Identification of a new plasmid-encoded sec-dependent bacteriocin produced by Listeria innocua 743

Injectisome T3SS subunits as potential chaperones in the extracellular export of Pectobacterium carotovorum subsp. carotovorum bacteriocins Carocin S1 and Carocin S3 secreted via flagellar T3SS

Pectobacterium carotovorum subsp. carotovorum (Pcc) causes soft-rot disease in a wide variety of plants resulting in economic losses worldwide. It produces various types of bacteriocin to compete against related plant pathogens. Studies on how bacteriocins are extracellularly secreted are conducted to understand the mechanism of interbacterial competition. In this study, the secretion of the low-molecular-weight bacteriocins (LMWB) Carocin S1 and Carocin S3 produced by a multiple-bacteriocin producing strain of Pcc, 89-H-4, was investigated. Tn5 insertional mutagenesis was used to generate a mutant, TH22–6, incapable of LMWBs secretion. Sequence and homology analyses of the gene disrupted by transposon Tn5 insertion revealed that the gene sctT, an essential component of the injectisome type III secretion machinery (T3aSS), is required for the secretion of the bacteriocins. This result raised a question regarding the nature of the secretion mechanism of Pcc bacteriocins which was previously discovered to be secreted via T3bSS, a system that utilizes the bacterial flagellum for extracellular secretions. Our previous report has shown that bacteriocin Carocin S1 cannot be secreted by mutants that are defective of T3bSS-related genes such as flhA, flhC, flhD and fliC. We knocked out several genes making up the significant structural components of both T3aSS and T3bSS. The findings led us to hypothesize the potential roles of the T3aSS-related proteins, SctT, SctU and SctV, as flagellar T3SS chaperones in the secretion of Pcc bacteriocins. This current discovery and the findings of our previous study helped us to conceptualize a unique Type III secretion system for bacteriocin extracellular export which is a hybrid of the injectisome and flagellar secretion systems.

Plasmidome of Listeria spp.-The repA-Family Business

Bacteria of the genus Listeria (phylum Firmicutes) include both human and animal pathogens, as well as saprophytic strains. A common component of Listeria spp. genomes are plasmids, i.e., extrachromosomal replicons that contribute to gene flux in bacteria. This study provides an in-depth insight into the structure, diversity and evolution of plasmids occurring in Listeria strains inhabiting various environments under different anthropogenic pressures. Apart from the components of the conserved plasmid backbone (providing replication, stable maintenance and conjugational transfer functions), these replicons contain numerous adaptive genes possibly involved in: (i) resistance to antibiotics, heavy metals, metalloids and sanitizers, and (ii) responses to heat, oxidative, acid and high salinity stressors. Their genomes are also enriched by numerous transposable elements, which have influenced the plasmid architecture. The plasmidome of Listeria is dominated by a group of related replicons encoding the RepA replication initiation protein. Detailed comparative analyses provide valuable data on the level of conservation of these replicons and their role in shaping the structure of the Listeria pangenome, as well as their relationship to plasmids of other genera of Firmicutes, which demonstrates the range and direction of flow of genetic information in this important group of bacteria.

Bacteriocins of Lactic Acid Bacteria as Potent Antimicrobial Peptides against Food Pathogens

An ever-growing demand for food products with minimal chemical additives has generated a necessity for exploring new alternatives for food preservation. In this context, more recently, bacteriocins, the peptides having antimicrobial property, synthesized ribosomally by numerous bacteria have been attracting a lot of attention. They are known to possess the potential to restrict the growth of microorganisms causing food spoilage without causing any harm to the bacteria themselves owing to the presence of self-defensive proteins. In particular, the bacteriocins of lactic acid bacteria have been considered harmless and safe for consumption and are indicated to evade the development of unwanted bacteria. Use of bacteriocins as biopreservatives has been studied in various food industries, and they have been established to elevate the shelf life of minimally processed food items by exerting killing mechanism. They restrict the growth of undesirable bacteria by breaking the target cell membrane and finally resulting into pore formation. The current article provides an insight on bacteriocins of lactic acid bacteria, their biosynthesis, mechanism of action, and promising applications of these antimicrobial peptides in the food sector.

