S-layer proteins from Lactobacillus sp. inhibit bacterial infection by blockage of DC-SIGN cell receptor

Interaction mechanism between surface layer protein and yeast mannan: Insights from multi-spectroscopic and molecular dynamics simulation analyses

Tibet kefir grain (TKG) formation is mainly dependent on the aggregation of lactobacillus and yeasts. The interaction of surface layer protein (SLP) and yeast mannan plays an important role in mediating the co-aggregation of Lactobacillus kefiri with Saccharomyces cerevisiae. The interaction mechanism of the two was researched through multispectral spectroscopy, morphology observation and silico approaches. Fluorescence spectra data revealed that mannan was bound to SLP through a spontaneous binding process. The particle size of the binding complex increased as the mannan concentration increased. Synchronous fluorescence spectroscopy and circular dichroism (CD) spectra showed the conformational and microenvironment alteration of SLP treated with mannan. Molecular docking results indicated that hydrophobic interactions played major roles in the formation of SLP-mannan complexes. These findings provide a deeper insight into the interactions of protein and polysaccharide, and this knowledge is valuable in the application of SLP and mannan in co-fermentation systems.

Host-microbe interaction and pathogen exclusion mediated by an aggregation-prone surface layer protein of Lactobacillus helveticus

Probiotic surface layer proteins (Slps) have multiple functions and bacterial adhesion to host cells is one of them. The precise role of Slps in cellular adhesion is not well understood due to its low native protein yield and self-aggregative nature. Here, we report the recombinant expression and purification of biologically active Slp of Lactobacillus helveticus NCDC 288 (SlpH) in high yield. SlpH is a highly basic protein (pI = 9.4), having a molecular weight of 45 kDa. Circular Dichroism showed a prevalence of beta-strands in SlpH structure and resistance to low pH. SlpH showed binding to human intestinal tissue, enteric Caco-2 cell line, and porcine gastric mucin, but not with fibronectin, collagen type IV and laminin. SlpH inhibited the binding of the enterotoxigenic E. coli by 70 % and 76 % and that of Salmonella Typhimurium SL1344 by 71 % and 75 % to enteric Caco-2 cell line in the exclusion and competition assays, respectively. The pathogen exclusion and competition activity and tolerance to harsh gastrointestinal conditions show the potential for developing SlpH as a prophylactic or therapeutic agent against enteric pathogens.

The metabolites of lactic acid bacteria: classification, biosynthesis and modulation of gut microbiota

Lactic acid bacteria (LAB) are ubiquitous microorganisms that can colonize the intestine and participate in the physiological metabolism of the host. LAB can produce a variety of metabolites, including organic acids, bacteriocin, amino acids, exopolysaccharides and vitamins. These metabolites are the basis of LAB function and have a profound impact on host health. The intestine is colonized by a large number of gut microorganisms with high species diversity. Metabolites of LAB can keep the balance and stability of gut microbiota through aiding in the maintenance of the intestinal epithelial barrier, resisting to pathogens and regulating immune responses, which further influence the nutrition, metabolism and behavior of the host. In this review, we summarize the metabolites of LAB and their influence on the intestine. We also discuss the underlying regulatory mechanisms and emphasize the link between LAB and the human gut from the perspective of health promotion.

S-layer proteins as immune players: tales from pathogenic and non-pathogenic bacteria

In bacteria, as in other microorganisms, surface compounds interact with different pattern recognition receptors expressed by host cells, which usually triggers a variety of cellular responses that result in immunomodulation. The S-layer is a two-dimensional macromolecular crystalline structure formed by (glyco)-protein subunits that covers the surface of many species of Bacteria and almost all Archaea. In Bacteria, the presence of S-layer has been described in both pathogenic and non-pathogenic strains. As surface components, special attention deserves the role that S-layer proteins (SLPs) play in the interaction of bacterial cells with humoral and cellular components of the immune system. In this sense, some differences can be predicted between pathogenic and non-pathogenic bacteria. In the first group, the S-layer constitutes an important virulence factor, which in turn makes it a potential therapeutic target. For the other group, the growing interest to understand the mechanisms of action of commensal microbiota and probiotic strains has prompted the studies of the role of the S-layer in the interaction between the host immune cells and bacteria bearing this surface structure. In this review, we aim to summarize the main latest reports and the perspectives of bacterial SLPs as immune players, focusing on those from pathogenic and commensal/probiotic most studied species.

