-motif-based assay was developed and applied. Given that there is an increasing demand for natural preservatives, we set out to obtain lactic acid bacteria (LAB) that produce bacteriocins against Gram-positive and Gram-negative bacteria. We here isolated 123 LAB strains from 5 types of traditional Chinese fermented food and screened them for the production of bacteriocins using the agar well diffusion assay (AWDA). Then, to acquire LAB producing class IIa bacteriocins, we used a YGNGV-motif-based assay that was based on 14 degenerate primers matching all class IIa bacteriocin-encoding genes currently deposited in NCBI. Eight of the LAB strains identified by AWDA could inhibit Gram-positive and Gram-negative bacteria; 5 of these were YGNGV-amplicon positive. Among these 5 isolates, amplicons from 2 strains (Y31 and Y33) matched class IIa bacteriocin genes. Strain Y31 demonstrated the highest inhibitory activity and the best match to a class IIa bacteriocin gene in NCBI, and was identified as Enterococcus faecium. The bacteriocin from Enterococcus avium Y33 was 100% identical to enterocin P. Both of these strains produced bacteriocins with strong antimicrobial activity against Listeria monocytogenes, Escherichia coli, and Bacillus subtilis, hence these bacteriocins hold promise as potential bio-preservatives in the food industry. These findings also indicated that the YGNGV-motif-based assay used in this study could identify novel class IIa bacteriocinogenic LAB, rapidly and specifically, saving time and labour by by-passing multiple separation and purification steps.

Purification and characterization of bacteriocin produced by oral Lactobacillus paracasei SD1

The present study aimed to purify and characterize the antimicrobial protein from Lactobacillus paracasei SD1, which is a strain from the human oral cavity. Antimicrobial activity was obtained from purifying the culture supernatant of L. paracasei SD1. Purification of the active compound was achieved with ammonium sulfate precipitation followed by chloroform and gel filtration chromatography. As revealed by SDS-PAGE, the active fraction was homogeneous, showing a protein with an approximate molecular weight of 25,000 Da. It was confirmed as having a molecular mass of 24,028.2 Da by mass spectrometry. The antimicrobial compound, named "paracasin SD1", exhibited a broad spectrum against oral pathogens. Paracasin SD1 was stable in a pH range between 3.0 and 8.0 at 100 °C for 5 min, and showed resistance to α-amylase, catalase, lysozyme and whole saliva. However, its activity was lost after proteinase K and trypsin treatment. The results obtained suggest the possibility of using paracasin SD1 for application in prevention/treatment of oral diseases.

Sonorensin: an antimicrobial peptide, belonging to the heterocycloanthracin subfamily of bacteriocins, from a new marine isolate, Bacillus sonorensis MT93

Marine environments are the greatest fronts of biodiversity, representing a resource of unexploited or unknown microorganisms and new substances having potential applications. Among microbial products, antimicrobial peptides (AMPs) have received great attention recently due to their applications as food preservatives and therapeutic agents. A new marine soil isolate producing an AMP was identified as Bacillus sonorensis based on 16S rRNA gene sequence analysis. It produced an AMP that showed a broad spectrum of activity against both Gram-positive and Gram-negative bacteria. The peptide, named sonorensin, was purified to homogeneity using a combination of chromatographic techniques. The intact molecular mass of the purified peptide, 6,274 Da, as revealed by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF), was in agreement with Tricine-SDS-PAGE analysis. A PCR array of primers was used to identify AMP structural genes, which allowed the successful amplification of the related genes from strain MT93. The putative open reading frame of sonorensin was amplified, cloned into the pET-32a(+) vector, expressed as a thioredoxin (Trx) fusion protein in Escherichia coli, and then purified. Sequence alignment analysis revealed that the bacteriocin being reported could belong to new subfamily of bacteriocins, heterocycloanthracin. The peptide indicated its potential as a biocontrol agent or food antimicrobial agent, due to its antimicrobial activity against bacteria such as Listeria monocytogenes and Staphylococcus aureus. This is the first report of the production, purification, and characterization of wild-type and recombinant bacteriocin by B. sonorensis and the first bacteriocin of the heterocycloanthracin subfamily to be characterized.

