Lactobacilli isolated from kefir grains: evidence of the presence of S-layer proteins

Functional Significance of Probiotic Bacterial Interactions with Milk Fat Globules in a Human Host

Dairy products often serve as matrices for delivering probiotic bacteria to humans through the diet; however, little is known about the impact of milk fat globules on the growth and survival of probiotic microorganisms. This review discusses current knowledge on the structure and functionality of the milk fat globule membrane (MFGM) and the structural components contributing to the mechanisms of interactions with probiotic bacteria. We analyzed studies published between 2001 and 2025 with reference to earlier foundational research on probiotics and MFGM structure to explore the functional significance of MFGM-probiotic interactions. Recent research indicates that the effects of MFGM interaction with bacteria are species-specific and may influence probiotic activity in the host, including enhancing probiotic viability during intestinal transit and modulating probiotic colonization. In general, research findings suggest that the MFGM holds potential for use as a probiotic carrier to the gut with beneficial health consequences.

Display of porcine epidemic diarrhea virus spike protein B-cell linear epitope on Lactobacillus mucosae G01 S-layer surface induce a robust immunogenicity in mice

The Porcine epidemic diarrhea virus (PEDV) presents a substantial risk to the domestic pig industry, resulting in extensive and fatal viral diarrhea among piglets. Recognizing the mucosal stimulation triggered by PEDV and harnessing the regulatory impact of lactobacilli on intestinal function, we have developed a lactobacillus-based vaccine that is carefully designed to elicit a strong mucosal immune response. Through bioinformatics analysis, we examined PEDV S proteins to identify B-cell linear epitopes that meet the criteria of being non-toxic, soluble, antigenic, and capable of neutralizing the virus. In this study, a genetically modified strain of Lactobacillus mucosae G01 (L.mucosae G01) was created by utilizing the S layer protein (SLP) as a scaffold for surface presentation. Chimeric immunodominant epitopes with neutralizing activity were incorporated at various sites on SLP. The successful expression of SLP chimeric immunodominant epitope 1 on the surface of L.mucosae G01 was confirmed through indirect immunofluorescence and transmission electron microscopy, revealing the formation of a transparent membrane. The findings demonstrate that the oral administration of L.mucosae G01, which expresses the SLP chimeric immunodominant gene epitope1, induces the production of secreted IgA in the intestine and feces of mice. Additionally, there is an elevation in IgG levels in the serum. Moreover, the levels of cytokines IL-2, IL-4, IFN-γ, and IL-17 are significantly increased compared to the negative control group. These results suggest that L. mucosae G01 has the ability to deliver exogenous antigens and elicit a specific mucosal immune response against PEDV. This investigation presents new possibilities for immunoprophylaxis against PEDV-induced diarrhea.

Multifunctional properties of the transmembrane LPxTG-motif protein derived from Limosilactobacillus reuteri SH-23

The LPxTG-motif protein is an important transmembrane protein with high hydrophilicity and stability, as evidenced by its stress tolerance and adhesion ability. In this study, a novel LPxTG-motif protein with esterase activity (LEP) was expressed, and the multifunctional properties such as adhesion properties and esterase activity were also investigated. When cocultured with Limosilactobacillus reuteri SH-23, the adhesion ability of L. reuteri SH-23 to HT-29 cells was improved, and this adhesion was further found relating to the potential target protein Pyruvate kinase M1/2 (PKM) of HT-29 cells. In addition, as a multifunctional protein, LEP can promote the hydrolysis of bovine milk lipids with its esterase activity, and the activity was enhanced in the presence of Zn2+ and Mn2+ at pH 7. Furthermore, the polyunsaturated fatty acids (PUFA) such as linoleic acid and eicosapentaenoic acid were found to increase during the hydrolyzing process. These unique properties of LEP provide a comprehensive understanding of the adhesion function and PUFA releasing properties of the multifunctional protein derived from L. reuteri SH-23 and shed light on the beneficial effect of this Lactobacillus strain on the colonization of the gastrointestinal tract.

The Evaluation of Activity of Selected Lactic Acid Bacteria for Bioconversion of Milk and Whey from Goat Milk to Release Biomolecules with Antibacterial Activity

The aim of the study was to assess the antibacterial features of functional macromolecules released during the fermentation of goat milk and whey from goat milk by selected lactic acid bacteria strains that are components of kefir grain microflora. Two milk sources were used: goat milk and whey from goat milk. The lactic acid bacteria (LAB) and indicator microorganisms used were Lactobacillus plantarum PCM 1386, Lactobacillus fermentum PCM 491, Lactobacillus rhamnosus PCM 2677, Lactobacillus acidophilus PCM 2499, Escherichia coli PCM 2793, Salmonella enteritidis PCM 2548, Micrococcus luteus PCM 525, and Proteus mirabilis PCM 1361. The metabolic activity of LAB was described by the Gompertz model, and the parameters proposed for this experiment were the maximum rate of change of electrical impedance and potential biodegradability. Antibacterial activity was examined using the culture method in a liquid medium, determination of the reduction in indicator microorganisms, and optical density changes. Results show that the selective LAB produced certain active biomolecules with antibacterial activity from whey, a by-product that is sometimes troublesome for goat milk processors to manage. Lactobacillus acidophilus is a microorganism that is characterized by the highest metabolic activity in goat milk and whey from goat milk. It has the possibility to produce macromolecules with antibacterial activity.