Uptake of Levilactobacillus brevis JCM 1059 by THP-1 Cells via Interaction between SlpB and CAP-1 Promotes Cytokine Production

Several probiotic lactic acid bacteria (LAB) exert immunomodulatory effects on the host. However, the reasons for the different effects of LAB have not been fully elucidated. To understand the different immunomodulatory effects of LAB, we evaluated the levels of critical molecules in differentiated monocytic THP-1 and dendritic cells (DCs) following the uptake of various LAB strains. Lactobacillus helveticus JCM 1120, Lactobacillus acidophilus JCM 1132, Levilactobacillus brevis JCM 1059, and Lentilactobacillus kefiri JCM 5818 showed significantly higher uptake among the 12 LAB species tested. The uptake of microbeads by THP-1 DC increased when coupled with the surface layer proteins (Slps) from the tested strains. SlpB was mainly observed in the L. brevis JCM 1059 Slps extract. The expected cell surface receptor for SlpB on THP-1 DC was purified using SlpB-coupled affinity resin and identified as adenylyl cyclase-associated protein 1 (CAP-1). SlpB binding to THP-1 DC decreased after the addition of anti-CAP-1 and anti-DC-SIGN antibodies but not after the addition of anti-macrophage-inducible C-type lectin (Mincle) antibody. These results suggest that SlpB on L. brevis JCM 1059 plays preferentially binds to CAP-1 on THP-1 DC and plays a crucial role in bacterial uptake by THP-1 cells as well as in subsequent interleukin-12 (IL-12) production.

Surface Layer Protein Pattern of Levilactobacillus brevis Strains Investigated by Proteomics

The outermost constituent of many bacterial cells is represented by an S-layer, i.e., a semiporous lattice-like layer composed of self-assembling protein subunits called S-layer proteins (Slps). These proteins are involved in several processes, such as protecting against environmental stresses, mediating bacterial adhesion to host cells, and modulating gut immune response. Slps may also act as a scaffold for the external display of additional cell surface proteins also named S-layer associated proteins (SLAPs). Levilactobacillus brevis is an S-layer forming lactic acid bacterium present in many different environments, such as sourdough, milk, cheese, and the intestinal tract of humans and animals. This microorganism exhibits probiotic features including the inhibition of bacterial infection and the improvement of human immune function. The potential role of Slps in its probiotic and biotechnological features was documented. A shotgun proteomic approach was applied to identify in a single experiment both the Slps and the SLAPs pattern of five different L. brevis strains isolated from traditional sourdoughs of the Southern Italian region. This study reveals that these closely related strains expressed a specific pattern of surface proteins, possibly affecting their peculiar properties.

Novel perspective on the regulation of food allergy by probiotic: The potential of its structural components

Food allergy (FA) is a global public health issue with growing prevalence. Increasing evidence supports the strong correlation between intestinal microbiota dysbiosis and food allergies. Probiotic intervention as a microbiota-based therapy could alleviate FA effectively. In addition to improving the intestinal microbiota disturbance and affecting microbial metabolites to regulate immune system, immune responses induced by the recognition of pattern recognition receptors to probiotic components may also be one of the mechanisms of probiotics protecting against FA. In this review, it is highlighted in detail about the regulatory effects on the immune system and anti-allergic potential of probiotic components including the flagellin, pili, peptidoglycan, lipoteichoic acid, exopolysaccharides, surface (S)-layer proteins and DNA. Probiotic components could enhance the function of intestinal epithelial barrier as well as regulate the balance of cytokines and T helper (Th) 1/Th2/regulatory T cell (Treg) responses. These evidences suggest that probiotic components could be used as nutritional or therapeutic agents for maintaining immune homeostasis to prevent FA, which will contribute to providing new insights into the resolution of FA and better guidance for the development of probiotic products.

Strain-dependent effectivity, and protective role against enzymes of S-layers in Lactiplantibacillus plantarum strains

The present study investigated whether the surface layer (S-layer), which is known to have a varying effect from strain to strain on aggregation, adhesion ability, also has an effect on the resistance of bacteria to digestive enzymes, phenol, lysozymes. The effect of S-layers on the resistance against various enzymes, aggregation and adhesion abilities, and strain specificity were determined of eight Lactiplantibacillus plantarum strains. Strains were treated with 5 M lithium chloride (LiCl) to extract the S-layers, the presence of this layer in those microorganisms was demonstrated by polyacrylamide gel electrophoresis. Scanning electron microscopy was used to visualize the separation of the S-layer, which surrounds the microorganism, from the microorganism by the LiCl. The images were taken three times, once at the beginning, once 30 min later, and once at the end of this process, which took 2 h in total. The effect against enzymes varied depending on the strain, but it was determined that all the tested strains had a serious loss of viability against phenol in the absence of an S-layer. Lpb. plantarum DA100 showed a maximum decrease against gastrointestinal system enzymes after the LiCl (96.48 ± 0.03% before and 66.46 ± 0.01% after LiCl). Lpb. plantarum DA255 showed a significant decrease against lysozyme (99.11 ± 0.00% before and 62.80 ± 0.0% after LiCl). Removal of the S-layer greatly affected the adhesion ability of some strains, while for others there was hardly any change. The results showed that the role of the S-layer may be strain-specific, the rate of effect can vary. The primary function of S-layer proteins is thought to contribute to the adhesion ability of bacteria. There are limited studies that have reported the protective property of this layer against various enzymes, however, our results showed that S-layer could be one of the resistance strategies developed by bacteria against enzymes.