Treatment of infectious disease: beyond antibiotics

Several antibiotics have been discovered following the discovery of penicillin. These antibiotics had been helpful in treatment of infectious diseases considered dread for centuries. The advent of multiple drug resistance in microbes has posed new challenge to researchers. The scientists are now evaluating alternatives for combating infectious diseases. This review focuses on major alternatives to antibiotics on which preliminary work had been carried out. These promising anti-microbial include: phages, bacteriocins, killing factors, antibacterial activities of non-antibiotic drugs and quorum quenching.

Surface-conjugated antimicrobial peptide leucocin a displays high binding to pathogenic gram-positive bacteria

Leucocin A, a representative class IIa bacteriocin, is a ribosomally synthesized antimicrobial peptide (AMP) that displays potent activity against specific gram-positive bacteria. The antibacterial activity of such peptides is preceded by the binding event that can be utilized for studying specific peptide-bacteria interactions. In this study, 37-residue Leucocin A (LeuA) was synthesized using solid-phase peptide synthesis and covalently immobilized on gold substrates from either the N- or C-terminal. Both the peptide monolayers on gold substrates were incubated separately with five strains of gram-positive bacteria and displayed differential binding to different strains with highest binding to pathogenic Listeria monocytogenes . The C-terminally immobilized LeuA showed higher bacterial binding compared to the N-terminally attached LeuA. The full length immobilized LeuA (37-residue) was active as well as displayed higher bacterial binding (73 ± 6 bacteria/100 μm(2)) compared to 24-residue inactive LeuA fragment (40 ± 8 bacteria/100 μm(2)) from the C-terminal region. The high and specific bacterial binding ability of LeuA functionalized surfaces support the potential use of class IIa bacteriocins in antimicrobial peptide-based diagnostic platforms.

Detection and partial characterization of a bacteriocin-like substance produced by Lactobacillus fermentum CS57 isolated from human vaginal secretions

Lactobacilli (150) from human vaginal secretions were tested for the production of antimicrobial substances which can provide a physiological defense against the pathogenic microorganisms in the vaginal area. Sixteen of the isolates (10.6%) showed antibacterial activity against one or several closely related microorganisms used as indicators. Lactobacillus fermentum CS57 was the best producer and secretes a bacteriocin-like substance (BLS) with antagonistic activity against Streptococcus agalactiae and Candida albicans. The compound was susceptible to the proteolytic enzymes and was heat labile. The mode of action was identified as bactericidal. The crude activity of the L. fermentum CS57 BLS was linked to a substance with a molecular weight larger than 30 kDa. Plasmid analysis of L. fermentum CS57 revealed the presence of a plasmid band with molecular weight of 54.7 kb. All L. fermentum CS57 non-producer variants (BLS-) obtained by curing experiments, showed loss of plasmid band and were susceptible to the BLS of the original strain. Therefore antimicrobial activity and immunity production seem to be linked to genes located on that same plasmid. Taking into account our results, L. fermentum CS57 could be considered a candidate for potential use as probiotic for the prophylaxis of vaginal human infections.

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.

The influence of the human microbiome and probiotics on cardiovascular health

Cardiovascular disease (CVD) is a major cause of death worldwide. Of the many etiological factors, microorganisms constitute one. From the local impact of the gut microbiota on energy metabolism and obesity, to the distal association of periodontal disease with coronary heart disease, microbes have a significant impact on cardiovascular health. In terms of the ability to modulate or influence the microbes, probiotic applications have been considered. These are live microorganisms which when administered in adequate amounts confer a benefit on the host. While a number of reports have established the beneficial abilities of certain probiotic bacterial strains to reduce cholesterol and hypertension, recent research suggests that their use could be more widely applied. This review presents an up-to-date summary of the known associations of the microbiome with CVD, and potential applications of probiotic therapy.

