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

Background

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

Results

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

Conclusion

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

     

Characterization of the collagen-binding S-layer protein CbsA of Lactobacillus crispatus

The cbsA gene of Lactobacillus crispatus strain JCM 5810, encoding a protein that mediates adhesiveness to collagens, was characterized and expressed in Escherichia coli. The cbsA open reading frame encoded a signal sequence of 30 amino acids and a mature polypeptide of 410 amino acids with typical features of a bacterial S-layer protein. The cbsA gene product was expressed as a His tag fusion protein, purified by affinity chromatography, and shown to bind solubilized as well as immobilized type I and IV collagens. Three other Lactobacillus S-layer proteins, SlpA, CbsB, and SlpnB, bound collagens only weakly, and sequence comparisons of CbsA with these S-layer proteins were used to select sites in cbsA where deletions and mutations were introduced. In addition, hybrid S-layer proteins that contained the N or the C terminus from CbsA, SlpA, or SlpnB as well as N- and C-terminally truncated peptides from CbsA were constructed by gene fusion. Analysis of these molecules revealed the major collagen-binding region within the N-terminal 287 residues and a weaker type I collagen-binding region in the C terminus of the CbsA molecule. The mutated or hybrid CbsA molecules and peptides that failed to polymerize into a periodic S-layer did not bind collagens, suggesting that the crystal structure with a regular array is optimal for expression of collagen binding by CbsA. Strain JCM 5810 was found to contain another S-layer gene termed cbsB that was 44% identical in sequence to cbsA. RNA analysis showed that cbsA, but not cbsB, was transcribed under laboratory conditions. S-layer-protein-expressing cells of strain JCM 5810 adhered to collagen-containing regions in the chicken colon, suggesting that CbsA-mediated collagen binding represents a true tissue adherence property of L. crispatus.     

Protein O-glucosylation in Lactobacillus buchneri

Based on the previous demonstration of surface (S-) layer protein glycosylation in Lactobacillus buchneri 41021/251 and because of general advantages of lactic acid bacteria for applied research, protein glycosylation in this bacterial species was investigated in detail. The cell surface of L. buchneri CD034 is completely covered with an oblique 2D crystalline array (lattice parameters, a?=?5.9 nm; b?=?6.2 nm; γ ~ 77°) formed by self-assembly of the S-layer protein SlpB. Biochemical and mass spectrometric analyses revealed that SlpB is the most abundant protein and that it is O-glycosylated at four serine residues within the sequence S₁₅₂-A-S₁₅₄-S₁₅₅-A-S₁₅₇ with, on average, seven Glc(α1-6) residues, each. Subcellular fractionation of strain CD034 indicated a sequential order of SlpB export and glucosylation as evidenced by lack of glucosylation of cytosolic SlpB. Protein glycosylation analysis was extended to strain L. buchneri NRRL B-30929 where an analogous glucosylation scenario could be detected, with the S-layer glycoprotein SlpN containing an O-glycosylation motif identical to that of SlpB. This corroborates previous data on S-layer protein glucosylation of strain 41021/251 and let us propose a species-wide S-layer protein O-glucosylation in L. buchneri targeted at the sequence motif S-A-S-S-A-S. Search of the L. buchneri genomes for the said glucosylation motif revealed one further ORF, encoding the putative glycosyl‐hydrolase LbGH25B and LbGH25N in L. buchneri CD034 and NRRL B-30929, respectively, for which we have indications of a glycosylation comparable to that of the S-layer proteins. These findings demonstrate the presence of a distinct protein O-glucosylation system in Gram-positive and beneficial microbes.     

Lactobacillus slpA promotes ESC growth through the ERK1/2 pathway

Bacterial surface layers (S-layers) are cell envelope structures ubiquitously found in gram-negative and gram-positive bacteria, including Lactobacillus. S-layers play a role in the determination and maintenance of cell shape as virulence factors, mediate cell adhesion, and regulate immature dendritic and T cells. In this study, we sought to understand the involvement of MAPK serine/threonine kinases in alterations in Endometrial epithelial cells (ESC) growth induced by Lactobacillus crispatus (L. crispatus) slpA, an S-layer protein. We applied various concentrations of L. crispatus to cultured ESCs and observed growth and changes in the phosphorylation status of ERK1/2, JNK, and p38. Similar experiments were conducted using L. crispatus lacking and overexpressing slpA. We found that ESC growth was altered by slpA primarily via ERK1/2. Our findings suggest that L. crispatus slpA promotes ESC growth mainly through an ERK1/2-dependent pathway.     

