Analogues of bacteriocins: antimicrobial specificity and interactions of leucocin A with its enantiomer, carnobacteriocin B2, and truncated derivatives

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.

Recent Progress in the Chemical Synthesis of Class II and S-Glycosylated Bacteriocins

A wide variety of antimicrobial peptides produced by lactic acid bacteria (LAB) have been identified and studied in the last decades. Known as bacteriocins, these ribosomally synthesized peptides inhibit the growth of a wide range of bacterial species through numerous mechanisms and show a great variety of spectrum of activity. With their great potential as antimicrobial additives and alternatives to traditional antibiotics in food preservation and handling, animal production and in veterinary and medical medicine, the demand for bacteriocins is rapidly increasing. Bacteriocins are most often produced by fermentation but, in several cases, the low isolated yields and difficulties associated with their purification seriously limit their use on a large scale. Chemical synthesis has been proposed for their production and recent advances in peptide synthesis methodologies have allowed the preparation of several bacteriocins. Moreover, the significant cost reduction for peptide synthesis reagents and building blocks has made chemical synthesis of bacteriocins more attractive and competitive. From a protein engineering point of view, the chemical approach offers many advantages such as the possibility to rapidly perform amino acid substitution, use unnatural or modified residues, and make backbone and side chain modifications to improve potency, modify the activity spectrum or increase the stability of the targeted bacteriocin. This review summarized synthetic approaches that have been developed and used in recent years to allow the preparation of class IIa bacteriocins and S-linked glycopeptides from LAB. Synthetic strategies such as the use of pseudoprolines, backbone protecting groups, microwave irradiations, selective disulfide bridge formation and chemical ligations to prepare class II and S-glycosylsated bacteriocins are discussed.

A combined solid- and solution-phase approach provides convenient access to analogues of the calcium-dependent lipopeptide antibiotics

A synthetic route combining solid- and solution-phase techniques allows for the rapid preparation of daptomycin analogues.

Peptide-bacteria interactions using engineered surface-immobilized peptides from class IIa bacteriocins

Specificity of the class IIa bacteriocin Leucocin A (LeuA), an antimicrobial peptide active against Gram-positive bacteria, including Listeria monocytogenes , is known to be dictated by the C-terminal amphipathic helical region, including the extended hairpin-like structure. However, its specificity when attached to a substrate has not been investigated. Exploiting properties of LeuA, we have synthesized two LeuA derivatives, which span the amphipathic helical region of the wild-type LeuA, consisting of 14- (14AA LeuA, CWGEAFSAGVHRLA) and 24-amino acid residues (24AA LeuA, CSVNWGEAFSAGVHRLANGGNGFW). The peptides were purified to >95% purity, as shown by analytical RP-HPLC and mass spectrometry. By including an N-terminal cysteine group, the tailored peptide fragments were readily immobilized at the gold interfaces. The resulting thickness and molecular orientation, determined by ellipsometry and grazing angle infrared spectroscopy, respectively, indicated that the peptides were covalently immobilized in a random helical orientation. The bacterial specificity of the anchored peptide fragments was tested against Gram-positive and Gram-negative bacteria. Our results showed that the adsorbed 14AA LeuA exhibited no specificity toward the bacterial strains, whereas the surface-immobilized 24AA LeuA displayed significant binding toward Gram-positive bacteria with various binding affinities from one strain to another. The 14AA LeuA did not show binding as this fragment is most likely too short in length for recognition by the membrane-bound receptor on the target bacterial cell membrane. These results support the potential use of class IIa bacteriocins as molecular recognition elements in biosensing platforms.

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.

Target recognition, resistance, immunity and genome mining of class II bacteriocins from Gram-positive bacteria

Due to their very potent antimicrobial activity against diverse food-spoiling bacteria and pathogens and their favourable biochemical properties, peptide bacteriocins from Gram-positive bacteria have long been considered promising for applications in food preservation or medical treatment. To take advantage of bacteriocins in different applications, it is crucial to have detailed knowledge on the molecular mechanisms by which these peptides recognize and kill target cells, how producer cells protect themselves from their own bacteriocin (self-immunity) and how target cells may develop resistance. In this review we discuss some important recent progress in these areas for the non-lantibiotic (class II) bacteriocins. We also discuss some examples of how the current wealth of genome sequences provides an invaluable source in the search for novel class II bacteriocins.