Functional genetic analysis of the GarML gene cluster in Lactococcus garvieae DCC43 gives new insights into circular bacteriocin biosynthesis

Garvicin ML (GarML) is a circular bacteriocin produced by Lactococcus garvieae DCC43. The recently published draft genome of this strain allowed determination of the genetic background for bacteriocin production. Bioinformatic analysis identified a gene cluster consisting of nine open reading frames likely involved in the production of and immunity to GarML. The garA gene encodes the bacteriocin precursor, garX a large transmembrane protein, garBCDE a putative immunity protein (garB) followed by an ATPase and two transmembrane proteins, and garFGH a putative ABC transporter complex. Functional genetic analysis revealed that deletion of garFGH had no effect on sensitivity to or production of GarML. In contrast, deletion of garBCDE or inactivation of garX resulted in high-level sensitivity to GarML and completely abolished production of active bacteriocin. Mass spectrometry of culture supernatants revealed that wild-type cultures contained the mature circular form as well as the linear forms of the bacteriocin, both with and without the three-amino-acid leader sequence, while bacteriocin-negative mutants contained only the linear forms. These results indicate that cleavage of the leader peptide precedes circularization and is likely performed by a functional entity separate from the GarML gene cluster. To our knowledge, this is the first conclusive evidence for these processes being separated in time. Loss of immunity and antimicrobial activity in addition to our inability to detect the circular bacteriocin in the ΔgarBCDE and garX::pCG47 mutants demonstrate that both these units are indispensable for GarML biosynthesis as well as immunity. Furthermore, the results indicate that these genes are implicated in the circularization of the bacteriocin and that their functions are probably interlinked.

A novel formulation of thiamine dilaurylsulphate and its preservative effect on apple juice and sterilised milk

Thiamine dilaurylsulphate (TLS), synthesised by a new method, was developed for a formulation with a wide antimicrobial spectrum and evaluated in apple juice and milk. The result of a disc diffusion method showed that TLS, which was synthesised from sodium lauryl sulphate and thiamine hydrochloride, had strong growth-inhibitory effects on Gram-positive bacteria, yeasts, and moulds, but had weak inhibitory effects on Gram-negative bacteria such as Escherichia coli. Moreover, the TLS antibacterial spectrum could be broadened to all microbes by formulation with organic acids, such as gluconic acid or acetic acid, since organic acids have synergistic effects on TLS bacteriostasis. Compared to TLS, the formulation had a better preservation in milk (pH 6.64) by viable microbial counting, although the antibacterial effect of the formulation with TLS showed no significant difference in apple juice (pH 3.98). Therefore, the TLS formulation has wide application and better potential, as a preservative, to enhance food safety.

Expression of bacteriocin divercin AS7 in Escherichia coli and its functional analysis

Bacteriocins are small peptides with antimicrobial activity, that are produced by bacteria. Four classes of bacteriocins produced by lactic acid bacteria have been defined. Class IIa bacteriocins are promising candidates for industrial applications due to their high biological activity and their physicochemical properties. Divercin AS7 is a class IIa bacteriocin produced by Carnobacterium divergens AS7. It shows antibacterial activity against pathogens and food spoilage flora including Listeria spp. Little is known about the impact of class IIa bacteriocins upon eukaryotic cells. The safe use of bacteriocins as food biopreservatives requires the absence of cytotoxicity to human cells. To analyze the impact of divercin AS7 on human enterocytes, we expressed the recombinant divercin AS7 in the Escherichia coli BL21DE3pLys strain and conducted in vitro studies to evaluate the safety of recombinant divercin AS7. No cytotoxic effect on differentiated monolayer Caco-2 cells and no apoptotic appearance were observed when recombinant divercin AS7 was used at a concentration of 2 μg ml⁻¹. In our study, divercin AS7 also did not interfere with differentiated Caco-2 cells monolayer integrity. The obtained results suggest that divercin AS7 is a promising peptide for the food industry.

