Inducible bacteriocin production in Lactobacillus is regulated by differential expression of the pln operons and by two antagonizing response regulators, the activity of which is enhanced upon phosphorylation

Antimicrobial activity of Lactiplantibacillus plantarum against shiga toxin-producing Escherichia coli

AIMS

The aim of the present work was to characterize the Lactiplantibacillus sp. LP5 strain, isolated from pork production, and identify bacteriocin-like inhibitory substances produced by this strain.

METHODS AND RESULTS

In this study, LP5 was identified by species-specific PCR and 16S rRNA sequencing. Additionally, bacterial growth kinetics, antimicrobial activity, the detection of genes related to plantaricin production, and the genetic expression of plantaricins were determined. Lactiplantibacillus sp. LP5 was identified as Lactiplantibacillus plantarum. The well-diffusion test using cell-free supernatants (CFS), neutralized CFS, CFS treated with catalase, and CFS treated with proteinase K showed that inhibitory effects on a Shiga toxin-producing Escherichia coli (STEC) strain were produced by bacteriocins. The PCR technique allowed the detection of genes encoding E/F plantaricins, as well as J/K and whole genome sequencing, and bacteriocin mining analysis allowed us to confirm the presence of these plantaricins.

CONCLUSIONS

We can conclude that the inhibitory effect of L. plantarum LP5 isolated from pigs against the STEC EDL933 strain could be associated with the bacteriocins production and represents a potential use as a probiotic strain.

Dose-Dependent Inhibitory Effect of Probiotic Lactobacillus plantarum on Streptococcus mutans-Candida albicans Cross-Kingdom Microorganisms

Dental caries is one of the most common chronic diseases worldwide. Streptococcus mutans and Candida albicans are two major pathogens associated with dental caries. Several recent studies revealed that Lactobacillus plantarum inhibits S. mutans and C. albicans in biofilms and in a rodent model of dental caries. The aim of this study was to investigate the dose-dependent effect of L. plantarum against S. mutans and C. albicans in a planktonic model that simulated a high-caries-risk clinical condition. Mono-, dual-, and multi-species models were utilized, with five doses of L. plantarum (ranging from 1.0 × 104 to 1.0 × 108 CFU/mL). Real-time PCR was used to assess the expression of the virulence genes of C. albicans and S. mutans and the genes of L. plantarum. Student's t-tests and one-way ANOVA, followed by post hoc tests, were employed to compare the cell viability and gene expression among groups. A dose-dependent inhibition on C. albicans and S. mutans was observed with increased dosages of L. plantarum. L. plantarum at 108 CFU/mL demonstrated the highest antibacterial and antifungal inhibitory effect in the dual- and multi-species models. Specifically, at 20 h, the growth of C. albicans and S. mutans was suppressed by 1.5 and 5 logs, respectively (p < 0.05). The antifungal and antibacterial effects were attenuated in lower doses of L. plantarum (104-107 CFU/mL). The expression of C. albicans HWP1 and ECE 1 genes and S. mutans lacC and lacG genes were significantly downregulated with an added 108 CFU/mL of L. plantarum (p < 0.05). The addition of 108 CFU/mL L. plantarum further inhibited the hyphae or pseudohyphae formation of C. albicans. In summary, L. plantarum demonstrated dose-dependent antifungal and antibacterial effects against C. albicans and S. mutans. L. plantarum emerged as a promising candidate for the creation of novel antimicrobial probiotic products targeting dental caries prevention. Further research is warranted to identify the functional metabolites produced by L. plantarum at different dosages when interacting with C. albicans and S. mutans.

Screening of Lactiplantibacillus plantarum Strains from Sourdoughs for Biosuppression of Pseudomonas syringae pv. syringae and Botrytis cinerea in Table Grapes

Grapes (Vitis vinifera L.) are an essential crop for fresh consumption and wine production. Vineyards are attacked by several economically important bacterial and fungal diseases that require regular pesticide treatment. Among them, Pseudomonas syringae pv. syringae (Ps. syringae) and Botrytis cinerea (B. cinerea) infections cause huge economic losses. The fresh fruit market has shifted to functional natural foodstuffs with clear health benefits and a reduced use of chemicals along the production chain. Lactic acid bacteria (LAB) have a biopreservative effect and are applied to ensure food safety in response to consumers' demands. In the present study, the possibilities of using microorganisms with a potential antimicrobial effect against Ps. syringae and B. cinerea in the production of table grapes were investigated. LAB of the genus Lactiplantibacillus can be a natural antagonist of pathogenic bacteria and fungi by releasing lactic acid, acetic acid, ethanol, carbon dioxide and bacteriocins in the medium. The present study focuses on the characterization of nine Lactiplantibacillus plantarum (Lp. plantarum) strains isolated from spontaneously fermented sourdoughs. Species-specific PCR identified the isolated LAB for partial recA gene amplification with an amplicon size of 318 bp. RAPD-PCR analysis showed the intraspecific diversity of the individual strains. Thirteen plantaricin-like peptides (PlnA, PlnB, PlnC, PlnD, PlnEF, PlnG, PlnI, PlnJ, PlnK, PlnN, PlnNC8, PlnS, and PlnW) produced by isolated Lp. plantarum strains were detected by PCR with gene-specific primers. The key features for future industrial applications were their antimicrobial properties. The culture medium and cell-free supernatant (CFS) were used to establish in vitro antimicrobial activities of Lp. plantarum strains against Ps. syringae and B. cinerea, and inhibition of phytopathogen development was observed. The inhibitory effect of the CFS (cell-free supernatant) of all strains was assessed by infecting table grapes with these pathogens in in vivo experiments. Lp. plantarum Q4 showed the most effective suppression of the pathogens both in vitro and in vivo, which indicates its potential use as a biocontrol agent against berry rot and grey rot on grapes, caused by Ps. syringae and B. cinerea.

Food and gut originated bacteriocins involved in gut microbe-host interactions

The gut microbes interact with each other as well as host, influencing human health and some diseases. Many gut commensals and food originated bacteria produce bacteriocins which can inhibit pathogens and modulate gut microbiota. Bacteriocins have comparable narrow antimicrobial spectrum and are attractive potentials for precision therapy of gut disorders. In this review, the bacteriocins from food and gut microbiomes and their involvement in the interaction between producers and gut ecosystem, along with their characteristics, types, biosynthesis, and functions are described and discussed. Bacteriocins are produced by many intestinal commensals and food microbes among which lactic acid bacteria (many are probiotics) has been paid more attention. Bacteriocin production has been generally regarded as a probiotic trait. They give a competitive advantage to bacteria, enabling their colonization in human gut, and mediating the interaction between the producers and host ecosystem. They fight against unwanted bacteria and pathogens without significant impact on the composition of commensal microbiota. Bacteriocins assist the producers to survive and colonize in the gut microbial populations. There is a great need to evaluate and utilize the potential of bacteriocins for improved therapeutic implications for intestinal health.