Development of Class IIa Bacteriocins as Therapeutic Agents

Class IIa bacteriocins have been primarily explored as natural food preservatives, but there is much interest in exploring the application of these peptides as therapeutic antimicrobial agents. Bacteriocins of this class possess antimicrobial activity against several important human pathogens. Therefore, the therapeutic development of these bacteriocins will be reviewed. Biological and chemical modifications to both stabilize and increase the potency of bacteriocins are discussed, as well as the optimization of their production and purification. The suitability of bacteriocins as pharmaceuticals is explored through determinations of cytotoxicity, effects on the natural microbiota, and in vivo efficacy in mouse models. Recent results suggest that class IIa bacteriocins show promise as a class of therapeutic agents.

Mutational analysis of residues in the helical region of the class IIa bacteriocin pediocin PA-1

A 15-mer fragment that is derived from the helical region in the C-terminal half of pediocin PA-1 inhibited the activity of pediocin PA-1. Of 13 other pediocin-like (hybrid) bacteriocins, only the hybrid bacteriocin Sak/Ped was markedly inhibited by the 15-mer fragment. Sak/Ped was the only one of these bacteriocins that had a sequence (in the C-terminal helix-containing half) identical to that of the 15-mer fragment, indicating that the fragment inhibits pediocin-like bacteriocins in a sequence-dependent manner. By replacing (one at a time) all 15 residues in the fragment with Ala or Leu, five residues (K1, A2, T4, N8, and A15) were identified as being especially important for the inhibitory action of the fragment. The results suggest that the corresponding residues (K20, A21, T23, N27, and A34, respectively) in pediocin PA-1 might be involved in interactions between pediocin PA-1 and its receptor. To characterize the environment surrounding these five residues when pediocin PA-1 interacts with target cells, these residues were replaced (one at a time) with a hydrophobic large (Leu) residue, a hydrophilic charged (Asp or Arg) residue, and a small (Ala or Gly) residue. The results revealed that residues A21 and A34 are in a spatially constrained environment, since the replacement with a small (Gly) residue was the only substitution that did not markedly reduce the bacteriocin activity. The positive charge in K20 and the polar amide group in N27 appeared to interact with electronegative groups, since the replacement of these two residues with a positive (Arg) residue was well tolerated, while replacement with a negative (Asp) residue was detrimental to the bacteriocin activity. K20 was in a less constrained environment than N27, since the replacement of K20 with a large hydrophobic (Leu) residue was tolerated fairly well and to a greater extent than N27. T23 seemed to be in an environment that was not restricted with respect to size, polarity, and charge, since replacements with large (Leu) and small (Ala) hydrophobic residues and a hydrophilic negative (Asp) residue were tolerated fairly well (2- to 6-fold reduction in activity). Moreover, the replacement of T23 with a large positive (Arg) residue resulted in wild-type or better-than-wild-type activity.

Identification and characterization of novel multiple bacteriocins produced by Leuconostoc pseudomesenteroides QU 15

AIM

To characterize novel multiple bacteriocins produced by Leuconostoc pseudomesenteroides QU 15.

METHODS AND RESULTS

Leuconostoc pseudomesenteroides QU 15 isolated from Nukadoko (rice bran bed) produced novel bacteriocins. By using three purification steps, four antimicrobial peptides termed leucocin A (ΔC7), leucocin A-QU 15, leucocin Q and leucocin N were purified from the culture supernatant. The amino acid sequences of leucocin A (ΔC7) and leucocin A-QU 15 were identical to that of leucocin A-UAL 187 belonging to class IIa bacteriocins, but leucocin A (ΔC7) was deficient in seven C-terminal residues. Leucocin Q and leucocin N are novel class IId bacteriocins. Moreover, the DNA sequences encoding three bacteriocins, leucocin A-QU 15, leucocin Q and leucocin N were obtained.

