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

The complex world of kefir: Structural insights and symbiotic relationships

Kefir milk, known for its high nutritional value and health benefits, is traditionally produced by fermenting milk with kefir grains. These grains are a complex symbiotic community of lactic acid bacteria, acetic acid bacteria, yeasts, and other microorganisms. However, the intricate coexistence mechanisms within these microbial colonies remain a mystery, posing challenges in predicting their biological and functional traits. This uncertainty often leads to variability in kefir milk's quality and safety. This review delves into the unique structural characteristics of kefir grains, particularly their distinctive hollow structure. We propose hypotheses on their formation, which appears to be influenced by the aggregation behaviors of the community members and their alliances. In kefir milk, a systematic colonization process is driven by metabolite release, orchestrating the spatiotemporal rearrangement of ecological niches. We place special emphasis on the dynamic spatiotemporal changes within the kefir microbial community. Spatially, we observe variations in species morphology and distribution across different locations within the grain structure. Temporally, the review highlights the succession patterns of the microbial community, shedding light on their evolving interactions.Furthermore, we explore the ecological mechanisms underpinning the formation of a stable community composition. The interplay of cooperative and competitive species within these microorganisms ensures a dynamic balance, contributing to the community's richness and stability. In kefir community, competitive species foster diversity and stability, whereas cooperative species bolster mutualistic symbiosis. By deepening our understanding of the behaviors of these complex microbial communities, we can pave the way for future advancements in the development and diversification of starter cultures for food fermentation processes.

Bio surfactants from lactic acid bacteria: an in-depth analysis of therapeutic properties and food formulation

Healthy humans and animals commonly harbor lactic acid bacteria (LAB) on their mucosal surfaces, which are often associated with food fermentation. These microorganisms can produce amphiphilic compounds, known as microbial surface-active agents, that exhibit remarkable emulsifying activity. However, the exact functions of these microbial surfactants within the producer cells remain unclear. Consequently, there is a growing urgency to develop biosurfactant production from nonpathogenic microbes, particularly those derived from LAB. This approach aims to harness the benefits of biosurfactants while ensuring their safety and applicability. This review encompasses a comprehensive analysis of native and genetically modified LAB biosurfactants, shedding light on microbial interactions, cell signaling, pathogenicity, and biofilm development. It aims to provide valuable insights into the applications of these active substances in therapeutic use and food formulation as well as their potential biological and other benefits. By synthesizing the latest knowledge and advancements, this review contributes to the understanding and utilization of LAB biosurfactants in the food and nutritional areas.

Kefir microbiota and metabolites stimulate intestinal mucosal immunity and its early development

Kefir consists of a large number of probiotics, which can regulate or shape the balance of intestinal microbiota, and enhance the host's immune response. Kefir microbiota can shape the mucosal immunity of the body through SCFAs, EPS, polypeptides, lactic acid, and other metabolites and microbial antigens themselves, and this shaping may have time windows and specific pathways. Kefir can regulate antibody SIgA and IL-10 levels to maintain intestinal homeostasis, and its secreted SIgA can shape the stable microbiota system by wrapping and binding different classes of microorganisms. The incidence of intestinal inflammation is closely linked to the development and maturation of intestinal mucosal immunity. Based on summarizing the existing research results on Kefir, its metabolites, and immune system development, this paper proposes to use Kefir, traditional fermented food with natural immune-enhancing components and stable functional microbiota, as an intervention method. Early intervention in the immune system may seize the critical window period of mucosal immunity and stimulate the development and maturation of intestinal mucosal immunity in time.

Towards understanding mechanisms and functional consequences of bacterial interactions with members of various kingdoms in complex biofilms that abound in nature

The microbial world represents a phenomenal diversity of microorganisms from different kingdoms of life which occupy an impressive set of ecological niches. Most, if not all, microorganisms once colonise a surface develop architecturally complex surface-adhered communities which we refer to as biofilms. They are embedded in polymeric structural scaffolds serve as a dynamic milieu for intercellular communication through physical and chemical signalling. Deciphering microbial ecology of biofilms in various natural or engineered settings has revealed co-existence of microorganisms from all domains of life, including Bacteria, Archaea and Eukarya. The coexistence of these dynamic microbes is not arbitrary, as a highly coordinated architectural setup and physiological complexity show ecological interdependence and myriads of underlying interactions. In this review, we describe how species from different kingdoms interact in biofilms and discuss the functional consequences of such interactions. We highlight metabolic advances of collaboration among species from different kingdoms, and advocate that these interactions are of great importance and need to be addressed in future research. Since trans-kingdom biofilms impact diverse contexts, ranging from complicated infections to efficient growth of plants, future knowledge within this field will be beneficial for medical microbiology, biotechnology, and our general understanding of microbial life in nature.

