Structural studies of the rhamnose-rich cell wall polysaccharide of Lactobacillus casei BL23

Genetic and functional diversity of β-N-acetylgalactosamine-targeting glycosidases expanded by deep-sea metagenome analysis

β-N-Acetylgalactosamine-containing glycans play essential roles in several biological processes, including cell adhesion, signal transduction, and immune responses. β-N-Acetylgalactosaminidases hydrolyze β-N-acetylgalactosamine linkages of various glycoconjugates. However, their biological significance remains ambiguous, primarily because only one type of enzyme, exo-β-N-acetylgalactosaminidases that specifically act on β-N-acetylgalactosamine residues, has been documented to date. In this study, we identify four groups distributed among all three domains of life and characterize eight β-N-acetylgalactosaminidases and β-N-acetylhexosaminidase through sequence-based screening of deep-sea metagenomes and subsequent searching of public protein databases. Despite low sequence similarity, the crystal structures of these enzymes demonstrate that all enzymes share a prototype structure and have diversified their substrate specificities (oligosaccharide-releasing, oligosaccharide/monosaccharide-releasing, and monosaccharide-releasing) through the accumulation of mutations and insertional amino acid sequences. The diverse β-N-acetylgalactosaminidases reported in this study could facilitate the comprehension of their structures and functions and present evolutionary pathways for expanding their substrate specificity.

Structural characterization of an exopolysaccharide produced by two strains of Lacticaseibacilluscasei

Exopolysaccharides (EPSs) were isolated and purified from Lacticaseibacillus casei strains type V and RW-3703M grown under various fermentation conditions (carbon source, incubation temperature, and duration). Identical 1H NMR spectra were obtained in all cases. The molar mass determined by size-exclusion chromatography coupled with multi-angle light scattering was different for the two strains and in different culture media. The primary structure was elucidated using chemical and spectroscopic techniques. Monosaccharide and absolute configuration analyses gave the following composition: d-Glc, 1; d-Gal, 2; l-Rha, 2; d-GlcNAc, 1. Methylation analysis indicated the presence of 4-linked Glc, terminal and 6-linked Gal, terminal and 3-linked Rha, and 3,4,6-linked GlcNAc. On the basis of one- and two-dimensional 1H and 13C NMR data, the structure of the EPS was consistent with the following hexasaccharide repeating unit: {4)[Rhap(α1-3)][Galp(α1-6)]GlcpNAc(β1-6)Galp(α1-3)Rhap(β1-4)Glcp(β1-}n. Complete 1H and 13C NMR assignments are reported.

Structural variations and roles of rhamnose-rich cell wall polysaccharides in Gram-positive bacteria

Rhamnose-rich cell wall polysaccharides (Rha-CWPSs) have emerged as crucial cell wall components of numerous Gram-positive, ovoid-shaped bacteria—including streptococci, enterococci, and lactococci—of which many are of clinical or biotechnological importance. Rha-CWPS are composed of a conserved polyrhamnose backbone with side-chain substituents of variable size and structure. Because these substituents contain phosphate groups, Rha-CWPS can also be classified as polyanionic glycopolymers, similar to wall teichoic acids, of which they appear to be functional homologs. Recent advances have highlighted the critical role of these side-chain substituents in bacterial cell growth and division, as well as in specific interactions between bacteria and infecting bacteriophages or eukaryotic hosts. Here, we review the current state of knowledge on the structure and biosynthesis of Rha-CWPS in several ovoid-shaped bacterial species. We emphasize the role played by multicomponent transmembrane glycosylation systems in the addition of side-chain substituents of various sizes as extracytoplasmic modifications of the polyrhamnose backbone. We provide an overview of the contribution of Rha-CWPS to cell wall architecture and biogenesis and discuss current hypotheses regarding their importance in the cell division process. Finally, we sum up the critical roles that Rha-CWPS can play as bacteriophage receptors or in escaping host defenses, roles that are mediated mainly through their side-chain substituents. From an applied perspective, increased knowledge of Rha-CWPS can lead to advancements in strategies for preventing phage infection of lactococci and streptococci in food fermentation and for combating pathogenic streptococci and enterococci.

Structural determination of the cell wall polysaccharide LCPS-1 in Lacticaseibacillus paracasei strain Shirota YIT 9029