CONCLUSIONS

These bacteriocins including two novel bacteriocins were identified from Leuc. pseudomesenteroides QU 15. They showed similar antimicrobial spectra, but their intensities differed. The C-terminal region of leucocin A-QU 15 was important for its antimicrobial activity. Leucocins Q and N were encoded by adjacent open reading frames (ORFs) in the same operon, but leucocin A-QU 15 was not.

SIGNIFICANCE AND IMPACT OF STUDY

These leucocins were produced concomitantly by the same strain. Although the two novel bacteriocins were encoded by adjacent ORFs, a characteristic of class IIb bacteriocins, they did not show synergistic activity.

Interactions between two carnobacteriocins Cbn BM1 and Cbn B2 from Carnobacterium maltaromaticum CP5 on target bacteria and Caco-2 cells

Two purified class IIa carnobacteriocins Cbn BM1 and Cbn B2, from Carnobacterium maltaromaticum CP5, were evaluated for antimicrobial activity against pathogenic, spoilage and lactic acid bacteria. Then, the presence of a synergistic mode of action of these two carnobacteriocins on Listeria sp., Enterococcus sp. and Carnobacterium sp. was investigated. A synergistic mode of action between Cbn BM1 and Cbn B2 on sensitive target bacteria was demonstrated using the FIC index method. Combinations of carnobacteriocins enhanced their antibacterial activities and MICs were significantly reduced, between 2- and 15-fold, by the addition of the second bacteriocin. To improve the safety of the bacteriocins as biopreservative agents, the cytotoxicity of the combination of theses two bacteriocins was determined on Caco-2 cell line. However, these two peptides used alone or in combination, at concentration 100-fold higher than those required for antimicrobial activity, were not cytotoxic. This suggests that the two carnobacteriocins produced by C. maltaromaticum CP5 could be potential natural agents for food preservation.

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.

Approaches to the discovery of new antibacterial agents based on bacteriocins

The development of antibiotic resistance in pathogenic bacteria has led to a search for novel classes of antimicrobial drugs. Bacteriocins are peptides that are naturally produced by bacteria and have considerable potential to fulfill the need for more effective bacteriocidal agents. In this mini-review, we describe research aimed at generating analogues of bacteriocins from lactic acid bacteria, with the goal of gaining a better understanding of structure-activity relationships in these peptides. In particular, we report recent findings on synthetic analogues of leucocin A, pediocin PA1, and lacticin 3147 A2, as well as on the significance of these results for the design and production of new antibiotics.

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.

Action of divergicin M35, a class IIa bacteriocin, on liposomes and Listeria

AIMS

The mode of action of divergicin M35, a class IIa bacteriocin, was studied against Listeria monocytogenes with sensitive (DivS) and resistant (DivM) phenotypes, as well as on synthetic phospholipid liposomes.

METHODS AND RESULTS

Divergicin-induced release of 1,6-diphenyl-1,3,5-hexatriene (DPH) from zwitterionic (DMPC) and anionic (DMPC/DMPG, 4:1) liposomes, divergicin binding to liposomes, intracellular ATP concentration, cation efflux, cell affinity for hydrocarbons and cell lysis were measured and cell damage was visualized by fluorescence imaging and transmission electron microscopy. Divergicin M35 at 5 microg ml(-1) induced DPH efflux from anionic and zwitterionic liposomes at rates of about 2.58% and 1.61% per minute, respectively. DPH efflux rate from anionic liposomes was reduced by about 1.83% and 2.1% per minute in the presence of Li+ and Ca2+, respectively. Binding affinity of divergicin M35 to anionic and zwitterionic liposomes was about 86% and 63%, respectively. Intracellular ATP decreased in the sensitive and the resistant strains by 96.7% and 72.8%, respectively after 20 min of exposure to 5 microg ml(-1) divergicin M35. Lysis of the sensitive strain reached 57% in 18 h at a concentration of 5 microg ml(-1) when compared with the lysis of the divergicin-resistant strain (38.8%). The K+ and Na+ efflux from the divergicin-sensitive strain reached 87% and 80% of the total ion content within 5 min of exposure. This strain also showed higher affinity for hydrocarbons.

CONCLUSIONS

The cell death of listerial strains upon addition of divergicin M35 could result from ATP depletion, K+ and Na+ efflux, and bacteriolysis. This triple biological effect was attenuated in the DivM strain.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study contributed to the understanding of the mode of action of divergicin M35, a pediocin-like bacteriocin.