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.

Adhesion and Colonization of the Probiotic Lactobacillus plantarum HC-2 in the Intestine of Litopenaeus Vannamei Are Associated With Bacterial Surface Proteins

Surface proteins are a type of proteins expressed on the surface of bacteria that play an important role in cell wall synthesis, maintenance of cell morphology, and signaling with the host. Our previous study showed that the probiotic Lactobacillus plantarum HC-2 improved the growth performance and immune response of Litopenaeus vannamei. To further investigate the probiotic mechanism, we determined the automatic aggregation ability of the bacteria and surface hydrophobicity of HC-2 after being treated with 5 M of lithium chloride (LiCl) and observed the morphology and adhesion of the bacteria to HCT116 cells. The results showed that with the removal of the HC-2 surface protein, the auto-aggregation ability and surface hydrophobicity of HC-2 decreased, and the crude mucus layer coated on the bacterial surface gradually dissociated. The adhesion rate of HC-2 to HCT116 cells decreased from 98.1 to 20.9%. Moreover, a total of 201 unique proteins were identified from the mixture of the surface proteins by mass spectrometry (MS). Several proteins are involved in transcription and translation, biosynthetic or metabolic process, cell cycle or division, cell wall synthesis, and emergency response. Meanwhile, a quantitative real-time PCR qPCR_ showed that HC-2 was mainly colonized in the midgut of shrimp, and the colonization numbers were 15 times higher than that in the foregut, while the colonization rate in the hindgut was lower. The adhesion activity measurement showed that the adhesion level of HC-2 to crude intestinal mucus of L. vannamei was higher than that of bovine serum albumin (BSA) and collagen, and the adhesion capacity of the bacterial cells decreased with the extension of LiCl-treatment time. Finally, we identified the elongation factor Tu, Type I glyceraldehyde-3-phosphate dehydrogenase, small heat shock protein, and 30S ribosomal protein from the surface proteins, which may be the adhesion proteins of HC-2 colonization in the shrimp intestine. The above results indicate that surface proteins play an important role in maintaining the cell structure stability and cell adhesion. Surface proteomics analysis contributes to describing potential protein-mediated probiotic-host interactions. The identification of some interacting proteins in this work may be beneficial to further understand the adhesion/colonization mechanism and probiotic properties of L. plantarum HC-2 in the shrimp intestine.

Effect of Surface Layer Proteins Derived from Paraprobiotic Kefir Lactic Acid Bacteria on Inflammation and High-Fat Diet-Induced Obesity

The antiobesity action of nonviable probiotic lactic acid bacteria (PLAB) may be attributed to bacterial cellular components recognized by host cells. The anti-inflammation and antiobesity properties of surface layer proteins (SLPs) that are cellular components isolated from kefir PLAB were determined in macrophage RAW 264.7 cells and obese mice. Kefir SLPs significantly decreased secretion of IL-6 and production of NF-kB p65 protein by LPS-stimulated RAW 264.7 cells in a dose-response manner. C57BL/6J mice were fed a high-fat (HF) diet with oral administration of either saline (CON) or kefir SLPs for 6 weeks. SLPs significantly improved body weight gain and adipose tissue weight, plasma triglyceride concentrations, and insulin resistance. Profiling of adipocyte gene expression showed that the antiobesity effect was significantly related to the expression of genes associated with adipogenesis, autophagy, and inflammatory/immune response, and fatty acid oxidation. Taken together, SLPs are a novel bioactive component in kefir PLABs to target obesity and obesity-related disorders.

Immunostimulation by Lactobacillus kefiri S-layer proteins with distinct glycosylation patterns requires different lectin partners

S-layer (glyco)-proteins (SLPs) form a nanostructured envelope that covers the surface of different prokaryotes and show immunomodulatory activity. Previously, we have demonstrated that the S-layer glycoprotein from probiotic Lactobacillus kefiri CIDCA 8348 (SLP-8348) is recognized by Mincle (macrophage inducible C-type lectin receptor), and its adjuvanticity depends on the integrity of its glycans. However, the glycan's structure has not been described so far. Herein, we analyze the glycosylation pattern of three SLPs, SLP-8348, SLP-8321, and SLP-5818, and explore how these patterns impact their recognition by C-type lectin receptors and the immunomodulatory effect of the L. kefiri SLPs on antigen-presenting cells. High-performance anion-exchange chromatography-pulse amperometric detector performed after β-elimination showed glucose as the major component in the O-glycans of the three SLPs; however, some differences in the length of hexose chains were observed. No N-glycosylation signals were detected in SLP-8348 and SLP-8321, but SLP-5818 was observed to have two sites carrying complex N-glycans based on a site-specific analysis and a glycomic workflow of the permethylated glycans. SLP-8348 was previously shown to enhance LPS-induced activation on both RAW264.7 macrophages and murine bone marrow-derived dendritic cells; we now show that SLP-8321 and SLP-5818 have a similar effect regardless of the differences in their glycosylation patterns. Studies performed with bone marrow-derived dendritic cells from C-type lectin receptor-deficient mice revealed that the immunostimulatory activity of SLP-8321 depends on its recognition by Mincle, whereas SLP-5818's effects are dependent on SignR3 (murine ortholog of human DC-SIGN). These findings encourage further investigation of both the potential application of these SLPs as new adjuvants and the protein glycosylation mechanisms in these bacteria.