Pediocin-Like Antimicrobial Peptides of Bacteria

Bacteriocins are bacterial antimicrobial peptides that, unlike classical peptide antibiotics, are products of ribosomal synthesis and usually have a narrow spectrum of antibacterial activity against species closely related to the producers. Pediocin-like bacteriocins (PLBs) belong to the class IIa of the bacteriocins of Gram-positive bacteria. PLBs possess high activity against pathogenic bacteria from Listeria and Enterococcus genera. Molecular target for PLBs is a membrane protein complex - bacterial mannose-phosphotransferase. PLBs can be synthesized by components of symbiotic microflora and participate in the maintenance of homeostasis in various compartments of the digestive tract and on the surface of epithelial tissues contacting the external environment. PLBs could give a rise to a new group of antibiotics of narrow spectrum of activity.

Lactolisterin BU, a Novel Class II Broad-Spectrum Bacteriocin from Lactococcus lactis subsp. lactis bv. diacetylactis BGBU1-4

Lactococcus lactis subsp. lactis bv. diacetylactis BGBU1-4 produces a novel bacteriocin, lactolisterin BU, with strong antimicrobial activity against many species of Gram-positive bacteria, including important food spoilage and foodborne pathogens, such as Listeria monocytogenes, Staphylococcus aureus, Bacillus spp., and streptococci. Lactolisterin BU was extracted from the cell surface of BGBU1-4 by 2-propanol and purified to homogeneity by C₁₈ solid-phase extraction and reversed-phase high-performance liquid chromatography. The molecular mass of the purified lactolisterin BU was 5,160.94 Da, and an internal fragment, AVSWAWQH, as determined by N-terminal sequencing, showed low-level similarity to existing antimicrobial peptides. Curing and transformation experiments revealed the presence of a corresponding bacteriocin operon on the smallest plasmid, pBU6 (6.2 kb), of strain BGBU1-4. Analysis of the bacteriocin operon revealed a leaderless bacteriocin of 43 amino acids that exhibited similarity to bacteriocin BHT-B (63%) from Streptococcus ratti, a bacteriocin with analogy to aureocin A.IMPORTANCE Lactolisterin BU, a broad-spectrum leaderless bacteriocin produced by L. lactis subsp. lactis bv. diacetylactis BGBU1-4, expresses strong antimicrobial activity against food spoilage and foodborne pathogens, such as Listeria monocytogenes, Staphylococcus aureus, Bacillus spp., and streptococci. Lactolisterin BU showed the highest similarity to aureocin-like bacteriocins produced by different bacteria. The operon for synthesis is located on the smallest plasmid, pBU6 (6.2 kb), of strain BGBU1-4, indicating possible horizontal transfer among producers.

Maltaricin CPN, a new class IIa bacteriocin produced by Carnobacterium maltaromaticum CPN isolated from mould-ripened cheese

AIMS

The purpose of this study was to isolate, characterize and determine the structure and the antibacterial activities of a bacteriocin produced by Carnobacterium maltaromaticum CPN, a strain isolated from unpasteurized milk Camembert cheese.

METHODS AND RESULTS

This bacteriocin, termed maltaricin CPN, was produced at higher amounts in MRS broth at temperatures between 15°C and 25°C. It was purified to homogeneity from culture supernatant by using a simple method consisting of cation-exchange and reversed-phase chromatographies. Mass spectrometry showed that maltaricin was a 4427·29 Da bacteriocin. Its amino acid sequence was determined by Edman degradation which showed that it had close similarity with bacteriocins of the class IIa. Maltaricin CPN consisted in fact of 44 unmodified amino acids including two cysteine residues at positions 9 and 14 linked by a disulphide bond. The antimicrobial activity of maltaricin CPN covered a range of bacteria, with strong activity against many species of Gram-positive bacteria, especially the food-borne pathogen Listeria monocytogenes, but no activity against Gram-negative ones.

CONCLUSIONS

In the studied conditions, C. maltaromaticum CPN produced a new class IIa bacteriocin with strong anti-Listeria activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study covers the purification and the structural characterization of a new bacteriocin produced by strain C. maltaromaticum CPN isolated from Camembert cheese. Its activity against strains of L. monocytogenes and higher production rates at relatively low temperatures show potential technological applications to improve the safety of refrigerated food.

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.

