Structural and functional analysis of the S-layer protein crystallisation domain of Lactobacillus acidophilus ATCC 4356: evidence for protein-protein interaction of two subdomains

Synbiotics: a New Route of Self-production and Applications to Human and Animal Health

Synbiotics are preparations in which prebiotics are added to probiotics to achieve superior performance and benefits on the host. A new route of their formation is to induce the prebiotic biosynthesis within the probiotic for synbiotic self-production or autologous synbiotics. The aim of this review paper is first to overview the basic concept and (updated) definitions of synergistic synbiotics, and then to focus particularly on the prebiotic properties of probiotic wall components while describing the environmental factors/stresses that stimulate autologous synbiotics, that is, the biosynthesis of prebiotic-forming microcapsule by probiotic bacteria, and finally to present some of their applications to human and animal health.

TRAIL/S-layer/graphene quantum dot nanohybrid enhanced stability and anticancer activity of TRAIL on colon cancer cells

Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL), known as a cytokine of the TNF superfamily, is considered a promising antitumor agent due to its ability to selectively induce apoptosis in a wide variety of cancer cells. However, failure of its successful translation into clinic has led to development of nano-based platforms aiming to improve TRAIL therapeutic efficacy. In this regard, we fabricated a novel TRAIL-S-layer fusion protein (S-TRAIL) conjugated with graphene quantum dots (GQDs) to benefit both the self-assembly of S-layer proteins, which leads to elevated TRAIL functional stability, and unique optical properties of GQDs. Noncovalent conjugation of biocompatible GQDs and soluble fusion protein was verified via UV–visible and fluorescence spectroscopy, size and ζ-potential measurements and transmission electron microscopy. The potential anticancer efficacy of the nanohybrid system on intrinsically resistant cells to TRAIL (HT-29 human colon carcinoma cells) was investigated by MTT assay and flow cytometry, which indicated about 80% apoptosis in cancer cells. These results highlight the potential of TRAIL as a therapeutic protein that can be extensively improved by taking advantage of nanotechnology and introduce S-TRAIL/GQD complex as a promising nanohybrid system in cancer treatment.

Functional role of surface layer proteins of Lactobacillus acidophilus L-92 in stress tolerance and binding to host cell proteins

Lactobacillus acidophilus surface layer proteins (SLPs) self-assemble into a monolayer that is non-covalently bound to the outer surface of the cells. There they are in direct contact with the environment, environmental stressors and gut components of the host in which the organism resides. The role of L. acidophilus SLPs is not entirely understood, although SLPs seem to be essential for bacterial growth. We constructed three L. acidophilus L-92 strains, each expressing a mutant of the most abundant SLP, SlpA. Each carried a 12-amino acid c-myc epitope substitution at a different position in the protein. A strain was also obtained that expressed the SlpA paralog SlpB from an originally silent slpB gene. All four strains behaved differently with respect to growth under various stress conditions, such as the presence of salt, ox gall or ethanol, suggesting that SlpA affects stress tolerance in L. acidophilus L-92. Also, the four mutants showed differential in vitro binding ability to human host cell proteins such as uromodulin or dendritic cell (DC)-specific intercellular adhesion molecule-3 grabbing non-integrin (DC-SIGN). Furthermore, co-culture of murine immature DCs with a mutant strain expressing one of the recombinant SlpA proteins changed the concentrations of the cytokines IL-10 and IL-12. Our data suggest that SlpA and SlpB of L. acidophilus participate in bacterial stress tolerance and binding to uromodulin or DC-SIGN, possibly leading to effective immune-modification.

Development of an Antigen Delivery Platform Using Lactobacillus acidophilus Decorated With Heterologous Proteins: A Sheep in Wolf's Clothing Story

S-layers are bacterial structures present on the surface of several Gram-positive and Gram-negative bacteria that play a role in bacterial protection. In Lactobacillus acidophilus (L. acidophilus ATCC 4356), the S-layer is mainly composed of the protein SlpA. A tandem of two copies of the protein domain SLP-A (pfam: 03217) was identified at the C-terminal of SlpA, being this double SLP-A protein domain (in short dSLP-A) necessary and sufficient for the association of the protein to the L. acidophilus cell wall. A variety of proteins fused to the dSLP-A domain were able to spontaneously associate with high affinity to the cell wall of L. acidophilus and Bacillus subtilis var. natto, in a process that we termed decoration. Binding of dSLP-A-containing-proteins to L. acidophilus was stable at conditions that mimic the gastrointestinal transit in terms of pH, proteases, and bile salts. To evaluate if protein decoration of L. acidophilus can be adapted to generate an oral vaccine platform, a chimeric antigen derived from the bacterial pathogen Shiga-toxin-producing Escherichia coli (STEC) was constructed by fusing the sequences encoding the polypeptides EspA^36–192, Intimin^653–953, Tir^240–378, and H7 flagellin^352–374 (EITH7) to the dSLP-A domain (EITH7-dSLP-A). Recombinantly expressed EITH7-dSLP-A protein was affinity purified and combined with L. acidophilus cultures to allow the association of the chimeric antigen to the bacterial surface. EITH7-decorated L. acidophilus was orally administered to BALB/c mice and the induction of anti-EITH7 specific antibodies in sera and feces determined by ELISA. Mice presenting significantly higher anti-EITH7 antibodies titers were able to control more efficiently an experimental STEC infection than mice that received the non-decorated L. acidophilus carrier, indicating that antigen-decorated L. acidophilus can be adapted as a mucosal immunization delivery platform to elicit a protective immune response for vaccine purposes.