The neutral polysaccharides LCPS-1 and LCPS-2 play functional roles in the cell wall of the lactic acid bacterium Lacticaseibacillus paracasei strain Shirota YIT 9029 (LcS; formerly Lactobacillus casei strain Shirota YIT 9029), which has long been used as a probiotic food product. Studies have shown that LCPS-1 is associated with the immunomodulatory functions of LcS. We hypothesized that the structure of LCPS-1 is crucial for elucidating the mechanism of action of LcS on host immune responses and aimed to solve the undetermined primary structure of LCPS-1. Our results showed that LCPS-1 has a molecular weight of >400 kDa and is composed of Glc, Rha, Gal, and GlcNAc, with a repeating structure. Using limited degradation reactions, including controlled Smith and deamination degradations, we obtained key fragments with low molecular weight. Subsequently, their structures were analyzed using NMR spectra and other analytical techniques. Further, we integrated the results for each key fragment to derive the complete structure of LCPS-1. Our results indicated that the most probable structure of LCPS-1 is composed of two types of units (X, Y), each with a basic structure of seven sugars in which the C2-position of Rha is substituted with an acetyl group. The structure of X is {6[Glcβ1-2] Galα1-3[2-OAc] Rhaβ1-4Glcβ1-4[Rhaα1-3] [Glcα1-6] Glcβ1-} and that of Y is {6[Glcβ1-2] Galα1-3[2-OAc] Rhaβ1-4Glcβ1-4[Rhaα1-3] [Glcα1-6)] GlcNAcβ1-}, which can be expressed as (X₆Y₁₂)_n. In this study, we identified the primary structure of LCPS-1, and our results may enable an improved understanding of the immunomodulatory abilities of LcS.

The Phagocytosis of Lacticaseibacillus casei and Its Immunomodulatory Properties on Human Monocyte-Derived Dendritic Cells Depend on the Expression of Lc-p75, a Bacterial Peptidoglycan Hydrolase

The human gut symbiont Lacticaseibacillus (L.) casei (previously Lactobacillus casei) is under intense research due to its wide range of immunomodulatory effects on the human host. Dendritic cells (DCs) are crucial players in the direct and indirect communication with lactobacilli in the gastrointestinal tract. Here, we demonstrate that human monocyte-derived DCs (moDCs) are able to engulf L. casei BL23, in which the intact bacterial cell wall and morphology have a key role. The absence of the bacterial cell-wall-degrading enzyme, Lc-p75, in L. casei cells causes remarkable morphological changes, which have important consequences in the phagocytosis of L. casei by moDCs. Our results showed that the Lc-p75 mutation induced defective internalization and impaired proinflammatory and T-cell-polarizing cytokine secretion by bacteria-exposed moDCs. The T helper (Th) 1 and Th17 cell activating capacity of moDCs induced by the mutant L. casei was consequently reduced. Moreover, inhibition of the phagocytosis of wild-type bacteria showed similar results. Taken together, these data suggested that formation of short bacterial chains helps to exert the potent immunomodulatory properties of L. casei BL23.

Characterization and Antioxidant Activity of Released Exopolysaccharide from Potential Probiotic Leuconostoc mesenteroides LM187

A released exopolysaccharide (rEPS)-producing strain (LM187) with good acid resistance, bile salt resistance, and cholesterol-lowering properties was isolated from Sichuan paocai and identified as Leuconostoc mesenteroides subsp. mesenteroides. The purified rEPS, designated as rEPS414, had a uniform molecular weight of 7.757 × 10⁵ Da. Analysis of the monosaccharide composition revealed that the molecule was mainly composed of glucose. The Fourier transform-infrared spectrum showed that rEPS414 contained both α-type and β-type glycosidic bonds. ¹H and ¹³C nuclear magnetic resonance spectra analysis showed that the purified rEPS contained arabinose, galactose, and rhamnose, but less uronic acid. Scanning electron microscopy demonstrated that the exopolysaccharide displayed a large number of scattered, fluffy, porous cellular network flake structures. In addition, rEPS414 exhibited strong in vitro antioxidant activity. These results showed that strain LM187 and its rEPS are promising probiotics with broad prospects in industry.

Advances in Separation and Purification of Bioactive Polysaccharides through High-speed Counter-Current Chromatography

Polysaccharides, with an extensive distribution in natural products, represent a group of natural bioactive substances having widespread applications in health-care food products and as biomaterials. Devising an efficient system for the separation and purification of polysaccharides from natural sources, hence, is of utmost importance in the widespread applicability and feasibility of research for the development of polysaccharide-based products. High-speed counter-current chromatography (HSCCC) is a continuous liquid-liquid partitioning chromatography with the ability to support a high loading amount and crude material treatment. Due to its flexible two-phase solvent system, HSCCC has been successfully used in the separation of many natural products. Based on HSCCC unique advantages over general column chromatography and its enhanced superiority in this regard when coupled to aqueous two-phase system (ATPS), this review summarizes the separation and purification of various bioactive polysaccharides through HSCCC and its coupling to ATPS as an aid in future research in this direction.

The biological activities of postbiotics in gastrointestinal disorders

According to outcomes from clinical studies, an intricate relationship occurs between the beneficial microbiota, gut homeostasis, and the host's health status. Numerous studies have confirmed the health-promoting effects of probiotics, particularly in gastrointestinal diseases. On the other hand, the safety issues regarding the consumption of some probiotics are still a matter of debate, thus to overcome the problems related to the application of live probiotic cells in terms of clinical, technological, and economic aspects, microbial-derived biomolecules (postbiotics) were introducing as a potential alternative agent. Presently scientific literature confirms that the postbiotic components can be used as promising tools for both prevention and treatment strategies in gastrointestinal disorders with less undesirable side-effects, particularly in infants and children. Future head-to-head trials are required to distinguish appropriate strains of parent cells, optimal dosages of postbiotics, and assessment of the cost-effectiveness of postbiotics compared to alternative drugs. This review provides an overview of the concept and safety issues regarding postbiotics, with emphasis on their biological role in the treatment of some important gastrointestinal disorders.