Expression of cbsA encoding the collagen-binding S-protein of Lactobacillus crispatus JCM5810 in Lactobacillus casei ATCC 393(T)

The cbsA gene encoding the collagen-binding S-layer protein of Lactobacillus crispatus JCM5810 was expressed in L. casei ATCC 393^T. The S-protein was not retained on the surface of the recombinant bacteria but was secreted into the medium. By translational fusion of CbsA to the cell wall sorting signal of the proteinase, PrtP, of L. casei, CbsA was presented at the surface, rendering the transformants able to bind to immobilized collagens.     

The S-Layer Proteins of Two Bacillus stearothermophilus Wild-Type Strains Are Bound via Their N-Terminal Region to a Secondary Cell Wall Polymer of Identical Chemical Composition

Two Bacillus stearothermophilus wild-type strains were investigated regarding a common recognition and binding mechanism between the S-layer protein and the underlying cell envelope layer. The S-layer protein from B. stearothermophilus PV72/p6 has a molecular weight of 130,000 and assembles into a hexagonally ordered lattice. The S-layer from B. stearothermophilus ATCC 12980 shows oblique lattice symmetry and is composed of subunits with a molecular weight of 122,000. Immunoblotting, peptide mapping, N-terminal sequencing of the whole S-layer protein from B. stearothermophilus ATCC 12980 and of proteolytic cleavage fragments, and comparison with the S-layer protein from B. stearothermophilus PV72/p6 revealed that the two S-layer proteins have identical N-terminal regions but no other extended structurally homologous domains. In contrast to the heterogeneity observed for the S-layer proteins, the secondary cell wall polymer isolated from peptidoglycan-containing sacculi of the different strains showed identical chemical compositions and comparable molecular weights. The S-layer proteins could bind and recrystallize into the appropriate lattice type on native peptidoglycan-containing sacculi from both organisms but not on those extracted with hydrofluoric acid, leading to peptidoglycan of the A1γ chemotype. Affinity studies showed that only proteolytic cleavage fragments possessing the complete N terminus of the mature S-layer proteins recognized native peptidoglycan-containing sacculi as binding sites or could associate with the isolated secondary cell wall polymer, while proteolytic cleavage fragments missing the N-terminal region remained unbound. From the results obtained in this study, it can be concluded that S-layer proteins from B. stearothermophilus wild-type strains possess an identical N-terminal region which is responsible for anchoring the S-layer subunits to a secondary cell wall polymer of identical chemical composition.     

GneZ,a UDP-GlcNAc 2-Epimerase,Is Required for S-Layer Assembly and Vegetative Growth of Bacillus anthracis

Bacillus anthracis, the causative agent of anthrax, forms an S-layer atop its peptidoglycan envelope and displays S-layer proteins and Bacillus S-layer-associated (BSL) proteins with specific functions to support cell separation of vegetative bacilli and growth in infected mammalian hosts. S-layer and BSL proteins bind via the S-layer homology (SLH) domain to the pyruvylated secondary cell wall polysaccharide (SCWP) with the repeat structure [→4)-β-ManNAc-(1→4)-β-GlcNAc-(1→6)-α-GlcNAc-(1→]_n, where α-GlcNAc and β-GlcNAc are substituted with two and one galactosyl residues, respectively. B. anthracis gneY (BAS5048) and gneZ (BAS5117) encode nearly identical UDP-GlcNAc 2-epimerase enzymes that catalyze the reversible conversion of UDP-GlcNAc and UDP-ManNAc. UDP-GlcNAc 2-epimerase enzymes have been shown to be required for the attachment of the phage lysin PlyG with the bacterial envelope and for bacterial growth. Here, we asked whether gneY and gneZ are required for the synthesis of the pyruvylated SCWP and for S-layer assembly. We show that gneZ, but not gneY, is required for B. anthracis vegetative growth, rod cell shape, S-layer assembly, and synthesis of pyruvylated SCWP. Nevertheless, inducible expression of gneY alleviated all the defects associated with the gneZ mutant. In contrast to vegetative growth, neither germination of B. anthracis spores nor the formation of spores in mother cells required UDP-GlcNAc 2-epimerase activity.     