Genetic characterisation and heterologous expression of leucocin C, a class IIa bacteriocin from Leuconostoc carnosum 4010

Leuconostoc carnosum 4010 is a protective culture for meat products. It kills the foodborne pathogen Listeria monocytogenes by producing two class IIa (pediocin-like) bacteriocins, leucocin A and leucocin C. The genes for leucocin A production have previously been characterised from Leuconostoc gelidum UAL 187, whereas no genetic studies about leucocin C has been published. Here, we characterised the genes for the production of leucocins A and C in L. carnosum 4010. In this strain, leucocin A and leucocin C operons were localised in different plasmids. Unlike in L. gelidum, leucocin A operon in L. carnosum 4010 only contained the structural and the immunity genes lcaAB without transporter genes lcaECD. On the contrary, leucocin C cluster included two intact operons. Novel genes lecCI encode the leucocin C precursor and the 97-aa immunity protein LecI, respectively. LecI shares 48 % homology with the immunity proteins of sakacin P and listeriocin. Another leucocin C operon lecXTS, encoding an ABC transporter and an accessory protein, was 97 % identical with the leucocin A transporter operon lcaECD of L. gelidum. For heterologous expression of leucocin C in Lactococcus lactis, the mature part of the lecC gene was fused with the signal sequence of usp45 in the secretion vector pLEB690. L. lactis secreted leucocin C efficiently, as shown by large halos on lawns of L. monocytogenes and Leuconostoc mesenteroides indicators. The function of LecI was then demonstrated by expressing the gene lecI in L. monocytogenes. LecI-producing Listeria was less sensitive to leucocin C than the vector strain, thus corroborating the immunity function of LecI.

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.

Characterization of putative class II bacteriocins identified from a non-bacteriocin-producing strain Lactobacillus casei ATCC 334

Several putative class II bacteriocin-like genes were identified in Lactobacillus casei ATCC 334, all of which might encode peptides with a double-glycine leader. Six peptides encoded by these genes were heterologously expressed in Escherichia coli and then partially purified in order to test their bacteriocin activity. The results revealed that the mature LSEI_2163 peptide was a class IId bacteriocin that exhibited antimicrobial activity against some lactobacilli and several Listeria species. Similarly, mature LSEI_2386 was a putative pheromone peptide that also had significant bacteriocin activity against several Listeria species. The activities of both peptides tolerated 121°C for 30 min but not treatment with proteinase K or trypsin. The two Cys residues located at positions 4 and 24 in the mature LSEI_2163 peptide were shown by mass spectrometry to form a disulfide bridge, which was required for optimal antibacterial activity. However, replacement of one or both Cys with Ser would cause significant reduction of the antibacterial activity, the reduction being greater when only one of the Cys residues (C4S) was replaced than when both (C4S/C24S) were replaced.

Insights into structure-activity relationships in the C-terminal region of divercin V41, a class IIa bacteriocin with high-level antilisterial activity

Divercin V41 (DvnV41) is a class IIa bacteriocin with potent antilisterial activity isolated from Carnobacterium divergens V41. Previously, we expressed from a synthetic gene, in Escherichia coli Origami, a recombinant DvnV41 designated DvnRV41, which possesses four additional amino acids (AMDP) in the N-terminal region that result from enzymatic cleavage and retains the initial DvnV41 activity. To unravel the relationship between the structure of DvnRV41 and its particularly elevated activity, we produced by site-directed mutagenesis eight variants in which a single amino acid replacement was specifically introduced into the sequence. The point mutations were designed to change either conserved residues in class IIa bacteriocins or residues specific to DvnV41 located mainly in the C-terminal region. The fusion proteins were purified from the cytosoluble fractions by immobilized affinity chromatography. DvnRV41 and its variants were released from the fusion proteins by enzymatic cleavage, using enterokinase. The purity of DvnRV41 and of the variants was checked by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, high-performance liquid chromatography, and mass spectrometry. The antibacterial activity of DvnRV41 and its variants was assessed using different indicator strains, including Listeria monocytogenes EGDe and Enterococcus faecalis JH2-2. The activity of all of the variants appeared to be less than the activity of DvnRV41. The decrease in activity did not appear to be related to a global conformational change, as determined by circular dichroism. Overall, the variants of DvnRV41 produced in the present study provide interesting insights into structure-activity relationships of class IIa bacteriocins.