Lactobacillus plantarum Disrupts S. mutans–C. albicans Cross-Kingdom Biofilms

Dental caries, an ecological dysbiosis of oral microflora, initiates from the virulent biofilms formed on tooth surfaces where cariogenic microorganisms metabolize dietary carbohydrates, producing acid that demineralizes tooth enamel. Forming cariogenic biofilms, Streptococcus mutans and Candida albicans are well-recognized and emerging pathogens for dental caries. Recently, probiotics have demonstrated their potential in treating biofilm-related diseases, including caries. However, limited studies have assessed their effect on cariogenic bacteria–fungi cross-kingdom biofilm formation and their underlying interactions. Here, we assessed the effect of four probiotic Lactobacillus strains (Lactobacillus rhamnosus ATCC 2836, Lactobacillus plantarum ATCC 8014, Lactobacillus plantarum ATCC 14917, and Lactobacillus salivarius ATCC 11741) on S. mutans and C. albicans using a comprehensive multispecies biofilm model that mimicked high caries risk clinical conditions. Among the tested probiotic species, L. plantarum demonstrated superior inhibition on the growth of C. albicans and S. mutans, disruption of virulent biofilm formation with reduced bacteria and exopolysaccharide (EPS) components, and formation of virulent microcolonies structures. Transcriptome analysis (RNA sequencing) further revealed disruption of S. mutans and C. albicans cross-kingdom interactions with added L. plantarum. Genes of S. mutans and C. albicans involved in metabolic pathways (e.g., EPS formation, carbohydrate metabolism, glycan biosynthesis, and metabolism) were significantly downregulated. More significantly, genes related to C. albicans resistance to antifungal medication (ERG4), fungal cell wall chitin remodeling (CHT2), and resistance to oxidative stress (CAT1) were also significantly downregulated. In contrast, Lactobacillus genes plnD, plnG, and plnN that contribute to antimicrobial peptide plantaricin production were significantly upregulated. Our novel study findings support further assessment of the potential role of probiotic L. plantarum for cariogenic biofilm control.

Characterization of a Lactiplantibacillus plantarum R23 Isolated from Arugula by Whole-Genome Sequencing and Its Bacteriocin Production Ability

Lactiplantibacillus plantarum is one of the lactic acid bacteria species most used as probiotics and starter cultures in food production. Bacteriocin-producers Lpb. plantarum are also promising natural food preservatives. This study aimed to characterize Lpb. plantarum R23 and its bacteriocins (R23 bacteriocins). The genome sequence of Lpb. plantarum R23 was obtained by whole-genome sequencing (WGS) in an Illumina NovaSeq platform. The activity of Lpb. plantarum R23-produced bacteriocin against two Listeria monocytogenes strains (L7946 and L7947) was evaluated, and its molecular size was determined by tricine-SDS-PAGE. No virulence or antibiotic resistance genes were detected. Four 100% identical proteins to the class II bacteriocins (Plantaricin E, Plantaricin F, Pediocin PA-1 (Pediocin AcH), and Coagulin A) were found by WGS analysis. The small (<6.5 kDa) R23 bacteriocins were stable at different pH values (ranging from 2 to 8), temperatures (between 4 and 100 °C), detergents (all, except Triton X-100 and Triton X-114 at 0.01 g/mL), and enzymes (catalase and α-amylase), did not adsorb to the producer cells, had a bacteriostatic mode of action and their maximum activity (AU/mL = 12,800) against two L. monocytogenes strains occurred between 15 and 21 h of Lpb. plantarum R23 growth. Lactiplantibacillus plantarum R23 showed to be a promising bio-preservative culture because, besides being safe, it produces a stable bacteriocin or bacteriocins (harbors genes encoding for the production of four) inhibiting pathogens as L. monocytogenes. Further studies in different food matrices are required to confirm this hypothesis and its suitability as a future starter culture.

Health-Promoting Role of Lactiplantibacillus plantarum Isolated from Fermented Foods

Fermentation processes have been used for centuries for food production and preservation. Besides the contribution of fermentation to food quality, recently, scientific interest in the beneficial nature of fermented foods as a reservoir of probiotic candidates is increasing. Fermented food microbes are gaining attention for their health-promoting potential and for being genetically related to human probiotic bacteria. Among them, Lactiplantibacillus (Lpb.) plantarum strains, with a long history in the food industry as starter cultures in the production of a wide variety of fermented foods, are being investigated for their beneficial properties which are similar to those of probiotic strains, and they are also applied in clinical interventions. Food-associated Lpb. plantarum showed a good adaptation and adhesion ability in the gastro-intestinal tract and the potential to affect host health through various beneficial activities, e.g., antimicrobial, antioxidative, antigenotoxic, anti-inflammatory and immunomodulatory, in several in vitro and in vivo studies. This review provides an overview of fermented-associated Lpb. plantarum health benefits with evidence from clinical studies. Probiotic criteria that fermented-associated microbes need to fulfil are also reported.

Oleuropein Transcriptionally Primes Lactobacillus plantarum to Interact With Plant Hosts

Oleuropein (OLE) is a secoiridoid unique to Oleaceae known to play a role in the plant–herbivore interaction. However, it is not clear how this molecule is induced to mediate plant responses to microbes and how microbes, in turn, withstand with OLE. To better understand how OLE affects the plant–microbe interaction, the contribution of differential gene expression in the adaptation to OLE was characterized by whole genome transcriptional profiling in Lactobacillus plantarum, a bacterium associated to the olive. OLE downregulated functions associated to rapid growth, remodeled membrane phospholipid biosynthesis pathways and markedly repressed the expression of several ABC transporters from L. plantarum. Genes encoding the plantaricin and lamABDCA quorum-sensing (QS) systems were down-regulated indicating the potential of OLE as a QS-antagonist. Notably, OLE diminished the expression of a set of genes encoding inmunomodulatory components and reoriented metabolic pathways to increase protein acetylation, probably to attenuate plant immunity. Responses were also triggered to repress the transport of acetoin and to buffer reactive oxygen species accumulation, two signals involved in plant development. The results suggest that OLE could act as a signaling molecule in the plant–microbe interaction and facilitate the accommodation of beneficial microbes such as L. plantarum by the plant host, via controlled expression of bacterial molecular players involved in this reciprocal interplay.

