Characterization, production, and purification of leucocin H, a two-peptide bacteriocin from Leuconostoc MF215B

Review: Bacteriocins of Lactic Acid Bacteria

During the last few years, a large number of new bacteriocins produced by lactic acid bacteria (LAB) have been identified and characterized. LAB-bacteriocins comprise a heterogeneous group of physicochemically diverse ribosomally-synthesized peptides or proteins showing a narrow or broad antimicrobial activity spectrum against Gram-positive bacteria. Bacteriocins are classified into separate groups such as the lantibiotics (Class I); the small (<10 kDa) heat-stable postranslationally unmodified non-lantibiotics (Class II), further subdivided in the pediocin-like and anti Listeria bacteriocins (subclass IIa), the two-peptide bacteriocins (subclass IIb), and the sec-dependent bacteriocins (subclass IIc); and the large (>30 kDa) heat-labile non-lantibiotics (Class III). Most bacteriocins characterized to date belong to Class II and are synthesized as precursor peptides (preprobacteriocins) containing an N-terminal double-glycine leader peptide, which is cleaved off concomitantly with externalization of biologically active bacteriocins by a dedicated ABC-transporter and its accessory protein. However, the recently identified sec-dependent bacteriocins contain an N-terminal signal peptide that directs bacteriocin secretion through the general secretory pathway (GSP). Most LAB-bacteriocins act on sensitive cells by destabilization and permeabilization of the cytoplasmic membrane through the formation of transitory poration complexes or ionic channels that cause the reduction or dissipation of the proton motive force (PMF). Bacteriocin producing LAB strains protect themselves against the toxicity of their own bacteriocins by the expression of a specific immunity protein which is generally encoded in the bacteriocin operon. Bacteriocin production in LAB is frequently regulated by a three-component signal transduction system consisting of an induction factor (IF), and histidine protein kinase (HPK) and a response regulator (RR). This paper presents an updated review on the general knowledge about physicochemical properties, molecular mode of action, biosynthesis, regulation and genetics of LAB-bacteriocins.

Bacteriocins: Novel Solutions to Age Old Spore-Related Problems?

Bacteriocins are ribosomally synthesized antimicrobial peptides produced by bacteria, which have the ability to kill or inhibit other bacteria. Many bacteriocins are produced by food grade lactic acid bacteria (LAB). Indeed, the prototypic bacteriocin, nisin, is produced by Lactococcus lactis, and is licensed in over 50 countries. With consumers becoming more concerned about the levels of chemical preservatives present in food, bacteriocins offer an alternative, more natural approach, while ensuring both food safety and product shelf life. Bacteriocins also show additive/synergistic effects when used in combination with other treatments, such as heating, high pressure, organic compounds, and as part of food packaging. These features are particularly attractive from the perspective of controlling sporeforming bacteria. Bacterial spores are common contaminants of food products, and their outgrowth may cause food spoilage or food-borne illness. They are of particular concern to the food industry due to their thermal and chemical resistance in their dormant state. However, when spores germinate they lose the majority of their resistance traits, making them susceptible to a variety of food processing treatments. Bacteriocins represent one potential treatment as they may inhibit spores in the post-germination/outgrowth phase of the spore cycle. Spore eradication and control in food is critical, as they are able to spoil and in certain cases compromise the safety of food by producing dangerous toxins. Thus, understanding the mechanisms by which bacteriocins exert their sporostatic/sporicidal activity against bacterial spores will ultimately facilitate their optimal use in food. This review will focus on the use of bacteriocins alone, or in combination with other innovative processing methods to control spores in food, the current knowledge and gaps therein with regard to bacteriocin-spore interactions and discuss future research approaches to enable spores to be more effectively targeted by bacteriocins in food settings.

Genetic characterization and expression of leucocin B, a class IId bacteriocin from Leuconostoc carnosum 4010

Leuconostoc carnosum 4010 is an antimicrobial strain used as a protective culture in vacuum-packed meats. In this study, we showed that, in addition to antilisterial class IIa bacteriocins leucocin A and C, the strain also produces class IId bacteriocin leucocin B, the antimicrobial activity of which is limited to the genera Leuconostoc and Weissella. Two novel genes, lebBI encoding the leucocin B precursor with a double-glycine-type leader and putative immunity protein LebI, were identified on L. carnosum 4010 plasmid pLC4010-1. LebI contains three transmembrane spans and shares 55% identity with the mesentericin B105 immunity protein. Genes lebBI were shown to be transcribed in 4010 by RT-PCR analysis. The secretion of leucocin B in L. carnosum 4010 was shown by spot-on-lawn and SDS-gel overlay methods with a Leuconostoc strain sensitive to leucocin B but resistant to leucocins A and C. In addition, leucocins A and B from L. carnosum 4010 were cloned as SSusp45 fusions in heterologous host Lactococcus lactis and the secretion of active bacteriocins was detected on indicator plates.