The importance of the S-layer on the adhesion and aggregation ability of Lactic acid bacteria

S-layer proteins in Lactic acid bacteria are not the only cell surface structures used for aggregation, but also plays significant role for intestinal tissue adhesion along with some other functional elements. In addition, it was determined that the properties of S-layer proteins differs not only between species but also the strains which belong to same species. In this work, presence and some functions of S-layer in lactic acid bacteria were determined, its effect on resistance to gastrointestinal enzymes, aggregation and adhesion ability were investigated as well. For this purpose S-layers of microorganisms were removed by 5 M LiCl treatment and size of the proteins were determined by SDS-PAGE analysis. The removal of S-layer proteins caused a change in the resistance of microorganisms to GIS enzymes. After the S-layer removal, two strains considerably lost their resistance to GIS enzymes. The strains mostly lost their aggregation ability in the absence of S-layer. The results showed that S-layer proteins are not the only structures involved in aggregation processes but, is a major mediator in Lactobacilli. Removal of S-layer had no effect on adhesion ability of W. cibaria DA28, the effect on L. casei DA4, L. coryniformis DA263 and L. plantarum DA140 was moderate, but the effect was high on L. plantarum DA100. The study showed that S-layer proteins play limited protection against GIS enzymes. In addition, absence of S-layer adversely affected aggregation and adhesion ability of strains.

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.

Slp-coated liposomes for drug delivery and biomedical applications: potential and challenges

Slp forms a crystalline array of proteins on the outermost envelope of bacteria and archaea with a molecular weight of 40-200 kDa. Slp can self-assemble on the surface of liposomes in a proper environment via electrostatic interactions, which could be employed to functionalize liposomes by forming Slp-coated liposomes for various applications. Among the molecular characteristics, the stability, adhesion, and immobilization of biomacromolecules are regarded as the most meaningful. Compared to plain liposomes, Slp-coated liposomes show excellent physicochemical and biological stabilities. Recently, Slp-coated liposomes were shown to specifically adhere to the gastrointestinal tract, which was attributed to the "ligand-receptor interaction" effect. Furthermore, Slp as a "bridge" can immobilize functional biomacromol-ecules on the surface of liposomes via protein fusion technology or intermolecular forces, endowing liposomes with beneficial functions. In view of these favorable features, Slp-coated liposomes are highly likely to be an ideal platform for drug delivery and biomedical uses. This review aims to provide a general framework for the structure and characteristics of Slp and the interactions between Slp and liposomes, to highlight the unique properties and drug delivery as well as the biomedical applications of the Slp-coated liposomes, and to discuss the ongoing challenges and perspectives.

Effects of oligosaccharides on the fermentation properties of Lactobacillus plantarum

In the present work, we studied the effects of different oligosaccharides on Lactobacillus plantarum ATCC14917, focusing on growth and adhesion characteristics and fermented milk flavor. The results showed that mannan-oligosaccharide (MOS) had the greatest proliferative effect on L. plantarum ATCC14917 in vitro. In terms of adhesive properties, the autoaggregation rate of L. plantarum cultured in MOS was 23.76%, adhesion to mucin was 24.65%, and adhesion to Caco-2 cells was 14.71%. These results for L. plantarum cultured with MOS were higher than those for L. plantarum cultured in fructo-oligosaccharides (FOS) or galacto-oligosaccharides (GOS). Furthermore, the surface consistency and viscosity scores of fermented milk of the MOS group was higher than that of milks cultured with FOS or GOS, although MOS had the lowest scores for fermented milk flavor.

Purification of Lactobacillus acidophilus surface-layer protein and its immunomodulatory effects on RAW264.7 cells

BACKGROUND

Surface-layer proteins (SLP) have been found in the outermost layer of the cell wall in many types of lactobacillus are considered to be an important factor with respect to intestinal immunity.

RESULTS

The present study compared the effects of SLP extracted by different concentrations of LiCl and carbamide, and subsequently identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, circular dichroism and differential scanning calorimetry. Furthermore, RAW 264.7 cells were used to evaluate the immunomodulatory effects of SLP. SLP were derived from Lactobacillus acidophilus CICC6074 with a molecular weight of 46 kDa, and consisted of 16.9% α-helix, 42.3% β-sheet, 20.8% β-turns and 22.5% random coils. SLP promoted NO secretion and higher quantities of NO were produced as the SLP concentrations increased. SLP concentrations over 50 µg mL^-1 significantly decreased the amount of tumor necrosis factor-α secreted by RAW264.7 cells.

CONCLUSION

SLP can trigger immunomodulatory effects in RAW 264.7 cells. This provides crucial information that will enable the further use of L. acidophilus in food, medicine and other products. © 2017 Society of Chemical Industry.

Biological activity of the non-microbial fraction of kefir: antagonism against intestinal pathogens

Kefir is a fermented milk obtained by the activity of kefir grains which are composed of lactic and acetic acid bacteria, and yeasts. Many beneficial health effects have been associated with kefir consumption such as stimulation of the immune system and inhibition of pathogenic microorganisms. The biological activity of kefir may be attributed to the presence of a complex microbiota as well as the microbial metabolites that are released during fermentation. The aim of this work was to characterise the non-microbial fraction of kefir and to study its antagonism againstEscherichia coli,Salmonellaspp. andBacillus cereus.During milk fermentation there was a production of organic acids, mainly lactic and acetic acid, with a consequent decrease in pH and lactose content. The non-microbial fraction of kefir added to nutrient broth at concentrations above 75% v/v induced a complete inhibition of pathogenic growth that could be ascribed to the presence of un-dissociated lactic acid. In vitro assays using an intestinal epithelial cell model indicated that pre-incubation of cells with the non-microbial fraction of kefir did not modify the association/invasion ofSalmonellawhereas pre-incubation ofSalmonellawith this fraction under conditions that did not affect their viability significantly decreased the pathogen's ability to invade epithelial cells. Lactate exerted a protective effect againstSalmonellain a mouse model, demonstrating the relevance of metabolites present in the non-microbial fraction of kefir produced during milk fermentation.