Functional Characterization of the Lactolisterin BU Gene Cluster of Lactococcus lactis subsp. lactis BGBU1-4

The gene cluster responsible for the production of the aureocin A53-like bacteriocin, lactolisterin BU, is located on plasmid pBU6 in Lactococcus lactis subsp. lactis BGBU1-4. Heterologous expression of pBU6 confirmed that production and limited immunity to lactolisterin BU were provided by the plasmid. Comparative analysis of aureocin A53-like operons revealed that the structural genes shared a low level of identity, while other genes were without homology, indicating a different origin. Subcloning and expression of genes located downstream of the structural gene, lliBU, revealed that the lactolisterin BU cluster consists of four genes: the structural gene lliBU, the abcT gene encoding an ABC transporter, the accL gene encoding an accessory protein and the immL gene which provides limited immunity to lactolisterin BU. Reverse transcription analysis revealed that all genes were transcribed as one polycistronic mRNA. Attempts to split the lactolisterin BU operon, even when both parts were under control of the PlliBU promoter, were unsuccessful indicating that expression of lactolisterin BU is probably precisely regulated at the translational level by translational coupling and is possible only when all genes of the operon are in cis constellation. Two ρ-independent transcription terminators were detected in the lactolisterin BU operon: the first in the intergenic region of the lliBU and abcT genes and the second at the end of operon. Deletion of the second transcription terminator did not influence production of the bacteriocin in lactococci.

Adaptation and Probiotic Potential of Lactobacilli, Isolated from the Oral Cavity and Intestines of Healthy People

The present study shows that, from 300 Lactobacillus strains isolated from the oral cavity and large intestine of 600 healthy people, only 9 had high antagonistic activity against pathogens and opportunistic pathogens. All antagonistic strains of lactobacilli have been identified by 16S rRNA sequencing and assigned to four species: Lactobacillus fermentum, Lactobacillus rhamnosus, Lactobacillus plantarum, and Lactobacillus casei. In addition, these lactobacilli appeared to be nonpathogenic and had some probiotic potential: the strains produced lactic acid and bacteriocins, showed high sensitivity to broad-spectrum antibiotics, and were capable of forming biofilms in vitro. With the help of PCR and specific primers, the presence of genes for prebacteriocins in L. plantarum (plnEF, plnJ, plnN) and L. rhamnosus (LGG_02380 and LGG_02400) has been revealed. It was found that intestinal strains of lactobacilli were resistant to hydrochloric acid and bile. Lactobacilli isolated from the oral cavity were characterized by a high degree of adhesion, whereas intestinal strains were characterized by average adhesion. Both types of lactobacilli had medium to high rates of auto-aggregation and hydrophobicity and could coaggregate with pathogens and opportunistic pathogens. Additionally, the ability of the lactobacilli strains to produce gasotransmitters, CH₄, CO₂, C₂H₆, CO, and NH₃, has been revealed.

Expression of Genes Involved in Bacteriocin Production and Self-Resistance in Lactobacillus brevis 174A Is Mediated by Two Regulatory Proteins

We have previously shown that the lactic acid bacterium Lactobacillus brevis 174A, isolated from Citrus iyo fruit, produces a bacteriocin designated brevicin 174A, which is comprised of two antibacterial polypeptides (designated brevicins 174A-β and 174A-γ). We have also found a gene cluster, composed of eight open reading frames (ORFs), that contains genes for the biosynthesis of brevicin 174A, self-resistance to its own bacteriocin, and two transcriptional regulatory proteins. Some lactic acid bacterial strains have a system to start the production of bacteriocin at an adequate stage of growth. Generally, the system consists of a membrane-bound histidine protein kinase (HPK) that senses a specific environmental stimulus and a corresponding response regulator (RR) that mediates the cellular response. We have previously shown that although the HPK- and RR-encoding genes are not found on the brevicin 174A biosynthetic gene cluster in the 174A strain, two putative regulatory genes, designated breD and breG, are in the gene cluster. In the present study, we demonstrate that the expression of brevicin 174A production and self-resistance is positively controlled by two transcriptional regulatory proteins, designated BreD and BreG. BreD is expressed together with BreE as the self-resistance determinant of L. brevis 174A. DNase I footprinting analysis and a promoter assay demonstrated that BreD binds to the breED promoter as a positive autoregulator. The present study also demonstrates that BreG, carrying a transmembrane domain, binds to the common promoter of breB and breC, encoding brevicins 174A-β and 174A-γ, respectively, for positive regulation.IMPORTANCE The problem of the appearance of bacteria that are resistant to practical antibiotics and the increasing demand for safe foods have increased interest in replacing conventional antibiotics with bacteriocin produced by the lactic acid bacteria. This antibacterial substance can inhibit the growth of pathogenic bacteria without side effects on the human body. The bacteriocin that is produced by a Citrus iyo-derived Lactobacillus brevis strain inhibits the growth of pathogenic bacteria such as Listeria monocytogenes, Staphylococcus aureus, and Streptococcus mutans In general, lactic acid bacterial strains have a system to start the production of bacteriocin at an adequate stage of growth, which is called a quorum-sensing system. The system consists of a membrane-bound histidine protein kinase that senses a specific environmental stimulus and a corresponding response regulator that mediates the cellular response. The present study demonstrates that the expression of the genes encoding bacteriocin biosynthesis and the self-resistance determinant is positively controlled by two transcriptional regulatory proteins.