Short peptides derived from the NH2-terminus of subclass IIa bacteriocin enterocin CRL35 show antimicrobial activity

OBJECTIVES

Subclass IIa bacteriocins are characterized by a hydrophilic N-terminal domain that shares a YGNGVxCxxxxC consensus and a variable hydrophobic C-terminus. Enterocin CRL35 is a 43-amino-acid heat stable peptide with antilisterial activity. Short synthetic peptides derived from the N-terminal half of enterocin CRL35 and other subclass IIa bacteriocins were evaluated for antimicrobial properties.

METHODS

In vitro activities of synthetic peptides were evaluated in complex, chemically defined and minimal media. MIC assays were performed by the agar well-diffusion method. Fluorescence assays to evaluate the dissipation of membrane potentials in intact cells were carried out. Time-kill kinetics of Listeria innocua cells with the active peptide were performed.

RESULTS AND CONCLUSIONS

A 15-mer peptide derived from enterocin CRL35 inhibited the growth of L. innocua and Listeria monocytogenes in synthetic/minimal media and dissipated the membrane potential of sensitive cells, with MICs of 10 and 50 microM, respectively. 15-mer derivatives from other class IIa bacteriocins (mesentericin Y105, pediocin PA-1 and piscicolin 126) also showed antimicrobial activities.

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.

Cell-surface alterations in class IIa bacteriocin-resistant Listeria monocytogenes strains

Strains of the food-borne pathogenListeria monocytogenes, showing either intermediate or high-level resistance to class IIa bacteriocins, were investigated to determine characteristics that correlated with their sensitivity levels. Two intermediate and one highly resistant spontaneous mutant ofL. monocytogenesB73, a highly resistant mutant ofL. monocytogenes412, and a highly resistant, defined (mptA) mutant ofL. monocytogenesEGDe were compared with their respective wild-type strains in order to investigate the contribution of different factors to resistance. Decreased mannose-specific phosphotransferase system gene expression (mptA, EIIABMancomponent) was implicated in all levels of resistance, confirming previous studies by the authors' group. However, a clear correlation betweend-alanine content in teichoic acid (TA), in particular the alanine : phosphorus ratio, and a more positive cell surface, as determined by cytochromecbinding, were found for the highly resistant strains. Furthermore, two of the three highly resistant strains showed a significant increase in sensitivity towardsd-cycloserine (DCS). However, real-time PCR of thedltA(d-alanine esterification), anddalandddlAgenes (peptidoglycan biosynthesis) showed no change in transcriptional levels. The link between DCS sensitivity and increasedd-alanine esterification of TA may be that DCS competes with alanine for transport via the alanine transporter. A possible tendency towards increased lysinylation of membrane phospholipid in the highly resistant strains was also found. A previous study reported that cell membranes of all the resistant strains, including the intermediate resistant strains, contained more unsaturated phosphatidylglycerol, which is an indication of a more fluid cell membrane. The results of that study correlate with the possible lysinylation, decreasedmptAexpression,d-alanine esterification of TA and more positive cell surface charge found in this study for resistant strains. The authors' findings strongly indicate that all these factors could contribute to class IIa bacteriocin resistance and that the combination and contribution of each of these factors determine the level of bacteriocin resistance.

Novel expression system for large-scale production and purification of recombinant class IIa bacteriocins and its application to piscicolin 126

Piscicolin 126 is a class IIa bacteriocin isolated from Carnobacterium piscicola JG126 that exhibits strong activity against Listeria monocytogenes. The gene encoding mature piscicolin 126 (m-pisA) was cloned into an Escherichia coli expression system and expressed as a thioredoxin-piscicolin 126 fusion protein that was purified by affinity chromatography. Purified recombinant piscicolin 126 was obtained after CNBr cleavage of the fusion protein followed by reversed-phase chromatography. Recombinant piscicolin 126 contained a single disulfide bond and had a mass identical to that of native piscicolin 126. This novel bacteriocin expression system generated approximately 26 mg of purified bacteriocin from 1 liter of E. coli culture. The purified recombinant piscicolin 126 acted by disruption of the bacterial cell membrane.

