S-layer protein gene of Lactobacillus brevis: cloning by polymerase chain reaction and determination of the nucleotide sequence

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.

Oral Delivery of Novel Recombinant Lactobacillus Elicit High Protection against Staphylococcus aureus Pulmonary and Skin Infections

Staphylococcus aureus is a leading cause of nosocomial and community-associated infection worldwide; however, there is no licensed vaccine available. S. aureus initiates infection via the mucosa; therefore, a mucosal vaccine is likely to be a promising approach against S. aureus infection. Lactobacilli, a non-pathogenic bacterium, has gained increasing interest as a mucosal delivery vehicle. Hence, we attempted to develop an oral S. aureus vaccine based on lactobacilli to cushion the stress of drug resistance and vaccine needs. In this study, we designed, constructed, and evaluated recombinant Lactobacillus strains synthesizing S. aureus nontoxic mutated α-hemolysins (Hla_H35L). The results from animal clinical trials showed that recombinant Lactobacillus can persist for at least 72 h and can stably express heterologous protein in vivo. Recombinant L. plantarum WXD234 (pNZ8148-Hla) could induce robust mucosal immunity in the GALT, as evidenced by a significant increase in IgA and IL-17 production and the strong proliferation of T-lymphocytes derived from Peyer’s patches. WXD234 (pNZ8148-Hla) conferred up to 83% protection against S. aureus pulmonary infection and significantly reduced the abscess size in a S. aureus skin infection model. Of particular interest is the sharp reduction of the protective effect offered by WXD234 (pNZ8148-Hla) vaccination in γδ T cell-deficient or IL-17-deficient mice. In conclusion, for the first time, genetically engineered Lactobacillus WXD234 (pNZ8148-Hla) as an oral vaccine induced superior mucosal immunity, which was associated with high protection against pulmonary and skin infections caused by S. aureus. Taken together, our findings suggest the great potential for a delivery system based on lactobacilli and provide experimental data for the development of mucosal vaccines for S. aureus.

Environmental factors influence the Haloferax volcanii S-layer protein structure

S-layers commonly cover archaeal cell envelopes and are composed of proteins that self-assemble into a paracrystalline surface structure. Despite their detection in almost all archaea, there are few reports investigating the structural properties of these proteins, with no reports exploring this topic for halophilic S-layers. The objective of the present study was to investigate the secondary and tertiary organization of the Haloferax volcanii S-layer protein. Such investigations were performed using circular dichroism, fluorescence spectroscopy, dynamic light scattering and transmission electron microscopy. The protein secondary structure is centered on β-sheets and is affected by environmental pH, with higher disorder in more alkaline conditions. The pH can also affect the protein's tertiary structure, with higher tryptophan side-chain exposure to the medium under the same conditions. The concentrations of Na, Mg and Ca ions in the environment also affect the protein structures, with small changes in α-helix and β-sheet content, as well as changes in tryptophan side chain exposure. These changes in turn influence the protein's functional properties, with cell envelope preparations revealing striking differences when in different salt conditions. Thermal denaturation assays revealed that the protein is stable. It has been reported that the S-layer protein N-glycosylation process is affected by external factors and the present study indicates for the first time changes in the protein structure.

Adhesion of Lactobacilli and their anti-infectivity potential

The probiotic potential of lactic acid bacteria primarily point toward colonizing ability of Lactobacilli as the most important attribute for endowing all the known beneficial effects in a host. Lactobacillus species exert health-promoting function in the gastrointestinal tract through various mechanisms such as pathogen exclusion, maintenance of microbial balance, immunomodulation, and other crucial functions. It has been seen that many surface layer proteins are involved in host adhesion, and play significant role in the modification of some signaling pathways within the host cells. Interaction between different bacterial cell surface proteins and host receptor has been imperative for a better understanding of the mechanism through which Lactobacilli exert their health-promoting functions.

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.

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

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

Colonization of the Intestinal Tract of the Polyphagous Pest Spodoptera littoralis with the GFP-Tagged Indigenous Gut Bacterium Enterococcus mundtii

The alkaline gut of Lepidopterans plays a crucial role in shaping communities of bacteria. Enterococcus mundtii has emerged as one of the predominant gut microorganisms in the gastrointestinal tract of the major agricultural pest, Spodoptera littoralis. Therefore, it was selected as a model bacterium to study its adaptation to harsh alkaline gut conditions in its host insect throughout different stages of development (larvae, pupae, adults, and eggs). To date, the mechanism of bacterial survival in insects' intestinal tract has been unknown. Therefore, we have engineered a GFP-tagged species of bacteria, E. mundtii, to track how it colonizes the intestine of S. littoralis. Three promoters of different strengths were used to control the expression of GFP in E. mundtii. The promoter ermB was the most effective, exhibiting the highest GFP fluorescence intensity, and hence was chosen as our main construct. Our data show that the engineered fluorescent bacteria survived and proliferated in the intestinal tract of the insect at all life stages for up to the second generation following ingestion.

Adhesion of indigenous Lactobacillus plantarum to gut extracellular matrix and its physicochemical characterization

Adhesion to the human intestinal epithelial cell is considered as one of the important selection criteria of lactobacilli for probiotic attributes. Sixteen Lactobacillus plantarum strains from human origins were subjected for adhesion to extracellular matrix (ECM) components, and their physiochemical characterization, incubation time course and effect of different pH on bacterial adhesion in vitro were studied. Four strains showed significant binding to both fibronectin and mucin. After pretreatment with pepsin and trypsin, the bacterial adhesion to ECM reduced to the level of 50 % and with lysozyme significantly decreased by 65-70 %. Treatment with LiCl also strongly inhibited (90 %) the bacterial adhesion to ECM. Tested strains showed highest binding efficacy at time course of 120 and 180 min. Additionally, the binding of Lp91 to ECM was highest at pH 6 (155 ± 2.90 CFU/well). This study proved that surface layer components are proteinaceous in nature, which contributed in adhesion of lactobacillus strains. Further, the study can provide a better platform for introduction of new indigenous probiotic strains having strong adhesion potential for future use.