Deleterious Metabolic Effects of High Fructose Intake: The Preventive Effect of Lactobacillus kefiri Administration

Modern lifestyle and diets have been associated with metabolic disorders and an imbalance in the normal gut microbiota. Probiotics are widely known for their health beneficial properties targeting the gut microbial ecosystem. The aim of our study was to evaluate the preventive effect of Lactobacillus kefiri (L. kefiri) administration in a fructose-rich diet (FRD) mice model. Mice were provided with tap water or fructose-added (20% w/v) drinking water supplemented or not with L. kefiri. Results showed that probiotic administration prevented weight gain and epidydimal adipose tissue (EAT) expansion, with partial reversion of the adipocyte hypertrophy developed by FRD. Moreover, the probiotic prevented the increase of plasma triglycerides and leptin, together with the liver triglyceride content. Leptin adipocyte secretion was also improved by L. kefiri, being able to respond to an insulin stimulus. Glucose intolerance was partially prevented by L. kefiri treatment (GTT) and local inflammation (TNFα; IL1β; IL6 and INFγ) was completely inhibited in EAT. L. kefiri supplementation generated an impact on gut microbiota composition, changing Bacteroidetes and Firmicutes profiles. Overall, our results indicate that the administration of probiotics prevents the deleterious effects of FRD intake and should therefore be promoted to improve metabolic disorders.

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.

The Adhesion of Lactobacillus salivarius REN to a Human Intestinal Epithelial Cell Line Requires S-layer Proteins

Lactobacillus salivarius REN, a novel probiotic isolated from Chinese centenarians, can adhere to intestinal epithelial cells and subsequently colonize the host. We show here that the surface-layer protein choline-binding protein A (CbpA) of L. salivarius REN was involved in adherence to the human colorectal adenocarcinoma cell line HT-29. Adhesion of a cbpA deletion mutant was significantly reduced compared with that of wild-type, suggesting that CbpA acts as an adhesin that mediates the interaction between the bacterium and its host. To identify the molecular mechanism of adhesion, we determined the crystal structure of a truncated form of CbpA that is likely involved in binding to its cell-surface receptor. The crystal structure identified CbpA as a peptidase of the M23 family whose members harbor a zinc-dependent catalytic site. Therefore, we propose that CbpA acts as a multifunctional surface protein that cleaves the host extracellular matrix and participates in adherence. Moreover, we identified enolase as the CbpA receptor on the surface of HT-29 cells. The present study reveals a new class of surface-layer proteins as well as the molecular mechanism that may contribute to the ability of L. salivarius REN to colonize the human gut.

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.

Characteristics of surface layer proteins from two new and native strains of Lactobacillus brevis

In this work, some important characteristics of surface layer (S-layer) proteins extracted from two new and native Lactobacillus strains, L.brevis KM3 and L.brevis KM7, were investigated. The presence of S-layer on the external surface of L.brevis KM3 was displayed by thin sectioning and negative staining. SDS-PAGE analysis were shown same dominant protein bands approximately around 48kDa for both S-layer proteins. Moreover, the S-layer reappeared when LiCl treated cells were allowed to grow again. Protein secondary structure and thermal behavior were evaluated by using circular dichroism (CD) and differential scanning calorimetry (DSC), respectively. Both S-layer proteins had high content of β-sheet and low amount of α-helix. The thermograms of lyophilized S-layer proteins of L.brevis KM3 and L.brevis KM7 showed one transition peak at 67.9°C and 59.14°C, respectively. To determine monodispersity of extracted S-layer proteins, dynamic light scattering (DLS) was used. The results indicated that the main population of S-layer molecules in two tested lactobacillus strains were composed of monomer with an expected diameter close to 10nm. Furthermore, Zeta potential measurements were showed positive potential for both S-layer proteins, as expected. Our results could be used as the basis for biotechnological applications of these two new S-layer proteins.

Diversity in S-layers

Surface layers, referred simply as S-layers, are the two-dimensional crystalline arrays of protein or glycoprotein subunits on cell surface. They are one of the most common outermost envelope components observed in prokaryotic organisms (Archaea and Bacteria). Over the past decades, S-layers have become an issue of increasing interest due to their ubiquitousness, special features and functions. Substantial work in this field provides evidences of an enormous diversity in S-layers. This paper reviews and illustrates the diversity from several different aspects, involving the S-layer-carrying strains, the structure of S-layers, the S-layer proteins and genes, as well as the functions of S-layers.