Expression of mptC of Listeria monocytogenes induces sensitivity to class IIa bacteriocins in Lactococcus lactis

Sensitivity to class IIa bacteriocins from lactic acid bacteria was recently associated with the mannose phosphotransferase system (PTS) permease, , in Listeria monocytogenes. To assess the involvement of this protein complex in class IIa bacteriocin activity, the mptACD operon, encoding , was heterologously expressed in an insensitive species, namely Lactococcus lactis, using the NICE double plasmid system. Upon induction of the cloned operon, the recombinant Lc. lactis became sensitive to leucocin A. Pediocin PA-1 and enterocin A also showed inhibitory activity against Lc. lactis cultures expressing mptACD. Furthermore, the role of the three genes of the mptACD operon was investigated. Derivative plasmids containing various combinations of these three genes were made from the parental mptACD plasmid by divergent PCR. The results showed that expression of mptC alone is sufficient to confer sensitivity to class IIa bacteriocins in Lc. lactis.

Enhancement of the enterocin CRL35 activity by a synthetic peptide derived from the NH2-terminal sequence

The enterocin CRL35 biosynthetic gene cluster was cloned and sequenced. The sequence was revealed to be highly identical to that of the mundticin KS gene cluster (S. Kawamoto, J. Shima, R. Sato, T. Eguchi, S. Ohmomo, J. Shibato, N. Horikoshi, K. Takeshita, and T. Sameshima, Appl. Environ. Microbiol. 68:3830-3840, 2002). Short synthetic peptides were designed based on the bacteriocin sequence and were evaluated in antimicrobial competitive assays. The peptide KYYGNGVSCNKKGCS produced an enhancement of enterocin CRL35 antimicrobial activity in a buffer system.

NMR solution structure of the precursor for carnobacteriocin B2, an antimicrobial peptide from Carnobacterium piscicola

Type IIa bacteriocins, which are isolated from lactic acid bacteria that are useful for food preservation, are potent antimicrobial peptides with considerable potential as therapeutic agents for gastrointestinal infections in mammals. They are ribosomally synthesized as precursors with an N-terminal leader, typically 18-24 amino acid residues in length, which is cleaved during export from the producing cell. We have chemically synthesized the full precursor of carnobacteriocin B2, precarnobacteriocin (preCbnB2), which has a C-terminal amide rather than a carboxyl, and also produced preCbnB2(1-64), which is missing two amino acid residues at the C-terminus (Arg65 and Pro66), via expression in Escherichia coli as a maltose-binding protein fusion that is then cut with Factor Xa. PreCbnB2(1-64) is readily labeled with (15)N and (13)C for NMR studies using the latter approach. Multidimensional NMR analysis of preCbnB2(1-64) shows that, like the parent bacteriocin, it exists as a random coil in water but assumes a defined conformation in water/trifluoroethanol mixtures. In 70 : 30 trifluoroethanol/water, the 3D structure of the preCbnB2 section corresponding to the mature bacteriocin is essentially the same as reported previously by us for carnobacteriocin B2 (CbnB2). This structure maintains the highly conserved alpha-helix corresponding to residues 20-38 of CbnB2 that is believed to be responsible for interaction with a target receptor in sensitive cells, including Listeria monocytogenes. PreCbnB2 also has a second alpha-helix from residues 3-13 (i.e. -15 to -5 relative to CbnB2) in the leader section of the peptide. This helix appears to be conserved in related type IIa bacteriocin precursors based on sequence analysis. It is likely to be a key recognition element for export and processing, and is probably responsible for the considerably reduced antimicrobial activity of preCbnB2. The latter effect may assist the producing cell in avoiding the toxic effects of the bacteriocin. This is the first 3D structure determined for a prebacteriocin from lactic acid bacteria.

Exploration of antimicrobial potential in LAB by genomics

A tremendous flow of information has been created through various genome sequencing projects worldwide. So far, 128 bacterial genome sequences have been completed and 391 are under way. Many of these bacteria, including several lactic acid bacteria (LAB), are used in the production and preservation of food and feed. The major antimicrobial and biopreservative substance produced by LAB is organic acid; however, some LAB produce additional antimicrobial compounds. Among these, the bacteriocins have demonstrated great potential as food preservatives. Additionally, antimicrobial compounds different from the bacteriocins have recently been identified, of which several display strong antifungal activity. The information obtained from genomics and related technologies will have great impact on the future identification and development of new antimicrobial agents. Developments will include the identification of pathways for the production of antimicrobials and genome mining for new antimicrobial peptides.