Protective effects of a food-grade recombinant Lactobacillus plantarum with surface displayed AMA1 and EtMIC2 proteins of Eimeria tenella in broiler chickens

Background

Avian coccidiosis posts a severe threat to poultry production. In addition to commercial attenuated vaccines, other strategies to combat coccidiosis are urgently needed. Lactobacillus plantarum has been frequently used for expression of foreign proteins as an oral vaccine delivery system using traditional erythromycin resistance gene (erm). However, antibiotic selection markers were often used during protein expression and they pose a risk of transferring antibiotic resistance genes to the environment, and significantly restricting the application in field production. Therefore, a food-grade recombinant L. plantarum vaccine candidate would dramatically improve its application potential in the poultry industry.

Results

In this study, we firstly replaced the erythromycin resistance gene (erm) of the pLp_1261Inv-derived expression vector with a non-antibiotic, asd-alr fusion gene, yielding a series of non-antibiotic and reliable, food grade expression vectors. In addition, we designed a dual-expression vector that displayed two foreign proteins on the surface of L. plantarum using the anchoring sequences from either a truncated poly-γ-glutamic acid synthetase A (pgsA′) from Bacillus subtilis or the L. acidophilus surface layer protein (SlpA). EGFP and mCherry were used as marker proteins to evaluate the surface displayed properties of recombinant L. plantarum strains and were inspected by western blot, flow cytometry and fluorescence microscopy. To further determine its application as oral vaccine candidate, the AMA1 and EtMIC2 genes of E. tenella were anchored on the surface of L. plantarum strain. After oral immunization in chickens, the recombinant L. plantarum strain was able to induce antigen specific humoral, mucosal, and T cell-mediated immune responses, providing efficient protection against coccidiosis challenge.

Conclusions

The novel constructed food grade recombinant L. plantarum strain with double surface displayed antigens provides a potential efficient oral vaccine candidate for coccidiosis.

Engineering Components of the Lactobacillus S-Layer for Biotherapeutic Applications

Lactic acid bacteria (LAB) are frequently harnessed for the delivery of biomolecules to mucosal tissues. Several species of Lactobacillus are commonly employed for this task, of which a subset are known to possess surface-layers (S-layers). S-layers are two-dimensional crystalline arrays of repeating proteinaceous subunits that form the outermost coating of many prokaryotic cell envelopes. Their periodicity and abundance have made them a target for numerous biotechnological applications. In the following review, we examine the multi-faceted S-layer protein (Slp), and its use in both heterologous protein expression systems and mucosal vaccine delivery frameworks, through its diverse genetic components: the strong native promoter, capable of synthesizing as many as 500 Slp subunits per second; the signal peptide that stimulates robust secretion of recombinant proteins; and the structural domains, which can be harnessed for both cell surface display of foreign peptides or adhesion enhancement of a host bacterium. Although numerous studies have established vaccine platforms based on one or more components of the Lactobacillus S-layer, this area of research still remains largely in its infancy, thus this review is meant to not only highlight past works, but also advocate for the future usage of Slps in biotherapeutic research.

Lactobacillus buchneri S-layer as carrier for an Ara h 2-derived peptide for peanut allergen-specific immunotherapy

Peanut allergy is an IgE-mediated severe hypersensitivity disorder. The lack of a treatment of this potentially fatal allergy has led to intensive research on vaccine development. Here, we describe the design and initial characterization of a carrier-bound peptide derived from the most potent peanut allergen, Ara h 2, as a candidate vaccine. Based on the adjuvant capability of bacterial surface (S-) layers, a fusion protein of the S-layer protein SlpB from Lactobacillus buchneri CD034 and the Ara h 2-derived peptide AH3a42 was produced. This peptide comprised immunodominant B-cell epitopes as well as one T cell epitope. The fusion protein SlpB-AH3a42 was expressed in E. coli, purified, and tested for its IgE binding capacity as well as for its ability to activate sensitized rat basophil leukemia (RBL) cells. The capacity of Ara h 2-specific IgG rabbit-antibodies raised against SlpB-AH3a42 or Ara h 2 to inhibit IgE-binding was determined by ELISA inhibition assays using sera of peanut allergic patients sensitized to Ara h 2. IgE specific to the SlpB-AH3a42 fusion protein was detected in 69% (25 of 36) of the sera. Despite the recognition by IgE, the SlpB-AH3a42 fusion protein was unable to induce β-hexosaminidase release from sensitized RBL cells at concentrations up to 100ng per ml. The inhibition of IgE-binding to the natural allergen observed after pre-incubation of the 20 sera with rabbit anti-SlpB-AH3a42 IgG was more than 30% for four sera, more than 20% for eight sera, and below 10% for eight sera. In comparison, anti-Ara h 2 rabbit IgG antibodies inhibited binding to Ara h 2 by 48% ±13.5%. Our data provide evidence for the feasibility of this novel approach towards the development of a peanut allergen peptide-based carrier-bound vaccine. Our experiments further indicate that more than one allergen-peptide will be needed to induce a broader protection of patients allergic to Ara h 2.