Genes encoding bile salt hydrolase differentially affect adhesion of Lactiplantibacillus plantarum AR113

BACKGROUND

Adhesion is considered important for Lactiplantibacillus to persist in the human gut and for it to exert probiotic effects. Lactiplantibacillus plantarum contains a considerable number and variety of genes encoding bile salt hydrolases (bsh), but their effects on microbial adhesion remain poorly understood. To clarify the effects of four bsh on adhesion, we tried to knock out bsh (Δbsh) of L. plantarum AR113 using the CRISPR-Cas9 method, and compared the growth, auto-aggregation (R_AA ), co-aggregation (R_CA ), surface hydrophobicity (A_HC ) of AR113 wild-type and Δbsh strains and their adhesion abilities to HT29 cells.

RESULTS

We first obtained the AR113 Δbsh1,3,2,4 strain with four bsh knocked out. Their growth was significantly slower than the wild-type strain cultured in De Man, Rogosa, and Sharpe medium (MRS) with 3.0 g L^-1 glyco- or tauro-conjugated bile acid. Bsh had no significant effect on the growth of ten strains cultured in MRS, but Δbsh1 inhibited their growth when cultured in MRS containing 3.0 g L^-1 sodium glycocholate, whereas Δbsh4 instead promoted their growth in MRS with 3.0 g L^-1 sodium glycocholate and sodium taurocholate. R_CA and R_AA were linearly positive for all strains except AR113 Δbsh2,4, and A_HC and R_AA were negatively correlated for most strains excluding AR113 Δbsh2, with R_AA  = 6.38-25.05%, R_CA  = 5.17-9.22%, and A_CH  = 3.22-47.71%. The adhesion ability of ten strains cultured in MRS was higher than that of strains cultured in MRS with 3.0 g L^-1 bovine bile, and it was related to bsh2.

CONCLUSION

Bsh differentially affected the adhesion of AR113 series strains. This adds to the available information about substrate-gene-performance, and provides new information to enable engineering to regulate the colonization of Lactiplantibacillus. © 2021 Society of Chemical Industry.

Characteristics of surface layer protein from Lactobacillus kefiri HBA20 and the role in mediating interactions with Saccharomyces cerevisiae Y8

In this study, the surface layer protein (SLP) from Lactobacillus kefiri HBA20 was characterized. The SLP was extracted by 5 M LiCl. The molecular mass of the SLP was approximately 64 kDa as analyzed via SDS-PAGE. The surface morphology and the adhesion potential of L. kefiri HBA20 in the absence and presence of SLP were measured by AFM. Moreover, the protein secondary structure was evaluated by using circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR), respectively. SLP had high β-sheet contents and low content of α-helix. Thermal analysis of SLP of Lactobacillus kefiri HBA20 exhibited one transition peak at 129.64 °C. Furthermore, SEM measurements were showed that after the SLP were removed from the cell surface, the coaggregation ability with Saccharomyces cerevisiae Y8 of the strain was significantly reduced. In conclusion, the SLP of Lactobacillus kefiri HBA20 has a stable structure and the ability of adhesion to yeast. Molecular docking study revealed that mannan bind with the hydrophobic residues of SLP. Our results will help further understanding of the new surface layer protein and the interaction between L. kefiri and S. cerevisiae.

Molecular and Cellular Mechanisms Influenced by Postbiotics

In recent years, commensal bacteria colonizing the human body have been recognized as important determinants of health and multiple pathologic conditions. Among the most extensively studied commensal bacteria are the gut microbiota, which perform a plethora of functions, including the synthesis of bioactive products, metabolism of dietary compounds, and immunomodulation, both through attenuation and immunostimulation. An imbalance in the microbiota population, i.e., dysbiosis, has been linked to many human pathologies, including various cancer types and neurodegenerative diseases. Targeting gut microbiota and microbiome-host interactions resulting from probiotics, prebiotics, and postbiotics is a growing opportunity for the effective treatment of various diseases. As more research is being conducted, the microbiome field is shifting from simple descriptive analysis of commensal compositions to more molecular, cellular, and functional studies. Insight into these mechanisms is of paramount importance for understanding and modulating the effects that microbiota, probiotics, and their derivatives exert on host health.

Potentiality of Food-Isolated Lentilactobacillus kefiri Strains as Probiotics: State-of-Art and Perspectives

Lentilactobacillus kefiri is one of the main lactic acid bacteria species in kefir and it was also isolated from other fermented foods. Numerous strains have been isolated and characterized regarding its potential as probiotics for the development of novel functional foods. To our knowledge this is the first review focused on highlighting safety aspects and health beneficial effects reported for L. kefiri strains. Several L. kefiri strains lack of transmissible antibiotic resistance genes, are tolerant to the harsh conditions of the gastrointestinal environment, and could resist different preservation procedures. Moreover, many of the isolated strains have shown antimicrobial activity against pathogens and their toxins, exhibited immunomodulatory activity as well as induced some beneficial effects at metabolic level. Regarding all the scientific evidence, certain L. kefiri strains emerge as excellent candidates to be applied to the development of both food supplements and new fermented foods with health-promoting properties. However, the availability of genomic information is still very limited, so much more work must be done in order to explore the potentiality of L. kefiri as a probiotic and a source of bioactive metabolites.