Leuconostoc citreumMB1 as biocontrol agent ofListeria monocytogenesin milk

Cell-free supernatant fromLeuconostoc citreumMB1 revealed specific antilisterial activity. Preliminary studies demonstrated the proteinaceous, heat-stable, bacteriocin-like trait of the antimicrobial components present in the supernatant. Determination of the genes encoding bacteriocins by PCR and DNA sequencing led to amplification products highly homologous with leucocin A (found in diverseLeuconostocspecies) and UviB (found inLeuc. citreumKM20) sequences. Additionally, antimicrobial activity of cell-free supernatant fromLeuc. citreumMB1 was revealed by an inhibition halo of the SDS-PAGE gel subjected to a direct detection usingListeria monocytogenesas indicator strain. Different assays were carried out to assess the capacity ofLeuc.citreumMB1 to controlList. monocytogenesgrowth: (i) inactivation kinetics of the pathogen by antilisterial compounds present in concentrated cell-free supernatant fromLeuc. citreumMB1, (ii) evaluation of optimalLeuc. citreumMB1 initial concentration to obtain maximumList. monocytogenesATCC 15313 inhibition, and (iii) biocontrol ofList. monocytogenesATCC 15313 withLeuc. citreumMB1 during growth in milk at refrigeration temperature. According to our results, it is unquestionable that at least one bacteriocin is active inLeuc. citreumMB1, since important antilisterial activity was verified either in its cell-free supernatant or in co-culture experiments. Co-culture tests showed that ∼107 CFU/mlLeuc. citreumMB1 was the optimal initial concentration to obtain maximum pathogen inhibition. Moreover,Leuc. citreumMB1 was able to delayList. monocytogenesgrowth at refrigerated temperature.

Structure and Mode-of-Action of the Two-Peptide (Class-IIb) Bacteriocins

This review focuses on the structure and mode-of-action of the two-peptide (class-IIb) bacteriocins that consist of two different peptides whose genes are next to each other in the same operon. Optimal antibacterial activity requires the presence of both peptides in about equal amounts. The two peptides are synthesized as preforms that contain a 15–30 residue double-glycine-type N-terminal leader sequence that is cleaved off at the C-terminal side of two glycine residues by a dedicated ABC-transporter that concomitantly transfers the bacteriocin peptides across cell membranes. Two-peptide bacteriocins render the membrane of sensitive bacteria permeable to a selected group of ions, indicating that the bacteriocins form or induce the formation of pores that display specificity with respect to the transport of molecules. Based on structure–function studies, it has been proposed that the two peptides of two-peptide bacteriocins form a membrane-penetrating helix–helix structure involving helix–helix-interacting GxxxG-motifs that are present in all characterized two-peptide bacteriocins. It has also been suggested that the membrane-penetrating helix–helix structure interacts with an integrated membrane protein, thereby triggering a conformational alteration in the protein, which in turn causes membrane-leakage. This proposed mode-of-action is similar to the mode-of-action of the pediocin-like (class-IIa) bacteriocins and lactococcin A (a class-IId bacteriocin), which bind to a membrane-embedded part of the mannose phosphotransferase permease in a manner that causes membrane-leakage and cell death.

Bacteriocin: safest approach to preserve food products

Start of the 21st century with its universal call to feed the hungry is an appropriate time to refocus attention on food security and especially the impact of biopatenting on poor communities who are the primary victims of hunger in our world. Antibacterial metabolites of lactic acid bacteria and Bacillus spp have potential as natural preservatives to control the growth of spoilage and pathogenic bacteria in food. Among them, bacteriocin is used as a preservative in food due to its heat stability, wider pH tolerance and its proteolytic activity. Due to thermo stability and pH tolerance it can withstand heat and acidity/alkanity of food during storage condition. Bacteriocin are ribosomally synthesized peptides originally defined as proteinaceous compound affecting growth or viability of closely related organisms. Research is going on extensively to explore the nascent field of biopreservation. Scientists all over the world are showing their keen interest to isolate different types of bacteriocin producing strains and characterize bacteriocin produced by them for food preservation.