Physiological Role of Two-Component Signal Transduction Systems in Food-Associated Lactic Acid Bacteria

Two-component systems (TCSs) are widespread signal transduction pathways mainly found in bacteria where they play a major role in adaptation to changing environmental conditions. TCSs generally consist of sensor histidine kinases that autophosphorylate in response to a specific stimulus and subsequently transfer the phosphate group to their cognate response regulators thus modulating their activity, usually as transcriptional regulators. In this review we present the current knowledge on the physiological role of TCSs in species of the families Lactobacillaceae and Leuconostocaceae of the group of lactic acid bacteria (LAB). LAB are microorganisms of great relevance for health and food production as the group spans from starter organisms to pathogens. Whereas the role of TCSs in pathogenic LAB (most of them belonging to the family Streptococcaceae) has focused the attention, the roles of TCSs in commensal LAB, such as most species of Lactobacillaceae and Leuconostocaceae, have been somewhat neglected. However, evidence available indicates that TCSs are key players in the regulation of the physiology of these bacteria. The first studies in food-associated LAB showed the involvement of some TCSs in quorum sensing and production of bacteriocins, but subsequent studies have shown that TCSs participate in other physiological processes, such as stress response, regulation of nitrogen metabolism, regulation of malate metabolism, and resistance to antimicrobial peptides, among others.

Characterization of antimicrobial substance from Lactobacillus salivarius KL-D4 and its application as biopreservative for creamy filling

Lactobacillus salivarius KL-D4 isolated from duck intestine produced bacteriocin which was stable at high temperature and a wide pH range of 3-10. Its cell free supernatant at pH 5.5 exhibited wide inhibitory spectrum against both G+ and G- bacteria. The highest bacteriocin production was obtained in MRS broth supplemented with 0.5 % (w/v) CaCO3 at 6 h by gentle shaking. PCR walking using specific primers at the conserved region of class-II bacteriocin resulted in 4 known genes of kld1, kld2, kld3 and kld4 with 100 % similarity to genes encoding for salivaricin α, β, induction peptide and histidine protein kinase of Lb. salivarius GJ-24 which did not previously report for bacteriocin characterization, while showing 94, 93, 59 and 62 % to other salivaricin gene cluster, respectively. The high activities of 25,600 AU/ml indicated a strong induction peptide expressed by kld3 which has low similarity to previous inducer reported. Based on operon analysis, only kld1, kld3 and kld4 could be expressed and subsequently elucidated that only salivaricin α like bacteriocin was produced and secreted out of the cells. Using protein purification, only a single peptide band obtained showed that this strain produced one bacteriocin which could be salivaricin α namely salivaricin KLD showing about 4.3 kDa on SDS-PAGE. Partial purification by 20 % ammonium sulfate precipitation of the product was tested on the artificial contamination of creamy filling by Bacillus cereus, Enterococcus faecalis, Pseudomonas stutzeri, Staphylococcus sp. and Stenotrophomonas sp. resulting the growth inhibitory efficiency of 4.45-66.9, 11.5-100, 100, 0-28.1 and 5-100 % respectively. Therefore, salivaricin KLD can be a tentative biopreservative for food industry in the future.

Insights into the DNA-binding mechanism of a LytTR-type transcription regulator

A combination of surface plasmon resonance (SPR) spectroscopy and interaction map® (IM) analysis was used to characterize binding of the LytTR-type response regulator YpdB to promoter DNA. YpdB follows an ‘AB-BA’ mechanism involving sequential and cooperative DNA binding followed by rapid successive promoter clearance.

Most bacterial response regulators (RRs) make contact with DNA through a recognition α-helix in their DNA-binding domains. An emerging class of RRs interacts with DNA via a relatively novel type of binding domain, called the LytTR domain, which is mainly composed of β-strands. YpdB belongs to this latter class, is part of a nutrient-sensing network in Escherichia coli and triggers expression of its only target gene, yhjX, in response to extracellular pyruvate. Expression of yhjX mainly occurs in the late exponential growth phase, and in a pulsed manner. Although the DNA-binding sites for YpdB are well defined, exactly how YpdB initiates pulsed gene expression has remained elusive. To address this question, we measured the binding kinetics of wild-type YpdB and the phosphomimetic variant YpdB-D53E to the yhjX promoter region (P_yhjX) using surface plasmon resonance (SPR) spectroscopy combined with interaction map® (IM) analysis. Both YpdB and YpdB-D53E bound as monomers to the tandem-repeat sequences in the promoter, with YpdB-D53E displaying a higher maximal binding rate than YpdB. Furthermore, we identified a high-affinity (A-site) and a low-affinity binding site (B-site) within the yhjX promoter. Only YpdB-D53E utilizes an ‘AB-BA’ DNA-binding mechanism, involving sequential and cooperative promoter binding, and rapid, successive promoter clearance. We propose that response regulator phosphorylation, in combination with the cycle of cooperative DNA binding and rapid promoter clearance just described, can account for pulsed gene expression.

Bacteriocins of lactic acid bacteria: extending the family

Lactic acid bacteria (LAB) constitute a heterogeneous group of microorganisms that produce lactic acid as the major product during the fermentation process. LAB are Gram-positive bacteria with great biotechnological potential in the food industry. They can produce bacteriocins, which are proteinaceous antimicrobial molecules with a diverse genetic origin, posttranslationally modified or not, that can help the producer organism to outcompete other bacterial species. In this review, we focus on the various types of bacteriocins that can be found in LAB and the organization and regulation of the gene clusters responsible for their production and biosynthesis, and consider the food applications of the prototype bacteriocins from LAB. Furthermore, we propose a revised classification of bacteriocins that can accommodate the increasing number of classes reported over the last years.

Quorum-sensing regulation of constitutive plantaricin by Lactobacillus plantarum strains under a model system for vegetables and fruits

This study aimed at investigating the regulatory system of bacteriocin synthesis by Lactobacillus plantarum strains in vegetables and fruits in a model system. Sterile and neutralized cell-free supernatant (CFS) from L. plantarum strains grown in MRS broth showed in vitro antimicrobial activities toward various indicator strains. The highest activity was that of L. plantarum C2. The antimicrobial activity was further assayed on vegetable and fruit agar plates (solid conditions) and in juices (liquid conditions). A regulatory mechanism of bacteriocin synthesis via quorum sensing was hypothesized. The synthesis of antimicrobial compounds seemed to be constitutive under solid conditions of growth on vegetable and fruit agar plates. In contrast, it depended on the size of the inoculum when L. plantarum C2 was grown in carrot juice. Only the inoculum of ca. 9.0 log CFU ml(-1) produced detectable activity. The genes plnA, plnEF, plnG, and plnH were found in all L. plantarum strains. The genes plnJK and plnN were detected in only three or four strains. Reverse-phase high-performance liquid chromatography purification and mass spectrometry analysis revealed the presence of a mixture of eight peptides in the most active fraction of the CFS from L. plantarum C2. Active peptides were encrypted into bacteriocin precursors, such as plantaricins PlnJ/K and PlnH and PlnG, which are involved in the ABC transport system. A real-time PCR assay showed an increase in the expression of plnJK and plnG during growth of L. plantarum C2 in carrot juice.