Probiotic Lactobacillus and Bifidobacterial Lectins Against Candida albicans and Staphylococcus aureus Clinical Strains: New Class of the Pathogen Biofilm Destructors

Preparations of probiotic bifidobacterial and lactobacillus lectins possessed system affinity to mannan and mucin-type polymers. It was shown that these lectins possess fungistatic and fungicidal activities against nystatin-resistant Candida albicans clinical strains. Lectins revealed destructive properties with respect to C. albicans and Staphylococcus aureus biofilms, depending on clinical strain origin and lectin preparation type. Synergistic antipathogen activities between lectins and between lectins and nystatin were observed. In the presence of lectins, pathogen biofilm degradation occurred in sequential steps, including biofilm refinement, appearance of edge cavities, segmentation, detachment of fragments and their lysis. Fungal response to lectins was more complex compared to that of staphylococci. Cold stress improved pictures of lectin antipathogen action. The data indicate that probiotic bacterial lectins are members of a new class of antimicrobials-destructors of pathogen biofilms.

Role of S-layer proteins in the biosorption capacity of lead by Lactobacillus kefir

The role of S-layer proteins (SLP) on the Pb(2+) sequestrant capacity by Lactobacillus kefir CIDCA 8348 and JCM 5818 was investigated. Cultures in the stationary phase were treated with proteinase K. A dot blot assay was carried out to assess the removal of SLP. Strains with and without SLP were exposed to 0-0.5 mM Pb(NO3)2. The maximum binding capacity (q max ) and the affinity coefficient (b) were calculated using the Langmuir equation. The structural effect of Pb(2+) on microorganisms with and without SLP was determined using Raman spectroscopy. The bacterial interaction with Pb(2+) led to a broadening in the phosphate bands (1,300-1,200 cm(-1) region) and strong alterations on amide and carboxylate-related bands (νCOO(-) as and νCOO(-) s). Microorganisms without SLP removed higher percentages of Pb(2+) and had higher q max than those bearing SLP. Isolated SLP had much lower q max and also removed lower percentages of Pb(2+) than the corresponding whole microorganisms. The hydrofobicity of both strains dramatically dropped when removing SLP. When bearing SLP, strains do not expose a large amount of charged groups on their surfaces, thus making less efficient the Pb(2+) removal. On the contrary, the extremely low hydrofobicity of microorganisms without SLP (and consequently, their higher capacity to remove Pb(2+)) can be explained on the basis of a greater exposure of charged chemical groups for the interaction with Pb(2+). The viability of bacteria without SLP was not significantly lower than that of bacteria bearing SLP. However, microorganisms without SLP were more prone to the detrimental effect of Pb(2+), thus suggesting that SLP acts as a protective rather than as a sequestrant layer.

Mechanism of algal aggregation by Bacillus sp. strain RP1137

Alga-derived biofuels are one of the best alternatives for economically replacing liquid fossil fuels with a fungible renewable energy source. Production of fuel from algae is technically feasible but not yet economically viable. Harvest of dilute algal biomass from the surrounding water remains one of the largest barriers to economic production of algal biofuel. We identified Bacillus sp. strain RP1137 in a previous study and showed that this strain can rapidly aggregate several biofuel-producing algae in a pH- and divalent-cation-dependent manner. In this study, we further characterized the mechanism of algal aggregation by RP1137. We show that aggregation of both algae and bacteria is optimal in the exponential phase of growth and that the density of ionizable residues on the RP1137 cell surface changes with growth stage. Aggregation likely occurs via charge neutralization with calcium ions at the cell surface of both algae and bacteria. We show that charge neutralization occurs at least in part through binding of calcium to negatively charged teichoic acid residues. The addition of calcium also renders both algae and bacteria more able to bind to hydrophobic beads, suggesting that aggregation may occur through hydrophobic interactions. Knowledge of the aggregation mechanism may enable engineering of RP1137 to obtain more efficient algal harvesting.

Adhesion properties of potentially probiotic Lactobacillus kefiri to gastrointestinal mucus

We investigated the mucus-binding properties of aggregating and non-aggregating potentially probiotic strains of kefir-isolated Lactobacillus kefiri, using different substrates. All the strains were able to adhere to commercial gastric mucin (MUCIN) and extracted mucus from small intestine (SIM) and colon (CM). The extraction of surface proteins from bacteria using LiCl or NaOH significantly reduced the adhesion of three selected strains (CIDCA 8348, CIDCA 83115 and JCM 5818); although a significant proportion (up to 50%) of S-layer proteins were not completely eliminated after treatments. The surface (S-layer) protein extracts from all the strains of Lb. kefiri were capable of binding to MUCIN, SIM or CM, and no differences were observed among them. The addition of their own surface protein extract increased adhesion of CIDCA 8348 and 83115 to MUCIN and SIM, meanwhile no changes in adhesion were observed for JCM 5818. None of the seven sugars tested had the ability to inhibit the adhesion of whole bacteria to the three mucus extracts. Noteworthy, the degree of bacterial adhesion reached in the presence of their own surface protein (S-layer) extract decreased to basal levels in the presence of some sugars, suggesting an interaction between the added sugar and the surface proteins. In conclusion, the ability of these food-isolated bacteria to adhere to gastrointestinal mucus becomes an essential issue regarding the biotechnological potentiality of Lb. kefiri for the food industry.

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.