Expression of bioactive porcine interferon-alpha in Lactobacillus casei

In this study, we constructed an expression cassette containing the inducible lac promoter and the secretion signal from an S-layer protein of Lactobacillus brevis for the expression of porcine interferon-alpha (IFN-α) in Lactobacillus casei (Lb. casei). Reverse-transcriptase PCR verified the presence of porcine IFN-α mRNA in the recombinant Lb. casei. The porcine IFN-α protein expressed in the recombinant Lb. casei was identified by both Western blot analysis and ELISA. We used various pH values and induction times to optimize the yield of IFN-α, and found that induction with 0.8% lactose for 16 h under anaerobic conditions produced the highest concentrations of IFN-α. Furthermore, the activity of porcine IFN-α in the cultural supernatant was evaluated on ST cells infected with pseudorabies virus. The results revealed that porcine IFN-α inhibited virus replication in vitro. The findings of our study indicate that recombinant Lb. casei producing porcine IFN-α has great potential for use as a novel oral antiviral agent in animal healthcare.

Lactobacillus brevis strains from fermented aloe vera survive gastroduodenal environment and suppress common food borne enteropathogens

Five novel Lactobacillus brevis strains were isolated from naturally fermented Aloe vera leaf flesh. Each strain was identified by Random Amplified Polymorphic DNA (RAPD) analysis and 16S rRNA sequence comparison. These strains were highly tolerant to acid, surviving in pH2.5 for up to 4 hours, and resistant to 5% bile salts at 37°C for 18 hours. Due to its tolerance to acid and bile salts, one strain passed through the gastric barrier and colonised the intestine after oral administration. All five strains inhibited the growth of many harmful enteropathogens without restraining most of normal commensals in the gut and hence named POAL (Probiotics Originating from Aloe Leaf) strains. Additionally, each strain exhibited discriminative resistance to a wide range of antibiotics. The L. brevis POAL strains, moreover, expressed high levels of the glutamate decarboxylase (GAD) gene which produces a beneficial neurotransmitter, γ-aminobutyric acid (GABA). These characteristics in all suggest that the novel L. brevis strains should be considered as potential food additives and resources for pharmaceutical research.

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.

Production, secretion, and cell surface display of recombinant Sporosarcina ureae S-layer fusion proteins in Bacillus megaterium

Monomolecular crystalline bacterial cell surface layers (S-layers) have broad application potential in nanobiotechnology due to their ability to generate functional supramolecular structures. Here, we report that Bacillus megaterium is an excellent host organism for the heterologous expression and efficient secretion of hemagglutinin (HA) epitope-tagged versions of the S-layer protein SslA from Sporosarcina ureae ATCC 13881. Three chimeric proteins were constructed, comprising the precursor, C-terminally truncated, and N- and C-terminally truncated forms of the S-layer SslA protein tagged with the human influenza hemagglutinin epitope. For secretion of fusion proteins, the open reading frames were cloned into the Escherichia coli-Bacillus megaterium shuttle vector pHIS1525. After transformation of the respective plasmids into Bacillus megaterium protoplasts, the recombinant genes were successfully expressed and the proteins were secreted into the growth medium. The isolated S-layer proteins are able to assemble in vitro into highly ordered, crystalline, sheetlike structures with the fused HA tag accessible to antibody. We further show by fluorescent labeling that the secreted S-layer fusion proteins are also clustered on the cell envelope of Bacillus megaterium, indicating that the cell surface can serve in vivo as a nucleation point for crystallization. Thus, this system can be used as a display system that allows the dense and periodic presentation of S-layer proteins or the fused tags.

A Collagen-Binding S-Layer Protein in Lactobacillus crispatus

Two S-layer-expressing strains, Lactobacillus crispatus JCM 5810 and Lactobacillus acidophilus JCM 1132, were assessed for adherence to proteins of the mammalian extracellular matrix. L. crispatus JCM 5810 adhered efficiently to immobilized type IV and I collagens, laminin, and, with a lower affinity, to type V collagen and fibronectin. Strain JCM 1132 did not exhibit detectable adhesiveness. Within the fibronectin molecule, JCM 5810 recognized the 120-kDa cell-binding fragment of the protein, while no bacterial adhesion to the amino-terminal 30-kDa or the gelatin-binding 40-kDa fragment was detected. JCM 5810 but not JCM 1132 also bound (sup125)I-labelled soluble type IV collagen, and this binding was efficiently inhibited by unlabelled type IV and I collagens and less efficiently by type V collagen, but not by laminin or fibronectin. L. crispatus JCM 5810 but not L. acidophilus JCM 1132 also adhered to Matrigel, a reconstituted basement membrane preparation from mouse sarcoma cells, as well as to the extracellular matrix prepared from human Intestine 407 cells. S-layers from both strains were extracted with 2 M guanidine hydrochloride, separated by electrophoresis, and transferred to nitrocellulose sheets. The S-layer protein from JCM 5810 bound (sup125)I-labelled type IV collagen, whereas no binding was seen with the S-layer protein from JCM 1132. Binding of (sup125)I-collagen IV to the JCM 5810 S-layer protein was effectively inhibited by unlabelled type I and IV collagens but not by type V collagen, laminin, or fibronectin. It was concluded that L. crispatus JCM 5810 has the capacity to adhere to human subintestinal extracellular matrix via a collagen-binding S-layer.