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

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

Polymicrobial interaction between Lactobacillus and Saccharomyces cerevisiae: coexistence-relevant mechanisms

The coordination of single or multiple microorganisms are required for the manufacture of traditional fermented foods, improving the flavour and nutrition of the food materials. However, both the additional economic benefits and safety concerns have been raised by microbiotas in fermented products. Among the fermented products, Lactobacillus and Saccharomyces cerevisiae are one of the stable microbiotas, suggesting their interaction is mediated by coexistence-relevant mechanisms and prevent to be excluded by other microbial species. Thus, aiming to guide the manufacture of fermented foods, this review will focus on interactions of coexistence-relevant mechanisms between Lactobacillus and S. cerevisiae, including metabolites communications, aggregation, and polymicrobial biofilm. Also, the molecular regulatory network of the coexistence-relevant mechanisms is discussed according to omics researches.

Antimicrobial mechanisms and applications of yeasts

Yeasts and humans have had a close relationship for millenia. Yeast have been used for food production since the first human societies. Since then, alternative uses have been discovered. Nowadays, antibiotic resistance constitutes a pressing need worldwide. In order to overcome this threat, one of the most important strategies is the search for new antimicrobials in natural sources. Moreover, biopreservation based on natural sources has emerged as an alternative to more common chemical preservatives. Yeasts constitute an underexploited source of antagonistic activity against other microorganisms. Here, we compile a summary of the antagonistic activity of yeast origin against other yeast and other microorganisms, such as bacteria or parasites. We present the mechanisms of action used by yeasts to display these activities. We also provide applications of these antagonistic activities in food industry and agriculture, medicine and veterinary, where yeast promise to play a pivotal role in the near future.

A Big World in Small Grain: A Review of Natural Milk Kefir Starters

Milk kefir is a traditional fermented milk product whose consumption is becoming increasingly popular. The natural starter for kefir production is kefir grain, which consists of various bacterial and yeast species. At the industrial scale, however, kefir grains are rarely used due to their slow growth, complex application, bad reproducibility and high costs. Instead, mixtures of defined lactic acid bacteria and sometimes yeasts are applied, which alter sensory and functional properties compared to natural grain-based milk kefir. In order to be able to mimic natural starter cultures for authentic kefir production, it is a prerequisite to gain deep knowledge about the nature of kefir grains, its microbial composition, morphologic structure, composition of strains on grains and the impact of environmental parameters on kefir grain characteristics. In addition, it is very important to deeply investigate the numerous multi-dimensional interactions among different species, which play important roles on the formation and the functionality of grains.

Effects of cerium oxide nanoparticles on bacterial growth and behaviors: induction of biofilm formation and stress response

In this paper, the effects of cerium oxide nanoparticles (CeO₂ NPs) on the group bacterial behaviors were elaborated. After 36-h cultivation, the biofilm biomass was enhanced by the sub-lethal concentrations of 0.5 and 2 mg/L CeO₂ NP exposure. Meanwhile, the promoted production of total amino acids in microbes further resulted in the increased surface hydrophobicity and percentage aggregation. To resist the CeO₂ NPs stress, the biofilm exhibited a double-layer microstructure, with the protein (PRO) and living cells occupying the bottom, the polysaccharide (PS), and dead cells dominating the top. The bacterial diversity was highly suppressed and Citrobacter and Pseudomonas from the phylum of γ-Proteobacteria strongly dominated the biofilm, indicating the selective and enriched effects of CeO₂ NPs on resistant bacteria. The stimulated inherent resistance of biofilm was reflected by the reduced adenosine triphosphate (ATP) content after 4 h exposure. The increased levels of reactive oxygen species (ROS) in the treatments of 8 h CeO₂ NP exposure led to the upregulated quorum sensing signals of acylated homoserine lactone (AHL) and autoinducer 2 (AI-2), beneficial to mitigating the environmental disturbance of CeO₂ NPs. These results provide evidences for the accelerating effects of CeO₂ NPs on biofilm formation through oxidative stress, which expand the understanding of the ecological effects of CeO₂ NPs.