Probiotic Gastrointestinal Transit and Colonization After Oral Administration: A Long Journey

Orally administered probiotics encounter various challenges on their journey through the mouth, stomach, intestine and colon. The health benefits of probiotics are diminished mainly due to the substantial reduction of viable probiotic bacteria under the harsh conditions in the gastrointestinal tract and the colonization resistance caused by commensal bacteria. In this review, we illustrate the factors affecting probiotic viability and their mucoadhesive properties through their journey in the gastrointestinal tract, including a discussion on various mucosadhesion-related proteins on the probiotic cell surface which facilitate colonization.

S-Layer From Lactobacillus brevis Modulates Antigen-Presenting Cell Functions via the Mincle-Syk-Card9 Axis

C-type lectin receptors (CLRs) are pattern recognition receptors that are crucial in the innate immune response. The gastrointestinal tract contributes significantly to the maintenance of immune homeostasis; it is the shelter for billions of microorganisms including many genera of Lactobacillus sp. Previously, it was shown that host-CLR interactions with gut microbiota play a crucial role in this context. The Macrophage-inducible C-type lectin (Mincle) is a Syk-coupled CLR that contributes to sensing of mucosa-associated commensals. In this study, we identified Mincle as a receptor for the Surface (S)-layer of the probiotic bacteria Lactobacillus brevis modulating GM-CSF bone marrow-derived cells (BMDCs) functions. We found that the S-layer/Mincle interaction led to a balanced cytokine response in BMDCs by triggering the release of both pro- and anti-inflammatory cytokines. In contrast, BMDCs derived from Mincle^−/−, CARD9^−/− or conditional Syk^−/− mice failed to maintain this balance, thus leading to an increased production of the pro-inflammatory cytokines TNF and IL-6, whereas the levels of the anti-inflammatory cytokines IL-10 and TGF-β were markedly decreased. Importantly, this was accompanied by an altered CD4⁺ T cell priming capacity of Mincle^−/− BMDCs resulting in an increased CD4⁺ T cell IFN-γ production upon stimulation with L. brevis S-layer. Our results contribute to the understanding of how commensal bacteria regulate antigen-presenting cell (APC) functions and highlight the importance of the Mincle/Syk/Card9 axis in APCs as a key factor in host-microbiota interactions.

Paraprobiotics and Postbiotics of Probiotic Lactobacilli, Their Positive Effects on the Host and Action Mechanisms: A Review

Lactobacilli comprise an important group of probiotics for both human and animals. The emerging concern regarding safety problems associated with live microbial cells is enhancing the interest in using cell components and metabolites derived from probiotic strains. Here, we define cell structural components and metabolites of probiotic bacteria as paraprobiotics and postbiotics, respectively. Paraprobiotics and postbiotics produced from Lactobacilli consist of a wide range of molecules including peptidoglycans, surface proteins, cell wall polysaccharides, secreted proteins, bacteriocins, and organic acids, which mediate positive effect on the host, such as immunomodulatory, anti-tumor, antimicrobial, and barrier-preservation effects. In this review, we systematically summarize the paraprobiotics and postbiotics derived from Lactobacilli and their beneficial functions. We also discuss the mechanisms underlying their beneficial effects on the host, and their interaction with the host cells. This review may boost our understanding on the benefits and molecular mechanisms associated with paraprobiotics and probiotics from Lactobacilli, which may promote their applications in humans and animals.

Lactobacillus Cell Surface Proteins Involved in Interaction with Mucus and Extracellular Matrix Components

The gut microbiota is a complex microbial ecosystem where bacteria, through mutual interactions, cooperate in maintaining of wellbeing and health. Lactobacilli are among the most important constituents of human and animal intestinal microbiota and include many probiotic strains. Their presence ensures protection from invasion of pathogens, as well as stimulation of the immune system and protection of the intestinal flora, often exerted through the ability to interact with mucus and extracellular matrix components. The main factors responsible for mediating adhesion of pathogens and commensals to the gut are cell surface proteins that recognize host targets, as mucus layer and extracellular matrix proteins. In the last years, several adhesins have been reported to be involved in lactobacilli–host interaction often miming the same mechanism used by pathogens.

Surface components and metabolites of probiotics for regulation of intestinal epithelial barrier

The gut microbiota can significantly affect the function of the intestinal barrier. Some intestinal probiotics (such as Lactobacillus, Bifidobacteria, a few Escherichia coli strains, and a new generation of probiotics including Bacteroides thetaiotaomicron and Akkermansia muciniphila) can maintain intestinal epithelial homeostasis and promote health. This review first summarizes probiotics' regulation of the intestinal epithelium via their surface compounds. Surface layer proteins, flagella, pili and capsular polysaccharides constitute microbial-associated molecular patterns and specifically bind to pattern recognition receptors, which can regulate signaling pathways to produce cytokines or inhibit apoptosis, thereby attenuating inflammation and enhancing the function of the gut epithelium. The review also explains the effects of metabolites (such as secreted proteins, organic acids, indole, extracellular vesicles and bacteriocins) of probiotics on host receptors and the mechanisms by which these metabolites regulate gut epithelial barrier function. Previous reviews summarized the role of the surface macromolecules or metabolites of gut microbes (including both probiotics and pathogens) in human health. However, these reviews were mostly focused on the interactions between these substances and the intestinal mucosal immune system. In the current review, we only focused on probiotics and discussed the molecular interaction between these bacteria and the gut epithelial barrier.