ComE/ComE~P interplay dictates activation or extinction status of pneumococcal X-state (competence)

Since 1996, induction of competence for genetic transformation of Streptococcus pneumoniae is known to be controlled by the ComD/ComE two-component regulatory system. The mechanism of induction is generally described as involving ComD autophosphorylation, transphosphorylation of ComE and transcriptional activation by ComE~P of the early competence (com) genes, including comX which encodes the competence-specific σ(X) . However, none of these features has been experimentally established. Here we document the autokinase activity of ComD proteins in vitro, and provide an estimate of the stoichiometry of ComD and ComE in vivo. We report that a phosphorylmimetic mutant, ComE(D58E), constructed because of the failure to detect transphosphorylation of purified ComE in vitro, displays full spontaneous competence in ΔcomD cells, an that in vitro ComE(D58E) exhibits significantly improved binding affinity for P(comCDE). We also provide evidence for a differential transcriptional activation and repression of P(comCDE) and P(comX). Altogether, these data support the model of ComE~P-dependent activation of transcription. Finally, we establish that ComE antagonizes expression of the early com genes and propose that the rapid deceleration of transcription from P(comCDE) observed even in cells lacking σ(X) is due to the progressive accumulation of ComE, which outcompetes ComE~P.

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.

An overview of the mosaic bacteriocin pln loci from Lactobacillus plantarum

The pln locus responsible for bacteriocin biosynthesis in Lactobacillus plantarum C11 was first unraveled about 15 years ago and since then different strains of L. plantarum (NC8, WCFS1, J23 and J51) have been found to harbor mosaic pln loci in their genomes. Each locus is of 18-19kb and contains 22-25 genes organized into 5-6 operons. Together these strains produce four different class IIb two-peptide bacteriocins, plantaricins EF, JK, NC8 and J51 and a pheromone peptide plantaricin A with antimicrobial activity. Their production has been found to be regulated through a quorum-sensing based network consisting of a secreted peptide pheromone, a membrane-located sensor and one or two transcription regulators. The individual loci each contain a set of semi-conserved regulated promoters with subtle differences necessary for the regulators to regulate their promoter activity individually with respect to timing and strength. These subtle differences in the promoters are highly conserved across the different pln loci, in a functionally related manner. In this review we will discuss various aspects of these bacteriocin loci with special focus on their mosaic genetic composition, gene regulation and mode of action. We also present a novel pln locus containing a transposon of the MULE superfamily, a mobile element which has not been described in L. plantarum before.

DNA binding kinetics of two response regulators, PlnC and PlnD, from the bacteriocin regulon of Lactobacillus plantarum C11

Background

Bacteriocin production in the lactic acid bacterium Lactobacillus plantarum C11 is regulated through a quorum sensing based pathway involving two highly homologous response regulators (59% identity and 76% similarity), PlnC as a transcriptional activator and PlnD as a repressor. Previous in vitro studies have shown that both regulators bind, as homodimers, to the same DNA regulatory repeats to exert their regulatory functions. As the genes for these two proteins are located on the same auto-regulatory operon, hence being co-expressed upon gene activation, it is plausible that their opposite functions must somehow be differentially regulated, either in terms of timing and/or binding kinetics, so that their activities do not impair each other in an uncontrolled manner. To understand the nature behind this potential differentiation, we have studied the binding kinetics of the two regulators on five target promoters (P_plnA, P_plnM, P_plnJ, P_plnE and P_plnG) from the bacteriocin regulon of L. plantarum C11.

Results

By using surface plasmon resonance spectroscopy we obtained parameters such as association rates, dissociation rates and dissociation constants, showing that the two regulators indeed differ greatly from each other in terms of cooperative binding and binding strength to the different promoters. For instance, cooperativity is very strong for PlnC binding to the promoter of the regulatory operon (P_plnA), but not to the promoter of the transport operon (P_plnG), while the opposite is seen for PlnD binding to these two promoters. The estimated affinity constants indicate that PlnC can bind to P_plnA to activate transcription of the key regulatory operon plnABCD without much interference from PlnD, and that the repressive function of PlnD might act through a different mechanism than repression of the regulatory operon.

Conclusion

We have characterised the DNA binding kinetics of the two regulators PlnC and PlnD from the bacteriocin locus in L. plantarum C11. Our data show that PlnC and PlnD, despite their strong homology to each other, differ greatly from each other in terms of binding affinity and cooperativity to the different promoters of the pln regulon.

Quorum sensing in staphylococci

The staphylococcal agr locus encodes a quorum sensing (QS) system that controls the expression of virulence and other accessory genes by a classical two-component signaling module. Like QS modalities in other Gram-positive bacteria, agr encodes an autoactivating peptide (AIP) that is the inducing ligand for AgrC, the agr signal receptor. Unlike other such systems, agr variants have arisen that show strong cross-inhibition in heterologous combinations, with important evolutionary implications. Also unlike other systems, the effector of global gene regulation in the agr system is a major regulatory RNA, RNAIII. In this review, we describe the functions of the agr system's elements, show how they interact to bring about the regulatory response, and discuss the role of QS in staphylococcal pathobiology. We conclude with the suggestion that agr autoactivation, unlike classical enzyme induction, can occur under suboptimal conditions and can distinguish self from non-self by inducing an exclusive and coordinated population wide response.

Structure of the Staphylococcus aureus AgrA LytTR domain bound to DNA reveals a beta fold with an unusual mode of binding

The LytTR domain is a DNA-binding motif found within the AlgR/AgrA/LytR family of transcription factors that regulate virulence factor and toxin gene expression in pathogenic bacteria. This previously uncharacterized domain lacks sequence similarity with proteins of known structure. The crystal structure of the DNA-binding domain of Staphylococcus aureus AgrA complexed with a DNA pentadecamer duplex has been determined at 1.6 A resolution. The structure establishes a 10-stranded beta fold for the LytTR domain and reveals its mode of interaction with DNA. Residues within loop regions of AgrA contact two successive major grooves and the intervening minor groove on one face of the oligonucleotide duplex, inducing a substantial bend in the DNA. Loss of DNA binding upon substitution of key interacting residues in AgrA supports the observed binding mode. This mode of protein-DNA interaction provides a potential target for future antimicrobial drug design.