Cell wall homeostasis in lactic acid bacteria: threats and defences

Lactic acid bacteria (LAB) encompasses industrially relevant bacteria involved in food fermentations as well as health-promoting members of our autochthonous microbiota. In the last years, we have witnessed major progresses in the knowledge of the biology of their cell wall, the outermost macrostructure of a Gram-positive cell, which is crucial for survival. Sophisticated biochemical analyses combined with mutation strategies have been applied to unravel biosynthetic routes that sustain the inter- and intra-species cell wall diversity within LAB. Interplay with global cell metabolism has been deciphered that improved our fundamental understanding of the plasticity of the cell wall during growth. The cell wall is also decisive for the antimicrobial activity of many bacteriocins, for bacteriophage infection and for the interactions with the external environment. Therefore, genetic circuits involved in monitoring cell wall damage have been described in LAB, together with a plethora of defence mechanisms that help them to cope with external threats and adapt to harsh conditions. Since the cell wall plays a pivotal role in several technological and health-promoting traits of LAB, we anticipate that this knowledge will pave the way for the future development and extended applications of LAB.

Genome Sequence and Characterization of Lactobacillus casei Phage, vB_LcaM_Lbab1 Isolated from Raw Milk

Introduction: Only a few Lactobacillus casei phages have so far been characterized. As several L. casei strains are part of probiotic formulations, bacteriophage outbreaks targeting these strains can lead to critical losses within the dairy industry. Materials and Methods: A new L. casei phage was isolated from raw milk obtained from a milking yard from the province of Buenos Aires. The phage genome was sequenced, annotated, and analyzed. Morphology was determined by electron microscopy and the host range was established. Results: Lactobacillus phage vB_LcaM_Lbab1 is a member of the Herelleviridae family and features a host range including L. casei/Lactobacillus paracasei and Lactobacillus kefiri strains. We further analyzed the baseplate proteins in silico and found putative carbohydrate binding modules that are responsible for host recognition in other Lactobacillus phages. Conclusions: A new Lactobacillus phage was isolated and characterized. The focus was made on its host recognition mechanism, pointing toward the development of future strategies to avoid deleterious infections in the dairy industry.

NDP-rhamnose biosynthesis and rhamnosyltransferases: building diverse glycoconjugates in nature

Rhamnose is an important 6-deoxy sugar present in many natural products, glycoproteins, and structural polysaccharides. Whilst predominantly found as the l-enantiomer, instances of d-rhamnose are also found in nature, particularly in the Pseudomonads bacteria. Interestingly, rhamnose is notably absent from humans and other animals, which poses unique opportunities for drug discovery targeted towards rhamnose utilizing enzymes from pathogenic bacteria. Whilst the biosynthesis of nucleotide-activated rhamnose (NDP-rhamnose) is well studied, the study of rhamnosyltransferases that synthesize rhamnose-containing glycoconjugates is the current focus amongst the scientific community. In this review, we describe where rhamnose has been found in nature, as well as what is known about TDP-β-l-rhamnose, UDP-β-l-rhamnose, and GDP-α-d-rhamnose biosynthesis. We then focus on examples of rhamnosyltransferases that have been characterized using both in vivo and in vitro approaches from plants and bacteria, highlighting enzymes where 3D structures have been obtained. The ongoing study of rhamnose and rhamnosyltransferases, in particular in pathogenic organisms, is important to inform future drug discovery projects and vaccine development.

Selection of Exopolysaccharide-Producing Lactobacillus Plantarum (Lactiplantibacillus Plantarum) Isolated from Algerian Fermented Foods for the Manufacture of Skim-Milk Fermented Products

The exopolysaccharide (EPS)-producing Lactobacillus plantarum (renamed as Lactiplantibacillus plantarum) LBIO1, LBIO14 and LBIO28 strains, isolated from fermented dairy products typical from Algeria, were characterized to evaluate the impact of the polymers in milk fermentations. Their genomes revealed the presence of two complete eps clusters of the four described for the reference strain WCFS1. Besides, the three strains presented identical sequences of eps3 and eps4 clusters, but LBIO1 and LBIO28 harbour three genes belonging to eps2 which are absent in the LBIO14 genome. The EPS purified from fermented skim-milks manufactured with the strains showed identical nuclear magnetic resonance (1H-NMR) and size exclusion chromatography coupled with a multiangle laser light scattering detector (SEC-MALLS) profiles for polymers LBIO1 and LBIO28, whereas LBIO14 EPS was different due to the lack of the high-molecular weight (HMW)-EPS and the absence of specific monosaccharide's peaks in the anomeric region of its proton NMR spectrum. The presence of the HMW-EPS correlated with optimal sensorial-physical characteristics of the fermented skim-milks (ropy phenotype). Their microstructures, studied by confocal scanning laser microscopy (CSLM), also showed differences in the organization of the casein-network and the distribution of the bacteria inside this matrix. Therefore, the strain LBIO1 can be proposed for the manufacture of dairy products that require high whey retention capability, whereas LBIO28 could be applied to increase the viscosity.