Mutational analysis of putative helix-helix interacting GxxxG-motifs and tryptophan residues in the two-peptide bacteriocin lactococcin G

The membrane-permeabilizing two-peptide bacteriocin lactococcin G consists of two different peptides, LcnG-alpha and LcnG-beta. The bacteriocin contains several tryptophan and tyrosine residues and three putative helix-helix interacting GxxxG-motifs, G 7xxxG 11 and G 18xxxG 22 in LcnG-alpha and G 18xxxG 22 in LcnG-beta. The tryptophan and tyrosine residues and residues in the GxxxG-motifs were altered by site-directed mutagenesis to analyze the structure and membrane-orientation of lactococcin G. Substituting the glycine residues at position 7 or 11 in the G 7xxxG 11-motif in LcnG-alpha with large hydrophobic or hydrophilic residues was highly detrimental, whereas small residues were tolerated. Qualitatively similar results were obtained for the G 18xxxG 22-motif in LcnG-beta. In contrast, replacement of the glycine residues in the middle of these two motifs with large hydrophilic residues was tolerated. All mutations in the G 18xxxG 22-motif in LcnG-alpha were relatively well-tolerated, indicating that this motif is not involved in helix-helix interactions. The four aromatic residues in the N-terminal part of LcnG-beta could individually be replaced by other aromatic residues, a hydrophilic positive residue, and a hydrophobic residue without a marked reduced activity, indicating that this region is structurally flexible and not embedded in a strictly hydrophobic or hydrophilic environment. The results are in accordance with a structural model where the G 7xxxG 11-motif in LcnG-alpha and the G 18xxxG 22-motif in LcnG-beta interact and allow the two peptides to form a parallel transmembrane helix-helix structure, with the tryptophan-rich N-terminal part of LcnG-beta positioned in the outer membrane interface and the cationic C-terminal end of LcnG-alpha inside the cell.

The Two-Peptide Class II Bacteriocins: Structure, Production, and Mode of Action

The two-peptide class II bacteriocins consist of two different unmodified peptides, both of which must be present in about equal amounts in order for these bacteriocins to exert optimal antimicrobial activity. These bacteriocins render the membrane of target cells permeable to various small molecules. The genes encoding the two peptides of two-peptide bacteriocins are adjacent to each other in the same operon and they are near the genes encoding (i) the immunity protein that protects the bacteriocin-producing bacteria from being killed by their own bacteriocin, (ii) a dedicated ABC transporter that transports the bacteriocin out of the bacteriocin-producing bacteria, and (iii) an accessory protein whose specific role is not known, but which also appears to be required for secretion of the bacteriocin. The production of some two-peptide bacteriocins is transcriptionally regulated through a three-component regulatory system that consists of a membrane-interacting peptide pheromone, a membrane-associated histidine protein kinase, and response regulators. Structure analysis of three two-peptide bacteriocins (plantaricin E/F, plantaricin J/K, and lactococcin G) by CD (and in part by NMR) spectroscopy reveal that these bacteriocins contain long amphiphilic alpha-helical stretches and that the two complementary peptides interact and structure each other when exposed to membrane-like entities. Lactococcin G shares about 55% sequence identity with enterocin 1071, but these two bacteriocins nevertheless kill different types of bacteria. The target-cell specificity of lactococcin G-enterocin 1071 hybrid bacteriocins that have been constructed by site-directed mutagenesis suggests that the beta-peptide is important for determining the target-cell specificity.

Characterization of mesentericin ST99, a bacteriocin produced by Leuconostoc mesenteroides subsp. dextranicum ST99 isolated from boza

Lactic acid bacteria isolated from Boza, a cereal-fermented beverage from Belogratchik, Bulgaria, were screened for the production of bacteriocins. With the first screening, 13 of the 52 isolates inhibited the growth of Listeria innocua and Lactobacillus plantarum. The cell-free supernatant of one of these strains, classified as Leuconostoc mesenteroides subsp. dextranicum ST99, inhibited the growth of Bacillus subtilis, Enterococcus faecalis, several Lactobacillus spp., Lactococcus lactis subsp. cremoris, Listeria innocua, Listeria monocytogenes, Pediococcus pentosaceus, Staphylococcus aureus and Streptococcus thermophilus. Clostridium spp., Carnobacterium spp., L. mesenteroides and Gram-negative bacteria were not inhibited. Maximum antimicrobial activity, i.e. 6,400 arbitrary units (AU)/ml, was recorded in MRS broth after 24 h at 30 degrees C. Incubation in the presence of protease IV and pronase E resulted in loss of antimicrobial activity, confirming that growth inhibition was caused by a bacteriocin, designated here as mesentericin ST99. No loss in activity was recorded after treatment with alpha-amylase, SDS, Tween 20, Tween 80, urea, Triton X-100, N-laurylsarcosin, EDTA and phenylmethylsulfonylfluoride. Mesentericin ST99 remained active after 30 min at 121 degrees C and after 2 h of incubation at pH 2 to 12. Metabolically active cells of L. innocua treated with mesentericin ST99 did not undergo lysis. Mesentericin ST99 did not adhere to the cell surface of strain ST99. Precipitation with ammonium sulfate (70% saturation), followed by Sep-Pack C18 chromatography and reverse-phase HPLC on a C18 Nucleosil column yielded one antimicrobial peptide.