Identification of an operon and inducing peptide involved in the production of lactacin B by Lactobacillus acidophilus

AIM

To determine if a 9.5-kb region on the Lactobacillus acidophilus NCFM genome, encoded the genetic determinants for regulation and production of lactacin B, a class II bacteriocin.

METHODS

Transcriptional analysis was used to identify a 9.5-kb polycistronic region suspected of encoding the lab operon. The 12 putative open reading frames (LBA1803-LBA1791) were organized into three clusters: a production and regulation cluster encoding a putative two-component signal transduction system; an export cluster encoding a putative ABC transporter and a final cluster composed of three unknown proteins. Seven genes were typical of bacteriocins, encoding small, cationic peptides, each with an N-terminal double-glycine leader motif. Inactivation of a predicted ABC transporter completely abolished bacteriocin activity. When cloned and expressed together, LBA1803-LBA1800 resulted in markedly higher levels of lactacin B activity. The four peptides were chemically synthesized but exhibited no bacteriocin activity, alone or in combination. Only LBA1800 induced lactacin B production in broth cultures.

CONCLUSIONS

Lactacin B production is encoded within the 9.5-kb lab operon of 12 genes that are transcribed in a single transcript. LBA1800 is an inducing peptide of bacteriocin production.

SIGNIFICANCE AND IMPACT OF THE STUDY

A three-component regulatory system common to class II bacteriocins regulates the production of this bacteriocin by Lact. acidophilus.

Characterization of a new organization of the plantaricin locus in the inducible bacteriocin-producing Lactobacillus plantarum J23 of grape must origin

Lactobacillus plantarum J23 was previously characterized as a bacteriocin-producer-strain when it was cocultured with other lactic acid bacteria. In this work, the genetic organization of the pln locus in the J23 strain was studied and compared with those of previously described L. plantarum C11, WCFS1 and NC8 strains. A new organization of the plantaricin locus was detected in the J23 strain. The sequenced fragment (20,266 bp) comprised plnJLR, plnMNOP, plnEFI, plnGHSTUVWXY, and plNC8IF-plNC8HK-plnD operons, as well as a new region that includes three new orfs (GenBank accession number DQ323671). When the J23 pln gene sequences were compared with those included in the GenBank database, the identity of the putative encoded proteins was in the range 67.1-100%. The regulatory system and the repertoire of putative bacteriocins of the J23 pln locus presented important differences with respect to the ones of C11, WCFS1 and NC8, such as the absence of plnK and the presence of a larger plnJ gene than the previously described for the other L. plantarum strains. The pln locus in L. plantarum strains seems to be a mosaic-like structure with different modules and reorganizations that presents highly conserved regions related to transport and bacteriocin maturation and variable regions related to regulation and bacteriocin production.

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.

Quorum-sensing based bacteriocin production is down-regulated by N-terminally truncated species of gene activators

Down-regulation of quorum-sensing based pathways is an important but yet poorly understood process in bacterial gene regulation. In this study, we show that the gene regulator plnC not only acts as an activator gene in the quorum-sensing based bacteriocin production in Lactobacillus plantarum C11, but it also concurrently codes for truncated forms that were shown to repress bacteriocin production. By amino acid N-terminal sequencing and DNA sequence analysis, the truncated species of PlnC are believed to be translated from alternative start codons located in the so-called receiver domain of the regulator. To analyse the structure-function relationship of truncated species of PlnC, we performed a series of systematic truncation mutations: ten in the receiver domain, one in the hinge region and two in the C-terminal DNA-binding domain. It was revealed that any truncation mutation containing a disrupted receiver domain together with an intact DNA-binding domain displayed a repressive effect on bacteriocin production. Such a gene repression mechanism mediated by truncated regulators was also found in two other quorum-sensing based bacteriocin systems (spp in L. sakei LTH673 and NC8-pln in L. plantarum NC8), suggesting that this mode of repression might represent a common means applied by bacteria to down-regulate certain quorum-sensing based pathways.

Stimulus perception in bacterial signal-transducing histidine kinases

Two-component signal-transducing systems are ubiquitously distributed communication interfaces in bacteria. They consist of a histidine kinase that senses a specific environmental stimulus and a cognate response regulator that mediates the cellular response, mostly through differential expression of target genes. Histidine kinases are typically transmembrane proteins harboring at least two domains: an input (or sensor) domain and a cytoplasmic transmitter (or kinase) domain. They can be identified and classified by virtue of their conserved cytoplasmic kinase domains. In contrast, the sensor domains are highly variable, reflecting the plethora of different signals and modes of sensing. In order to gain insight into the mechanisms of stimulus perception by bacterial histidine kinases, we here survey sensor domain architecture and topology within the bacterial membrane, functional aspects related to this topology, and sequence and phylogenetic conservation. Based on these criteria, three groups of histidine kinases can be differentiated. (i) Periplasmic-sensing histidine kinases detect their stimuli (often small solutes) through an extracellular input domain. (ii) Histidine kinases with sensing mechanisms linked to the transmembrane regions detect stimuli (usually membrane-associated stimuli, such as ionic strength, osmolarity, turgor, or functional state of the cell envelope) via their membrane-spanning segments and sometimes via additional short extracellular loops. (iii) Cytoplasmic-sensing histidine kinases (either membrane anchored or soluble) detect cellular or diffusible signals reporting the metabolic or developmental state of the cell. This review provides an overview of mechanisms of stimulus perception for members of all three groups of bacterial signal-transducing histidine kinases.

Isolation of bacteriocinogenic Lactobacillus plantarum strains from ben saalga, a traditional fermented gruel from Burkina Faso

A collection of lactic acid bacteria isolated from ben saalga, a traditional fermented gruel from Burkina Faso, was screened for bacteriocin production. Seven isolates were selected for their broad antimicrobial spectra, which overall included strains of Bacillus cereus, Bacillus licheniformis, Enterococcus faecalis, Listeria innocua, Listeria monocytogenes, Staphylococcus aureus, Escherichia coli and Salmonella enterica. Cluster analysis of RAPD-PCR patterns revealed that six of the isolates represent different strains. The six selected strains were identified as Lactobacillus plantarum by 16S rDNA sequencing, species-specific PCR and multiplex PCR of the recA gene. PCR amplification revealed the presence of genes of the plantaricin cluster described in L. plantarum C11. Among them, strain 5.2.2 carried the largest number of genes from this cluster.