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

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

Mechanistic understanding of cerium oxide nanoparticle-mediated biofilm formation in Pseudomonas aeruginosa

In this study, the biofilm formation of Pseudomonas aeruginosa in the presence of cerium oxide nanoparticles (CeO₂ NPs) was investigated. With the addition of 0.1 mg/L and 1 mg/L CeO₂ NPs, the biofilm development was substantially enhanced. During the attachment process, the enhanced surface hydrophobicity and excess production of mannosan and rhamnolipids in CeO₂ NP treatments were detected, which were conductive to the colonization of bacterial cells. During the maturation period, the biofilm biomass was accelerated by the improved aggregation percentage as well as the secretion of extracellular DNA and pyocyanin. The reactive oxygen species (ROS) generated by CeO₂ NPs were found to activate the N-butyryl homoserine lactone (C₄-HSL) and quinolone signals secreted by Pseudomonas aeruginosa. Moreover, the quorum sensing (QS) systems of rhl and pqs were initiated, reflected by the stimulated expression levels of biofilm formation-related genes rhlI-rhlR, rhlAB, and pqsR-pqsA. The addition of a quorum quencher, furanone C-30, significantly declined the activities of QS-controlled catalase and superoxide dismutase. A dose of antioxidant, ascorbic acid, effectively relieved the accelerating effects of NPs on biofilm formation. These results indicated that CeO₂ NPs could accelerate biofilm formation through the interference of QS system by generating ROS, which provides possible targets for controlling biofilm growth in the NP exposure environments.

Lactobacillus hordei dextrans induce Saccharomyces cerevisiae aggregation and network formation on hydrophilic surfaces

Water kefir granules are supposed to mainly consist of dextrans produced by Lactobacillus (L.) hilgardii. Still, other microorganisms such as L. hordei, L. nagelii, Leuconostoc (Lc.) citreum and Saccharomyces (S.) cerevisiae are commonly isolated from water kefir granules, while their contribution to the granule formation remains unknown. We studied putative functions of these microbes in granule formation, upon development of a simplified model system containing hydrophilic object slides, which mimics the hydrophilic surface of a growing kefir granule. We found that all tested lactic acid bacteria produced glucans, while solely those isolated from the four different L. hordei strains induced yeast aggregation on the hydrophilic slides. Therefore, structural differences between these glucans were investigated with respect to their size distributions and their linkage types. Beyond the finding that all glucans were identified as dextrans, those of the four L. hordei strains were highly similar among each other regarding portions of linkage types and size distributions. Thus, our study suggests the specific size and structural organization of the dextran produced by L. hordei as the main cause for inducing S. cerevisiae aggregation and network formation on hydrophilic surfaces and thus as crucial initiation of the stepwise water kefir granule growth.

Biological and physicochemical properties of biosurfactants produced by Lactobacillus jensenii P6A and Lactobacillus gasseri P65

Background

Lactobacillus species produce biosurfactants that can contribute to the bacteria’s ability to prevent microbial infections associated with urogenital and gastrointestinal tracts and the skin. Here, we described the biological and physicochemical properties of biosurfactants produced by Lactobacillus jensenii P_6A and Lactobacillus gasseri P₆₅.