Surface display on lactic acid bacteria without genetic modification: strategies and applications

Microbial cell surface display has attracted greater attention than ever and has numerous potential applications in biotechnology. With the safety and probiotic properties, lactic acid bacteria (LAB) have been used widely in food and industrial applications. In order to circumvent using genetically modified microorganisms which face low public acceptance and severe regulatory scrutiny, surface-engineered LAB without genetical modification are more preferred. According to the way used to obtain the fusion protein containing the passenger molecule and anchoring domain, the genetic or chemical approaches can be used to construct these surface-engineered LAB. In addition to the viable wide-type LAB, non-living bacterial-like particles (BLP) can be attached by these fusion proteins added from outside. Compared to the living LAB, BLP have a higher binding capacity and less anticarrier response. Mucosal vaccines are the predominant application of these surface-engineered LAB with no genetical modification.

Influence of osmotic stress on the profile and gene expression of surface layer proteins in Lactobacillus acidophilus ATCC 4356

In this work, we studied the role of surface layer (S-layer) proteins in the adaptation of Lactobacillus acidophilus ATCC 4356 to the osmotic stress generated by high salt. The amounts of the predominant and the auxiliary S-layer proteins SlpA and SlpX were strongly influenced by the growth phase and high-salt conditions (0.6 M NaCl). Changes in gene expression were also observed as the mRNAs of the slpA and slpX genes increased related to the growth phase and presence of high salt. A growth stage-dependent modification on the S-layer protein profile in response to NaCl was observed: while in control conditions, the auxiliary SlpX protein represented less than 10 % of the total S-layer protein, in high-salt conditions, it increased to almost 40 % in the stationary phase. The increase in S-layer protein synthesis in the stress condition could be a consequence of or a way to counteract the fragility of the cell wall, since a decrease in the cell wall thickness and envelope components (peptidoglycan layer and lipoteichoic acid content) was observed in L. acidophilus when compared to a non-S-layer-producing species such as Lactobacillus casei. Also, the stationary phase and growth in high-salt medium resulted in increased release of S-layer proteins to the supernatant medium. Overall, these findings suggest that pre-growth in high-salt conditions would result in an advantage for the probiotic nature of L. acidophilus ATCC 4356 as the increased amount and release of the S-layer might be appropriate for its antimicrobial capacity.

Characteristics of the Clostridium difficile cell envelope and its importance in therapeutics

Clostridium difficile infection (CDI) is a challenging threat to human health. Infections occur after disruption of the normal microbiota, most commonly through the use of antibiotics. Current treatment for CDI largely relies on the broad‐spectrum antibiotics vancomycin and metronidazole that further disrupt the microbiota resulting in frequent recurrence, highlighting the need for C. difficile‐specific antimicrobials. The cell surface of C. difficile represents a promising target for the development of new drugs. C. difficile possesses a highly deacetylated peptidoglycan cell wall containing unique secondary cell wall polymers. Bound to the cell wall is an essential S‐layer, formed of SlpA and decorated with an additional 28 related proteins. In addition to the S‐layer, many other cell surface proteins have been identified, including several with roles in host colonization. This review aims to summarize our current understanding of these different C. difficile cell surface components and their viability as therapeutic targets.

Display of recombinant proteins at the surface of lactic acid bacteria: strategies and applications

Lactic acid bacteria (LAB) are promising vectors of choice to deliver active molecules to mucosal tissues. They are recognized as safe by the World Health Organization and some strains have probiotic properties. The wide range of potential applications of LAB-driven mucosal delivery includes control of inflammatory bowel disease, vaccine delivery, and management of auto-immune diseases. Because of this potential, strategies for the display of proteins at the surface of LAB are gaining interest. To display a protein at the surface of LAB, a signal peptide and an anchor domain are necessary. The recombinant protein can be attached to the membrane layer, using a transmembrane anchor or a lipoprotein-anchor, or to the cell wall, by a covalent link using sortase mediated anchoring via the LPXTG motif, or by non-covalent liaisons employing binding domains such as LysM or WxL. Both the stability and functionality of the displayed proteins will be affected by the kind of anchor used. The most commonly surfaced exposed recombinant proteins produced in LAB are antigens and antibodies and the most commonly used LAB are lactococci and lactobacilli. Although it is not necessarily so that surface-display is the preferred localization in all cases, it has been shown that for certain applications, such as delivery of the human papillomavirus E7 antigen, surface-display elicits better biological responses, compared to cytosolic expression or secretion. Recent developments include the display of peptides and proteins targeting host cell receptors, for the purpose of enhancing the interactions between LAB and host. Surface-display technologies have other potential applications, such as degradation of biomass, which is of importance for some potential industrial applications of LAB.

Mucosal Immunogenicity of Genetically Modified Lactobacillus acidophilus Expressing an HIV-1 Epitope within the Surface Layer Protein

Surface layer proteins of probiotic lactobacilli are theoretically efficient epitope-displaying scaffolds for oral vaccine delivery due to their high expression levels and surface localization. In this study, we constructed genetically modified Lactobacillus acidophilus strains expressing the membrane proximal external region (MPER) from human immunodeficiency virus type 1 (HIV-1) within the context of the major S-layer protein, SlpA. Intragastric immunization of mice with the recombinants induced MPER-specific and S-layer protein-specific antibodies in serum and mucosal secretions. Moreover, analysis of systemic SlpA-specific cytokines revealed that the responses appeared to be Th1 and Th17 dominant. These findings demonstrated the potential use of the Lactobacillus S-layer protein for development of oral vaccines targeting specific peptides.