The antilisterial effect of Leuconostoc carnosum 4010 and leucocins 4010 in the presence of sodium chloride and sodium nitrite examined in a structured gelatin system

To further enhance biopreservation of meat products, the antilisterial effect of the newly described protective culture Leuconostoc carnosum 4010 and its bacteriocins, leucocins 4010, was examined in the presence of sodium chloride and sodium nitrite in a solid matrix using a structured gelatin system. Interaction between Listeria monocytogenes 4140 and Leuc. carnosum 4010 or the leucocins 4010-resistant mutant L. monocytogenes 4140P showed that the inhibitory effect of Leuc. carnosum 4010 in the gelatin system was caused by the production and activity of leucocins 4010. The presence of sodium chloride (2.5% w/v) and sodium nitrite (60 mg/l) reduced the antilisterial effect of Leuc. carnosum 4010 in the structured gel system compared to the use of Leuc. carnosum 4010 alone. Investigations carried out at 10 degrees C showed that the lag phase of L. monocytogenes 4140 in the presence of Leuc. carnosum 4010 was reduced from 71 to 58 h by the addition of sodium chloride and to 40 h by the addition of sodium nitrite. Addition of sodium chloride increased the maximum specific growth rate of L. monocytogenes 4140 in the presence of Leuc. carnosum 4010 from 0.02 to 0.06 h(-1), whereas no change was observed by the addition of sodium nitrite. Compared to the antilisterial effect of leucocins 4010 alone, the addition of sodium chloride (2.5%, w/v) decreased the antilisterial effect at high concentrations of leucocins 4010 (5.3 and 10.6 AU/ml) as measured after 11 days of incubation at 10 degrees C. In gels with added leucocins 4010, the most pronounced reduction in growth of L. monocytogenes 4140 was observed at the highest concentration of leucocins 4010 (10.6 AU/ml) together with sodium nitrite (60 mg/l). More detailed information on the lag phase and the maximum specific growth rate of single colonies of L. monocytogenes 4140 in the presence of leucocins 4010 was obtained using microscopy and image analysis. No pronounced difference in the growth of single colonies was observed in the gel system. Real-time measurements of colony growth at 10 degrees C in the gelatin matrix showed that the growth inhibiting effect of leucocins 4010, including a longer lag phase as well as a lower maximum specific growth rate for L. monocytogenes 4010, was negated in the presence of 2.5% (w/v) sodium chloride.

Two-peptide bacteriocins produced by lactic acid bacteria

Bacteriocins from lactic acid bacteria are ribosomally produced peptides (usually 30-60 amino acids) that display potent antimicrobial activity against certain other Gram-positive organisms. They function by disruption of the membrane of their targets, mediated in at least some cases by interaction of the peptide with a chiral receptor molecule (e.g., lipid II or sugar PTS proteins). Some bacteriocins are unmodified (except for disulfide bridges), whereas others (i.e. lantibiotics) possess extensive post-translational modifications which include multiple monosulfide (lanthionine) bridges and dehydro amino acids as well as possible keto amide residues at the N-terminus. Most known bacteriocins are biologically active as single peptides. However, there is a growing class of two peptide systems, both unmodified and lantibiotic, which are fully active only when both partners are present (usually 1:1). In some cases, neither peptide has activity by itself, whereas in others, the activity of one is enhanced by the other. This review discusses the classification, structure, production, regulation, biological activity, and potential applications of such two-peptide bacteriocins.

Synergy and duality in peptide antibiotic mechanisms

The molecular mechanisms by which peptide antibiotics disrupt bacterial DNA synthesis, protein biosynthesis, cell wall biosynthesis, and membrane integrity are diverse, yet historically have been understood to follow a theme of one antibiotic, one inhibitory mechanism. In the past year, mechanistic and structural studies have shown a rich diversity in peptide antibiotic mechanism. Novel secondary targeting mechanisms for peptide antibiotics have recently been discovered, and the mechanisms of peptide antibiotics involved in synergistic relationships with antibiotics and proteins have been more clearly defined. In apparent response to selective pressures, antibiotic-producing organisms have elegantly integrated multiple functions and cooperative interactions into peptide antibiotic design for the purpose of improving antimicrobial success.