S-Layer Protein of Lactobacillus helveticus SBT2171 Promotes Human β-Defensin 2 Expression via TLR2-JNK Signaling

Antimicrobial peptides that contribute to innate immunity are among the most important protective measures against infection in many organisms. Several substances are known to regulate the expression of antimicrobial peptides. In this study, we investigated the factors in lactic acid bacteria (LAB) that induce antimicrobial peptide expression in the host. We found that Lactobacillus helveticus SBT2171 (LH2171) induced the expression of human β-defensin (hBD)2 in Caco-2 human colonic epithelial cells. Specifically, surface layer protein (SLP) of LH2171 stimulated hBD2 expression by activating c-Jun N-terminal kinase (JNK) signaling via Toll-like receptor (TLR)2 in Caco-2 cells. SLPs extracted from other lactobacilli similarly increased hBD2 expression, suggesting that this stimulatory effect is common feature of Lactobacillus SLPs. Interestingly, Lactobacillus strains that strongly induced hBD2 expression also potently activated JNK signaling. Thus, upregulation of hBD2 induced by TLR2–JNK signaling contributes to protection of the host against infection.

S-Layer Glycoprotein From Lactobacillus kefiri Exerts Its Immunostimulatory Activity Through Glycan Recognition by Mincle

The development of new subunit vaccines has promoted the rational design of adjuvants able to induce a strong T-cell activation by targeting specific immune receptors. The S-layer is a (glyco)-proteinaceous envelope constituted by subunits that self-assemble to form a two-dimensional lattice that covers the surface of different species of Bacteria and Archaea. Due to their ability to self-assemble in solution, they are attractive tools to be used as antigen/hapten carriers or adjuvants. Recently, we have demonstrated that S-layer glycoprotein from Lactobacillus kefiri CIDCA 8348 (SLP-8348) enhanced the LPS-induced response on macrophages in a Ca²⁺-dependent manner, but the receptors involved in these immunomodulatory properties remain unknown. Therefore, we aim to determine the C-type lectin receptors (CLRs) recognizing this bacterial surface glycoprotein as well as to investigate the role of glycans in both the immunogenicity and adjuvant capacity of SLP-8348. Here, using a mild periodate oxidation protocol, we showed that loss of SLP-8348 glycan integrity impairs the cell-mediated immune response against the protein. Moreover, our data indicate that the adjuvant capacity of SLP-8348 is also dependent of the biological activity of the SLP-8348 glycans. In order to evaluate the CLRs involved in the interaction with SLP-8348 an ELISA-based method using CLR–hFc fusion proteins showed that SLP-8348 interacts with different CLRs such as Mincle, SingR3, and hDC-SIGN. Using BMDCs derived from CLR-deficient mice, we show that SLP-8348 uptake is dependent of Mincle. Furthermore, we demonstrate that the SLP-8348-induced activation of BMDCs as well as its adjuvant capacity relies on the presence of Mincle and its signaling adaptor CARD9 on BMDCs, since SLP-8348-activated BMDCs from Mincle^−/− or CARD9^−/− mice were not capable to enhance OVA-specific response in CD4⁺ T cells purified from OT-II mice. These findings significantly contribute to the understanding of the role of glycans in the immunomodulation elicited by bacterial SLPs and generate a great opportunity in the search for new adjuvants derived from non-pathogenic microorganisms.

Genomic Comparison of Lactobacillus helveticus Strains Highlights Probiotic Potential

Lactobacillus helveticus belongs to the large group of lactic acid bacteria (LAB), which are the major players in the fermentation of a wide range of foods. LAB are also present in the human gut, which has often been exploited as a reservoir of potential novel probiotic strains, but several parameters need to be assessed before establishing their safety and potential use for human consumption. In the present study, six L. helveticus strains isolated from natural whey cultures were analyzed for their phenotype and genotype in exopolysaccharide (EPS) production, low pH and bile salt tolerance, bile salt hydrolase (BSH) activity, and antibiotic resistance profile. In addition, a comparative genomic investigation was performed between the six newly sequenced strains and the 51 publicly available genomes of L. helveticus to define the pangenome structure. The results indicate that the newly sequenced strain UC1267 and the deposited strain DSM 20075 can be considered good candidates for gut-adapted strains due to their ability to survive in the presence of 0.2% glycocholic acid (GCA) and 1% taurocholic and taurodeoxycholic acid (TDCA). Moreover, these strains had the highest bile salt deconjugation activity among the tested L. helveticus strains. Considering the safety profile, none of these strains presented antibiotic resistance phenotypically and/or at the genome level. The pangenome analysis revealed genes specific to the new isolates, such as enzymes related to folate biosynthesis in strains UC1266 and UC1267 and an integrated phage in strain UC1035. Finally, the presence of maltose-degrading enzymes and multiple copies of 6-phospho-β-glucosidase genes in our strains indicates the capability to metabolize sugars other than lactose, which is related solely to dairy niches.