Distinctive proteolytic activity of cell envelope proteinase of Lactobacillus helveticus isolated from airag, a traditional Mongolian fermented mare's milk

Airag is a traditional fermented milk of Mongolia that is usually made from raw mare's milk. Lactobacillus helveticus is one of the lactic acid bacteria most frequently isolated from airag. In this study, we investigated the genetic and physiological characteristics of L. helveticus strains isolated from airag and clarified their significance in airag by comparing them with strains from different sources. Six strains of L. helveticus were isolated from five home-made airag samples collected from different regions of Mongolia. The optimal temperature for acidification in skim milk was 30 to 35°C for all the Mongolian strains, which is lower than those for the reference strains (JCM 1554 and JCM 1120(T)) isolated from European cheeses. All of the strains had a prtH1-like gene encoding a variant type of cell envelope proteinase (CEP). The CEP amino acid sequence in Snow Brand Typeculture (SBT) 11087 isolated from airag shared 71% identity with PrtH of L. helveticus CNRZ32 (AAD50643.1) but 98% identity with PrtH of Lactobacillus kefiranofaciens ZW3 (AEG40278.1) isolated from a traditional fermented milk in Tibet. The proteolytic activities of the CEP from SBT11087 on artificial substrate (N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide) and pure casein were measured using an intact-cell degradation assay. The activity of the CEP from SBT11087 was observed to be weak and exhibited a lower optimal temperature (40°C) than those from the reference strains (45-50°C). The specificity of the SBT11087 CEP for αS1-casein was typical of the CEPs previously reported in L. helveticus, as determined through the degradation profiles obtained through gel electrophoresis and mass spectrometry analyses. In contrast, the degradation profile of β-casein revealed that the CEP of SBT11087 primarily hydrolyzes its C-terminal domain and hydrolyzed nine of the 16 cleavage sites shared among the CEPs of other L. helveticus strains. Thus, the CEP of SBT11087 is distinct from those from previously reported L. helveticus strains in terms of its optimal temperature and its degradation of β-casein. Therefore, the Mongolian L. helveticus strains differ from other strains of the species in different collections and are specifically suited for the natural lactic acid bacterial population in airag.

S-layers: principles and applications

Monomolecular arrays of protein or glycoprotein subunits forming surface layers (S-layers) are one of the most commonly observed prokaryotic cell envelope components. S-layers are generally the most abundantly expressed proteins, have been observed in species of nearly every taxonomical group of walled bacteria, and represent an almost universal feature of archaeal envelopes. The isoporous lattices completely covering the cell surface provide organisms with various selection advantages including functioning as protective coats, molecular sieves and ion traps, as structures involved in surface recognition and cell adhesion, and as antifouling layers. S-layers are also identified to contribute to virulence when present as a structural component of pathogens. In Archaea, most of which possess S-layers as exclusive wall component, they are involved in determining cell shape and cell division. Studies on structure, chemistry, genetics, assembly, function, and evolutionary relationship of S-layers revealed considerable application potential in (nano)biotechnology, biomimetics, biomedicine, and synthetic biology.

Construction of a food-grade cell surface display system for Lactobacillus casei

In this study, a food-grade cell surface display host/vector system for Lactobacillus casei was constructed. The food-grade host L. casei Q-5 was a lactose-deficient derivative of L. casei ATCC 334 obtained by plasmid elimination. The food-grade cell surface display vector was constructed based on safe DNA elements from lactic acid bacteria containing the following: pSH71 replicon from Lactococcus lactis, lactose metabolism genes from L. casei ATCC 334 as complementation markers, and surface layer protein gene from Lactobacillus acidophilus ATCC 4356 for cell surface display. The feasibility of the new host/vector system was verified by the expression of green fluorescent protein (GFP) on L. casei. Laser scanning confocal microscopy and immunofluorescence analysis using anti-GFP antibody confirmed that GFP was anchored on the surface of the recombinant cells. The stability of recombinant L. casei cells in artificial gastrointestinal conditions was verified, which is beneficial for oral vaccination applications. These results indicate that the food-grade host/vector system can be an excellent antigen delivery vehicle in oral vaccine construction.

Lactobacillus surface layer proteins: structure, function and applications

Bacterial surface (S) layers are the outermost proteinaceous cell envelope structures found on members of nearly all taxonomic groups of bacteria and Archaea. They are composed of numerous identical subunits forming a symmetric, porous, lattice-like layer that completely covers the cell surface. The subunits are held together and attached to cell wall carbohydrates by non-covalent interactions, and they spontaneously reassemble in vitro by an entropy-driven process. Due to the low amino acid sequence similarity among S-layer proteins in general, verification of the presence of an S-layer on the bacterial cell surface usually requires electron microscopy. In lactobacilli, S-layer proteins have been detected on many but not all species. Lactobacillus S-layer proteins differ from those of other bacteria in their smaller size and high predicted pI. The positive charge in Lactobacillus S-layer proteins is concentrated in the more conserved cell wall binding domain, which can be either N- or C-terminal depending on the species. The more variable domain is responsible for the self-assembly of the monomers to a periodic structure. The biological functions of Lactobacillus S-layer proteins are poorly understood, but in some species S-layer proteins mediate bacterial adherence to host cells or extracellular matrix proteins or have protective or enzymatic functions. Lactobacillus S-layer proteins show potential for use as antigen carriers in live oral vaccine design because of their adhesive and immunomodulatory properties and the general non-pathogenicity of the species.