Production of piscicolin 126 by Carnobacterium maltaromaticum UAL26 is controlled by temperature and induction peptide concentration

Carnobacterium maltaromaticum UAL26 produces the antimicrobial peptides (bacteriocins) piscicolin 126, first isolated from C. maltaromaticum JG126, and carnobacteriocin BM1, first isolated from C. maltaromaticum LV17. C. maltaromaticum UAL26 is especially inhibitory to strains of Listeria monocytogenes. Bacteriocin activity is not observable in the supernatant of cultures of UAL26 grown in liquid media at 25 degrees C, but at temperatures less than 19 degrees C bacteriocin activity can be detected. In contrast to JG126, the piscicolin 126 operon is downregulated in UAL26 at higher temperature, and piscicolin 126 mRNA is not detected when UAL26 is grown at 25 degrees C. Bacteriocin production in UAL26 grown at 15 degrees C can be induced by addition of 10(-10) M of chemically synthesized piscicolin 126 induction peptide (PisN). However, induction of bacteriocin production in UAL26 grown at 25 degrees C requires 10(-7) M of PisN. The sequence of the piscicolin 126 operon in UAL26 contains 34 single nucleotide differences compared with the piscicolin 126 operon in JG126, including single nucleotide differences in the immunity, histidine kinase, dedicated ABC-transporter and accessory genes, as well as a single nucleotide deletion in the transport accessory gene. This deletion causes a frameshift, resulting in truncation of the PisE transport accessory protein in UAL26.

Data mining and characterization of a novel pediocin-like bacteriocin system from the genome of Pediococcus pentosaceus ATCC 25745

The genome of Pediococcus pentosaceus ATCC 25745 contains a gene cluster that resembles a regulated bacteriocin system. The gene cluster has an operon-like structure consisting of a putative pediocin-like bacteriocin gene (termed penA) and a potential immunity gene (termed peiA). Genetic determinants involved in bacteriocin transport and regulation are also found in proximity to penA and peiA but the so-called accessory gene involved in transport and the inducer gene involved in regulation are missing. Consequently, this bacterium is a poor bacteriocin producer. To analyse the potency of the putative bacteriocin operon, the two genes penA-peiA were heterologously expressed in a Lactobacillus sakei host that contains the complete apparatus for gene activation, maturation and externalization of bacteriocins. It was demonstrated that the heterologous host expressing penA and peiA produced a strong bacteriocin activity; in addition, the host became immune to its own bacteriocin, identifying the gene pair penA-peiA as a potent bacteriocin system. The novel pediocin-like bacteriocin, termed penocin A, has an isotopic mass [M+H]+ of 4684.6 Da as determined by mass spectrometry; this value corresponds well to the expected size of the mature 42 aa peptide containing a disulfide bridge. The bacteriocin is heat-stable but protease-sensitive and has a calculated pI of 9.45. Penocin A has a relatively broad inhibition spectrum, including pathogenic Listeria and Clostridium species. Immediately upstream of the regulatory genes reside some features that resemble remnants of a disrupted inducer gene. This degenerate gene was restored and shown to encode a double-glycine leader-containing peptide. Furthermore, expression of the restored gene triggered high bacteriocin production in P. pentosaceus ATCC 25745, thus confirming its role as an inducer in the pen regulon.

Improved expression and purification of the correctly folded response regulator PlnC from lactobacilli

The response regulator PlnC is part of the signal transduction system that plays a key role in the regulation of bacteriocin production in Lactobacillus plantarum C11. In this study, we wanted to express high levels of the response regulator PlnC in a soluble and native form for purification and further studies. The protein was expressed as a fusion protein (fPlnC) containing an N-terminal Flag-tag to facilitate detection and purification. When the fusion gene, fplnC, was expressed in Escherichia coli BL21, nearly all (99%) of the recombinant protein ended up inside inclusion bodies as an incorrectly folded protein. By utilizing two different Gram-positive expression systems (SIP and NICE) in L. plantarum NC8 and Lactobacillus sakei Lb790, the expression of the soluble fPlnC was significantly increased, being 20-40 times more than that in E. coli BL21. Using the N-terminal tag, the expressed protein was purified by immunoprecipitation. By DNA-binding study (EMSA), we demonstrated that the fusion protein purified from the soluble pool was correctly folded as judged by its ability to bind specifically on regulated promoters. Using our approach, we estimate that about 1 mg of fPlnC can be purified from 11 of the bacterial culture.

Purification and sequencing of cerein 7B, a novel bacteriocin produced by Bacillus cereus Bc7

Cerein 7B is a new bacteriocin produced simultaneously with cerein 7A by Bacillus cereus Bc7 in liquid brain heart infusion cultures. Both bacteriocins are not synergistic. The two peptides have been purified to homogeneity by hydrophobic interaction, cation exchange and reverse-phase liquid chromatography. They can be distinguished by their N-terminal amino acid sequences N-Gly-Trp-Gly-Asp-Val-Leu (7A) and N-Gly-Trp-Trp-Asn-Ser-Trp-Gly-Lys (7B). Pre-cerein 7B is 74 amino acids long and contains an 18 aminoacid double-glycine type leader sequence that is removed to produce the mature bacteriocin. The leader peptide sequence is related to that of sec-independent secretion signals suggesting that cerein 7B belongs to class II sec-independent bacteriocins.

Bacteriocins: developing innate immunity for food

Bacteriocins are bacterially produced antimicrobial peptides with narrow or broad host ranges. Many bacteriocins are produced by food-grade lactic acid bacteria, a phenomenon which offers food scientists the possibility of directing or preventing the development of specific bacterial species in food. This can be particularly useful in preservation or food safety applications, but also has implications for the development of desirable flora in fermented food. In this sense, bacteriocins can be used to confer a rudimentary form of innate immunity to foodstuffs, helping processors extend their control over the food flora long after manufacture.

Plasmid p256 from Lactobacillus plantarum represents a new type of replicon in lactic acid bacteria, and contains a toxin-antitoxin-like plasmid maintenance system

Lactobacillus plantarum NC7 harbours a single 7.2 kb plasmid called p256. This report describes the complete nucleotide sequence and annotation of p256, as well as the identification of the minimal replicon of the plasmid. Based on sequence features in the unusually small (0.7 kb) minimal replicon, and the absence of a gene for a replication-relevant protein, p256 seems to represent a hitherto unknown type of theta replicon in lactic acid bacteria (LAB), with a relatively low copy-number. In addition, a putative toxin-antitoxin (TA) locus was identified. Experiments with variants of p256 indicated that the TA system was involved in plasmid maintenance. Furthermore, controlled expression of the TA genes stabilized vectors derived from the p256 replicon. To the authors' knowledge, this is the first time a TA locus with a demonstrated plasmid maintenance function has been identified in LAB. Transformation of several LAB with plasmids derived from p256 indicated that it has a narrow host range. Several effective expression vectors based on the p256 replicon have been constructed.