Characterization and separate activities of the two promoters of the Lactobacillus brevis S-layer protein gene

Lactobacillus brevis ATCC 8287 possesses a surface (S)-layer protein SlpA, the gene of which is very efficiently expressed. To study the expression signals of the slpA gene, several different reporter plasmids, based on the low-copy-number vector pKTH2121 derived from pGK12, were constructed. In the reporter plasmids, only one of the two consecutive slpA promoters (P1, P2) was placed upstream of the Lactobacillus helveticus proline iminopeptidase (pepI) gene, and defined parts of the sequences upstream of the promoter were deleted. As indicated by reporter enzyme activities, both promoters were efficiently recognized at different growth stages in L. brevis. An upstream region important for the full activity of P1 was identified. The quantification of pepI-specific mRNA in L. brevis and SDS-PAGE indicated that slpA expression is not regulated at the post-transcriptional level and revealed no regulation of slpA promoters under the conditions tested. The high expression levels of both slpA and the reporter gene in L. brevis were found to remain at a high level after the addition of bile or pancreatin in the growth medium or after a change of the carbon source, which is advantageous for the potential use of SlpA as a carrier in live oral vaccines.

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.

Surface location of individual residues of SlpA provides insight into the Lactobacillus brevis S-layer

Bacterial surface layer (S-layer) proteins are excellent candidates for in vivo and in vitro nanobiotechnological applications because of their ability to self-assemble into two-dimensional lattices that form the outermost layer of many Eubacteria and most Archaea species. Despite this potential, knowledge about their molecular architecture is limited. In this study, we investigated SlpA, the S-layer protein of the potentially probiotic bacterium Lactobacillus brevis ATCC 8287 by cysteine-scanning mutagenesis and chemical modification. We generated a series of 46 mutant proteins by replacing single amino acids with cysteine, which is not present in the wild-type protein. Most of the replaced amino acids were located in the self-assembly domain (residues 179 to 435) that likely faces the outer surface of the lattice. As revealed by electron microscopy, all the mutant proteins were able to form self-assembly products identical to that of the wild type, proving that this replacement does not dramatically alter the protein conformation. The surface accessibility of the sulfhydryl groups introduced was studied with two maleimide-containing marker molecules, TMM(PEG)(12) (molecular weight [MW], 2,360) and AlexaFluor488-maleimide (MW = 720), using both monomeric proteins in solution and proteins allowed to self-assemble on cell wall fragments. Using the acquired data and available domain information, we assigned the mutated residues into four groups according to their location in the protein monomer and lattice structure: outer surface of the lattice (9 residues), inner surface of the lattice (9), protein interior (12), and protein-protein interface/pore regions (16). This information is essential, e.g., in the development of therapeutic and other health-related applications of Lactobacillus S-layers.

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).

Genes and molecules of lactobacilli supporting probiotic action

Lactobacilli have been crucial for the production of fermented products for centuries. They are also members of the mutualistic microbiota present in the human gastrointestinal and urogenital tract. Recently, increasing attention has been given to their probiotic, health-promoting capacities. Many human intervention studies demonstrating health effects have been published. However, as not all studies resulted in positive outcomes, scientific interest arose regarding the precise mechanisms of action of probiotics. Many reported mechanistic studies have addressed mainly the host responses, with less attention being focused on the specificities of the bacterial partners, notwithstanding the completion of Lactobacillus genome sequencing projects, and increasing possibilities of genomics-based and dedicated mutant analyses. In this emerging and highly interdisciplinary field, microbiologists are facing the challenge of molecular characterization of probiotic traits. This review addresses the advances in the understanding of the probiotic-host interaction with a focus on the molecular microbiology of lactobacilli. Insight into the molecules and genes involved should contribute to a more judicious application of probiotic lactobacilli and to improved screening of novel potential probiotics.

Identification and characterization of domains responsible for self-assembly and cell wall binding of the surface layer protein of Lactobacillus brevis ATCC 8287

BACKGROUND

Lactobacillus brevis ATCC 8287 is covered by a regular surface (S-) layer consisting of a 435 amino acid protein SlpA. This protein is completely unrelated in sequence to the previously characterized S-layer proteins of Lactobacillus acidophilus group.

RESULTS

In this work, the self-assembly and cell wall binding domains of SlpA were characterized. The C-terminal self-assembly domain encompassed residues 179-435 of mature SlpA, as demonstrated by the ability of N-terminally truncated recombinant SlpA to form a periodic structure indistinguishable from that formed by full length SlpA. Furthermore, a trypsin degradation analysis indicated the existence of a protease resistant C-terminal domain of 214 amino acids. By producing a set of C-terminally truncated recombinant SlpA (rSlpA) proteins the cell wall binding region was mapped to the N-terminal part of SlpA, where the first 145 amino acids of mature SlpA alone were sufficient for binding to isolated cell wall fragments of L. brevis ATCC 8287. The binding of full length rSlpA to the cell walls was not affected by the treatment of the walls with 5% trichloroacetic acid (TCA), indicating that cell wall structures other than teichoic acids are involved, a feature not shared by the Lactobacillus acidophilus group S-layer proteins characterized so far. Conserved carbohydrate binding motifs were identified in the positively charged N-terminal regions of six Lactobacillus brevis S-layer proteins.

CONCLUSION

This study identifies SlpA as a two-domain protein in which the order of the functional domains is reversed compared to other characterized Lactobacillus S-layer proteins, and emphasizes the diversity of potential cell wall receptors despite similar carbohydrate binding sequence motifs in Lactobacillus S-layer proteins.

Surface-bound proteins of Lactobacillus plantarum 423 that contribute to adhesion of Caco-2 cells and their role in competitive exclusion and displacement of Clostridium sporogenes and Enterococcus faecalis

Elongation factor Tu (EF-Tu), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and triosephosphate isomerase (TPI) are surface-bound proteins with a role in adhesion of Lactobacillus plantarum 423 to Caco-2 cells. Removal of surface-bound proteins from L. plantarum 423 (treated with 4M guanidine-HCl) reduced adhesion to Caco-2 cells by 40%. In a competitive exclusion experiment where all three strains were given an equal chance to adhere to Caco-2 cells, L. plantarum 423 prevented 71% of cells of Clostridium sporogenes LMG 13570 and 89% of cells of Enterococcus faecalis LMG 13566 from adhering. Cells of L. plantarum 423, from which surface-bound proteins were removed, prevented 49% of cells of C. sporogenes LMG 13570 and 70% of cells of E. faecalis LMG 13566 from adhering to Caco-2 cells, suggesting that factors other than surface-bound proteins are involved in adhesion. Colonization of L. plantarum 423 to Caco-2 cells prevented adhesion of 74% of cells of C. sporogenes LMG 13570 and 62% of cells of E. faecalis LMG 13566. Furthermore, L. plantarum 423 displaced 81% of cells of C. sporogenes LMG 13570 and 91% of cells of E. faecalis LMG 13566 from Caco-2 cells. L. plantarum 423 is a potential probiotic strain.