Exploring lectin-like activity of the S-layer protein of Lactobacillus acidophilus ATCC 4356

The surface layer (S-layer) protein of Lactobacillus acidophilus is a crystalline array of self-assembling, proteinaceous subunits non-covalently bound to the outmost bacterial cell wall envelope and is involved in the adherence of bacteria to host cells. We have previously described that the S-layer protein of L. acidophilus possesses anti-viral and anti-bacterial properties. In this work, we extracted and purified S-layer proteins from L. acidophilus ATCC 4356 cells to study their interaction with cell wall components from prokaryotic (i.e., peptidoglycan and lipoteichoic acids) and eukaryotic origin (i.e., mucin and chitin), as well as with viruses, bacteria, yeast, and blood cells. Using chimeric S-layer fused to green fluorescent protein (GFP) from different parts of the protein, we analyzed their binding capacity. Our results show that the C-terminal part of the S-layer protein presents lectin-like activity, interacting with different glycoepitopes. We further demonstrate that lipoteichoic acid (LTA) serves as an anchor for the S-layer protein. Finally, a structure for the C-terminal part of S-layer and possible binding sites were predicted by a homology-based model.

Surface (S) Layer Proteins of Lactobacillus acidophilus Block Virus Infection via DC-SIGN Interaction

Alphaviruses and flaviviruses are important human pathogens that include Chikungunya virus (CHIKV), Dengue virus (DENV), and Zika virus (ZIKV), which can cause diseases in humans ranging from arthralgia to hemorrhagic fevers and microcephaly. It was previously shown that treatment with surface layer (S-layer) protein, present on the bacterial cell-envelope of Lactobacillus acidophilus, is able to inhibit viral and bacterial infections by blocking the pathogen’s interaction with DC-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN), a trans-membrane protein that is a C-type calcium-dependent lectin. DC-SIGN is known to act as an attachment factor for several viruses including alphaviruses and flaviviruses. In the present study, we used alphaviruses as a model system to dissect the mechanism of S-layer inhibition. We first evaluated the protective effect of S-layer using 3T3 cells, either wild type or stably expressing DC-SIGN, and infecting with the alphaviruses Semliki Forest virus (SFV) and CHIKV and the flaviviruses ZIKV and DENV. DC-SIGN expression significantly enhanced infection by all four viruses. Treatment of the cells with S-layer prior to infection decreased infectivity of all viruses only in cells expressing DC-SIGN. In vitro ELISA experiments showed a direct interaction between S-layer and DC-SIGN; however, confocal microscopy and flow cytometry demonstrated that S-layer binding to the cells was independent of DC-SIGN expression. S-layer protein prevented SFV binding and internalization in DC-SIGN-expressing cells but had no effect on virus binding to DC-SIGN-negative cells. Inhibition of virus binding occurred in a time-dependent manner, with a significant reduction of infection requiring at least a 30-min pre-incubation of S-layer with DC-SIGN-expressing cells. These results suggest that S-layer has a different mechanism of action compared to mannan, a common DC-SIGN-binding compound that has an immediate effect in blocking viral infection. This difference could reflect slower kinetics of S-layer binding to the DC-SIGN present at the plasma membrane (PM). Alternatively, the S-layer/DC-SIGN interaction may trigger the activation of signaling pathways that are required for the inhibition of viral infection. Together our results add important information relevant to the potential use of L. acidophilus S-layer protein as an antiviral therapy.

Lactobacillus acidophilus S-layer protein-mediated inhibition of PEDV-induced apoptosis of Vero cells

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Lactobacillus S-layer protein plays an inhibitory role during PEDV infection.

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In Vero cells infected with PEDV, apoptosis was mediated by caspase-8/3 activation.

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Lactobacillus S-layer protein inhibited PEDV-induced apoptosis in Vero cells.

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S-layer protein reduced caspase-8/3 activation against PEDV-induced apoptosis.

To gain insight into the mechanism of Lactobacillus acidophilus (L. acidophilus) S-layer protein antiviral activity, we examined how S-layer protein impacts porcine epidemic diarrhea virus (PEDV) infection and PEDV-induced apoptosis of Vero cells. Pretreatment (exclusion assay), coincubation (competition assay), and post-treatment (displacement assay) of PEDV-infected Vero cells with the S-layer protein was examined. Interestingly, significant inhibition of PEDV by S-layer protein was only observed in the exclusion assay. In Vero cells infected with PEDV, we found that apoptosis was mediated by activation of caspase-8 and caspase-3 in the late stage of infection. When PEDV-infected Vero cells were pretreated with S-layer protein, rates of Vero cell apoptosis were markedly decreased and cell damage was significantly reduced, as evaluated by flow cytometry and microscopy. Detailed analyses showed that the S-layer protein inhibited caspase-8 and caspase-3 activity. Taken together, our results suggest that L. acidophilus S-layer protein plays an inhibitory role during PEDV infection of Vero cells, and that the antagonistic activity of the protein is not via competition with PEDV for binding sites. In addition, the findings suggest that L. acidophilus S-layer protein protects against PEDV-induced apoptosis through reduced caspase-8 and caspase-3 activation in the later stages of infection. This mechanism may represent a novel approach for antagonizing PEDV and other viruses.