Antagonistic activities of lactobacilli and bifidobacteria against microbial pathogens

The gastrointestinal tract is a complex ecosystem that associates a resident microbiota and cells of various phenotypes lining the epithelial wall expressing complex metabolic activities. The resident microbiota in the digestive tract is a heterogeneous microbial ecosystem containing up to 1 x 10(14) colony-forming units (CFUs) of bacteria. The intestinal microbiota plays an important role in normal gut function and maintaining host health. The host is protected from attack by potentially harmful microbial microorganisms by the physical and chemical barriers created by the gastrointestinal epithelium. The cells lining the gastrointestinal epithelium and the resident microbiota are two partners that properly and/or synergistically function to promote an efficient host system of defence. The gastrointestinal cells that make up the epithelium, provide a physical barrier that protects the host against the unwanted intrusion of microorganisms into the gastrointestinal microbiota, and against the penetration of harmful microorganisms which usurp the cellular molecules and signalling pathways of the host to become pathogenic. One of the basic physiological functions of the resident microbiota is that it functions as a microbial barrier against microbial pathogens. The mechanisms by which the species of the microbiota exert this barrier effect remain largely to be determined. There is increasing evidence that lactobacilli and bifidobacteria, which inhabit the gastrointestinal microbiota, develop antimicrobial activities that participate in the host's gastrointestinal system of defence. The objective of this review is to analyze the in vitro and in vivo experimental and clinical studies in which the antimicrobial activities of selected lactobacilli and bifidobacteria strains have been documented.

Identification of a region involved in the pheromone receptor function of the histidine kinase PlnB

Bacteriocin biosynthesis in Lactobacillus plantarum is an inducible process, triggered by the secreted inducer peptide pheromone IP-C11. The environmental concentration of IP-C11 is monitored by the membrane-bound histidine protein kinase PlnB, which is part of a two-component signal transduction pathway. Upon interaction with IP-C11, PlnB phosphorylates the cognate response regulator PlnC. This regulator subsequently activates transcription of the bacteriocin genes. PlnB belongs to the HPK(10) subfamily of peptide-pheromone-activated histidine kinases. All members of this subfamily have an unusual polytopic membrane domain that previously has been shown to contain the peptide pheromone receptor. Employing an in vivo reporter assay, the present work investigated the receptor functionality of various mutagenized PlnB membrane domains. The results indicated that important determinants for receptor function locate to the most N-terminal extracytoplasmatic loop of the membrane domain. In addition, this region appears to be involved in the peptide pheromone interaction of ComD, another member of the HPK(10) subfamily.

Induction of plantaricin production in Lactobacillus plantarum NC8 after coculture with specific gram-positive bacteria is mediated by an autoinduction mechanism

Plantaricin NC8 (PLNC8), a coculture-inducible two-peptide bacteriocin from Lactobacillus plantarum NC8, has recently been purified and genetically characterized. Analysis of an 8.1-kb NC8 DNA region downstream of the PLNC8 operon revealed the presence of at least four operons involved in bacteriocin production, showing high homology to the plantaricin cluster in L. plantarum C11. However, we found a three-component regulatory operon involving a quorum-sensing mechanism. Two of these components, the induction factor (PLNC8IF) and the histidine kinase, are novel, while the response regulator is identical to PlnD from C11. Homologous expression of plNC8IF in NC8 allowed constitutive bacteriocin production. Heterologous expression of this gene in Lactococcus lactis MG1363 produced supernatants which promoted bacteriocin production in NC8. Reverse transcription-PCR studies indicated that cocultivation of NC8 with inducing cells promoted transcription of the bacteriocin and regulatory operons in NC8. An identical result was obtained after addition of an external source of PLNC8IF. We propose that the presence of specific bacteria could act as an environmental signal that is able to switch on bacteriocin production in L. plantarum NC8 via a quorum-sensing mechanism mediated by PLNC8IF.

Structural analysis of the peptide pheromone receptor PlnB, a histidine protein kinase from Lactobacillus plantarum

Intercellular communication plays a key role in the regulation of several physiological processes in gram-positive bacteria. Cell-cell communication is often mediated by secreted inducer peptide pheromones (IPs), which upon reaching a threshold concentration in the environment specifically activate a cognate membrane-localized histidine protein kinase (HPK). Interestingly, the majority of IP-activated HPKs fall into one distinct subfamily (HPK(10)). As part of an effort to study the mechanism underlying pheromone-mediated activation of the HPK(10) subfamily, the present work investigated the membrane topology of PlnB from Lactobacillus plantarum. Gene fusion experiments with Escherichia coli and Lactobacillus sakei, using alkaline phosphatase, beta-lactamase, and beta-galactosidase reporter fusions, suggested that PlnB is anchored to the cytoplasmic membrane via seven transmembrane segments. By domain switching between HPK(10) members, it was demonstrated that the determinants for pheromone binding and specificity are contained within the transmembrane domain. The results also indicate that the mechanism of signal transduction, in which the final transmembrane segment apparently plays a key role, is conserved between members of the HPK(10) subfamily.

Autoinduction and signal transduction in the regulation of staphylococcal virulence

The accessory genes of Staphylococcus aureus, including those involved in pathogenesis, are controlled by a complex regulatory network that includes at least four two-component systems, one of which, agr, is a quorum sensor, an alternative sigma factor and a large set of transcription factors, including at least two of the superantigen genes, tst and seb. These regulatory genes are hypothesized to act in a time- and population density-dependent manner to integrate signals received from the external environment with the internal metabolic machinery of the cell, in order to achieve the production of particular subsets of accessory/virulence factors at the time and in quantities that are appropriate to the needs of the organism at any given location. From the standpoint of pathogenesis, the regulatory agenda is presumably tuned to particular sites in the host organism. To address this hypothesis, it will be necessary to understand in considerable detail the regulatory interactions among the organism's numerous controlling systems. This review is an attempt to integrate a large body of data into the beginnings of a model that will hopefully help to guide research towards a full-scale test.