Adherence factors of Lactobacillus in the human gastrointestinal tract

Despite the increasing number of scientific reports describing adhesion of Lactobacillus to components of the human intestinal mucosa, information on the surface molecules mediating this adhesion and their corresponding receptors is fragmentary. This MiniReview compiles present knowledge of the genetically and functionally characterized Lactobacillus factors responsible for mediating adhesion to different components of the human gastrointestinal tract. In addition, for the proteins among these factors, the domain structure is discussed, and where appropriate the results of in silico analyses are reported.

The S-layer proteins of Lactobacillus crispatus strain ZJ001 is responsible for competitive exclusion against Escherichia coli O157:H7 and Salmonella typhimurium

Lactobacillus crispatus ZJ001, isolated from pig intestines and identified by sequencing analysis based on partial 16S rRNA gene, was examined in vitro for probiotic activity exerted by the surface layer proteins (S-layer). The characteristics of L. crispatus ZJ001 were compared to Lactobacillus acidophilus ATCC 4356 from the same genus which also produces the S-layer proteins. The strain ZJ001 was resistant to acidic condition and bile salt. Its antagonistic properties such as adhesion, inhibition of the pathogen growth and competitive exclusion against Escherichia coli O157:H7 and Salmonella typhimurium were apparently advantageous over L. acidophilus ATCC 4356. SDS-PAGE analysis of cell surface proteins revealed the presence of S-layer proteins, approximately at 42 kDa in L. crispatus ZJ001. Removal of the S-layer proteins reduced autoaggregation and adhesion to HeLa cells. The functional role of the S-layer proteins in adhesion was also confirmed by the antibody-mediated inhibition assay using the polyclonal antibody against the S-layer protein. The S-layer proteins from L. crispatus ZJ001 inhibited adhesion of S. typhimurium and E. coli O157:H7 to HeLa cells. These results suggest that L. crispatus ZJ001 possesses probiotic properties and the S-layer proteins are involved in the adhesion and competitive exclusion of pathogens to HeLa cells.

The surface (S)-layer gene cspB of Corynebacterium glutamicum is transcriptionally activated by a LuxR-type regulator and located on a 6 kb genomic island absent from the type strain ATCC 13032

The surface (S)-layer gene region of the Gram-positive bacterium Corynebacterium glutamicum ATCC 14067 was identified on fosmid clones, sequenced and compared with the genome sequence of C. glutamicum ATCC 13032, whose cell surface is devoid of an ordered S-layer lattice. A 5·97 kb DNA region that is absent from the C. glutamicum ATCC 13032 chromosome was identified. This region includes cspB, the structural gene encoding the S-layer protomer PS2, and six additional coding sequences. PCR experiments demonstrated that the respective DNA region is conserved in different C. glutamicum wild-type strains capable of S-layer formation. The DNA region is flanked by a 7 bp direct repeat, suggesting that illegitimate recombination might be responsible for gene loss in C. glutamicum ATCC 13032. Transfer of the cloned cspB gene restored the PS2− phenotype of C. glutamicum ATCC 13032, as confirmed by visualization of the PS2 proteins by SDS-PAGE and imaging of ordered hexagonal S-layer lattices on living C. glutamicum cells by atomic force microscopy. Furthermore, the promoter of the cspB gene was mapped by 5′ rapid amplification of cDNA ends PCR and the corresponding DNA fragment was used in DNA affinity purification assays. A 30 kDa protein specifically binding to the promoter region of the cspB gene was purified. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry and peptide mass fingerprinting of the purified protein led to the identification of the putative transcriptional regulator Cg2831, belonging to the LuxR regulatory protein family. Disruption of the cg2831 gene in C. glutamicum resulted in an almost complete loss of PS2 synthesis. These results suggested that Cg2831 is a transcriptional activator of cspB gene expression in C. glutamicum.

Sequence, organization, transcription and regulation of lactose and galactose operons in Lactobacillus rhamnosus TCELL-1

AIMS

Understanding the metabolism of lactose and galactose and their regulation in Lactobacillus rhamnosus.

METHODS AND RESULTS

A gene cluster containing nine open reading frames (ORFs) involved in the metabolism of lactose and galactose in Lact. rhamnosus TCELL-1 was sequenced and characterized. The order of the ORFs was lacTEGF and galKETRM. Northern blotting experiments revealed that the gene cluster could be transcribed as one lacTEGF-galKETRM mRNA though three major transcripts (lacTEGF, galKETRM and galETRM) were detected for the gene cluster. The transcription of the lac or gal operon was independently induced in the presence of lactose or galactose. Northern blotting and primer extension experiments found the presence of four putative promoters upstream from the ORFs lacT (lacTp), galK (galKp1 and galKp2) and galE (galEp). The measurements of enzymatic activities of GalK, GalE and GalT suggested that the expression of the gal operon was subjected to a galactose activation and glucose repression mechanism.

CONCLUSIONS

In Lact. rhamnosus TCELL-1, the galactose moiety of lactose could be metabolized by two alternative pathways (the Leloir and the tagatose 6-phosphate pathways) whereas galactose metabolism could be mediated by the Leloir pathway.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work provides important information about sugar metabolism in Lact. rhamnosus.