Probiotics and its functionally valuable products-a review

During the past two decades probiotic bacteria have been increasingly proposed as health promoting bacteria in variety of food system, because of its safety, functional, and technological characteristics. Commonly, Lactobacillus spp., Bifidobacterium spp., Saccharomyces boulardii, and some other microorganisms have been considered as probiotic strains. Possibly these bacterial strains exerted several beneficial effects into gastrointestinal tract of host while administered with variety of food system. Lactic acid bacteria (LAB) usually produce antimicrobial substances like bacteriocin which have broad spectrum of antagonist effect against closely related Gram positive and Gram negative pathogens. LAB strains often produce polymeric substances such as exopolysaccharides (EPS) which increase the colonization of probiotic bacteria by cell-cell interactions in gastrointestinal tract. LAB also produces biosurfactant which showed that the wide range of antimicrobial activity against bacterial pathogen as well as its antiadhesive properties reduces the adhesion of pathogens into gastric wall membrane. Furthermore, LAB strains have also been reported for production of antioxidants which are ability to scavenge the free radicals such as superoxide anions and hydroxyl radicals. For this sense, this review article is mainly focused on the ecology, biosynthesis, genetics, target sites, and applications of bacteriocins and EPS from LAB strains. Moreover, this review discusses about the production and functions of nutritive essential element folate and iron chelating agent such as siderophores from LAB.

Bactofencin A, a new type of cationic bacteriocin with unusual immunity

Bacteriocin production is an important probiotic trait of intestinal bacteria. In this study, we identify a new type of bacteriocin, bactofencin A, produced by a porcine intestinal isolate Lactobacillus salivarius DPC6502, and assess its potency against pathogenic species including Staphylococcus aureus and Listeria monocytogenes. Genome sequencing of the bacteriocin producer revealed bfnA, which encodes the mature and highly basic (pI 10.59), 22-amino-acid defensin-like peptide. Matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectral analysis determined that bactofencin A has a molecular mass of 2,782 Da and contains two cysteine residues that form an intramolecular disulfide bond. Although an ABC transporter and transport accessory protein were also present within the bacteriocin gene cluster, a classical bacteriocin immunity gene was not detected. Interestingly, a dltB homologue was identified downstream of bfnA. DltB is usually encoded within the dlt operon of many Gram-positive bacteria. It is responsible for d-alanylation of teichoic acids in the cell wall and has previously been associated with bacterial resistance to cationic antimicrobial peptides. Heterologous expression of this gene conferred bactofencin A-specific immunity on sensitive strains of L. salivarius and S. aureus (although not L. monocytogenes), establishing its role in bacteriocin immunity. An analysis of the distribution of bfnA revealed that it was present in four additional isolates derived from porcine origin and absent from five human isolates, suggesting that its distribution is host specific. Given its novelty, we anticipate that bactofencin A represents the prototype of a new class of bacteriocins characterized as being cationic, with a DltB homologue providing a cognate immunity function.

This study describes the identification, purification, and characterization of bactofencin A, a novel type of bacteriocin produced by L. salivarius DPC6502. Interestingly, bactofencin A is not similar to any other known bacteriocin but instead shares similarity with eukaryotic cationic antimicrobial peptides, and here, we demonstrate that it inhibits two medically significant pathogens. Genome sequence analysis of the producing strain also revealed the presence of an atypical dltB homologue in the bacteriocin gene cluster, which was lacking a classical bacteriocin immunity gene. Furthermore, cloning this gene rendered sensitive strains resistant to the bacteriocin, thereby establishing its role in providing cognate bacteriocin immunity. Four additional L. salivarius isolates, also of porcine origin, were found to contain the bacteriocin biosynthesis genes and successfully produced bactofencin A, while these genes were absent from five human-derived strains investigated.