Crystal structure and mutagenic analysis of a bacteriocin immunity protein, Mun-im

Bacteriocin-producing lactic acid bacteria (LAB) possess a self-protection factor, which is generally called an immunity protein. In this study, we determine the crystal structure of an immunity protein, designated Mun-im, which was classified into subgroup B immunity proteins for class IIa bacteriocins. The Mun-im protein takes a left-turning antiparallel four-helix bundle structure with the flexible N- and C-terminal parts. Although the amino acid sequences of the subgroup B immunity proteins are distinguished from those of the subgroup A, the core structure of Mun-im is well-superimposed with that of the subgroup A immunity protein, EntA-im, and the C-terminus of both proteins is flexible. However, the C-terminus of Mun-im is obviously shorter than that of the subgroup A. We found through mutagenic study of Mun-im that the C-terminus and the K86 residue on the helix 4 in the immunity protein molecule are important for expression of the immunity activity on the subgroup B immunity proteins.

Mutational analysis of the class IIa bacteriocin curvacin A and its orientation in target cell membranes

To analyze the orientation in target cell membranes of the pediocin-like bacteriocin (antimicrobial peptide) curvacin A, 55 variants were generated by site-directed mutagenesis and their potencies against four different target cells determined. The result suggest that the somewhat hydrophilic short central helix (residues 19 to 24), along with the N-terminal beta-sheet-like structure (residues 1 to 16), inserts in the interface region of the target cell membrane, with Ala22 close to the hydrophobic core of the membrane. The following hinge region, with Gly28 as an important residue, may then form a turn wherein Gly28 becomes positioned near the border between the interface and the hydrophobic regions, thus permitting the longer and more-hydrophobic C-terminal helix (residues 29 to 41) to insert into the hydrophobic core of the membrane. This helix contains three glycine residues (G33, G37, and G40) that form a putative helix-helix-interacting GxxxGxxG motif. The replacement of any of these glycines with a larger residue was very detrimental, suggesting their possible involvement in helix-helix interactions with a membrane-embedded receptor protein.

Nuclear magnetic resonance solution structure of PisI, a group B immunity protein that provides protection against the type IIa bacteriocin piscicolin 126, PisA

Lactic acid bacteria produce and secrete bacteriocins. These bacteriocins are potent antimicrobial peptides that are active against other closely related bacteria. As a means of self-protection, producer organisms also express immunity proteins. Immunity proteins are generally located on the same genetic locus and are cotranscribed with the bacteriocin. Although some cross immunity between bacteriocins has been observed, immunity proteins are typically highly specific. Immunity proteins for the type IIa bacteriocins range from 81 to 115 amino acids in length and display substantial variation in their sequences. Nonetheless, such immunity proteins have been classified into three groupings (groups A, B, and C) according to sequence homology. The structures of a group C (ImB2) and two group A (EntA-im and PedB) immunity proteins have previously been reported. We herein report the nuclear magnetic resonance solution structure of the remaining class of the type IIa immunity proteins. PisI, a 98-amino acid protein, is a group B immunity protein conferring immunity against piscicolin 126 (PisA). Like ImB2, EntA-im, and PedB, PisI folds into a globular protein in aqueous solution and contains an antiparallel four-helix bundle. Compared to ImB2 and EntA-im, PisI has a substantially longer and more flexible N-terminus, but a shorter C-terminus. No direct interaction between the bacteriocin and immunity protein is observed by NMR in either aqueous or membrane mimicking environments. This further suggests that the mechanism that mediates immunity is not due to a direct bacteriocin-immunity protein interaction but rather is receptor-mediated. It has now been confirmed that the four-helix bundle is indeed a structural motif among the type IIa immunity proteins.