Screening of Dairy Yeast Strains for Probiotic Applications

To evaluate the potential of yeasts of dairy origin as probiotics, we tested 8 species including Candida humilis, Debaryomyces hansenii, Debaryomyces occidentalis, Kluyveromyces lactis, Kluyveromyces lodderae, Kluyveromyces marxianus, Saccharomyces cerevisiae, and Yarrowia lipolytica, isolated from commercial blue cheese and kefir. Strains were randomly selected from each species and tested for their ability to adhere to human enterocyte-like Caco-2 cells in culture. Among the 8 species, K. lactis showed higher adhesive ability than K. marxianus, K. lodderae, and D. hansenii. The other 4 species were poorly adhesive. All species other than K. marxianus and C. humilis were resistant to acidic conditions. In the presence of bile acid, growth inhibition was undetectable when incubation was carried out at 27 degrees C; however, it was evident for C. humilis and a strain of D. occidentalis when incubated at 37 degrees C. Moreover, the influence of proteinase treatment of living cells of K. lactis and K. lodderae on their adhesion to Caco-2 cells was evaluated. Although a slight reduction was recognized when K. lactis was treated with proteinase K, the influence of intestinal protease treatments of pepsin followed by trypsin was negligible. These results indicated that a proteinaceous factor was unlikely to be involved in adhesion of K. lactis and K. lodderae to Caco-2 cells. No stimulation of IL-8 synthesis by Caco-2 cells was recognized in the presence of K. lactis. In conclusion, K. lactis was the most attractive to continue study for use as probiotic microorganisms.

Receptor binding domain of Escherichia coli F18 fimbrial adhesin FedF can be both efficiently secreted and surface displayed in a functional form in Lactococcus lactis

Adherence of F18 fimbrial Escherichia coli to porcine intestinal epithelial cells is mediated by the adhesin (FedF) of F18 fimbriae. In a previous study, we demonstrated the specificity of the amino acid residues between 60 and 109 as the receptor binding domain of FedF. In this study, different expression, secretion, and anchoring systems for the receptor binding domain of the FedF adhesin in Lactococcus lactis were evaluated. Two partially overlapping receptor binding domains (42 and 62 amino acid residues) were expressed as fusions with L. lactis subsp. cremoris protein PrtP for evaluation of secretion efficiency. To evaluate the cell surface display of these FedF-PrtP fusions, they were further combined with different lengths of PrtP spacers fused with either the L. lactis AcmA anchor or the PrtP cell wall binding domain. An HtrA-defective L. lactis NZ9000 mutant was constructed to determine its effect on the level of secreted or anchored fusion proteins. Recombinant L. lactis clones secreting the receptor binding domain of F18 fimbriae as a fusion with the H domains of L. lactis protein PrtP were first constructed by using two different signal peptides. FedF-PrtP fusions, directed by the signal sequence of L. brevis SlpA, were throughout found to be secreted at significantly higher quantities than corresponding fusions with the signal peptide of L. lactis Usp45. In the surface display systems tested, the L. lactis AcmA anchor performed significantly better, particularly in the L. lactis NZ9000DeltahtrA strain, compared to the L. lactis PrtP anchor region. Of the cell surface display constructs with the AcmA anchor, only those with the longest PrtP spacer regions resulted in efficient binding of recombinant L. lactis cells to porcine intestinal epithelial cells. These results confirmed that it is possible to efficiently produce the receptor binding domain of the F18 adhesin in a functionally active form in L. lactis.

Synthesis and Characterization of Bioconjugates of S-Layer Proteins

The self-assembling proteins that form crystalline surface layers (S-layers) on many microbial species have found numerous applications due to their nanostructured nature. To devise a new method to construct surface displays that exploit S-layer self-assembly activity and nanostructural properties, we have constructed polymer bioconjugates of S-layer proteins. The conjugates formed are similar in function to the monomer alkanethiols that form self-assembled monolayers (SAMs) on gold surfaces. However, the self-assembly is driven by the protein "headgroup" that positions polymer-tethered endgroups on a surface. This paper examines the integration of protein purification, conjugation, and surface assembly that has led to the development of this new method for the formation of nanostructured surfaces. Purified S-layer proteins from Lactobacillus brevis were conjugated with small molecule probes and polymers using amine-based reactions. To keep multiple labeling of protein amine groups to acceptable levels, the conjugations were performed at pH 6.5, allowing for limited yields (24-39%) as determined by mass spectrometry and SDS-polyacrylamide gel electrophoresis. As the presence of high levels of unlabeled S-layer proteins is undesired, we have developed a protocol for further purification that employs monomeric avidin affinity chromatography. The surface self-assembly of the polymer bioconjugates onto amine-terminated microspheres was studied using epi-fluorescence, confocal, and scanning electron microscopy. The surfaces obtained exhibited homogeneous distributions of tethered molecules. Also, in cases where the modular assembly of two distinct types of tethered endgroups was accomplished, there was no evidence for phase separation in the surfaces. The modular assembly method will provide a potential route to controlling surface display density as the starting assembly conditions guide displayed endgroup concentrations in mixed molecular monolayers.

Development of an inducible gene expression system for Lactobacillus sakei

AIM

To develop an inducible gene expression system for Lactobacillus sakei, based on the regulatory system of sakacin A production.

METHODS AND RESULTS

A Lactobacillus/Escherichia coli shuttle vector; pKRV3, was constructed including the signal transducing system genes of the bacteriocin sakacin A. The gusA gene fused to PsapA promoter, cloned in this vector allowed for inducible beta-glucuronidase expression in L. sakei and L. plantarum following the addition of the sakacin A inducing peptide. PsapA appeared to be a strong and tightly controlled promoter when compared with known promoters.

CONCLUSION

The pKRV3 system can be used as an inducible gene expression system in lactobacilli.

SIGNIFICANCE AND IMPACT OF THE STUDY

A novel, inducible gene expression system has been developed for lactic acid bacteria relevant in food fermentations.