Differences in susceptibility of Listeria monocytogenes strains to sakacin P, sakacin A, pediocin PA-1, and nisin

Two hundred strains of Listeria monocytogenes collected from food and the food industry were analyzed for susceptibility to the class IIa bacteriocins sakacin P, sakacin A, and pediocin PA-1 and the class I bacteriocin nisin. The individual 50% inhibitory concentrations (IC(50)) were determined in a microtiter assay and expressed in nanograms per milliliter. The IC(50) of sakacin P ranged from 0.01 to 0.61 ng ml(-1). The corresponding values for pediocin PA-1, sakacin A, and nisin were 0.10 to 7.34, 0.16 to 44.2, and 2.2 to 781 ng ml(-1), respectively. The use of a large number of strains and the accuracy of the IC(50) determination revealed patterns not previously described, and for the first time it was shown that the IC(50) of sakacin P divided the L. monocytogenes strains into two distinct groups. Ten strains from each group were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of whole-cell proteins and amplified fragment length polymorphism. The results from these studies essentially confirmed the grouping based on the IC(50) of sakacin P. A high correlation was found between the IC(50) of sakacin P and that of pediocin PA-1 for the 200 strains. Surprisingly, the correlation between the IC(50) of the two class IIa bacteriocins sakacin A and sakacin P was lower than the correlation between the IC(50) of sakacin A and the class I bacteriocin nisin.

Purification and characterization of brochocin A and brochocin B(10-43), a functional fragment generated by heterologous expression in Carnobacterium piscicola

Brochothrix campestris ATCC 43754 produces a heat-stable, two-component, nonlantibiotic, class IIb bacteriocin, brochocin C (BrcC), that is active against a broad range of gram-positive bacteria, including spores of Clostridium botulinum. An improved purification method was developed for BrcC, in which n-butanol and chloroform extraction are used. Mass spectral characterization of the two components, brochocin A (BrcA) and brochocin B (BrcB), showed that both components are excreted into the medium by B. campestris as mature peptides consisting of 59 and 43 amino acids, respectively. Separate expression clones of BrcA and BrcB were constructed previously in Carnobacterium piscicola LV17C, but the products were not chemically characterized. Purification by the new protocol showed that BrcA is expressed as the mature 59-amino-acid peptide but that BrcB is produced by C. piscicola as a fragment, BrcB(10-43), which is cleaved at an internal Gly-Gly site. This fragment is not antimicrobial by itself, but in combination with BrcA it displays the full activity of the BrcC complex. Circular dichroism measurements revealed a high beta-sheet content in the secondary structure of both BrcA and BrcB(10-43), as well as in a 1:1 BrcA-BrcB(10-43) mixture. Separate expression clones of brcA and brcB were also constructed in Escherichia coli, but these clones only produced multiple fragments of the desired peptides with little or no activity.

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.

Membranes of class IIa bacteriocin-resistant Listeria monocytogenes cells contain increased levels of desaturated and short-acyl-chain phosphatidylglycerols