The Cheese Matrix Modulates the Immunomodulatory Properties of Propionibacterium freudenreichii CIRM-BIA 129 in Healthy Piglets

Propionibacterium freudenreichii is a beneficial bacterium, used as a cheese starter, which presents versatile probiotic properties. These properties are strain-dependent. We hypothesized they may also be delivery vehicle-dependent. In this study, we thus explored in healthy piglets how the cheese matrix affects the immunomodulatory properties of P. freudenreichii. During 2 weeks, three groups of weaned piglets consumed, respectively, P. freudenreichii as a liquid culture (PF-culture), P. freudenreichii under the form of a cheese (PF-cheese), or a control sterile cheese matrix (Cheese-matrix). The in vivo metabolic activity of P. freudenreichii was assessed by determining short chain fatty acids (SCFA) concentration and bifidobacteria population in feces. Whatever the delivery vehicle, P. freudenreichii was metabolically active in piglets' colon and enhanced both bifidobacteria and SCFA in feces. P. freudenreichii consumption decreased the secretion of TNFα and of IL-10 by peripheral blood mononuclear cells (PBMC). It did not alter IL-10, IFNγ, IL-17, and TNFα secretion in mesenteric lymph node immune cells (MLNC). PF-cheese enhanced significantly Treg phenotype, while PF-culture decreased significantly Th17 phenotype in PBMC and MLNC. Remarkably, only PF-cheese induced an increase of Th2 phenotype in PBMC and MLNC. Ex vivo stimulation of PBMC and MLNC by Lipopolysaccharides and Concanavalin A emphasized the difference in the immunomodulatory responses between PF-culture and PF-cheese group, as well as between PBMC and MLNC. This study shows the importance to consider the delivery vehicle for probiotic administration. It confirms the anti-inflammatory potential of P. freudenreichii. It opens new perspectives for the use propionibacteria-fermented products as preventive agents for inflammatory bowel diseases and intestinal infectious diseases.

Caloric restriction promotes functional changes involving short-chain fatty acid biosynthesis in the rat gut microbiota

Caloric restriction (CR) is known to promote health and longevity, likely via modification of the gut microbiota (GM). However, functional and metabolic changes induced in the GM during CR are still unidentified. Here, we investigated the short- and long-term effects of CR on the rat GM using a metaproteogenomic approach. We show that a switch from ad libitum (AL) low fat diet to CR in young rats is able to induce rapid and deep changes in their GM metaproteomic profile, related to a reduction of the Firmicutes/Bacteroidetes ratio and an expansion of lactobacilli. Specifically, we observed a significant change in the expression of the microbial enzymes responsible for short-chain fatty acid biosynthesis, with CR boosting propionogenesis and limiting butyrogenesis and acetogenesis. Furthermore, these CR-induced effects were maintained up to adulthood and started to be reversed after a short-term diet change. We also found that CR alters the abundance of an array of host proteins released in stool, mainly related to epithelial barrier integrity and inflammation. Hence, our results provide thorough information about CR-induced modifications to GM and host functional activity, and might constitute the basis for novel GM-based approaches aimed at monitoring the effectiveness of dietary interventions.

Roles and applications of probiotic Lactobacillus strains

Lactobacilli are recognized as probiotics on account of their health-promoting effects in the host. The aim of this review is to summarize current knowledge of the mechanisms of the adaption factors and main functions of lactobacilli that exert health-promoting effects in the host and to discuss important applications in animal and human health. The adaption mechanisms of lactobacilli facilitate interactions with the host and directly contribute to the beneficial nutritional, physiological, microbiological, and immunological effects in the host. Besides, the application of probiotic lactobacilli will increase our understanding of practical uses based on the roles of these organisms in immunoregulation, antipathogenic activities, and enhancement of the epithelial barrier.

Extractable Bacterial Surface Proteins in Probiotic-Host Interaction

Some Gram-positive bacteria, including probiotic ones, are covered with an external proteinaceous layer called a surface-layer. Described as a paracrystalline layer and formed by the self-assembly of a surface-layer-protein (Slp), this optional structure is peculiar. The surface layer per se is conserved and encountered in many prokaryotes. However, the sequence of the corresponding Slp protein is highly variable among bacterial species, or even among strains of the same species. Other proteins, including surface layer associated proteins (SLAPs), and other non-covalently surface-bound proteins may also be extracted with this surface structure. They can be involved a various functions. In probiotic Gram-positives, they were shown by different authors and experimental approaches to play a role in key interactions with the host. Depending on the species, and sometime on the strain, they can be involved in stress tolerance, in survival within the host digestive tract, in adhesion to host cells or mucus, or in the modulation of intestinal inflammation. Future trends include the valorization of their properties in the formation of nanoparticles, coating and encapsulation, and in the development of new vaccines.

Draft Genome Sequence of Lactobacillus helveticus ATCC 12046

Lactobacillus helveticus is a lactic acid bacterium used traditionally in the dairy industry, especially in the manufacture of cheeses. We present here the 2,141,841-bp draft genome sequence of L. helveticus strain ATCC 12046, a potential starter strain for improving cheese production.