S-layer protein mediates the stimulatory effect of Lactobacillus helveticus MIMLh5 on innate immunity

The ability to positively affect host health through the modulation of the immune response is a feature of increasing importance in measuring the probiotic potential of a bacterial strain. However, the identities of the bacterial cell components involved in cross talk with immune cells remain elusive. In this study, we characterized the dairy strain Lactobacillus helveticus MIMLh5 and its surface-layer protein (SlpA) using in vitro and ex vivo analyses. We found that MIMLh5 and SlpA exert anti-inflammatory effects by reducing the activation of NF-κB on the intestinal epithelial Caco-2 cell line. On the contrary, MIMLh5 and SlpA act as stimulators of the innate immune system by triggering the expression of proinflammatory factors tumor necrosis factor alpha and COX-2 in the human macrophage cell line U937 via recognition through Toll-like receptor 2. In the same experiments, SlpA protein did not affect the expression of the anti-inflammatory cytokine interleukin-10. A similar response was observed following stimulation of macrophages isolated from mouse bone marrow or the peritoneal cavity. These results suggest that SlpA plays a major role in mediating bacterial immune-stimulating activity, which could help to induce the host's defenses against and responses toward infections. This study supports the concept that the viability of bacterial cells is not always essential to exert immunomodulatory effects, thus permitting the development of safer therapies for the treatment of specific diseases according to a paraprobiotic intervention.

Tight controlled expression and secretion of Lactobacillus brevis SlpA in Lactococcus lactis

Prokaryotes commonly present outer cell wall structures composed of a crystalline array of proteinaceous subunits, known as surface layers (S-layers). The ORF encoding the S-layer protein (SlpA) of Lactobacillus brevis was cloned into Lactococcus lactis under the transcriptional control of the xylose-inducible expression system (XIES). SlpA was secreted into the extracellular medium, as determined by immunoblotting, and assays on the kinetics of SlpA production revealed that repression of the system with glucose did not require the depletion of xylose from the medium that allows transitory ORF expression. The successful use of XIES to express S-layer proteins in the versatile and generally recognized as safe species L. lactis opens new possibilities for an efficient production and isolation of SlpA S-layer protein for its various applications in biotechnology and importantly as an antigen-carrying vehicle.

Heterologous protein display on the cell surface of lactic acid bacteria mediated by the s-layer protein

BACKGROUND

Previous studies have revealed that the C-terminal region of the S-layer protein from Lactobacillus is responsible for the cell wall anchoring, which provide an approach for targeting heterologous proteins to the cell wall of lactic acid bacteria (LAB). In this study, we developed a new surface display system in lactic acid bacteria with the C-terminal region of S-layer protein SlpB of Lactobacillus crispatus K2-4-3 isolated from chicken intestine.

RESULTS

Multiple sequence alignment revealed that the C-terminal region (LcsB) of Lb. crispatus K2-4-3 SlpB had a high similarity with the cell wall binding domains SA and CbsA of Lactobacillus acidophilus and Lb. crispatus. To evaluate the potential application as an anchoring protein, the green fluorescent protein (GFP) or beta-galactosidase (Gal) was fused to the N-terminus of the LcsB region, and the fused proteins were successfully produced in Escherichia coli, respectively. After mixing them with the non-genetically modified lactic acid bacteria cells, the fused GFP-LcsB and Gal-LcsB were functionally associated with the cell surface of various lactic acid bacteria tested. In addition, the binding capacity could be improved by SDS pretreatment. Moreover, both of the fused proteins could simultaneously bind to the surface of a single cell. Furthermore, when the fused DNA fragment of gfp:lcsB was inserted into the Lactococcus lactis expression vector pSec:Leiss:Nuc, the GFP could not be secreted into the medium under the control of the nisA promoter. Western blot, in-gel fluorescence assay, immunofluorescence microscopy and SDS sensitivity analysis confirmed that the GFP was successfully expressed onto the cell surface of L. lactis with the aid of the LcsB anchor.

CONCLUSION

The LcsB region can be used as a functional scaffold to target the heterologous proteins to the cell surfaces of lactic acid bacteria in vitro and in vivo, and has also the potential for biotechnological application.

Dissimilar properties of two recombinant Lactobacillus acidophilus strains displaying Salmonella FliC with different anchoring motifs

Display of heterologous antigens on the cell surface is considered a useful technique for vaccine delivery by recombinant lactobacilli. In this study, two recombinant Lactobacillus acidophilus derivatives displaying Salmonella flagellin (FliC) were constructed using different anchor motifs. In one instance, the FliC protein was fused to the C-terminal region of a cell envelope proteinase (PrtP) and was bound to the cell wall by electrostatic bonds. In the other case, the same antigen was conjugated to the anchor region of mucus binding protein (Mub) and was covalently associated with the cell wall by an LPXTG motif. These two recombinant L. acidophilus cell surface displays resulted in dissimilar maturation and cytokine production by human myeloid dendritic cells. The surface-associated antigen was highly sensitive to simulated gastric and small intestinal juices. By supplementation with bicarbonate buffer and soybean trypsin inhibitor, the cell surface antigen was protected from proteolytic enzymes during gastric challenge in vitro. The protective reagents also increased the viability of the L. acidophilus cells upon challenge with simulated digestive juices. These results demonstrate the importance of protecting cells and their surface-associated antigens during oral immunization.