A major concern in the use of class IIa bacteriocins as food preservatives is the well-documented resistance development in target Listeria strains. We studied the relationship between leucocin A, a class IIa bacteriocin, and the composition of the major phospholipid, phosphatidylglycerol (PG), in membranes of both sensitive and resistant L. monocytogenes strains. Two wild-type strains, L. monocytogenes B73 and 412, two spontaneous mutants of L. monocytogenes B73 with intermediate resistance to leucocin A (+/-2.4 and +/-4 times the 50% inhibitory concentrations [IC50] for sensitive strains), and two highly resistant mutants of each of the wild-type strains (>500 times the IC50 for sensitive strains) were analyzed. Electrospray mass spectrometry analysis showed an increase in the ratios of unsaturated to saturated and short- to long-acyl-chain species of PG in all the resistant L. monocytogenes strains in our study, although their sensitivities to leucocin A were significantly different. This alteration in membrane phospholipids toward PGs containing shorter, unsaturated acyl chains suggests that resistant strains have cells with a more fluid membrane. The presence of this phenomenon in a strain (L. monocytogenes 412P) which is resistant to both leucocin A and pediocin PA-1 may indicate a link between membrane composition and class IIa bacteriocin resistance in some L. monocytogenes strains. Treatment of strains with sterculic acid methyl ester (SME), a desaturase inhibitor, resulted in significant changes in the leucocin A sensitivity of the intermediate-resistance strains but no changes in the sensitivity of highly resistant strains. There was, however, a decrease in the amount of unsaturated and short-acyl-chain PGs after treatment with SME in one of the intermediate and both of the highly resistant strains, but the opposite effect was observed for the sensitive strains. It appears, therefore, that membrane adaptation may be part of a resistance mechanism but that several resistance mechanisms may contribute to a resistance phenotype and that levels of resistance vary according to the type of mechanisms present.

High-level resistance to class IIa bacteriocins is associated with one general mechanism in Listeria monocytogenes

Class IIa bacteriocins may be used as natural food preservatives, yet resistance development in the target organisms is still poorly understood. In this study, the understanding of class IIa resistance development in Listeria monocytogenes is extended, linking the seemingly diverging results previously reported. Eight resistant mutants having a high resistance level (at least a 10(3)-fold increase in MIC), originating from five wild-type listerial strains, were independently isolated following exposure to four different class IIa bacteriocin-producing lactic acid bacteria (including pediocin PA-1 and leucocin A producers). Two of the mutants were isolated from food model systems (a saveloy-type sausage at 10 degrees C, and salmon juice at 5 degrees C). Northern blot analysis showed that the eight mutants all had increased expression of EII(Bgl) and a phospho-beta-glucosidase homologue, both originating from putative beta-glucoside-specific phosphoenolpyruvate-dependent phosphotransferase systems (PTSs). However, disruption of these genes in a resistant mutant did not confer pediocin sensitivity. Comparative two-dimensional gel analysis of proteins isolated from mutant and wild-type strains showed that one spot was consistently missing in the gels from mutant strains. This spot corresponded to the MptA subunit of the mannose-specific PTS, found only in the gels of wild-type strains. The mptACD operon was recently shown to be regulated by the sigma(54) transcription factor in conjunction with the activator ManR. Class IIa bacteriocin-resistant mutants having defined mutations in mpt or manR also exhibited the two diverging PTS expression changes. It is suggested here that high-level class IIa resistance in L. monocytogenes and at least some other Gram-positive bacteria is developed by one prevalent mechanism, irrespective of wild-type strain, class IIa bacteriocin, or the tested environmental conditions. The changes in expression of the beta-glucoside-specific and the mannose-specific PTS are both influenced by this mechanism. The current understanding of the actual cause of class IIa resistance is discussed.

Mode of action of modified and unmodified bacteriocins from Gram-positive bacteria

The antibiotic activity of bacteriocins from Gram-positive bacteria, whether they are modified (class I bacteriocins, lantibiotics) or unmodified (class II), is based on interaction with the bacterial membrane. However, recent work has demonstrated that for many bacteriocins, generalised membrane disruption models as elaborated for amphiphilic peptides (e.g. tyriodal pore or carpet model) cannot adequately describe the bactericidal action. Rather, specific targets seem to be involved in pore formation and other activities. For the nisin and epidermin family of lantibiotics, the membrane-bound cell wall precursor lipid II has recently been identified as target. The duramycin family of lantibiotics binds specifically to phosphoethanolamine which results in inhibition of phospholipase A2 and various other cellular functions. Most of the class II bacteriocins dissipate the proton motive force (PMF) of the target cell, via pore formation. The subclass IIa bacteriocin activity likely depends on a mannose permease of the phosphotransferase system (PTS) as specific target. The subclass IIb bacteriocins (two-component) also induce dissipation of the PMF by forming cation- or anion-specific pores; specific targets have not yet been identified. Finally, the subclass IIc comprises miscellaneous peptides with various modes of action such as membrane permeabilization, specific inhibition of septum formation and pheromone activity.