Genetic analysis of bacteriocin 43 of vancomycin-resistant Enterococcus faecium

A total of 636 vancomycin-resistant Enterococcus faecium (VRE) isolates obtained between 1994 and 1999 from the Medical School Hospital of the University of Michigan were tested for bacteriocin production. Of the 277 (44%) bacteriocinogenic strains, 21 were active against E. faecalis, E. faecium, E. hirae, E. durans, and Listeria monocytogenes. Of those 21 strains, a representative bacteriocin of strain VRE82, designated bacteriocin 43, was found to be encoded on mobilizable plasmid pDT1 (6.2 kbp). Nine open reading frames (ORFs), ORF1 to ORF9, were presented on pDT1 and were oriented in the same direction. The bacteriocin 43 locus (bac43) consists of the bacteriocin gene bacA (ORF1) and the immunity gene bacB (ORF2). The deduced bacA product is 74 amino acids in length with a putative signal peptide of 30 amino acids at the N terminus. The bacB gene encodes a deduced 95-amino-acid protein without a signal sequence. The predicted mature BacA protein (44 amino acids) showed sequence homology with the membrane-active class IIa bacteriocins of lactic acid bacteria and showed 86% homology with bacteriocin 31 from E. faecalis YI717 and 98% homology with bacteriocin RC714. Southern analysis with a bac43 probe of each plasmid DNA from the 21 strains showed hybridization to a specific fragment corresponding to the 6.2-kbp EcoRI fragment, suggesting that the strains harbored the pDT1-like plasmid (6.2 kb) which encoded the bacteriocin 43-type bacteriocin. The bac43 determinant was not identified among non-VRE clinical isolates.

The continuing story of class IIa bacteriocins

Many bacteria produce antimicrobial peptides, which are also referred to as peptide bacteriocins. The class IIa bacteriocins, often designated pediocin-like bacteriocins, constitute the most dominant group of antimicrobial peptides produced by lactic acid bacteria. The bacteriocins that belong to this class are structurally related and kill target cells by membrane permeabilization. Despite their structural similarity, class IIa bacteriocins display different target cell specificities. In the search for new antibiotic substances, the class IIa bacteriocins have been identified as promising new candidates and have thus received much attention. They kill some pathogenic bacteria (e.g., Listeria) with high efficiency, and they constitute a good model system for structure-function analyses of antimicrobial peptides in general. This review focuses on class IIa bacteriocins, especially on their structure, function, mode of action, biosynthesis, bacteriocin immunity, and current food applications. The genetics and biosynthesis of class IIa bacteriocins are well understood. The bacteriocins are ribosomally synthesized with an N-terminal leader sequence, which is cleaved off upon secretion. After externalization, the class IIa bacteriocins attach to potential target cells and, through electrostatic and hydrophobic interactions, subsequently permeabilize the cell membrane of sensitive cells. Recent observations suggest that a chiral interaction and possibly the presence of a mannose permease protein on the target cell surface are required for a bacteria to be sensitive to class IIa bacteriocins. There is also substantial evidence that the C-terminal half penetrates into the target cell membrane, and it plays an important role in determining the target cell specificity of these bacteriocins. Immunity proteins protect the bacteriocin producer from the bacteriocin it secretes. The three-dimensional structures of two class IIa immunity proteins have been determined, and it has been shown that the C-terminal halves of these cytosolic four-helix bundle proteins specify which class IIa bacteriocin they protect against.

Mutational analysis and membrane-interactions of the β-sheet-like N-terminal domain of the pediocin-like antimicrobial peptide sakacin P

To gain insight into how the N-terminal three-stranded beta-sheet-like domain in pediocin-like antimicrobial peptides positions itself on membranes, residues in the well-conserved (Y)YGNGV-motif in the domain were substituted and the effect of the substitutions on antimicrobial activity and binding of peptides to liposomes was determined. Peptide-liposome interactions were detected by measuring tryptophan-fluorescence upon exposing liposomes to peptides in which a tryptophan residue had been introduced in the N-terminal domain. The results revealed that the N-terminal domain associates readily with anionic liposomes, but not with neutral liposomes. The electrostatic interactions between peptides and liposomes facilitated the penetration of some of the peptide residues into the liposomes. Measuring the antimicrobial activity of the mutated peptides revealed that the Tyr2Leu and Tyr3Leu mutations resulted in about a 10-fold reduction in activity, whereas the Tyr2Trp, Tyr2Phe, Tyr3Trp and Tyr3Phe mutations were tolerated fairly well, especially the mutations in position 3. The Val7Ile mutation did not have a marked detrimental effect on the activity. The Gly6Ala mutation was highly detrimental, consistent with Gly6 being in one of the turns in the beta-sheet-like N-terminal domain, whereas the Gly4Ala mutation was tolerated fairly well. All mutations involving Asn5, including the conservative mutations Asn5Gln and Asn5Asp, were very deleterious. Thus, both the polar amide group on the side chain of Asn5 and its exact position in space were crucial for the peptides to be fully active. Taken together, the results are consistent with Val7 positioning itself in the hydrophobic core of target membranes, thus forcing most of the other residues in the N-terminal domain into the membrane interface region: Tyr3 and Asn5 in the lower half with their side chains pointing downward and approaching the hydrophobic core, Tyr2, Gly4 and His8 and 12 in the upper half, Lys1 near the middle of the interface region, and the side chain of Lys11 pointing out toward the membrane surface.