Results

The biosurfactants produced by L. jensenii P_6A and L. gasseri P₆₅ reduced the water surface tension from 72 to 43.2 mN m⁻¹ and 42.5 mN m⁻¹ as their concentration increased up to the critical micelle concentration (CMC) values of 7.1 and 8.58 mg mL⁻¹, respectively. Maximum emulsifying activity was obtained at concentrations of 1 and 5 mg mL⁻¹ for the P_6A and P₆₅ strains, respectively. The Fourier transform infrared spectroscopy data revealed that the biomolecules consist of a mixture of carbohydrates, lipids and proteins. The gas chromatography-mass spectrum analysis of L. jensenii P_6A biosurfactant showed a major peak for 14-methypentadecanoic acid, which was the main fatty acid present in the biomolecule; conversely, eicosanoic acid dominated the biosurfactant produced by L. gasseri P₆₅. Although both biosurfactants contain different percentages of the sugars galactose, glucose and ribose; rhamnose was only detected in the biomolecule produced by L. jensenii P_6A. Emulsifying activities were stable after a 60-min incubation at 100 °C, at pH 2–10, and after the addition of potassium chloride and sodium bicarbonate, but not in the presence of sodium chloride. The biomolecules showed antimicrobial activity against clinical isolates of Escherichia coli and Candida albicans, with MIC values of 16 µg mL⁻¹, and against Staphylococcus saprophyticus, Enterobacter aerogenes and Klebsiella pneumoniae at 128 µg mL⁻¹. The biosurfactants also disrupted preformed biofilms of microorganisms at varying concentrations, being more efficient against E. aerogenes (64%) (P_6A biosurfactant), and E. coli (46.4%) and S. saprophyticus (39%) (P₆₅ biosurfactant). Both strains of lactobacilli could also co-aggregate pathogens.

Conclusions

This report presents the first characterization of biosurfactants produced by L. jensenii P_6A and L. gasseri P₆₅. The antimicrobial properties and stability of these biomolecules indicate their potential use as alternative antimicrobial agents in the medical field for applications against pathogens that are responsible for infections in the gastrointestinal and urogenital tracts and the skin.

Saccharomyces cerevisiae from Brazilian kefir-fermented milk: An in vitro evaluation of probiotic properties

The therapeutic use of probiotics for supporting the antibiotic action against gastrointestinal disorders is a current trend and emerging applications have gained popularity because of their support for various microbiological activities in digestive processes. Microorganisms isolated from kefir with great probiotic properties, in addition to high resistance to harsh environmental conditions, have been widely researched. Administration of probiotic yeasts offers a number of advantages, when compared to bacteria, because of particular characteristics as their larger cell size. In the present study, 28 strains of Saccharomyces cerevisiae were isolated, after in vitro digestion of kefir-fermented milk, and identified by molecular based approaches. A screening was performed to determine important quality requirements for probiotics including: antagonistic and antioxidant activities, β-galactosidase synthesis, autoaggregation, surface hydrophobicity and adhesion to epithelial cells. The results showed strains: with antagonistic activity against microbial pathogens such as Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, Bacillus subtilis; able to produce β-galactosidase; with antioxidant activity levels higher than 90%; with hydrophobicity activity and autoaggregation ability (evaluated by adhesion test, where all the strains presented adhesion to mice ileal epithelial cells). These findings are relevant and the strains are recommended for further in vivo studies as well as for potential therapeutic applications.

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

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

SIGNIFICANCE

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

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

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

Differential protection by cell wall components of Lactobacillus amylovorus DSM 16698Tagainst alterations of membrane barrier and NF-kB activation induced by enterotoxigenic F4+ Escherichia coli on intestinal cells

BACKGROUND

The role of Lactobacillus cell wall components in the protection against pathogen infection in the gut is still largely unexplored. We have previously shown that L. amylovorus DSM 16698^T is able to reduce the enterotoxigenic F4⁺ Escherichia coli (ETEC) adhesion and prevent the pathogen-induced membrane barrier disruption through the regulation of IL-10 and IL-8 expression in intestinal cells. We have also demonstrated that L. amylovorus DSM 16698^T protects host cells through the inhibition of NF-kB signaling. In the present study, we investigated the role of L. amylovorus DSM 16698^T cell wall components in the protection against F4⁺ETEC infection using the intestinal Caco-2 cell line.

METHODS

Purified cell wall fragments (CWF) from L. amylovorus DSM 16698^T were used either as such (uncoated, U-CWF) or coated with S-layer proteins (S-CWF). Differentiated Caco-2/TC7 cells on Transwell filters were infected with F4⁺ETEC, treated with S-CWF or U-CWF, co-treated with S-CWF or U-CWF and F4⁺ETEC for 2.5 h, or pre-treated with S-CWF or U-CWF for 1 h before F4⁺ETEC addition. Tight junction (TJ) and adherens junction (AJ) proteins were analyzed by immunofluorescence and Western blot. Membrane permeability was determined by phenol red passage. Phosphorylated p65-NF-kB was measured by Western blot.