The CWPS Rubik's cube: Linking diversity of cell wall polysaccharide structures with the encoded biosynthetic machinery of selected Lactococcus lactis strains

The biosynthetic machinery for cell wall polysaccharide (CWPS) production in lactococci is encoded by a large gene cluster, designated cwps. This locus displays considerable variation among lactococcal genomes, previously prompting a classification into three distinct genotypes (A-C). In the present study, the cwps loci of 107 lactococcal strains were compared, revealing the presence of a fourth cwps genotype (type D). Lactococcal CWPSs are comprised of two saccharidic structures: a peptidoglycan-embedded rhamnan backbone polymer to which a surface-exposed, poly/oligosaccharidic side-chain is covalently linked. Chemical structures of the side-chain of seven lactococcal strains were elucidated, highlighting their diverse and strain-specific nature. Furthermore, a link between cwps genotype and chemical structure was derived based on the number of glycosyltransferase-encoding genes in the cwps cluster and the presence of conserved genes encoding the presumed priming glycosyltransferase. This facilitates predictions of several structural features of lactococcal CWPSs including (a) whether the CWPS possesses short oligo/polysaccharide side-chains, (b) the number of component monosaccharides in a given CWPS structure, (c) the order of monosaccharide incorporation into the repeating units of the side-chain (for C-type strains), (d) the presence of Galf and phosphodiester bonds in the side-chain, and (e) the presence of glycerol phosphate substituents in the side-chain.

Analysis of infant microbiota composition and the relationship with breast milk components in the Asian elephant (Elephas maximus) at the zoo

The prevention of diseases through health control is essential at zoos. Here, we investigated the gut microbiota formation during infancy in an Asian elephant and compared the composition between infant and mother. Besides, we analyzed the components of breast milk and examined the correlation with the infant gut microbiota. Analysis revealed the gut microbiota of the infant contained high amount of Lactobacillales and its diversity was relatively low compared to that of the mother. We found several milk components, showed a positive correlation with the change of Lactobacillales. The present study revealed the mechanism of gut microbiota formation during infancy in an Asian elephant and provides important insights into the health control of Asian elephants in zoos.

Complete Structure of the Enterococcal Polysaccharide Antigen (EPA) of Vancomycin-Resistant Enterococcus faecalis V583 Reveals that EPA Decorations Are Teichoic Acids Covalently Linked to a Rhamnopolysaccharide Backbone

Enterococci are opportunistic pathogens responsible for hospital- and community-acquired infections. All enterococci produce a surface polysaccharide called EPA (enterococcal polysaccharide antigen) required for biofilm formation, antibiotic resistance, and pathogenesis. Despite the critical role of EPA in cell growth and division and as a major virulence factor, no information is available on its structure. Here, we report the complete structure of the EPA polymer produced by the model strain E. faecalis V583. We describe the structure of the EPA backbone, made of a rhamnan hexasaccharide substituted by Glc and GlcNAc residues, and show that teichoic acids are covalently bound to this rhamnan chain, forming the so-called “EPA decorations” essential for host colonization and pathogenesis. This report represents a key step in efforts to identify the structural properties of EPA that are essential for its biological activity and to identify novel targets to develop preventive and therapeutic approaches against enterococci.

All enterococci produce a complex polysaccharide called the enterococcal polysaccharide antigen (EPA). This polymer is required for normal cell growth and division and for resistance to cephalosporins and plays a critical role in host-pathogen interaction. The EPA contributes to host colonization and is essential for virulence, conferring resistance to phagocytosis during the infection. Recent studies revealed that the “decorations” of the EPA polymer, encoded by genetic loci that are variable between isolates, underpin the biological activity of this surface polysaccharide. In this work, we investigated the structure of the EPA polymer produced by the high-risk enterococcal clonal complex Enterococcus faecalis V583. We analyzed purified EPA from the wild-type strain and a mutant lacking decorations and elucidated the structure of the EPA backbone and decorations. We showed that the rhamnan backbone of EPA is composed of a hexasaccharide repeat unit of C2- and C3-linked rhamnan chains, partially substituted in the C3 position by α-glucose (α-Glc) and in the C2 position by β-N-acetylglucosamine (β-GlcNAc). The so-called “EPA decorations” consist of phosphopolysaccharide chains corresponding to teichoic acids covalently bound to the rhamnan backbone. The elucidation of the complete EPA structure allowed us to propose a biosynthetic pathway, a first essential step toward the design of antimicrobials targeting the synthesis of this virulence factor.