Concentrations and tracking of listeria monocytogenes strains in a seafood-processing environment using a most-probable-number enrichment procedure and randomly amplified polymorphic DNA analysis

Concentrations of environmental microflora and Listeria monocytogenes were monitored at multiple environmental locations within a seafood-processing facility over the course of 6 months. Concentrations of L. monocytogenes were determined using a most-probable-number (MPN) enrichment procedure. Two floor drains had persistent low concentrations of L. monocytogenes (0.03 to >1,100 MPN/cm2). In comparison, concentrations of the other organisms in the drain were much higher (heterotrophic plate count range of 10(5) to 10(8) CFU/cm2). Concentrations of environmental organisms (heterotrophic aerobic plate counts and counts of pseudomonads, Shewanella spp., Aeromonas hydrophila, and coliforms) were not correlated with concentrations of L. monocytogenes. The 178 confirmed L. monocytogenes isolates from the MPN procedure were further characterized by randomly amplified polymorphic DNA analysis. Sixteen different banding patterns were identified, and nine of the patterns were identified from samples collected on two or more collection dates. From all locations, banding type A was observed in 98 confirmed isolates (55%). Although present, L. monocytogenes was a relatively minor component in the ecosystem of the floor drains in this seafood-processing facility.

Pediocin-like antimicrobial peptides (class IIa bacteriocins) and their immunity proteins: biosynthesis, structure, and mode of action

Pediocin-like antimicrobial peptides (AMPs) form a group of lactic acid bacteria produced, cationic membrane-permeabilizing peptides with 37 to 48 residues. Upon exposure to membrane-mimicking entities, their hydrophilic, cationic, and highly conserved N-terminal region forms a three-stranded antiparallel beta-sheet supported by a conserved disulfide bridge. This N-terminal beta-sheet region is followed by a central amphiphilic alpha-helix and this in most (if not all) of these peptides is followed by a rather extended C-terminal tail that folds back onto the central alpha-helix, thereby creating a hairpin-like structure in the C-terminal half. There is a flexible hinge between the beta-sheet N-terminal region and the hairpin C-terminal region and one thus obtains two domains that may move relative to each other. The cationic N-terminal beta-sheet domain mediates binding of the pediocin-like AMPs to the target-cell surface through electrostatic interactions, while the more hydrophobic and amphiphilic C-terminal hairpin domain penetrates into the hydrophobic part of the target-cell membrane, thereby mediating leakage through the membrane. The hinge provides the structural flexibility that enables the C-terminal hairpin domain to dip into the hydrophobic part of the membrane. Despite extensive sequence similarities, these AMPs differ markedly in their target-cell specificity, and results obtained with hybrid AMPs indicate that the membrane-penetrating hairpin-like C-terminal domain is the major specificity determinant. Bacteria that produce pediocin-like AMPs also produce a 11-kDa cognate immunity protein that protects the producer. The immunity proteins are well-structured, 4-helix bundle cytosolic proteins. They show a high degree of specificity in that they largely recognize and confer immunity only to their cognate AMP and in some cases to a few AMPs that are closely related to their cognate AMP. The C-terminal half of the immunity proteins contains a domain that is involved in specific recognition of the C-terminal membrane-penetrating specificity-determining hairpin domain of the cognate AMP.