Probiotic Properties of Lactobacillus crispatus 2,029: Homeostatic Interaction with Cervicovaginal Epithelial Cells and Antagonistic Activity to Genitourinary Pathogens

Lactobacillus crispatus 2029 isolated upon investigation of vaginal lactobacilli of healthy women of reproductive age was selected as a probiotic candidate. The aim of the present study was elucidation of the role of L. crispatus 2029 in resistance of the female reproductive tract to genitourinary pathogens using cervicovaginal epithelial model. Lactobacillus crispatus 2029 has surface layers (S-layers), which completely surround cells as the outermost component of their envelope. S-layers are responsible for the adhesion of lactobacilli on the surface of cervicovaginal epithelial cells. Study of interactions between L. crispatus 2029 and a type IV collagen, a major molecular component of epithelial cell extracellular matrix, showed that 125I-labeled type IV collagen binds to lactobacilli with high affinity (Kd = (8.0 ± 0.7) × 10^?10 M). Lactobacillus crispatus 2029 consistently colonized epithelial cells. There were no toxicity, epithelial damage and apoptosis after 24 h of colonization. Electronic microscope images demonstrated intimate association between L. crispatus 2029 and epithelial cells. Upon binding to epithelial cells, lactobacilli were recognized by toll-like 2/6 receptors. Lactobacillus crispatus induced NF-κB activation in epithelial cells and did not induce expression of innate immunity mediators IL-8, IL-1β, IL-1α and TNF-α. Lactobacillus crispatus 2029 inhibited IL-8 production in epithelial cells induced by MALP-2 and increased production of anti-inflammatory cytokine IL-6, maintaining the homeostasis of female reproductive tract. Lactobacillus crispatus 2029 produced H₂O₂ and provided wide spectrum of antagonistic activity increasing colonization resistance to urinary tract infections by bacterial vaginosis and vulvovaginal candidiasis associated agents.     

Nanoscale mono- and multi-layer cylinder structures formed by recombinant S-layer proteins of mosquitocidal Bacillus sphaericus C3-41

The mature surface layer (S-layer) protein SlpC of mosquitocidal Bacillus sphaericus C3-41 comprises amino acids 31–1,176 and could recrystallize in vitro. The N-terminal SLH domain is responsible for binding function. Deletion of this part, S-layer proteins could not bind to the cell wall sacculi. To investigate the self-assembly ability of SlpC from B. sphaericus, nine truncations were constructed and their self-assembly properties were compared with the recombinant mature S-layer protein rSlpC_31–1,176. The results showed that rSbsC_31–1,176 and truncations rSlpC_211–1,176, rSlpC_278–1,176, rSlpC_31–1,100, and rSlpC_31–1,050 could assemble into multilayer cylinder structures, while N-terminal truncations rSlpC_338–1,176, rSlpC_438–1,176, and rSlpC_498–1,176 mainly showed monolayer cylinders in recombinant Escherichia coli BL21 (DE3) cells. Growth phase analysis of the self-assembly process revealed that rSlpC_498–1,176 mainly formed monolayer cylinders in the early stage (0.5 and 1 h induction of expression), but few double-layer or multilayer cylinders were also found with the cells growing, while rSlpC_31–1,176 could formed multilayer cylinders in all the growth stage in the E. coli cells. It is concluded that the deletion of the C-terminal 126 aa or the N-terminal 497 aa did not interfere with the self-assembly process, the fragment (amino acids 278 to 337) is essential for the multilayer cylinder formation in E. coli BL21 (DE3) cells in the early stage and the fragment (amino acids 338 to 497) is related to monolayer cylinder formation. The information is important for further studies on the assembly mechanism of S-layer proteins and forms a basis for further studies concerning surface display and nanobiotechnology.     

Improvement of protein production in lactic acid bacteria using 5′-untranslated leader sequence of slpA from Lactobacillus acidophilus

The 5′-untranslated leader sequence (UTLS) of the slpA gene from Lactobacillus acidophilus contributes to mRNA stabilization by producing a 5′ stem and loop structure, and a high-level expression system for the lactic acid bacteria was developed using the UTLS in this study. A plasmid, which expresses α-amylase under the control of the ldh promoter, was constructed by integrating the core promoter sequence with the UTLS. The role of the UTLS in increasing the copies of the α-amylase mRNA was proved by measuring α-amylase activity in the culture supernatant and the relative expression of α-amylase mRNA was determined by the quantitative real-time PCR analysis. Moreover, several expression systems were constructed by combining the core promoter sequence with the UTLS or with the partially deleted UTLS and the expression level was evaluated. The use of the UTLS led to the success in improving α-amylase expression in the two strains of Lactobacillus casei and Lactococcus lactis. The current study showed that the improvement in protein production using the UTLS could be applied to the expression system in the lactic acid bacteria.     