RESULTS

We showed that both the pre-treatment with S-CWF and the co- treatment of S-CWF with the pathogen protected the cells from F4⁺ETEC induced TJ and AJ injury, increased membrane permeability and activation of NF-kB expression. Moreover, the U-CWF pre-treatment, but not the co-treatment with F4⁺ETEC, inhibited membrane damage and prevented NF-kB activation.

CONCLUSIONS

The results indicate that the various components of L. amylovorus DSM 16698^T cell wall may counteract the damage caused by F4⁺ETEC through different mechanisms. S-layer proteins are essential for maintaining membrane barrier function and for mounting an anti-inflammatory response against F4⁺ETEC infection. U-CWF are not able to defend the cells when they are infected with F4⁺ETEC but may activate protective mechanisms before pathogen infection.

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.

Enhancement of bifidobacteria survival by Williopsis saturnus var. saturnus in milk

The viability of three strains of probiotic Bifidobacterium lactis that were inoculated into UHT milk was examined with and without the presence of the yeast, Williopsis saturnus var. saturnus NCYC 22, in polypropylene tubes at 30 °C. The B. lactis viable cell count for strains HN019 and BB-12 remained above 6.0 Log cfu/ml, while strain B94 had 5.7 Log cfu/ml after six weeks of incubation in the presence of the co-inoculated yeast. Incubating the bifidus milk without added yeast under anaerobic condition did not improve the survival of B. lactis HN019, indicating that oxygen removal may not be responsible for W. saturnus NCYC 22’s viability enhancing property. The addition of yeast supernatant or non-viable yeast also did not show any stabilising effects, suggesting that physical contact and/or interaction between viable W. saturnus and B. lactis plays an important role in sustaining the viability of the probiotic. W. saturnus NCYC 22 could increase the survival of B. lactis in bifidus milk under ambient temperature regardless of the initial concentration of yeast cells inoculated due to yeast growth. This study demonstrated the viability enhancing effect of viable W. saturnus NCYC 22 on B. lactis HN019, which could help towards extending the shelf-life of dairy beverages containing probiotic bifidobacteria.

Russian Kefir Grains Microbial Composition and Its Changes during Production Process

By combining DGGE-PCR method, classical microbiological analysis and light- and electron microscopic observations, it was found that the composition of microbial communities of central Russia regions kefir grains, starter and kefir drink include bacteria of the genera Lactobacillus, Leuconostoc and Lactococcus, and yeast anamorphs of the genera Saccharomyces, Kazachstania and Gibellulopsis. Fifteen prokaryotic and four eukaryotic pure cultures of microorganisms were isolated and identified from kefir grains. It has been shown that members of the genus Lactobacillus prevailed in kefir grains, whereas strains Leuconostoc pseudomesenteroides and Lactococcus lactis dominated in the final product - kefir drink. Yeasts contained in kefir grains in small amounts have reached a significant number of cells in the process of development of this dairy product. The possibility of reverse cell aggregation has been attempted in a mixed cultivation of all isolated pure cultures, but full formation kefir grains is not yet observed after 1.5 years of observation and reinoculations.

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

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

Screening of lactic acid bacteria that can form mixed-species biofilm with Saccharomyces cerevisiae

The abilities of lactic acid bacteria (LAB) to form mixed-species biofilm with Saccharomyces cerevisiae in a static co-culture were investigated out of 168 LAB stock cultures, and two Lactobacillus plantarum strains (D71 and E31) and one Leuconostoc mesenteroides strain K01 were found to form mixed-species biofilm with S. cerevisiae BY4741. SEM observation showed that there was no significant difference in morphological properties among these three mixed-species biofilms and they resembled that formed by S. cerevisiae with L. plantarum ML11-11 previously isolated from a brewing sample of Fukuyama pot vinegar. The co-aggregation assays showed that L. plantarum D71 and L. plantarum E31 could co-aggregate with S. cerevisiae similarly to L. plantarum ML11-11, while L. mesenteroides K01 had no ability to co-aggregate with yeast. The above results indicate that aggregation followed by direct cell-to-cell contact is required for mixed-species biofilm formation between these L. plantarum strains and S. cerevisiae, though some different mechanism may be involved in biofilm formation between L. mesenteroides strain and S. cerevisiae.