Probiotic and milk technological properties of Lactobacillus brevis

Two Lactobacillus brevis strains ATCC 8287 and ATCC 14869(T), were evaluated for their applicability as putative probiotics in dairy products. The strains expressed good in vitro adherence to human Caco-2 and Intestine 407 cells and tolerated well low pH, bile acids and pancreatic fluid under in vitro conditions. In antimicrobial activity assays, strain ATCC 8287 showed inhibitory properties toward selected potential harmful microorganisms, particularly against Bacillus cereus. Both L. brevis strains were resistant to vancomycin, which is typical for the genus Lactobacillus. The L. brevis strains were not able to acidify milk to yoghurt but were suitable as supplement strains in yoghurts. This was shown by producing a set of yoghurt products and analysing their rheological and sensory properties during a cold storage period of 28 days. Survival of the strains through human intestine was examined in 1-week feeding trials. Despite its human origin, L. brevis ATCC 14869(T) could not survive through the gastrointestinal (GI) tract, whereas L. brevis ATCC 8287 was detected in the faecal samples taken during and immediately after ingestion of the strain. In conclusion, L. brevis ATCC 8287 is a promising candidate as a probiotic supplement in dairy products.

Surface display of the receptor-binding region of the Lactobacillus brevis S-layer protein in Lactococcus lactis provides nonadhesive lactococci with the ability to adhere to intestinal epithelial cells

Lactobacillus brevis is a promising lactic acid bacterium for use as a probiotic dietary adjunct and a vaccine vector. The N-terminal region of the S-layer protein (SlpA) of L. brevis ATCC 8287 was recently shown to mediate adhesion to various human cell lines in vitro. In this study, a surface display cassette was constructed on the basis of this SlpA receptor-binding domain, a proteinase spacer, and an autolysin anchor. The cassette was expressed under control of the nisA promoter in Lactococcus lactis NZ9000. Western blot assay of lactococcal cell wall extracts with anti-SlpA antibodies confirmed that the SlpA adhesion domain of the fusion protein was expressed and located within the cell wall layer. Whole-cell enzyme-linked immunosorbent assay and immunofluorescence microscopy verified that the SlpA adhesion-mediating region was accessible on the lactococcal cell surface. In vitro adhesion assays with the human intestinal epithelial cell line Intestine 407 indicated that the recombinant lactococcal cells had gained an ability to adhere to Intestine 407 cells significantly greater than that of wild-type L. lactis NZ9000. Serum inhibition assay further confirmed that adhesion of recombinant lactococci to Intestine 407 cells was indeed mediated by the N terminus-encoding part of the slpA gene. The ability of the receptor-binding region of SlpA to adhere to fibronectin was also confirmed with this lactococcal surface display system. These results show that, with the aid of the receptor-binding region of the L. brevis SlpA protein, the ability to adhere to gut epithelial cells can indeed be transferred to another, nonadhesive, lactic acid bacterium.

Identification and characterization of novel surface proteins in Lactobacillus johnsonii and Lactobacillus gasseri

We have identified and sequenced the genes encoding the aggregation-promoting factor (APF) protein from six different strains of Lactobacillus johnsonii and Lactobacillus gasseri. Both species harbor two apf genes, apf1 and apf2, which are in the same orientation and encode proteins of 257 to 326 amino acids. Multiple alignments of the deduced amino acid sequences of these apf genes demonstrate a very strong sequence conservation of all of the genes with the exception of their central regions. Northern blot analysis showed that both genes are transcribed, reaching their maximum expression during the exponential phase. Primer extension analysis revealed that apf1 and apf2 harbor a putative promoter sequence that is conserved in all of the genes. Western blot analysis of the LiCl cell extracts showed that APF proteins are located on the cell surface. Intact cells of L. johnsonii revealed the typical cell wall architecture of S-layer-carrying gram-positive eubacteria, which could be selectively removed with LiCl treatment. In addition, the amino acid composition, physical properties, and genetic organization were found to be quite similar to those of S-layer proteins. These results suggest that APF is a novel surface protein of the Lactobacillus acidophilus B-homology group which might belong to an S-layer-like family.

Surface display of foreign epitopes on the Lactobacillus brevis S-layer

So far, the inability to establish viable Lactobacillus surface layer (S-layer) null mutants has hampered the biotechnological applications of Lactobacillus S-layers. In this study, we demonstrate the utilization of Lactobacillus brevis S-layer subunits (SlpA) for the surface display of foreign antigenic epitopes. With an inducible expression system, L. brevis strains producing chimeric S-layers were obtained after testing of four insertion sites in the slpA gene for poliovirus epitope VP1, that comprises 10 amino acids. The epitope insertion site allowing the best surface expression was used for the construction of an integration vector carrying the gene region encoding the c-Myc epitopes from the human c-myc proto-oncogene, which is composed of 11 amino acids. A gene replacement system was optimized for L. brevis and used for the replacement of the wild-type slpA gene with the slpA-c-myc construct. A uniform S-layer, displaying on its surface the desired antigen in all of the S-layer protein subunits, was obtained. The success of the gene replacement and expression of the uniform SlpA-c-Myc recombinant S-layer was confirmed by PCR, Southern blotting MALDI-TOF mass spectrometry, whole-cell enzyme-linked immunosorbent assay, and immunofluorescence microscopy. Furthermore, the integrity of the recombinant S-layer was studied by electron microscopy, which indicated that the S-layer lattice structure was not affected by the presence of c-Myc epitopes. To our knowledge, this is the first successful expression of foreign epitopes in every S-layer subunit of a Lactobacillus S-layer while still maintaining the S-layer lattice structure.