A glycoproteomic approach reveals that the S-layer glycoprotein of Lactobacillus kefiri CIDCA 83111 is O- and N-glycosylated

In Gram-positive bacteria, such as lactic acid bacteria, general glycosylation systems have not been documented so far. The aim of this work was to characterize in detail the glycosylation of the S-layer protein of Lactobacillus kefiri CIDCA 83111. A reductive β-elimination treatment followed by anion exchange high performance liquid chromatography analysis was useful to characterize the O-glycosidic structures. MALDI-TOF mass spectrometry analysis confirmed the presence of oligosaccharides bearing from 5 to 8 glucose units carrying galacturonic acid. Further nanoHPLC-ESI analysis of the glycopeptides showed two O-glycosylated peptides: the peptide sequence SSASSASSA already identified as a signature glycosylation motif in L. buchneri, substituted on average with eight glucose residues and decorated with galacturonic acid and another O-glycosylated site on peptide 471-476, with a Glc_5-8GalA₂ structure. As ten characteristic sequons (Asn-X-Ser/Thr) are present in the S-layer amino acid sequence, we performed a PNGase F digestion to release N-linked oligosaccharides. Anion exchange chromatography analysis showed mainly short N-linked chains. NanoHPLC-ESI in the positive and negative ion modes were useful to determine two different peptides substituted with short N-glycan structures. To our knowledge, this is the first description of the structure of N-glycans in S-layer glycoproteins from Lactobacillus species.

SIGNIFICANCE

A detailed characterization of protein glycosylation is essential to establish the basis for understanding and investigating its biological role. It is known that S-layer proteins from kefir-isolated L. kefiri strains are involved in the interaction of bacterial cells with yeasts present in kefir grains and are also capable to antagonize the adverse effects of different enteric pathogens. Therefore, characterization of type and site of glycosidic chains in this protein may help to understand these important properties. Furthermore, this is the first description of N-glycosidic chains in S-layer glycoprotein from Lactobacillus spp.

Lactobacillus reuteri Surface Mucus Adhesins Upregulate Inflammatory Responses Through Interactions With Innate C-Type Lectin Receptors

The vertebrate gut symbiont Lactobacillus reuteri exhibits strain-specific adhesion and health-promoting properties. Here, we investigated the role of the mucus adhesins, CmbA and MUB, upon interaction of L. reuteri ATCC PTA 6475 and ATCC 53608 strains with human monocyte-derived dendritic cells (moDCs). We showed that mucus adhesins increased the capacity of L. reuteri strains to interact with moDCs and promoted phagocytosis. Our data also indicated that mucus adhesins mediate anti- and pro-inflammatory effects by the induction of interleukin-10 (IL-10), tumor necrosis factor alpha (TNF-α), IL-1β, IL-6, and IL-12 cytokines. L. reuteri ATCC PTA 6475 and ATCC 53608 were exclusively able to induce moDC-mediated Th1 and Th17 immune responses. We further showed that purified MUB activates moDCs and induces Th1 polarized immune responses associated with increased IFNγ production. MUB appeared to mediate these effects via binding to C-type lectin receptors (CLRs), as shown using cell reporter assays. Blocking moDCs with antibodies against DC-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN) or Dectin-2 did not affect the uptake of the MUB-expressing strain, but reduced the production of TNF-α and IL-6 by moDCs significantly, in line with the Th1 polarizing capacity of moDCs. The direct interaction between MUB and CLRs was further confirmed by atomic force spectroscopy. Taken together these data suggest that mucus adhesins expressed at the cell surface of L. reuteri strains may exert immunoregulatory effects in the gut through modulating the Th1-promoting capacity of DCs upon interaction with C-type lectins.

Lactobacillus kefiri shows inter-strain variations in the amino acid sequence of the S-layer proteins

The S-layer is a proteinaceous envelope constituted by subunits that self-assemble to form a two-dimensional lattice that covers the surface of different species of Bacteria and Archaea, and it could be involved in cell recognition of microbes among other several distinct functions. In this work, both proteomic and genomic approaches were used to gain knowledge about the sequences of the S-layer protein (SLPs) encoding genes expressed by six aggregative and sixteen non-aggregative strains of potentially probiotic Lactobacillus kefiri. Peptide mass fingerprint (PMF) analysis confirmed the identity of SLPs extracted from L. kefiri, and based on the homology with phylogenetically related species, primers located outside and inside the SLP-genes were employed to amplify genomic DNA. The O-glycosylation site SASSAS was found in all L. kefiri SLPs. Ten strains were selected for sequencing of the complete genes. The total length of the mature proteins varies from 492 to 576 amino acids, and all SLPs have a calculated pI between 9.37 and 9.60. The N-terminal region is relatively conserved and shows a high percentage of positively charged amino acids. Major differences among strains are found in the C-terminal region. Different groups could be distinguished regarding the mature SLPs and the similarities observed in the PMF spectra. Interestingly, SLPs of the aggregative strains are 100% homologous, although these strains were isolated from different kefir grains. This knowledge provides relevant data for better understanding of the mechanisms involved in SLPs functionality and could contribute to the development of products of biotechnological interest from potentially probiotic bacteria.