Adhesion properties of potentially probiotic Lactobacillus kefiri to gastrointestinal mucus

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

Significance of microbial symbiotic coexistence in traditional fermentation

Symbiosis has long been a central theme in microbiology. There have been many studies on the symbioses between microorganisms and higher organisms such as plants and animals. There also have been some studies on the symbiosis or coexistence of microorganisms, such as yeasts, lactic acid bacteria (LAB), acetic acid bacteria (AAB) and koji molds, in traditional fermentation (brewing). These microorganisms are considered to interact and cooperate with each other in various natural environments, such as dropped cereal crops or ripe fruits. Human beings have taken advantage of these microbial interactions for producing various fermented foods.

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.

The role of cell surface architecture of lactobacilli in host-microbe interactions in the gastrointestinal tract

Lactobacillus species can exert health promoting effects in the gastrointestinal tract (GIT) through many mechanisms, which include pathogen inhibition, maintenance of microbial balance, immunomodulation, and enhancement of the epithelial barrier function. Different species of the genus Lactobacillus can evoke different responses in the host, and not all strains of the same species can be considered beneficial. Strain variations may be related to diversity of the cell surface architecture of lactobacilli and the bacteria's ability to express certain surface components or secrete specific compounds in response to the host environment. Lactobacilli are known to modify their surface structures in response to stress factors such as bile and low pH, and these adaptations may help their survival in the face of harsh environmental conditions encountered in the GIT. In recent years, multiple cell surface-associated molecules have been implicated in the adherence of lactobacilli to the GIT lining, immunomodulation, and protective effects on intestinal epithelial barrier function. Identification of the relevant bacterial ligands and their host receptors is imperative for a better understanding of the mechanisms through which lactobacilli exert their beneficial effects on human health.

In vitro characterization of aggregation and adhesion properties of viable and heat-killed forms of two probiotic Lactobacillus strains and interaction with foodborne zoonotic bacteria, especially Campylobacter jejuni

Bacterial aggregation and/or adhesion are key factors for colonization of the digestive ecosystem and the ability of probiotic strains to exclude pathogens. In the present study, two probiotic strains, Lactobacillus rhamnosus CNCM-I-3698 and Lactobacillus farciminis CNCM-I-3699, were evaluated as viable or heat-killed forms and compared with probiotic reference Lactobacillus strains (Lb. rhamnosus GG and Lb. farciminis CIP 103136). The autoaggregation potential of both forms was higher than that of reference strains and twice that of pathogenic strains. The coaggregation potential of these two beneficial micro-organisms was evaluated against several pathogenic agents that threaten the global safety of the feed/food chain: Escherichia coli, Salmonella spp., Campylobacter spp. and Listeria monocytogenes. The strongest coaggregative interactions were demonstrated with Campylobacter spp. by a coaggregation test, confirmed by electron microscopic examination for the two forms. Viable forms were investigated for the nature of the bacterial cell-surface molecules involved, by sugar reversal tests and chemical and enzymic pretreatments. The results suggest that the coaggregation between both probiotic strains and C. jejuni CIP 70.2(T) is mediated by a carbohydrate-lectin interaction. The autoaggregation potential of the two probiotics decreased upon exposure to proteinase, SDS or LiCl, showing that proteinaceous components on the surface of the two lactobacilli play an important role in this interaction. Adhesion abilities of both Lactobacillus strains were also demonstrated at significant levels on Caco-2 cells, mucin and extracellular matrix material. Both viable and heat-killed forms of the two probiotic lactobacilli inhibited the attachment of C. jejuni CIP 70.2(T) to mucin. In conclusion, in vitro assays showed that Lb. rhamnosus CNCM-I-3698 and Lb. farciminis CNCM-I-3699, as viable or heat-killed forms, are adherent to different intestinal matrix models and are highly aggregative in vitro with pathogens, especially Campylobacter spp., the most commonly reported zoonotic agent in the European Union. This study supports the need for further in vivo investigations to demonstrate the potential food safety benefits of Lb. rhamnosus CNCM-I-3698 and Lb. farciminis CNCM-I-3699, live or heat-killed, in the global feed/food chain.