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.     

Heterogeneity of S-layer proteins from aggregating and non-aggregating <Emphasis Type="Italic">Lactobacillus kefir</Emphasis> 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.     

Species-Specific Cell Wall Binding Affinity of the S-Layer Proteins of Mosquitocidal Bacterium Bacillus sphaericus C3-41

The binding affinities and specificities of six truncated S-layer homology domain (SLH) polypeptides of mosquitocidal Bacillus sphaericus strain C3-41 with the purified cell wall sacculi have been assayed. The results indicated that the SLH polypeptide comprised of amino acids 31 to 210 was responsible for anchoring the S-layer subunits to the rigid cell wall layer via a distinct type of secondary cell wall polymer and that a motif of the recombinant SLH polypeptide comprising amino acids 152 to 210 (rSLH_152-210) was essential for the stable binding of the S-layer with the bacterial cell walls. The quantitative assays revealed that the K_D (equilibrium dissociation constant) values of rSLH_152-210 and rSLH_31-210 with purified cell wall sacculi were 1.11 × 10⁻⁶ M and 1.40 × 10⁻⁶ M, respectively. The qualitative assays demonstrated that the SLH domain of strain C3-41 could bind only to the cell walls or the cells treated with 5 M guanidinium hydrochloride of both toxic and nontoxic B. sphaericus strains but not to those from other bacteria, indicating the species-specific binding of the SLH polypeptide of B. sphaericus with bacterial cell walls.Crystalline bacterial cell surface layers (S-layers) cover the cell surfaces of many bacteria and archaea during all stages of growth and division. S-layers are composed of identical protein or glycoprotein subunits, which can assemble into two-dimensional crystalline arrays and exhibit oblique, square, or hexagonal symmetry (27, 28, 30). S-layers play key roles in the interaction between bacterial cells and environment as protective coats, molecular sieves, ion traps, cell adhesion mediators, and attachment structures (4, 21, 26, 29). Many S-layer proteins possess an N-terminal region with highly conserved amino acid sequences, which is called an S-layer homology (SLH) domain. An SLH domain contains one, two, or three repeating SLH motifs (6, 16). Each SLH motif is composed of about 55 amino acids containing 10 to 15 conserved residues (6, 17). It is suggested that the SLH domain of S-layer proteins is responsible for the binding of the S-layer subunits to the rigid cell wall layer (6, 15, 17, 19, 25), while the middle and C-terminal parts include the domains which are involved in the self-assembly process (27). In the case of Bacillaceae, secondary cell wall polymers (SCWP) are responsible for binding with SLH domains (13, 18, 19), but the SLH domains of some other bacteria have an affinity for peptidoglycan (33).Bacillus sphaericus is a gram-positive soil bacterium that represents a strictly aerobic group of mesophilic endospore-forming bacteria. Due to its specific toxicity to target mosquito larvae and the limited environment impact, some strains of this bacterium have been successfully used worldwide in integrated mosquito control programs. Previous studies revealed that some nontoxic strains of B. sphaericus contained S-layer proteins, and the S-layer proteins of B. sphaericus NCTC 9602, JG-A12, P1, and CCM 2177 have been studied in detail elsewhere (3, 7-9, 12, 22).B. sphaericus C3-41, a highly active strain isolated from a mosquito-breeding site in China in 1987, has different levels of toxicity against Culex spp., Anopheles spp., and Aedes spp. This strain belongs to the flagella serotype H5a5b, like strains 2362 and 1593 (32), and it has been developed as a commercial larvicide (JianBao) for mosquito larva control in China during the last decade (31). The genomic analysis of strain C3-41 revealed that an S-layer protein gene (slpC) (GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"EF535606","term_id":"145975949","term_text":"EF535606"}}EF535606) exists on the chromosomal genome and its sequence is identical to the S-layer protein of B. sphaericus 2362 (1, 10), composed of 3,531 bp encoding a protein of 1,176 amino acids with a molecular size of 125 kDa. Although the binding function of S-layers has been identified in some nontoxic B. sphaericus strains (6, 11), it is not well documented in mosquitocidal B. sphaericus strains, and there are few reports on the binding function of each SLH motif and the binding specificity.In this study, the binding affinities and specificities of each SLH motif of S-layer protein from mosquitocidal B. sphaericus C3-41 alone and in combination with the different cell wall preparations have been investigated, and the species-specific binding of SLH polypeptide with bacterial cell walls has been demonstrated.     