Isolation of three new surface layer protein genes (slp) from Lactobacillus brevis ATCC 14869 and characterization of the change in their expression under aerated and anaerobic conditions

Two new surface layer (S-layer) proteins (SlpB and SlpD) were characterized, and three slp genes (slpB, slpC, and slpD) were isolated, sequenced, and studied for their expression in Lactobacillus brevis neotype strain ATCC 14869. Under different growth conditions, L. brevis strain 14869 was found to form two colony types, smooth (S) and rough (R), and to express the S-layer proteins differently. Under aerobic conditions R-colony type cells produced SlpB and SlpD proteins, whereas under anaerobic conditions S-colony type cells synthesized essentially only SlpB. Anaerobic and aerated cultivations of ATCC 14869 cells in rich medium also resulted in S-layer protein patterns similar to those of the S- and R-colony type cells, respectively. Electron microscopy suggested the presence of only a single S-layer with an oblique structure on the cells of both colony forms. The slpB and slpC genes were located adjacent to each other, whereas the slpD gene was not closely linked to the slpB-slpC gene region. Northern analyses confirmed that both slpB and slpD formed a monocistronic transcription unit and were effectively expressed, but slpD expression was induced under aerated conditions. slpC was a silent gene under the growth conditions tested. The amino acid contents of all the L. brevis ATCC 14869 S-layer proteins were typical of S-layer proteins, whereas their sequence similarities with other S-layer proteins were negligible. The interspecies identity of the L. brevis S-layer proteins was mainly restricted to the N-terminal regions of those proteins. Furthermore, Northern analyses, expression of a PepI reporter protein under the control of the slpD promoter, and quantitative real-time PCR analysis of slpD expression under aerated and anaerobic conditions suggested that, in L. brevis ATCC 14869, the variation of S-layer protein content involves activation of transcription by a soluble factor rather than DNA rearrangements that are typical for most of the S-layer phase variation mechanisms known.

Domains in the S-layer protein CbsA of Lactobacillus crispatus involved in adherence to collagens, laminin and lipoteichoic acids and in self-assembly

The protein regions in the S-layer protein CbsA of Lactobacillus crispatus JCM 5810, needed for binding to collagens and laminin, anchoring to bacterial cell wall, as well as self-assembly, were mapped by deletion analysis of His-tagged peptides isolated from Escherichia coli and by heterologous expression on Lactobacillus casei. Mature CbsA is 410 amino acids long, and stepwise genetic truncation at both termini revealed that the region 32-271 carries the infor-mation for self-assembly of CbsA into a periodic structure. The lactobacillar S-layer proteins exhibit sequence variation in their assembly domain, but the border regions 30-34 and 269-274 in CbsA are conserved in valine-rich short sequences. Short deletions or substitutions at these regions affected the morphology of His-CbsA polymers, which varied from sheet-like to cylindrical tubular polymers, and further truncation beyond the DNA encoding residues 32 and 271 leads to a non-periodic aggregation. The self-assembly of the truncated peptides, as seen by electron microscopy, was correlated with their behaviour in a cross-linking study. The shorter peptides not forming a regular polymer were observed by the cross-linking study and mass spectrometry to form dimers, trimers and tetramers, whereas the other peptides were cross-linked to large multimers only. Binding of solubilized type I and IV collagens was observed with the His-CbsA peptides 1-274 and 31-287, but not with the smaller peptides regardless of their ability to form regular polymers. Strain JCM 5810 also adheres to immobilized laminin and, in order to analyse the possible laminin binding by CbsA, cbsA and its fragments were expressed on the surface of L. casei. Expression of the CbsA peptides 1-274, 1-287, 28-287 and 31-287 on L. casei conferred adhesiveness to both laminin and collagen immobilized on glass as well as to laminin- and collagen-containing regions in chicken colon and ileum. The C-terminal peptides 251-410 and 288-410 bound to L. crispatus JCM 5810 cells from which the S-layer had been depleted by chemical extraction, whereas no binding was seen with the His-CbsA peptides 1-250 or 1-269 or to cells with an intact S-layer. The His-CbsA peptides 251-410 and 288-410 bound to teichoic acids of several bacterial species. The results show that CbsA is an adhesive complex with an N-terminal assembly domain exhibiting affinity for pericellular tissue components and a cationic C-terminal domain binding to negatively charged cell wall components.

Inhibition of the adherence of Escherichia coli strains to basement membrane by Lactobacillus crispatus expressing an S-layer

AIMS

This study aimed to evaluate the efficiency with which Lactobacillus crispatus JCM 5810 inhibited the adhesion of enteric pathogens to a synthetic basement membrane and to elucidate the mechanism underlying the inhibition.

METHODS AND RESULTS

Lactobacillus crispatus JCM 5810 inhibited the adhesion of three diarrhoeagenic Escherichia coli strains to a reconstituted basement membrane preparation called Matrigel, used as a model of a damaged intestinal tissue site. Inhibition was also observed with the use of immobilized laminin, a major component of Matrigel, but diminished after the removal of S-layer protein (CbsA) from JCM 5810 cells. The isolated CbsA inhibited the adhesion of E. coli to both Matrigel and immobilized laminin. Lactobacillus crispatus JCM 5810 and CbsA seem to inhibit pathogenic E. coli from adhering to basement membrane via competition with laminin molecules for binding sites.

CONCLUSIONS

These results suggested that not only Lact. crispatus JCM 5810 cells but CbsA alone might prevent pathogens from colonizing damaged intestinal tissues.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study to show the applied aspect of Lactobacillus S-layer protein.