Kefir-isolated Lactococcus lactis subsp. lactis inhibits the cytotoxic effect of Clostridium difficile in vitro

Kefir is a dairy product obtained by fermentation of milk with a complex microbial population and several health-promoting properties have been attributed to its consumption. In this work, we tested the ability of different kefir-isolated bacterial and yeast strains (Lactobacillus kefir, Lb. plantarum, Lactococcus lactis subps. lactis, Saccharomyces cerevisiae and Kluyveromyces marxianus) or a mixture of them (MM) to antagonise the cytopathic effect of toxins from Clostridium difficile (TcdA and TcdB). Cell detachment assays and F-actin network staining using Vero cell line were performed. Although incubation with microbial cells did not reduce the damage induced by C. difficile spent culture supernatant (SCS), Lc. lactis CIDCA 8221 and MM supernatants were able to inhibit the cytotoxicity of SCS to Vero cells. Fraction of Lc. lactis CIDCA 8221 supernatant containing components higher than 10 kDa were responsible for the inhibitory activity and heating of this fraction for 15 min at 100 °C completely abrogated this ability. By dot-blot assay with anti-TcdA or anti-TcdB antibodies, concentration of both toxins seems to be reduced in SCS treated with Lc. lactis CIDCA 8221 supernatant. However, protective effect was not affected by treatment with proteases or proteases-inhibitors tested. In conclusion, we demonstrated that kefir-isolated Lc. lactis CIDCA 8221 secreted heat-sensitive products able to protect eukaryotic cells from cytopathic effect of C. difficile toxins in vitro. Our findings provide new insights into the probiotic action of microorganisms isolated from kefir against virulence factors from intestinal pathogens.

Lactobacillus plantarum isolated from kefir protects vero cells from cytotoxicity by type-II shiga toxin from Escherichia coli O157:H7

Kefir is a fermented-milk beverage originating and widely consumed in the Caucasus as well as in Eastern Europe and is a source of bacteria with potential probiotic properties. Enterohaemorrhagic Escherichia coli producing Shiga toxin is commonly associated with food-transmitted diseases; the most prevalent serotype causing epidemics is Esch. coli O157:H7. The aim of this study was to evaluate the antagonism of Lactobacillus plantarum isolated from kefir against the action on Vero cells of supernatants of the Esch. coli O157:H7 strain 69160 expressing the type-II Shiga toxin (Stx2) and to study the role of the Lactobacillus cell wall in that inhibition. Spent culture supernatants of Esch. coli O157:H7 strain 69160 led to cytotoxic effects on cultured eukaryotic cells as evidenced by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-bromide-cleavage assay or by lactate-dehyrogenase release. Lb. plantarum CIDCA 83114 reduced the cytotoxic activity of Stx present in strain-69160 supernatants, and this protection was markedly higher than those of Lactobacillus kefir CIDCA 83113 and 8348 and Lb. delbrueckii subsp. bulgaricus CIDCA 333. This antagonism of cytotoxicity was mimicked by Lb. plantarum cell walls but was reduced after heating or protease treatments, thus indicating a protein or peptide as being involved in the protection mechanism. The cell surface of the lactobacilli bound the subunit B of Stx thereby decreasing the cytotoxicity. These interactions could constitute the first step in preventing the damage induced by Esch. coli O157:H7 supernatants, thus representing a valuable means of potentially mitigating the noxious effects of this food pathogen.

Physicochemical, microbiological and sensory profiles of fermented milk containing probiotic strains isolated from kefir

A two-strain starter culture containing Lactobacillus plantarum CIDCA 83114, a potential probiotic strain isolated from kefir grains, and Streptococcus thermophilus CIDCA 321 was tested for the preparation of a fermented milk product. Kluyveromyces marxianus CIDCA 8154, a yeast with immunomodulatory properties was included to formulate a three-strain starter culture. Supernatants of enterohaemorragic Escherichia coli, shiga-toxin-producing strain, along with a two-strain or a three-strain starter culture were included in the medium of Vero-cell surface cultures. The results demonstrated that these combinations of microorganisms antagonize the cytopathic action of shiga toxins. The cell concentration of Lb. plantarum did not decrease during fermentation, indicating that the viability of this strain was not affected by low pH, nor did the number of viable bacteria change during 21 days of storage in either fermented products. The number of viable yeasts increases during fermentation and storage. Trained assessors analyzed the general acceptability of fresh fermented milks and considered both acceptable. The milk fermented with the two-strain starter culture was considered acceptable after two week of storage, while the product fermented with the three-strain starter culture remained acceptable for less than one week. The main changes in sensory attributes detected by the trained panel were in sour taste, milky taste and also in fermented attributes. The correlation between different sensory attributes and acceptability indicated that the panel was positively influenced by milky attributes (taste, odour, and flavour) as well as the intensity of flavour. In conclusion, the two-strain starter culture would be the more promising alternative for inclusion of that potential probiotic lactobacillus in a fermented milk product.

Lectins of beneficial microbes: system organisation, functioning and functional superfamily

In this review our last results and proposals with respect to general aspects of lectin studies are summarised and compared. System presence, organisation and functioning of lectins are proposed, and accents on beneficial symbiotic microbial lectins studies are presented. The proposed general principles of lectin functioning allows for a comparison of lectins with other carbohydrate-recognition systems. A new structure-functional superfamily of symbiotic microbial lectins is proposed and its main properties are described. The proposed superfamily allows for extended searches of the biological activities of any microbial member. Prospects of lectins of beneficial symbiotic microorganisms are discussed.