Characterization of a new bacteriocin operon in sakacin P-producing Lactobacillus sakei, showing strong translational coupling between the bacteriocin and immunity genes

Previous studies of genes involved in the production of sakacin P by Lactobacillus sakei Lb674 revealed the presence of an inducible promoter downstream of the known spp gene clusters. We show here that this promoter drives the expression of an operon consisting of a bacteriocin gene (sppQ), a cognate immunity gene (spiQ), another gene with an unknown function (orf4), and a pseudoimmunity gene containing a frameshift mutation (orf5). The leader peptide of the new one-peptide bacteriocin sakacin Q contains consensus elements that are typical for so-called "double-glycine" leader peptides. The mature bacteriocin shows weak similarity to the BrcA peptide of the two-peptide bacteriocin brochocin C. Sakacin Q has an antimicrobial spectrum that differs from that of sakacin P, thus expanding the antimicrobial properties of the producer strain. The genes encoding sakacin Q and its cognate immunity protein showed strong translational coupling, which was investigated in detail by analyzing the properties of a series of beta-glucuronidase fusions. Our results provide experimental evidence that production of the bacteriocin and production of the cognate immunity protein are tightly coregulated at the translational level.

Influence of amino acid substitutions in the leader peptide on maturation and secretion of mesentericin Y105 by Leuconostoc mesenteroides

By site-specific mutagenesis, the hydrophobic conserved amino acids and the C-terminal GG doublet of the leader peptide of pre-mesentericin Y105 were demonstrated to be critical for optimal secretion of mesentericin Y105, as well as for the maturation of the pre-bacteriocin by the protease portion of the ABC transporter MesD.

Mutational analysis of mesentericin y105, an anti-Listeria bacteriocin, for determination of impact on bactericidal activity, in vitro secondary structure, and membrane interaction

Mesentericin Y105 is a 37-residue bacteriocin produced by Leuconostoc mesenteroides Y105 that displays antagonistic activity against gram-positive bacteria such as Enterococcus faecalis and Listeria monocytogenes. It is closely related to leucocin A, an antimicrobial peptide containing beta-sheet and alpha-helical structures. To analyze structure-function relationships and the mode of action of this bacteriocin, we generated a collection of mesentericin derivatives. Mutations were obtained mostly by PCR random mutagenesis, and the peptides were produced by an original system of heterologous expression recently described. Ten derivatives were obtained displaying modifications at eight different positions in the mesentericin Y105 sequence. Purified peptides were incorporated into lysophosphatidylcholine micelles and analyzed by circular dichroism. The alpha-helical contents of these peptides were compared and related to their respective bactericidal activities. Moreover, studies of the intrinsic fluorescence of tryptophan residues naturally occurring at positions 18 and 37 revealed information about insertion of the peptides in micelles. A model for the mode of action of mesentericin Y105 and related bacteriocins is proposed.

Butyrivibriocin AR10, a new cyclic bacteriocin produced by the ruminal anaerobe Butyrivibrio fibrisolvens AR10: characterization of the gene and peptide

The gene (bviA) encoding the ruminal bacteriocin butyrivibriocin AR10 was cloned from an EcoRI library by using an oligonucleotide probe based on a partial peptide sequence of the previously isolated peptide. The gene encoded an 80 amino acid prebacteriocin that demonstrated significant identity with the cyclic bacteriocin gassericin A. Negative ion time of flight mass spectroscopic analysis (ESI/MS) indicated a mass of 5981.5 Da for the isolated bacteriocin, a molecular mass that could not be generated by removal of a leader peptide alone. However, an N- to C-terminal cyclization of the predicted mature bacteriocin resulted in a peptide that conformed to the determined mass and charge characteristics. Northern blotting confirmed that expression of bviA mirrored the production of the bacteriocin in both liquid and solid media.

Functional analysis of the gene cluster involved in production of the bacteriocin circularin A by Clostridium beijerinckii ATCC 25752

A region of 12 kb flanking the structural gene of the cyclic antibacterial peptide circularin A of Clostridium beijerinckii ATCC 25752 was sequenced, and the putative proteins involved in the production and secretion of circularin A were identified. The genes are tightly organized in overlapping open reading frames. Heterologous expression of circularin A in Enterococcus faecalis was achieved, and five genes were identified as minimally required for bacteriocin production and secretion. Two of the putative proteins, CirB and CirC, are predicted to contain membrane-spanning domains, while CirD contains a highly conserved ATP-binding domain. Together with CirB and CirC, this ATP-binding protein is involved in the production of circularin A. The fifth gene, cirE, confers immunity towards circularin A when expressed in either Lactococcus lactis or E. faecalis and is needed in order to allow the bacteria to produce bacteriocin. Additional resistance against circularin A is conferred by the activity of the putative transporter consisting of CirB and CirD.