Prolonged Heat Stress of Lactobacillus paracasei GCRL163 Improves Binding to Human Colorectal Adenocarcinoma HT-29 Cells and Modulates the Relative Abundance of Secreted and Cell Surface-Located Proteins

Lactobacillus casei group bacteria improve cheese ripening and may interact with host intestinal cells as probiotics, where surface proteins play a key role. Three complementary methods [trypsin shaving (TS), LiCl-sucrose (LS) extraction, and extracellular culture fluid precipitation] were used to analyze cell surface proteins of Lactobacillus paracasei GCRL163 by label-free quantitative proteomics after culture to the mid-exponential phase in bioreactors at pH 6.5 and temperatures of 30-45 °C. A total of 416 proteins, including 300 with transmembrane, cell wall anchoring, and secretory motifs and 116 cytoplasmic proteins, were quantified as surface proteins. Although LS caused significantly greater cell lysis as growth temperature increased, higher numbers of extracytoplasmic proteins were exclusively obtained by LS treatment. Together with the increased positive surface charge of cells cultured at supra-optimal temperatures, proteins including cell wall hydrolases Msp1/p75 and Msp2/p40, α-fucosidase AlfB, SecA, and a PspC-domain putative adhesin were upregulated in surface or secreted protein fractions, suggesting that cell adhesion may be altered. Prolonged heat stress (PHS) increased binding of L. paracasei GCRL163 to human colorectal adenocarcinoma HT-29 cells, relative to acid-stressed cells. This study demonstrates that PHS influences cell adhesion and relative abundance of proteins located on the surface, which may impact probiotic functionality, and the detected novel surface proteins likely linked to the cell cycle and envelope stress.

A cell wall-associated polysaccharide is required for bacteriophage adsorption to the Streptococcus thermophilus cell surface

Streptococcus thermophilus strain ST64987 was exposed to a member of a recently discovered group of S. thermophilus phages (the 987 phage group), generating phage-insensitive mutants, which were then characterized phenotypically and genomically. Decreased phage adsorption was observed in selected bacteriophage-insensitive mutants, and was partnered with a sedimenting phenotype and increased cell chain length or aggregation. Whole genome sequencing of several bacteriophage-insensitive mutants identified mutations located in a gene cluster presumed to be responsible for cell wall polysaccharide production in this strain. Analysis of cell surface-associated glycans by methylation and NMR spectroscopy revealed a complex branched rhamno-polysaccharide in both ST64987 and phage-insensitive mutant BIM3. In addition, a second cell wall-associated polysaccharide of ST64987, composed of hexasaccharide branched repeating units containing galactose and glucose, was absent in the cell wall of mutant BIM3. Genetic complementation of three phage-resistant mutants was shown to restore the carbohydrate and phage resistance profiles of the wild-type strain, establishing the role of this gene cluster in cell wall polysaccharide production and phage adsorption and, thus, infection.

Ability of prebiotic polysaccharides to activate a HIF1α-antimicrobial peptide axis determines liver injury risk in zebrafish

Natural polysaccharides have received much attention for their ability to ameliorate hepatic steatosis induced by high-fat diet. However, the potential risks of their use have been less investigated. Here, we show that the exopolysaccharides (EPS) from Lactobacillus rhamnosus GG (LGG) and L. casei BL23 reduce hepatic steatosis in zebrafish fed a high-fat diet, while BL23 EPS, but not LGG EPS, induce liver inflammation and injury. This is due to the fact that BL23 EPS induces gut microbial dysbiosis, while LGG EPS promotes microbial homeostasis. We find that LGG EPS, but not BL23 EPS, can directly activate intestinal HIF1α, and increased HIF1α boosts local antimicrobial peptide expression to facilitate microbial homeostasis, explaining the distinct compositions of LGG EPS- and BL23 EPS-associated microbiota. Finally, we find that liver injury risk is not confined to Lactobacillus-derived EPS but extends to other types of commonly used natural polysaccharides, depending on their HIF1α activation efficiency.

Health Benefits of Heat-Killed (Tyndallized) Probiotics: An Overview

Nowadays, the oral use of probiotics is widespread. However, the safety profile with the use of live probiotics is still a matter of debate. Main risks include: Cases of systemic infections due to translocation, particularly in vulnerable patients and pediatric populations; acquisition of antibiotic resistance genes; or interference with gut colonization in neonates. To avoid these risks, there is an increasing interest in non-viable microorganisms or microbial cell extracts to be used as probiotics, mainly heat-killed (including tyndallized) probiotic bacteria (lactic acid bacteria and bifidobacteria). Heat-treated probiotic cells, cell-free supernatants, and purified key components are able to confer beneficial effects, mainly immunomodulatory effects, protection against enteropathogens, and maintenance of intestinal barrier integrity. At the clinical level, products containing tyndallized probiotic strains have had a role in gastrointestinal diseases, including bloating and infantile coli-in combination with mucosal protectors-and diarrhea. Heat-inactivated probiotics could also have a role in the management of dermatological or respiratory allergic diseases. The reviewed data indicate that heat-killed bacteria or their fractions or purified components have key probiotic effects, with advantages versus live probiotics (mainly their safety profile), positioning them as interesting strategies for the management of common prevalent conditions in a wide variety of patients´ characteristics.