Identification by flagellum display of an epithelial cell- and fibronectin-binding function in the SlpA surface protein of Lactobacillus brevis

Depletion of the SlpA protein from the bacterial surface greatly reduced the adhesion of Lactobacillus brevis ATCC 8287 to the human intestinal cell lines Caco-2 and Intestine 407, the endothelial cell line EA-hy926, and the urinary bladder cell line T24, as well as immobilized fibronectin. For functional analysis of the SlpA surface protein, different regions of the slpA gene were expressed as internal in-frame fusions in the variable region of the fliC(H7) gene of Escherichia coli. The resulting chimeric flagella carried inserts up to 275 amino acids long from the mature S-layer protein, which is 435 amino acids in size. The expression of the SlpA fragments on the chimeric flagella was assessed by immunoelectron microscopy and Western blotting using anti-SlpA antibodies, and their binding to human cells was assessed by indirect immunofluorescence. Chimeric flagella harboring inserts that represented the N-terminal part of the S-layer protein bound to the epithelial cell lines, whereas the C-terminal part of the S-layer protein did not confer binding on the flagella. The shortest S-layer peptide capable of detectable binding was 81 amino acid residues in size and represented residues 96 through 176 in the unprocessed S-layer protein. The bacteria and the chimeric flagella did not show detectable binding to erythrocytes, whereas the SlpA-expressing ATCC 8287 cells as well as the chimeric SlpA 96-245/FliC flagella bound to immobilized fibronectin. The N-terminal SlpA peptide 96-176 or 96-200 fused to FliC was not recognized in Western blotting or immunoelectron microscopy by a polyclonal serum raised against the S-layer protein; the antiserum, however, reacted in immunofluorescence with the ATCC 8287 cells. In contrast, an antiserum raised against the His-tagged peptide 96-245 of SlpA bound to the hybrid flagella with the N-terminal SlpA inserts but did not react with ATCC 8287 cells. The results identify the S-layer of L. brevis ATCC 8287 as an adhesin with affinity for human epithelial cells and fibronectin and locate the receptor-binding region within a fragment of 81 amino acids in the N-terminal part of the molecule, which in native S-layer seems inaccessible to antibodies.

Complete genome sequence of a virulent isolate of Streptococcus pneumoniae

The 2,160,837-base pair genome sequence of an isolate of Streptococcus pneumoniae, a Gram-positive pathogen that causes pneumonia, bacteremia, meningitis, and otitis media, contains 2236 predicted coding regions; of these, 1440 (64%) were assigned a biological role. Approximately 5% of the genome is composed of insertion sequences that may contribute to genome rearrangements through uptake of foreign DNA. Extracellular enzyme systems for the metabolism of polysaccharides and hexosamines provide a substantial source of carbon and nitrogen for S. pneumoniae and also damage host tissues and facilitate colonization. A motif identified within the signal peptide of proteins is potentially involved in targeting these proteins to the cell surface of low-guanine/cytosine (GC) Gram-positive species. Several surface-exposed proteins that may serve as potential vaccine candidates were identified. Comparative genome hybridization with DNA arrays revealed strain differences in S. pneumoniae that could contribute to differences in virulence and antigenicity.

The S-layer protein of Lactobacillus acidophilus ATCC 4356: identification and characterisation of domains responsible for S-protein assembly and cell wall binding

Lactobacillus acidophilus, like many other bacteria, harbors a surface layer consisting of a protein (S(A)-protein) of 43 kDa. S(A)-protein could be readily extracted and crystallized in vitro into large crystalline patches on lipid monolayers with a net negative charge but not on lipids with a net neutral charge. Reconstruction of the S-layer from crystals grown on dioleoylphosphatidylserine indicated an oblique lattice with unit cell dimensions (a=118 A; b=53 A, and gamma=102 degrees ) resembling those determined for the S-layer of Lactobacillus helveticus ATCC 12046. Sequence comparison of S(A)-protein with S-proteins from L. helveticus, Lactobacillus crispatus and the S-proteins encoded by the silent S-protein genes from L. acidophilus and L. crispatus suggested the presence of two domains, one comprising the N-terminal two-thirds (SAN), and another made up of the C-terminal one-third (SAC) of S(A)-protein. The sequence of the N-terminal domains is variable, while that of the C-terminal domain is highly conserved in the S-proteins of these organisms and contains a tandem repeat. Proteolytic digestion of S(A)-protein showed that SAN was protease-resistant, suggesting a compact structure. SAC was rapidly degraded by proteases and therefore probably has a more accessible structure. DNA sequences encoding SAN or Green Fluorescent Protein fused to SAC (GFP-SAC) were efficiently expressed in Escherichia coli. Purified SAN could crystallize into mono and multi-layered crystals with the same lattice parameters as those found for authentic S(A)-protein. A calculated S(A)-protein minus SAN density-difference map revealed the probable location, in projection, of the SAC domain, which is missing from the truncated SAN peptide. The GFP-SAC fusion product was shown to bind to the surface of L. acidophilus, L. helveticus and L. crispatus cells from which the S-layer had been removed, but not to non-stripped cells or to Lactobacillus casei.

Leaky Lactococcus cultures that externalize enzymes and antigens independently of culture lysis and secretion and export pathways

A novel system that leaks beta-galactosidase (beta-gal) without a requirement for secretion or export signals was developed in Lactococcus lactis by controlled expression of integrated phage holin and lysin cassettes. The late promoter of the lytic lactococcal bacteriophage phi31 is an 888-bp fragment (P(15A10)) encoding the transcriptional activator. When a high-copy-number P(15A10)::lacZ.st fusion was introduced into L. lactis strains C10, ML8, NCK203, and R1/r1t, high levels of the resultant beta-gal activity were detected in the supernatant (approximately 85% of the total beta-gal activity for C10, ML8, and NCK203 and 45% for R1/r1t). Studies showed that the phenotype resulted from expression of Tac31A from the P(15A10) fragment, which activated a homologous late promoter in prophages harbored by the lactococcal strains. Despite the high levels of beta-gal obtained in the supernatant, the growth of the strains was not significantly affected, nor was there any evidence of severe membrane damage as determined by using propidium iodide or transmission electron microscopy. Integration of the holin-lysin cassette of phage r1t, under the control of the phage phi31 late promoter, into the host genome of MG1363 yielded a similar "leaky" phenotype, indicating that holin and lysin might play a critical role in the release of beta-gal into the medium. In addition to beta-gal, tetanus toxin fragment C was successfully delivered into the growth medium by this system. Interestingly, the X-prolyl dipeptidyl aminopeptidase PepXP (a dimer with a molecular mass of 176 kDa) was not delivered at significant levels outside the cell. These findings point toward the development of bacterial strains able to efficiently release relevant proteins and enzymes outside the cell in the absence of known secretion and export signals.

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.