Identifying assembly-inhibiting and assembly-tolerant sites in the SbsB S-layer protein from Geobacillus stearothermophilus

Surface layer (S-layer) proteins self-assemble into two-dimensional crystalline lattices that cover the cell wall of all archaea and many bacteria. We have generated assembly-negative protein variants of high solubility that will facilitate high-resolution structure determination. Assembly-negative versions of the S-layer protein SbsB from Geobacillus stearothermophilus PV72/p2 were obtained using an insertion mutagenesis screen. The haemagglutinin epitope tag was inserted at 23 amino acid positions known to be located on the monomer protein surface from a previous cysteine accessibility screen. Limited proteolysis, circular dichroism, and fluorescence were used to probe whether the epitope insertion affected the secondary and tertiary structures of the monomer, while electron microscopy and size-exclusion chromatography were employed to examine proteins' ability to self-assemble. The screen not only identified assembly-compromised mutants with native fold but also yielded correctly folded, self-assembling mutants suitable for displaying epitopes for biomedical and biophysical applications, as well as cryo-electron microscopy imaging. Our study marks an important step in the analysis of the S-layer structure. In addition, the approach of concerted insertion and cysteine mutagenesis can likely be applied for other supramolecular assemblies.

S layer protein A of Lactobacillus acidophilus NCFM regulates immature dendritic cell and T cell functions

Dendritic cells (DCs) are antigen-presenting cells that play an essential role in mucosal tolerance. They regularly encounter beneficial intestinal bacteria, but the nature of these cellular contacts and the immune responses elicited by the bacteria are not entirely elucidated. Here, we examined the interactions of Lactobacillus acidophilus NCFM and its cell surface compounds with DCs. L. acidophilus NCFM attached to DCs and induced a concentration-dependent production of IL-10, and low IL-12p70. We further demonstrated that the bacterium binds to DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN), a DC- specific receptor. To identify the DC-SIGN ligand present on the bacterium, we took advantage of a generated array of L. acidophilus NCFM mutants. A knockout mutant of L. acidophilus NCFM lacking the surface (S) layer A protein (SlpA) was significantly reduced in binding to DC-SIGN. This mutant incurred a chromosomal inversion leading to dominant expression of a second S layer protein, SlpB. In the SlpB-dominant strain, the nature of the interaction of this bacterium with DCs changed dramatically. Higher concentrations of proinflammatory cytokines such as IL-12p70, TNFalpha, and IL-1beta were produced by DCs interacting with the SlpB-dominant strain compared with the parent NCFM strain. Unlike the SlpA-knockout mutant, T cells primed with L. acidophilus NCFM stimulated DCs produced more IL-4. The SlpA-DC-SIGN interaction was further confirmed as purified SlpA protein ligated directly to the DC-SIGN. In conclusion, the major S layer protein, SlpA, of L. acidophilus NCFM is the first probiotic bacterial DC-SIGN ligand identified that is functionally involved in the modulation of DCs and T cells functions.

Murein hydrolase activity in the surface layer of Lactobacillus acidophilus ATCC 4356

We describe a new enzymatic functionality for the surface layer (S-layer) of Lactobacillus acidophilus ATCC 4356, namely, an endopeptidase activity against the cell wall of Salmonella enterica serovar Newport, assayed via zymograms and identified by Western blotting. Based on amino acid sequence comparisons, the hydrolase activity was predicted to be located at the C terminus. Subsequent cloning and expression of the C-terminal domain in Bacillus subtilis resulted in the functional verification of the enzymatic activity.

The surface location of individual residues in a bacterial S-layer protein

Bacterial surface layer (S-layer) proteins self-assemble into large two-dimensional crystalline lattices that form the outermost cell-wall component of all archaea and many eubacteria. Despite being a large class of self-assembling proteins, little is known about their molecular architecture. We investigated the S-layer protein SbsB from Geobacillus stearothermophilus PV72/p2 to identify residues located at the subunit-subunit interface and to determine the S-layer's topology. Twenty-three single cysteine mutants, which were previously mapped to the surface of the SbsB monomer, were subjected to a cross-linking screen using the photoactivatable, sulfhydryl-reactive reagent N-[4-(p-azidosalicylamido)butyl]-3'-(2'-pyridyldithio)propionamide. Gel electrophoretic analysis on the formation of cross-linked dimers indicated that 8 out of the 23 residues were located at the interface. In combination with surface accessibility data for the assembled protein, 10 residues were assigned to positions at the inner, cell-wall-facing lattice surface, while 5 residues were mapped to the outer, ambient-exposed lattice surface. In addition, the cross-linking screen identified six positions of intramolecular cross-linking within the assembled protein but not in the monomeric S-layer protein. Most likely, these intramolecular cross-links result from conformational changes upon self-assembly. The results are an important step toward the further structural elucidation of the S-layer protein via, for example, X-ray crystallography and cryo-electron microscopy. Our approach of identifying the surface location of residues is relevant to other planar supramolecular protein assemblies.

Isolation of surface (S) layer protein carrying Lactobacillus species from porcine intestine and faeces and characterization of their adhesion properties to different host tissues

Surface-layer proteins (Slps) of lactobacilli have been shown to confer tissue adherence. This study aimed to isolate and identify Slps carrying Lactobacillus species from the porcine intestine and faeces and to characterize these S-layer-expressing strains for their ability to adhere to the pig and human intestinal cells and to extracellular matrix (ECM) proteins. In total 99 strains, putatively belonging to the genus Lactobacillus, were isolated as pure cultures. SDS-PAGE and a gene probe specific for the Lactobacillus brevis ATCC 8287 S-layer protein gene (slpA) were used to screen the presence of strains possessing putative Slps. Eight of the 99 pure cultures exhibited Slps according to the SDS-PAGE analyses. In these strains the presence of genes encoding Slps was confirmed by PCR and partial sequencing. Only one isolate of the 99 strains gave a positive hybridisation signal with the L. brevis slpA probe but did not appear to produce S-layer protein. Their taxonomic identification, based on phenotyping and the 16S rRNA sequences, revealed that the eight S-layer protein-producing strains were closely related to Lactobacillus amylovorus, Lactobacillus sobrius and Lactobacillus crispatus. The strain with the slpA positive hybridisation result was identified as Lactobacillus mucosae. The SDS-extractable protein profile, the size of the putative S-layer protein and binding capability of the strains varied greatly, even among the isolates belonging to the same Lactobacillus cluster. Removal of the intact Slps from the bacterial surface by extraction with guanidine hydrochloride reduced the adhesion of some strains to fibronectin and laminin, whereas, the adhesiveness to laminin increased with some strains.