Differential Sensitivity of Lactobacillus spp. to Inhibition by Candidate Topical Microbicides

Preclinical evaluation of vaginal microbicides includes screening against lactobacilli. However, there is no consensus regarding the species to be tested. This study was carried out to determine if results with one species would apply to other species, and to evaluate the utility of turbidometry as a screening tool. One current (PPCM; previously designated sulfuric acid-modified mandelic acid, SAMMA) and two former (cellulose sulfate, CS; and polystyrene sulfonate, PSS) candidate microbicides were evaluated. Bacterial growth was measured turbidometrically and by direct cell count. No microbicide affected Lact. gasseri, measured by either method. Apparent inhibition of Lact. jensenii by CS, PSS, and PPCM, and of Lact. crispatus by CS, occurred with turbidometric measurement. This was not substantiated with direct cell count. PSS and PPCM inhibited Lact. crispatus and Lact. acidophilus with both methods. These findings agree with results from vaginal isolates, which included Lact. gasseri, jensenii, acidophillus, crispatus, rhamnosis, casei, and paracasei. We conclude that sensitivities of similar lactobacilli to at least three microbicides are different. A single species is inadequate for screening vaginal products. Turbidometric evaluation is a sensitive, but not specific, screening method. We recommend that this method be used to screen candidate microbicides against several species of prevalent Lactobacillus species as an initial measure of microbicide safety evaluation.     

The Capsule and S-Layer: Two Independent and Yet Compatible Macromolecular Structures in Bacillus anthracis

Bacillus anthracis, the etiological agent of anthrax, is a gram-positive spore-forming bacterium. Fully virulent bacilli are toxinogenic and capsulated. Two abundant surface proteins, including the major antigen, are components of the B. anthracis surface layer (S-layer). The B. anthracis paracrystalline S-layer has previously only been found in noncapsulated vegetative cells. Here we report that the S-layer proteins are also synthesized under conditions where the poly-γ-d-glutamic acid capsule is present. Structural and immunological analyses show that the capsule is exterior to and completely covers the S-layer proteins. Nevertheless, analysis of single and double S-layer protein mutants shows that the presence of these proteins is not required for normal capsulation of the bacilli. Similarly, the S-layer proteins assemble as a two-dimensional crystal, even in the presence of the capsule. Thus, both structures are compatible, and yet neither is required for the correct formation of the other.

Bacillus anthracis, a gram-positive spore-forming bacterium, is the causative agent of anthrax. This disease, to which many animals, including humans, are susceptible, involves toxemia and septicemia. In the mammalian host, B. anthracis bacilli synthesize two toxins (lethal and edema toxins) (31) and a capsule (18) encoded by two large plasmids, pXO1 and pXO2, respectively (12, 21). The capsule is composed of poly-γ-d-glutamic acid and has antiphagocytic properties (13, 31, 37). Although unusual, a similar capsule is also found on Bacillus licheniformis bacilli (9). In the absence of pXO2 or the inducer bicarbonate, the cell does not produce a capsule and the cell wall appears layered. These layers are composed of fragments displaying a highly patterned ultrastructure (10, 16). This type of cell surface is now referred to as the surface layer (S-layer).S-layers are present on the surfaces of many archaea and bacteria (for reviews, see references 29 and 30). Most are formed by noncovalent, entropy-driven assembly of a single (glyco)protein protomer on the bacterial surface, giving rise to proteinaceous paracrystalline layers. Generally, a single S-layer is present, constituting 5 to 10% of total cell protein. Its synthesis is thus presumably energy consuming for the bacterium. Numerous bacteria have S-layers, suggesting that they play important roles in the interaction between the cell and its environment. Various functions have been proposed for S-layers, including shape maintenance and molecular sieving, and they can serve in phage fixation. The S-layer may be a virulence factor, protecting pathogenic bacteria against complement killing, facilitating binding of bacteria to host molecules, or enhancing their ability to associate with macrophages (for reviews, see references 27 and 29).Some bacteria, such as cyanobacteria or Azotobacter spp., possess both a capsule and an S-layer; however, to our knowledge, their structural relationships have not been analyzed through simultaneous genetic and cytologic studies. Both of these features have been independently described for the surface of the pathogenic bacterium B. anthracis. The components of the B. anthracis S-layer are two abundant surface proteins, EA1 and Sap (6, 20). Previous analyses of the B. anthracis S-layer used plasmid-cured strains; consequently, the interaction, if any, between the capsule and the S-layer could not be studied. Temporal or environmental regulation could be such that only one or the other structure is ever present at the cell surface. However, we show that S-layer proteins are synthesized under conditions where the bacilli are capsulated. We determined the localizations of capsule and S-layer components and analyzed whether the S-layer is necessary for proper capsulation. Finally, the assembly of the S-layer proteins in a two-dimensional crystal was examined in the presence of the capsule.     