Targeted and Repetitive Chromosomal Integration Enables High-Level Heterologous Gene Expression in Lactobacillus casei

Lactobacillus casei is a potential cell factory for the production of enzymes and bioactive molecules using episomal plasmids, which suffer from genetic instability. While chromosomal integration strategies can provide genetic stability of recombinant proteins, low expression yields limit their application. To address this problem, we developed a two-step integration strategy in Lb. casei by combination of the LCABL_13040-50-60 recombineering system (comprised of LCABL_1340, LCABL_13050, and LCABL_13060) with the Cre/loxP site-specific recombination system, with an efficiency of ∼3.7 × 10³ CFU/µg DNA. A gfp gene was successfully integrated into six selected chromosomal sites, and the relative fluorescence intensities (RFUs) of the resulting integrants varied up to ∼3.7-fold depending on the integrated site, among which the LCABL_07270 site gfp integration showed the highest RFU. However, integrants with gfp gene(s) integrated into the LCABL_07270 site showed various RFUs, ranging from 993 ± 89 to 7,289 ± 564 and corresponding to 1 to 13.68 ± 1.08 copies of gfp gene integration. Moreover, the integrant with 13.68 ± 1.08 copies of the gfp gene had a more stable RFU after 63 generations compared to that of a plasmid-engineered strain. To investigate the feasibility of this system for bioactive molecules with high expression levels, the fimbrial adhesin gene, faeG, from Escherichia coli was tested and successfully integrated into the LCABL_07270 site with 5.51 ± 0.25 copies, and the integrated faeG achieved stable expression. All results demonstrate that this two-step integration system could achieve a high yield of heterologous gene expression by repetitive integration at a targeted chromosomal location in Lb. casei IMPORTANCE Lactic acid bacteria (LAB), including Lactobacillus casei, have the potential for overexpression of heterologous proteins, such as bioactive molecules and enzymes. However, traditional genetic tools for expression of these proteins show genetic instability or low yields of the desired product. In this study, we provide a procedure for repetitive integration of genes at various chromosomal locations, achieving high-level and stable expression of proteins in Lb. casei without selective pressure. The protocol developed in this study provides an essential reference for chromosomal overexpression of proteins or bioactive molecules in LAB.

Structure and biological activities of a hexosamine-rich cell wall polysaccharide isolated from the probiotic Lactobacillus farciminis

Lactobacillus farciminis CIP 103136 is a bacterial strain with recognized probiotic properties. However, the mechanisms underlying such properties have only been partially elucidated. In this study, we isolated and purified a cell-wall associated polysaccharide (CWPS), and evaluated its biological role in vitro. The structure of CWPS and responses from stimulation of (i) human macrophage-like THP-1 cells, (ii) human embryonal kidney (HEK293) cells stably transfected with Toll-like receptors (TLR2 or TLR4) and (iii) human colonocyte-like T84 intestinal epithelial cells, upon exposure to CWPS were studied. The structure of the purified CWPS from L. farciminis CIP 103136 was analyzed by nuclear magnetic resonance (NMR), MALDI-TOF-TOF MS, and methylation analyses in its native form and following Smith degradation. It was shown to be a novel branched polysaccharide, composed of linear backbone of trisaccharide repeating units of: [→6αGlcpNAc1 → 4βManpNAc1 → 4βGlcpNAc1→] highly substituted with single residues of αGlcp, αGalp and αGlcpNAc. Subsequently, the lack of pro- or anti-inflammatory properties of CWPS was established on macrophage-like THP-1 cells. In addition, CWPS failed to modulate cell signaling pathways dependent of TLR2 and TLR4 in transfected HEK-cells. Finally, in T84 cells, CWPS neither influenced intestinal barrier integrity under basal conditions nor prevented TNF-α/IFN-γ cytokine-mediated epithelium impairment.

Simple Protocol to Purify Cell Wall Polysaccharide from Gram-Positive Bacteria and Assess Its Structural Integrity

Cell wall polysaccharides (CWPS), which are usually covalently bound to the peptidoglycan and are closely associated with the cell wall, are considered as ubiquitous components of the cell envelope of gram-positive bacteria and play an important role as mediators of bacterial interactions with the environment. Here, we describe a simple method for purifying CWPS by extraction of bacterial cells with consecutive acid treatments. Purified CWPS are obtained by gel-filtration chromatography following treatment with HF. We also provide the methodology to easily assess the integrity of CWPS using high-resolution magic-angle spinning (HR-MAS) NMR.