Cellular injuries of spray-dried Lactobacillus spp. isolated from kefir and their impact on probiotic properties

The injuries caused by spray drying (SD) of three potential probiotic lactobacilli isolated from kefir grains and the impact on some probiotic properties, were evaluated. Results demonstrated that Lactobacillus plantarum 83114 and L. kefir 8321 showed a slight reduction of viability (0.11 and 0.29 log CFU/ml respectively) after SD process, and L. kefir 8348 was found to be more sensitive to the process with a reduction in viability of 0.70 log CFU/ml. Neither membrane damage, evaluated by increased sensitivity to NaCl, lysozyme, bile salt and penicillin G, nor changes in acidifying activity in MRS and milk by lactobacilli were detected after SD. L. plantarum 83114 and L. kefir 8321 after SD did not lose their capacity to adhere to intestinal cells. Nevertheless, L. kefir 8348 showed a significant loss of adhesion capacity after SD. In addition, rehydrated spray-dried L. kefir 8321 retained the ability to protect against Salmonella invasion of intestinal cells. This effect was observed when L. kefir is co-incubated with Salmonella before invasion assay. This work shows that the membrane integrity evaluated by indirect methods and some probiotic properties of lactobacilli isolated from kefir did not change significantly after SD, and these powders could be used in functional foods applications.

Effect of freeze-drying on viability andin vitroprobiotic properties of a mixture of lactic acid bacteria and yeasts isolated from kefir

The effect of freeze-drying on viability and probiotic properties of a microbial mixture containing selected bacterial and yeast strains isolated from kefir grains (Lactobacillus kefir,Lactobacillus plantarum,Lactococcus lactis,Saccharomyces cerevisiaeandKluyveromyces marxianus) was studied. The microorganisms were selected according to their potentially probiotic propertiesin vitroalready reported. Two types of formulations were performed, a microbial mixture (MM) suspended in milk and a milk product fermented with MM (FMM). To test the effect of storage on viability of microorganisms, MM and FMM were freeze-dried and maintained at 4°C for six months. After 180 days of storage at 4°C, freeze-dried MM showed better survival rates for each strain than freeze-dried FMM. The addition of sugars (trehalose or sucrose) did not improve the survival rates of any of the microorganisms after freeze-drying. Freeze-drying did not affect the capacity of MM to inhibit growth ofShigella sonnei in vitro, since the co-incubation of this pathogen with freeze-dried MM produced a decrease of 2 log inShigellaviability. The safety of freeze-dried MM was tested in mice and non-translocation of microorganisms to liver or spleen was observed in BALB/c mice feedad libitumduring 7 or 20 days. To our knowledge, this is the first report about the effect of freeze-drying on viability,in vitroprobiotic properties and microbial translocation of a mixture containing different strains of both bacteria and yeasts isolated from kefir.

The s-layer glycome-adding to the sugar coat of bacteria

The amazing repertoire of glycoconjugates present on bacterial cell surfaces includes lipopolysaccharides, capsular polysaccharides, lipooligosaccharides, exopolysaccharides, and glycoproteins. While the former are constituents of Gram-negative cells, we review here the cell surface S-layer glycoproteins of Gram-positive bacteria. S-layer glycoproteins have the unique feature of self-assembling into 2D lattices providing a display matrix for glycans with periodicity at the nanometer scale. Typically, bacterial S-layer glycans are O-glycosidically linked to serine, threonine, or tyrosine residues, and they rely on a much wider variety of constituents, glycosidic linkage types, and structures than their eukaryotic counterparts. As the S-layer glycome of several bacteria is unravelling, a picture of how S-layer glycoproteins are biosynthesized is evolving. X-ray crystallography experiments allowed first insights into the catalysis mechanism of selected enzymes. In the future, it will be exciting to fully exploit the S-layer glycome for glycoengineering purposes and to link it to the bacterial interactome.

Evaluation of adhesion capacity, cell surface traits and immunomodulatory activity of presumptive probiotic Lactobacillus strains

Twelve lactobacilli previously isolated from newborn infants' gastrointestinal tract and Feta cheese were further characterized by pulse field gel eletrophoresis (PFGE). All strains exhibited distinct PFGE genotypic patterns with the exception of DC421 and DC423 strains possessing identical patterns. The strains DC421, 2035 and 2012 were found to posses certain cell surface traits such as hydrophobicity, autoaggregation and/or high adhesive capacity suggesting potential immunomodulatory activity. However, application of the dorsal mouse air pouch system revealed that only the DC421, DC429 and 2035 strains exhibited strong immunostimulatory activity such as increased chemotaxis of polymorphonuclear (PMN) cells in association with increased phagocytosis and cytokine production. The same strains also induced immunomodulatory activity in the gut associated lymphoid tissue in mice in the absence of any inflammatory response. All strains induced IgA production while reduced TNFalpha production by small intestine cells. The strains DC421 and DC429 exerted their effect on the intestine through Toll-like receptor TLR2/TLR4/TLR9 mediated signalling events leading to secretion of a certain profile of cytokines in which gamma interferon (IFN-gamma), interleukin (IL)-5, IL-6 and IL-10 are included. The strain 2035 induced similar cytokine profile through the synergy of TLR2/TLR4. This study further supports the eligibility of the air pouch model to discriminate presumptive probiotic Lactobacillus strains exhibiting immunostimulatory activity in the gut. Furthermore, evidence is provided that the cell surface traits examined may not be the only criteria but an alternative and important component of a complex mechanism that enables a microorganism to interact with the host gut to exert its immunoregulatory activity.