Bacillusanthracis Surface-Layer Proteins Assemble by Binding to the Secondary Cell Wall Polysaccharide in a Manner that Requires csaB and tagO

Bacillus anthracis, the causative agent of anthrax, requires surface (S)-layer proteins for the pathogenesis of infection. Previous work characterized S-layer protein binding via the surface layer homology domain to a pyruvylated carbohydrate in the envelope of vegetative forms. The molecular identity of this carbohydrate and the mechanism of its display in the bacterial envelope are still unknown. Analyzing acid-solubilized, purified carbohydrates by mass spectrometry and NMR spectroscopy, we identify secondary cell wall polysaccharide (SCWP) as the ligand of S-layer proteins. In agreement with the model that surface layer homology domains bind to pyruvylated carbohydrate, SCWP was observed to be linked to pyruvate in a manner requiring csaB, the only structural gene known to be required for S-layer assembly. B. anthracis does not elaborate wall teichoic acids; however, its genome harbors tagO and tagA, genes responsible for the synthesis of the linkage unit that tethers teichoic acids to the peptidoglycan layer. The tagO gene appears essential for B. anthracis growth and complements the tagO mutant phenotypes of staphylococci. Tunicamycin-mediated inhibition of TagO resulted in deformed, S-layer-deficient bacilli. Together, these results suggest that tagO-mediated assembly of linkage units tethers pyruvylated SCWP to the B. anthracis envelope, thereby enabling S-layer assembly and providing for the pathogenesis of anthrax infections.     

Unique Insights in the Cervicovaginal Lactobacillus iners and L. crispatus Proteomes and Their Associations with Microbiota Dysbiosis

Background

A Lactobacillus-dominated cervicovaginal microbiota (VMB) protects women from adverse reproductive health outcomes, but the role of L. iners in the VMB is poorly understood. Our aim was to explore the association between the cervicovaginal L. iners and L. crispatus proteomes and VMB composition.

Methods

The vaginal proteomes of 50 Rwandan women at high HIV risk, grouped into four VMB groups (based on 16S rDNA microarray results), were investigated by mass spectrometry using cervicovaginal lavage (CVL) samples. Only samples with positive 16S results for L. iners and/or L. crispatus within each group were included in subsequent comparative protein analyses: Lactobacillus crispatus-dominated VMB cluster (with 16S-proven L. iners (n_i) = 0, and with 16S-proven L. crispatus (n_c) = 5), L. iners-dominated VMB cluster (n_i = 11, n_c = 4), moderate dysbiosis (n_i = 12, n_c = 2); and severe dysbiosis (n_i = 8, n_c = 2). The relative abundances of proteins that were considered specific for L. iners and L. crispatus were compared among VMB groups.

Results

Forty Lactobacillus proteins were identified of which 7 were specific for L. iners and 11 for L. crispatus. The relative abundances of L. iners DNA starvation/stationary phase protection protein (DPS), and the glycolysis enzymes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and glucose-6-phosphate isomerase (GPI), were significantly decreased in women with L. iners-containing dysbiosis compared to women with a L. iners-dominated VMB, independent of vaginal pH and L. iners abundance. Furthermore, L. iners DPS, GAPDH, GPI, and fructose-bisphosphate aldolase (ALDO) were significantly negatively associated with vaginal pH. Glycolysis enzymes of L. crispatus showed a similar negative, but nonsignificant, trend related to dysbiosis.

Conclusions

Most identified Lactobacillus proteins had conserved intracellular functions, but their high abundance in CVL supernatant might imply an additional extracellular (moonlighting) role. Our findings suggest that these proteins can be important in maintaining a Lactobacillus-dominated VMB. Functional studies are needed to investigate their roles in vaginal bacterial communities and whether they can be used to prevent vaginal dysbiosis.