Molecular organization of selected prokaryotic S-layer proteins

Regular crystalline surface layers (S-layers) are widespread among prokaryotes and probably represent the earliest cell wall structures. S-layer genes have been found in approximately 400 different species of the prokaryotic domains bacteria and archaea. S-layers usually consist of a single (glyco-)protein species with molecular masses ranging from about 40 to 200 kDa that form lattices of oblique, tetragonal, or hexagonal architecture. The primary sequences of hyperthermophilic archaeal species exhibit some characteristic signatures. Further adaptations to their specific environments occur by various post-translational modifications, such as linkage of glycans, lipids, phosphate, and sulfate groups to the protein or by proteolytic processing. Specific domains direct the anchoring of the S-layer to the underlying cell wall components and transport across the cytoplasma membrane. In addition to their presumptive original role as protective coats in archaea and bacteria, they have adapted new functions, e.g., as molecular sieves, attachment sites for extracellular enzymes, and virulence factors.

Detection, characterization, and in vitro and in vivo expression of genes encoding S-proteins in Lactobacillus gallinarum strains isolated from chicken crops

Thirty-eight isolates of Lactobacillus gallinarum cultured from the crops of broiler chickens were screened for the presence of genes encoding S-layer proteins. All of the isolates had two S-protein genes, which were designated Lactobacillus gallinarum S-protein (lgs) genes. One gene in each isolate was either lgsA or lgsB. The Lactobacillus isolates were further characterized by pulsed-field gel electrophoresis of DNA digests, which grouped the isolates into 17 genotypes (strains). The second gene in each of eight representative strains was sequenced and shown to differ among strains (lgsC, lgsD, lgsE, lgsF, lgsG, lgsH, and lgsI). The genome of each strain thus encoded a common S-protein (encoded by either lgsA or lgsB) and a strain-specific S-protein. The extraction of cell surface proteins from cultures of the eight strains showed that each strain produced a single S-protein that was always encoded by the strain-specific lgs gene. Two of the strains were used to inoculate chickens maintained in a protected environment which were Lactobacillus-free prior to inoculation. DNAs and RNAs extracted from the digesta of the chickens were used for PCR and reverse transcription-PCR, respectively, to demonstrate the presence and transcription of lgs genes in vivo. In both cases, only the strain-specific gene was transcribed. Both of the strains adhered to the crop epithelium, consistent with published data predicting that S-proteins of lactobacilli are adhesins. The results of this study provide a basis for the investigation of gene duplication and sequence variation as mechanisms by which bacterial strains of the same species can share the same habitat.

Heterogeneity of putative surface layer proteins in Lactobacillus helveticus

The S-layer-encoding genes of 21 Lactobacillus helveticus strains were characterized. Phylogenetic analysis based on the identified S-layer genes revealed two main clusters, one which includes a sequence similar to that of the slpH1 gene of L. helveticus CNRZ 892 and a second cluster which includes genes similar to that of prtY. These results were further confirmed by Southern blot hybridization. This study demonstrates S-layer gene variability in the species L. helveticus.

Importance of S-layer proteins in probiotic activity of Lactobacillus acidophilus M92

AIMS

To investigate the functional role of surface layer proteins (S-layer) in probiotic strain Lactobacillus acidophilus M92, especially its influence on adhesiveness to mouse ileal epithelial cells.

METHODS AND RESULTS

Sodium dodecyl sulphate polyacrylamide gel electrophoresis of cell surface proteins revealed the presence of potential surface layer (S-layer) proteins, ca at 45 kDa in L. acidophilus M92. Southern blot with pBK1 plasmid, containing slpA gene, gave a positive signal, suggesting that L. acidophilus M92 has a slpA gene coding for the S-layer proteins. S-layer proteins of this strain are present during all phases of growth. The S-layer proteins appeared when cells treated with 5 mol l(-1) LiCl were allowed to grow again. Removal of the S-layer proteins reduced adhesion of L. acidophilus M92 to mouse ileal epithelial cells. Furthermore, the viability of cells without S-layer were reduced in simulated gastric juice at low pH range (2, 2.5, 3) and simulated pancreatic juice with bile salts (1.5 and 3 g l(-1)). S-layer proteins of L. acidophilus M92 were resistant to pepsin and pancreatin, in contrast, the treatment with proteinase K led to a significant proteolysis of the S-layer proteins.

CONCLUSIONS

These results demonstrated functional role of S-layer; it is responsible for adhesiveness of Lactobacillus acidophilus M92 to mouse ileal epithelial cells and has a protective role for this strain.

SIGNIFICANCE AND IMPACT OF THE STUDY

S-layer proteins have an important role in the establishment of probiotic strain Lactobacillus acidophilus M92 in the gastrointestinal tract.