Surface and adhesion properties of lactobacilli

The surface properties of lactobacilli are of significant technological importance as they determine the interaction of the bacterial cells with the gastrointestinal mucosa, and therefore influence their location in the gut and their functionality. Studying the surface of the bacteria is critical for understanding the adhesion process better. This review compiles the knowledge from studies on the characterization Lactobacillus surfaces and evaluates the potential relationship between the cells' physicochemical characteristics and their adhesive abilities. It also discusses the effect that the production processes, such as fermentation and drying, can exert on the surface properties and adhesion abilities of lactobacilli.

Heterogeneity of S-layer proteins from aggregating and non-aggregating Lactobacillus kefir strains

Since the presence of S-layer protein conditioned the autoaggregation capacity of some strains of Lactobacillus kefir, S-layer proteins from aggregating and non-aggregating L. kefir strains were characterized by immunochemical reactivity, MALDI-TOF spectrometry and glycosylation analysis. Two anti-S-layer monoclonal antibodies (Mab5F8 and Mab1F8) were produced; in an indirect enzyme-linked immunosorbent assay Mab1F8 recognized S-layer proteins from all L. kefir tested while Mab5F8 recognized only S-layer proteins from aggregating strains. Periodic Acid-Schiff staining of proteins after polyacrylamide gel electrophoresis under denaturing conditions revealed that all L. kefir S-layer proteins tested were glycosylated. Growth of bacteria in the presence of the N-glycosylation inhibitor tunicamycin suggested the presence of glycosydic chains O-linked to the protein backbone. MALDI-TOF peptide map fingerprint for S-layer proteins from 12 L. kefir strains showed very similar patterns for the aggregating strains, different from those for the non-aggregating ones. No positive match with other protein spectra in MSDB Database was found. Our results revealed a high heterogeneity among S-layer proteins from different L. kefir strains but also suggested a correlation between the structure of these S-layer glycoproteins and the aggregation properties of whole bacterial cells.

Expression of the Mucus Adhesion Gene Mub, Surface Layer Protein Slp and Adhesion-Like Factor EF-TU of Lactobacillus acidophilus ATCC 4356 Under Digestive Stress Conditions, as Monitored with Real-Time PCR

Expression of the mucus adhesion gene Mub, surface layer protein Slp and adhesion-like factor EF-Tu by Lactobacillus acidophilus ATCC 4356 grown in the presence of mucin, bile and pancreatin and at low pH was studied using real-time PCR. None of the genes were up-regulated under increasing concentrations of mucin, while Slp and EF-Tu were up-regulated in the presence of bile and pancreatin at normal concentrations (0.3%, w/v) and under stress conditions (1.0%, w/v).

Interaction between Lactobacillus kefir and Saccharomyces lipolytica isolated from kefir grains: evidence for lectin-like activity of bacterial surface proteins

Several microbial interactions involving yeast and lactobacilli have been suggested in fermented products. Co-aggregation between Lactobacillus kefir and yeast Saccharomyces lipolytica isolated from kefir grains was studied by scanning electron microscopy and aggregation assays. Six out of twenty Lb. kefir strains were able to co-aggregate with Sacch. lipolytica CIDCA 812 and showed thermolabile non-covalently bound surface molecules involved in this interaction. Co-aggregation inhibition after Lb. kefir pre-treatment with 5 m-LiCl or 20 g SDS/l showed that bacterial S-layer proteins play an important role in this interaction. Presence of different sugar (mannose, sucrose and fructose) or yeast pre-treatment with sodium periodate inhibited co-aggregation between Lb. kefir and Sacch. lipolytica. Co-aggregating Lb. kefir strains were also able to agglutinate with human red blood cells and they lost this ability after treatment with 5 m-LiCl. These results and the capacity of purified S-layer proteins of Lb. kefir to haemagglutinate, strongly suggest that a lectin-like activity of bacterial surface proteins (S-layer) mediates the aggregation with yeast cells.

Protective action of Lactobacillus kefir carrying S-layer protein against Salmonella enterica serovar Enteritidis

Eight Lactobacillus kefir strains isolated from different kefir grains were tested for their ability to antagonize Salmonella enterica serovar Enteritidis (Salmonella enteritidis) interaction with epithelial cells. L. kefir surface properties such as autoaggregation and coaggregation with Salmonella and adhesion to Caco-2/TC-7 cells were evaluated. L. kefir strains showed significantly different adhesion capacities, six strains were able to autoaggregate and four strains coaggregated with Salmonella. Coincubation of Salmonella with coaggregating L. kefir strains significantly decreased its capacity to adhere to and to invade Caco-2/TC-7 cells. This was not observed with non coaggregating L. kefir strains. Spent culture supernatants of L. kefir contain significant amounts of S-layer proteins. Salmonella pretreated with spent culture supernatants (pH 4.5-4.7) from all tested L. kefir strains showed a significant decrease in association and invasion to Caco-2/TC-7 cells. Artificially acidified MRS containing lactic acid to a final concentration and pH equivalent to lactobacilli spent culture supernatants did not show any protective action. Pretreatment of this pathogen with spent culture supernatants reduced microvilli disorganization produced by Salmonella. In addition, Salmonella pretreated with S-layer proteins extracted from coaggregating and non coaggregating L. kefir strains were unable to invade Caco-2/TC-7 cells. After treatment, L. kefir S-layer protein was detected associated with Salmonella, suggesting a protective role of this protein on association and invasion.