Interactions of Surface Exopolysaccharides From Bifidobacterium and Lactobacillus Within the Intestinal Environment

Exopolysaccharides (EPS) are surface carbohydrate polymers present in most bacteria acting as a protective surface layer but also interacting with the surrounding environment. This review discusses the roles of EPS synthesized by strains of Lactobacillus and Bifidobacterium, many of them with probiotic characteristics, in the intestinal environment. Current knowledge on genetics and biosynthesis pathways of EPS in lactic acid bacteria and bifidobacteria, as well as the development of genetic tools, has created possibilities to elucidate the interplay between EPS and host intestinal mucosa. These include the microbiota that inhabits this ecological niche and the host cells. Several carbohydrate recognition receptors located in the intestinal epithelium could be involved in the interaction with bacterial EPS and modulation of immune response; however, little is known about the receptors recognizing EPS from lactobacilli or bifidobacteria and the triggered response. On the contrary, it has been clearly demonstrated that EPS play a relevant role in the persistence of the producing bacteria in the intestinal tract. Indeed, some authors postulate that some of the beneficial actions of EPS-producing probiotics could be related to the formation of a biofilm layer protecting the host against injury, for example by pathogens or their toxins. Nevertheless, the in vivo formation of biofilms by probiotics has not been proved to date. Finally, EPS produced by probiotic strains are also able to interact with the intestinal microbiota that populates the gut. In fact, some of these polymers can be used as carbohydrate fermentable source by some gut commensals thus being putatively involved in the release of bacterial metabolites that exert positive benefits for the host. In spite of the increasing knowledge about the role that these surface molecules play in the interaction of probiotic bacteria with the gut mucosal actors, both intestinal receptors and microbiota, the challenging issue is to demonstrate the functionality of EPS in vivo, which will open an avenue of opportunities for the application of EPS-producing probiotics to improve health.

Molecular Basis of Bacterial Host Interactions by Gram-Positive Targeting Bacteriophages

The inherent ability of bacteriophages (phages) to infect specific bacterial hosts makes them ideal candidates to develop into antimicrobial agents for pathogen-specific remediation in food processing, biotechnology, and medicine (e.g., phage therapy). Conversely, phage contaminations of fermentation processes are a major concern to dairy and bioprocessing industries. The first stage of any successful phage infection is adsorption to a bacterial host cell, mediated by receptor-binding proteins (RBPs). As the first point of contact, the binding specificity of phage RBPs is the primary determinant of bacterial host range, and thus defines the remediative potential of a phage for a given bacterium. Co-evolution of RBPs and their bacterial receptors has forced endless adaptation cycles of phage-host interactions, which in turn has created a diverse array of phage adsorption mechanisms utilizing an assortment of RBPs. Over the last decade, these intricate mechanisms have been studied intensely using electron microscopy and X-ray crystallography, providing atomic-level details of this fundamental stage in the phage infection cycle. This review summarizes current knowledge surrounding the molecular basis of host interaction for various socioeconomically important Gram-positive targeting phage RBPs to their protein- and saccharide-based receptors. Special attention is paid to the abundant and best-characterized Siphoviridae family of tailed phages. Unravelling these complex phage-host dynamics is essential to harness the full potential of phage-based technologies, or for generating novel strategies to combat industrial phage contaminations.

Host recognition by lactic acid bacterial phages

Bacteriophage infection of lactic acid bacteria (LAB) is one of the most significant causes of inconsistencies in the manufacture of fermented foods, affecting production schedules and organoleptic properties of the final product. Consequently, LAB phages, and particularly those infecting Lactococcus lactis, have been the focus of intensive research efforts. During the past decade, multidisciplinary scientific approaches have uncovered molecular details on the exquisite process of how a lactococcal phage recognises and binds to its host. Such approaches have incorporated genomic/molecular analyses and their partnership with phage structural analysis and host cell wall biochemical studies are discussed in this review, which will also provide our views on future directions of this research field.

Coupling the recombineering to Cre-lox system enables simplified large-scale genome deletion in Lactobacillus casei

Background

Lactobacillus casei is widely used in the dairy and pharmaceutical industries and a promising candidate for use as cell factories. Recently, genome sequencing and functional genomics provide the possibility for reducing L. casei genome. However, it was still limited by the inefficient and laborious genome deletion methods.

Results

Here, we proposed a genome minimization strategy based on LCABL_13040-50-60 recombineering and Cre-lox site-specific recombination system in L. casei. The LCABL_13040-50-60 recombineering system was used to introduce two lox sites (lox66 and lox71) into 5′ and 3′ ends of the targeted region. Subsequently, the targeted region was excised by Cre recombinase. The robustness of the strategy was demonstrated by single-deletion of a nonessential ~ 39.3 kb or an important ~ 12.8 kb region and simultaneous deletion of two non-continuous genome regions (5.2 and 6.6 kb) with 100% efficiency. Furthermore, a cyclical application of this strategy generated a double-deletion mutant of which 1.68% of the chromosome was sequentially excised. Moreover, biological features (including growth rate, electroporation efficiency, cell morphology or heterologous protein productivity) of these mutants were characterized.