Genetic analysis of Bacillus anthracis Sap S-layer protein crystallization domain

Bacillus anthracis, the aetiological agent of anthrax, synthesizes two surface-layer (S-layer) proteins. S-layers are two-dimensional crystalline arrays that completely cover bacteria. In rich medium, the B. anthracis S-layer consists of Sap during the exponential growth phase. Sap is a modular protein composed of an SLH (S-layer homology)-anchoring domain followed by a putative crystallization domain (Sapc). A projection map of the two-dimensional Sap array has been established on deflated bacteria. In this work, the authors used two approaches to investigate whether Sapc is the crystallization domain. The purified Sapc polypeptide (604 aa) was sufficient to form a crystalline structure, as illustrated by electron microscopy. Consistent with this result, the entire Sapc domain promoted auto-interaction in a bacterial two-hybrid screen developed for the present study. The screen was derived from a system that takes advantage of the Bordetella pertussis cyclase subdomain structure to enable one to identify peptides that interact. A screening strategy was then employed to study Sapc subdomains that mediate interaction. A random library, derived from the Sapc domain, was constructed and screened. The selected polypeptides interacting with the complete Sapc were all larger (155 aa and above) than the mean size of the randomly cloned peptides (approx. 60 residues). This result suggests that, in contrast with observations for other interactions studied with this two-hybrid system, large fragments were required to ensure efficient interaction. It was noteworthy that only one polypeptide, which spanned aa 148–358, was able to interact with less than the complete Sapc, in fact, with itself.

Dynamic cell surface hydrophobicity of Lactobacillus strains with and without surface layer proteins

Variations in surface hydrophobicity of six Lactobacillus strains with and without an S-layer upon changes in ionic strength are derived from contact angle measurements with low- and high-ionic-strength aqueous solutions. Cell surface hydrophobicity changed in response to changes in ionic strength in three out of the six strains, offering these strains a versatile mechanism to adhere to different surfaces. The dynamic behavior of the cell surface hydrophobicity could be confirmed for two selected strains by measuring the interaction force between hydrophobic and hydrophilic tips with use of atomic force microscopy.

Peptide surface display and secretion using two LPXTG-containing surface proteins from Lactobacillus fermentum BR11

A locus encoding two repetitive proteins that have LPXTG cell wall anchoring signals from Lactobacillus fermentum BR11 has been identified by using an antiserum raised against whole L. fermentum BR11 cells. The first protein, Rlp, is similar to the Rib surface protein from Streptococcus agalactiae, while the other protein, Mlp, is similar to the mucus binding protein Mub from Lactobacillus reuteri. It was shown that multiple copies of mlp exist in the genome of L. fermentum BR11. Regions of Rlp, Mlp, and the previously characterized surface protein BspA were used to surface display or secrete heterologous peptides in L. fermentum. The peptides tested were 10 amino acids of the human cystic fibrosis transmembrane regulator protein and a six-histidine epitope (His(6)). The BspA promoter and secretion signal were used in combination with the Rlp cell wall sorting signal to express, export, and covalently anchor the heterologous peptides to the cell wall. Detection of the cell surface protein fusions revealed that Rlp was a significantly better surface display vector than BspA despite having lower cellular levels (0.7 mg per liter for the Rlp fusion compared with 4 mg per liter for the BspA fusion). The mlp promoter and encoded secretion signal were used to express and export large (328-kDa at 10 mg per liter) and small (27-kDa at 0.06 mg per liter) amino-terminal fragments of the Mlp protein fused to the His(6) and CFTR peptides or His(6) peptide, respectively. Therefore, these newly described proteins from L. fermentum BR11 have potential as protein production and targeting vectors.

Relationship between interaction sites in the gut, hydrophobicity, mucosal immunomodulating capacities and cell wall protein profiles in indigenous and exogenous bacteria

AIMS

To investigate whether there is a relationship between interaction sites in the gut, hydrophobicity, mucosal immunomodulating capacities and cell wall protein profiles in lactobacilli, bifidobacteria and enterococci.

METHODS AND RESULTS

Hydrophobicity, cell wall protein profiles and sites of interaction in the gut (by using fluorescein isothiocyanate-labelled bacteria) were determined for Lactobacillus casei, L. acidophilus, L. fermentum, Bifidobacterium bifidum, B. animalis and Enterococcus faecalis. We also determined the number of immunoglobulin (Ig)A+, tumour necrosis factor (TNF)alpha+, interleukin (IL)-6+ and IL-10+ cells after oral administration of the above bacteria to BALB/c mice. All strains assessed were found to interact with the sites of induction of the immune response in the gut. No correlation with hydrophobicity was observed. When some strains at certain doses were administered to mice, bacterial translocation to liver was observed. The oral administration of indigenous (104 cells day(-1)) and exogenous (107 cells day(-1)) bifidobacteria and lactobacilli for 5 consecutive days activated the systemic and intestinal mucosal immune response in a strain-specific way, independently whether the strain was indigenous or exogenous in relation to the host. The differences in the immunopotentiating capacity of the various strains might be related to the differences in their cell wall protein profiles.

CONCLUSIONS

Indigenous bacteria activated the mucosal immune response at a dose significantly smaller than the one required for probiotic exogenous bacteria. However, probiotic exogenous bacteria can be used at high concentrations in fermented dairy products with a great impact on the immune system, favouring its immunomodulation.

SIGNIFICANCE AND IMPACT OF THE STUDY

The immunomodulation capacity of probiotic bacteria is strain specific and independent of the specificity of the host. The ability of certain strains to down-regulate the production and release of IL-6 by IL-10 may have potential implications in their use in cases in which cytokine deregulation or excessive production at the mucosal level can be the cause of tissue damage.