Novel mechanism of bacteriocin secretion and immunity carried out by lactococcal multidrug resistance proteins

A natural isolate of Lactococcus lactis was shown to produce two narrow spectrum class II bacteriocins, designated LsbA and LsbB. The cognate genes are located on a 5.6-kb plasmid within a gene cluster specifying LmrB, an ATP-binding cassette-type multidrug resistance transporter protein. LsbA is a hydrophobic peptide that is initially synthesized with an N-terminal extension. The housekeeping surface proteinase HtrA was shown to be responsible for the cleavage of precursor peptide to yield the active bacteriocin. LsbB is a relatively hydrophilic protein synthesized without an N-terminal leader sequence or signal peptide. The secretion of both polypeptides was shown to be mediated by LmrB. An L. lactis strain lacking plasmid-encoded LmrB and the chromosomally encoded LmrA is unable to secrete either of the two bacteriocins. Complementation of the strain with an active LmrB protein resulted in restored export of the two polypeptides across the cytoplasmic membrane. When expressed in an L. lactis strain that is sensitive to LsbA and LsbB, LmrB was shown to confer resistance toward both bacteriocins. It does so, most likely, by removing the two polypeptides from the cytoplasmic membrane. This is the first report in which a multidrug transporter protein is shown to be involved in both secretion and immunity of antimicrobial peptides.

Identification and characterization of two novel clostridial bacteriocins, circularin A and closticin 574

Two novel antibacterial peptides of clostridial species were purified, N-terminally sequenced, and characterized. Moreover, their structural genes were identified. Closticin 574 is an 82-amino-acid bacteriocin produced by Clostridium tyrobutyricum ADRIAT 932. The supernatant of the producing strain showed a high level of activity against the indicator strain C. tyrobutyricum. The protein is synthesized as a preproprotein that is possibly secreted via the general secretion pathway, after which it is hydrolyzed at an Asp-Pro site. Circularin A is produced by Clostridium beijerinckii ATCC 25752 as a prepeptide of 72 amino acids. Cleavage of the prepeptide between the third leucine and fourth valine residues followed by a head-to-tail ligation between the N and C termini creates a circular antimicrobial peptide of 69 amino acids. The unusually small circularin A leader peptide of three amino acids is cleaved off in this process. The supernatant of C. beijerinckii ATCC 25752 showed a broad antibacterial activity range.

Comparative studies of immunity proteins of pediocin-like bacteriocins

Genes encoding pediocin-like bacteriocins are usually co-transcribed with a gene encoding a cognate immunity protein. To investigate the functionality and specificity of immunity proteins, immunity genes belonging to the bacteriocins curvacin A, enterocin A, enterocin P, leucocin A, pediocin PA-1 and sakacin P, as well as a putative immunity gene, orfY, were expressed in three bacteriocin-sensitive lactic acid bacteria (Lactobacillus sake, Carnobacterium piscicola and Enterococcus faecalis). The transformed indicator strains, each containing one of the immunity genes, were tested for sensitivity towards seven different purified bacteriocins (curvacin A, enterocin A, enterocin P, leucocin A, leucocin C, pediocin PA-1 and sakacin P). Cross-immunity was observed almost exclusively in situations where either the bacteriocins or the immunity proteins belonged to the same sequence-based subgroup. In a few cases, the functionality of immunity proteins was strain-dependent; e.g. the leucocin A immunity gene provided immunity to enterocin A, pediocin PA-1 and leucocin A in Ent. faecalis, whereas in the other two indicators, this gene provided immunity to leucocin A only. The orfY gene, which is transcribed without a cognate bacteriocin, was shown to encode a functional immunity protein that expands the bacteriocin resistance of the strain possessing this gene. The results show that the bacteriocin sensitivity of a lactic acid bacterium strain can depend on (1) the presence of immunity genes in connection with its own bacteriocin production, (2) the presence of extra immunity genes and (3) more general properties of the strain such as the membrane composition or the presence of receptors.