Conclusions

To our knowledge, this strategy is the first instance of sequential deletion of large-scale genome regions in L. casei. We expected this efficient and inexpensive tool can help for rapid genome streamlining and generation restructured L. casei strains used as cell factories.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0872-4) contains supplementary material, which is available to authorized users.

EPSP of L. casei BL23 Protected against the Infection Caused by Aeromonas veronii via Enhancement of Immune Response in Zebrafish

Aquaculture is the fastest-growing food production sector in the world, and it supplies nearly 50% of the global food fish supply. However, disease outbreaks have become a major problem in the fish farming industry. The beneficial contribution of probiotic bacteria to aquatic animals' health has been widely described, and they have been widely used in aquaculture for disease control and growth promotion. However, the action of probiotic bacterial components and mechanisms underlying protection against pathogens afforded by probiotic bacteria remain poorly understood. In the present study, we pre-colonized zebrafish larvae (before hatching) with 17 potential probiotic bacterial strains and screened for those possessing anti-infective effects against Aeromonas veronii. We found that Lactobacillus casei BL23 significantly increased the survival of zebrafish larvae upon A. veronii infection. Using a germ-free (GF) zebrafish model and gut microbiota transplant experiment, we showed that L. casei BL23 per se has anti-infective effects in zebrafish larvae, which does not involve microbiota. Furthermore, we identified an exopolysaccharide-protein complex (EPSP) extracted from L. casei BL23 cells, which consisted of a 40-45 KD size protein and an exopolysaccharide composed of α-Rha, α-Glc, β-GlcNAc, and β-GalNAc. EPSP significantly increased the survival rate of GF zebrafish at a dose of 10-20 μg/ml after A. veronii infection (P < 0.01). In addition, the EPSP induced a higher expression of TLR1 and TLR2, and modulated the expression profile of pro-inflammatory and anti-inflammatory cytokines in zebrafish liver (ZFL) cells. Our data indicated that the anti-infective effect of EPSP from L. casei BL23 was mediated by enhancement of immune responses in zebrafish, which might involve the TLR1/TLR2 signal pathway.

Another Brick in the Wall: a Rhamnan Polysaccharide Trapped inside Peptidoglycan of Lactococcus lactis

Polysaccharides are ubiquitous components of the Gram-positive bacterial cell wall. In Lactococcus lactis, a polysaccharide pellicle (PSP) forms a layer at the cell surface. The PSP structure varies among lactococcal strains; in L. lactis MG1363, the PSP is composed of repeating hexasaccharide phosphate units. Here, we report the presence of an additional neutral polysaccharide in L. lactis MG1363 that is a rhamnan composed of α-l-Rha trisaccharide repeating units. This rhamnan is still present in mutants devoid of the PSP, indicating that its synthesis can occur independently of PSP synthesis. High-resolution magic-angle spinning nuclear magnetic resonance (HR-MAS NMR) analysis of whole bacterial cells identified a PSP at the surface of wild-type cells. In contrast, rhamnan was detected only at the surface of PSP-negative mutant cells, indicating that rhamnan is located underneath the surface-exposed PSP and is trapped inside peptidoglycan. The genetic determinants of rhamnan biosynthesis appear to be within the same genetic locus that encodes the PSP biosynthetic machinery, except the gene tagO encoding the initiating glycosyltransferase. We present a model of rhamnan biosynthesis based on an ABC transporter-dependent pathway. Conditional mutants producing reduced amounts of rhamnan exhibit strong morphological defects and impaired division, indicating that rhamnan is essential for normal growth and division. Finally, a mutation leading to reduced expression of lcpA, encoding a protein of the LytR-CpsA-Psr (LCP) family, was shown to severely affect cell wall structure. In lcpA mutant cells, in contrast to wild-type cells, rhamnan was detected by HR-MAS NMR, suggesting that LcpA participates in the attachment of rhamnan to peptidoglycan.IMPORTANCE In the cell wall of Gram-positive bacteria, the peptidoglycan sacculus is considered the major structural component, maintaining cell shape and integrity. It is decorated with other glycopolymers, including polysaccharides, the roles of which are not fully elucidated. In the ovococcus Lactococcus lactis, a polysaccharide with a different structure between strains forms a layer at the bacterial surface and acts as the receptor for various bacteriophages that typically exhibit a narrow host range. The present report describes the identification of a novel polysaccharide in the L. lactis cell wall, a rhamnan that is trapped inside the peptidoglycan and covalently bound to it. We propose a model of rhamnan synthesis based on an ABC transporter-dependent pathway. Rhamnan appears as a conserved component of the lactococcal cell wall playing an essential role in growth and division, thus highlighting the importance of polysaccharides in the cell wall integrity of Gram-positive ovococci.