Bacterial behaviors associated with the quorum-sensing peptide pheromone ('alarmone') in streptococci

Streptococci are among the predominant bacterial species living in the human body. They are normally harmless bacteria, but have the ability to cause diverse infections, ranging from mild (e.g., tooth decay and sore throat) to life-threatening (e.g., endocarditis and meningitis). Streptococci have evolved various means of coping with the deleterious effects of environmental stressors and avoiding the host immune system. Recently, several studies have shown that streptococci colonizing the mouth and upper respiratory tract are able to mount complex stress responses in order to persist and successfully survive competition in their ecological niche. Using a small quorum-sensing peptide pheromone acting as a stress-inducible 'alarmone', oral streptococci synchronize the gene expression of a specific group of cells to coordinate important biological activities.

The HtrA protease of Streptococcus pneumoniae controls density-dependent stimulation of the bacteriocin blp locus via disruption of pheromone secretion

All fully sequenced strains of Streptococcus pneumoniae (pneumococcus) contain a version of the blp locus which is responsible for the regulation and secretion of a variable repertoire of pneumococcal bacteriocins called pneumocins and their associated immunity proteins. Pneumocins mediate intra- and interspecies competition in vitro and have been shown to provide a competitive advantage in vivo. Pneumocin production is stimulated by extracellular accumulation of the peptide pheromone, BlpC. Both BlpC and the functional pneumocins are secreted out of the cell via the Blp transporter, BlpAB. The conserved surface-expressed serine protease, HtrA, has been shown to limit activation of the locus and secretion of functional pneumocins. In this work, we demonstrate that htrA mutants stimulate the blp locus at lower cell density and to a greater extent than strains expressing wild-type HtrA. This effect is not due to direct proteolytic degradation of secreted pheromone by the protease, but instead is a result of HtrA-mediated disruption of peptide processing and secretion. Because pneumocins are secreted through the same transporter as the pheromone, this finding explains why pheromone supplementation cannot completely restore pneumocin inhibition to strains expressing high levels of HtrA despite restoration of blp transcriptional activity. HtrA restricts pneumocin production to high cell density by limiting the rate of accumulation of BlpC in the environment. Importantly, HtrA does not interfere with the ability of a strain to sense environmental pheromones, which is necessary for the induction of protective immunity in the face of pneumocin-secreting competitors.

Bifunctional gene cluster lnqBCDEF mediates bacteriocin production and immunity with differential genetic requirements

A comprehensive gene disruption of lacticin Q biosynthetic cluster lnqQBCDEF was carried out. The results demonstrated the necessity of the complete set of lnqQBCDEF for lacticin Q production, whereas immunity was flexible, with LnqEF (ABC transporter) being essential for and LnqBCD partially contributing to immunity.

Class IIa bacteriocins: diversity and new developments

Class IIa bacteriocins are heat-stable, unmodified peptides with a conserved amino acids sequence YGNGV on their N-terminal domains, and have received much attention due to their generally recognized as safe (GRAS) status, their high biological activity, and their excellent heat stability. They are promising and attractive agents that could function as biopreservatives in the food industry. This review summarizes the new developments in the area of class IIa bacteriocins and aims to provide uptodate information that can be used in designing future research.

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.

New type non-lantibiotic bacteriocins: circular and leaderless bacteriocins

Bacteriocins are antimicrobial peptides that are ribosomally synthesised by bacteria. Bacteriocins produced by Gram-positive bacteria, including lactic acid bacteria, are under focus as the next generation of safe natural biopreservatives and as therapeutic alternatives to antibiotics. Recently, two novel types of non-lantibiotic class II bacteriocins have been reported with unique characteristics in their structure and biosynthesis mechanism. One is a circular bacteriocin that contains a head-to-tail structure in the mature form, and the other is a leaderless bacteriocin without an N-terminal extension in the precursor peptide. A circular structure can provide the peptide with remarkable stability against various stresses; indeed, circular bacteriocins are known to possess higher stability than general linear bacteriocins. Leaderless bacteriocins are distinct from general bacteriocins, because they do not contain N-terminal leader sequences, which are responsible for the recognition process during secretion and for inactivation of bacteriocins inside producer cells. Leaderless bacteriocins do not require any post-translational processing for activity. These two novel types of bacteriocins are promising antimicrobial compounds, and their biosynthetic mechanisms are expected to be applied in synthetic biology to design new peptides and for new mass production systems. However, many questions remain about their biosynthesis. In this review, we introduce recent studies on these types of bacteriocins and their potential to open a new world of antimicrobial peptides.

In vitro and in vivo characterization and strain safety of Lactobacillus reuteri NCIMB 30253 for probiotic applications

Lactobacillus reuteri NCIMB 30253 was shown to have potential as a probiotic by reducing the proinflammatory chemokine interleukin-8. Moreover, this strain was evaluated, by in vitro and in vivo techniques, for its safety for human consumption. The identity of the strain was investigated by metabolic profiling and 16S rRNA gene sequencing, and in vitro safety evaluations were performed by molecular and metabolic techniques. Genetic analysis was confirmed by assessing the minimum inhibitory concentration to a panel of antibiotics, showing that the strain was susceptible to 8 antibiotics tested. The ability of the strain to produce potentially harmful by-products and antimicrobial compounds was evaluated, showing that the strain does not produce biogenic amines and does not show bacteriocin activity or reuterin production. A 28-day repeated oral dose study was conducted in normal Sprague–Dawley rats to support the in vivo strain safety. Oral administration of the strain resulted in no changes in general condition and no clinically significant changes to biochemical and haematological markers of safety relative to vehicle control treated animals. This comprehensive assessment of safety of L. reuteri NCIMB 30253 supports the safety of the strain for use as a probiotic.

Antibacterial activity of class IIa bacteriocin Cbn BM1 depends on the physiological state of the target bacteria

Carnobacteriocin BM1 (Cbn BM1) is a class IIa bacteriocin produced by Carnobacterium maltaromaticum CP5 isolated from a French mold ripened cheese. Numerous studies highlight variations in numerous parameters, such as bacterial membrane composition and potential, according to physiological changes. In this work, the mechanism of action of an oxidized form of Cbn BM1 was studied on C. maltaromaticum DSM20730 in log and stationary growth phases. Membrane integrity assessment and high resolution imaging by atomic force microscopy confirmed the link between physiological state and bacterial sensitivity to Cbn BM1. Indeed, these approaches enable visualizing morphological damage of C. maltaromaticum DSM20730 only in an active dividing state. To specifically address the interaction between peptide and bacterial membrane, fluorescence anisotropy measurements were conducted. Results revealed strong modifications in membrane fluidity by Cbn BM1 only for C. maltaromaticum DSM20730 in log growth phase. In a similar way, the Δψ component, but not the ΔpH component of the proton-motive force, was perturbed only for bacteria in log growth phase. These results clearly show that a class IIa bacteriocin antimicrobial mechanism of action can be modulated by the physiological state of its target bacteria.

Purification and characterization of multiple bacteriocins and an inducing peptide produced by Enterococcus faecium NKR-5-3 from Thai fermented fish

Enterocins NKR-5-3A, B, C, and D were purified from the culture supernatant of Enterococcus faecium NKR-5-3 and characterized. Among the four purified peptides, enterocin NKR-5-3A (5242.3 Da) was identical to brochocin A, produced by Brochothrix campestris ATCC 43754, in mature peptides, and its putative synergistic peptide, enterocin NKR-5-3Z, was found to be encoded in ent53Z downstream of ent53A, encoding enterocin NKR-5-3A. Enterocin NKR-5-3B (6316.4 Da) showed a broad antimicrobial spectrum, and enterocin NKR-5-3C (4512.8 Da) showed high activity against Listeria. Enterocin NKR-5-3D (2843.5 Da), showing high homology to an inducing peptide produced by Lactobacillus sakei 5, induced the production of the enterocins. The enterocins showed different antimicrobial spectra and intensities. E. faecium NKR-5-3 concomitantly produced enterocins NKR-5-3A, B, C, and D which probably belong to different classes of bacteriocins. Furthermore, NKR-5-3 production was induced by enterocin NKR-5-3D.