Antimicrobial and probiotic properties of yeasts: from fundamental to novel applications

The yeasts constitute a large and heterogeneous group of microorganisms that are currently attracting increased attention from scientists and industry. Numerous and diverse biological activities make them promising candidates for a wide range of applications not limited to the food sector. In addition to their major contribution to flavor development in fermented foods, their antagonistic activities toward undesirable bacteria, and fungi are now widely known. These activities are associated with their competitiveness for nutrients, acidification of their growth medium, their tolerance of high concentrations of ethanol, and release of antimicrobial compounds such as antifungal killer toxins or "mycocins" and antibacterial compounds. While the design of foods containing probiotics (microorganisms that confer health benefits) has focused primarily on Lactobacillus and Bifidobacterium, the yeast Saccharomyces cerevisiae var. boulardii has long been known effective for treating gastroenteritis. In this review, the antimicrobial activities of yeasts are examined. Mechanisms underlying this antagonistic activity as well as recent applications of these biologically active yeasts in both the medical and veterinary sectors are described.

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

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

Investigation of microorganisms involved in biosynthesis of the kefir grain

The purpose of this study was to understand the significance of each microorganism in grain formation by evaluating their microbial aggregation and cell surface properties during co-aggregation of LAB and yeasts together with an investigation of biofilm formation. Non-grain forming strains from viili were also evaluated as a comparison. Results indicated that the kefir grain strains, Lactobacillus kefiranofaciens and Saccharomyces turicensis possess strong auto-aggregation ability and that Lactobacillus kefiri shows significant biofilm formation properties. Significant co-aggregation was noted when S. turicensis and kefir LAB strains (Lb. kefiranofaciens and Lb. kefiri) were co-cultured. Most of the tested LAB strains are hydrophilic and had a negative charge on their cell surface. Only the kefir LAB strains, Lb. kefiranofaciens HL1 and Lb. kefiri HL2, possessed very high hydrophobicity and had a positive cell surface charge at pH 4.2. In contrast, the LAB and yeasts in viili did not show any significant self-aggregation or biofilm formation. Based on the above results, we propose that grain formation begins with the self-aggregation of Lb. kefiranofaciens and S. turicensis to form small granules. At this point, the biofilm producer, Lb. kefiri, then begins to attach to the surface of granules and co-aggregates with other organisms and components in the milk to form the grains. On sub-culturing, more organisms attach to the grains resulting in grain growth. When investigated by scanning electron microscopy, it was found that short-chain lactobacilli such as Lb. kefiri occupy the surface, while long-chain lactobacilli such as Lb. kefiranofaciens have aggregated towards the center of the kefir grains. These findings agree with the above hypothesis on the formation of grains. Taken together, this study demonstrates the importance of cell surface properties together with fermentation conditions to the formation of grains in kefir.

Fluorescent proteins as in-vivo and in-situ reporters to study the development of a Saccharomyces cerevisiae yeast biofilm and its invasion by the bacteria Escherichia coli

This work deals with the bacterial contamination of yeast, both as biofilm and in the planktonic phase. A model continuous system using self-fluorescent microorganisms was proposed to perform in vivo and in situ studies of a mixed biofilm. The yeast strain was inoculated first while the bacteria were added few days later to mimic a contamination. Supports sampled during the experiment were observed by scanning confocal laser microscopy. The behaviour of the microorganisms in real process conditions was then analysed without any treatment that could modify their physiology and/or damage the community structure. Using image analysis, the characteristics of biofilm development (microorganism ratio, 3D-organisation, growth rates) were studied and compared to the behaviour of the suspended cells in the bioreactor. Based on the biovolumes (volume occupied by each microorganism), the growth rates in biofilm for the bacteria and the yeasts were determined at 0.10 and 0.03 h(-1) respectively, while the imposed dilution rate was 0.10 h(-1). Even though the ability of yeast to develop biofilm was demonstrated, its capacity remained very low compared to that of the bacteria which quickly invaded and covered the whole yeast biofilm. This approach makes an original and powerful tool to study the competition phenomena occurring in model biofilms.

The importance of inter-species cell-cell co-aggregation between Lactobacillus plantarum ML11-11 and Saccharomyces cerevisiae BY4741 in mixed-species biofilm formation

Cells of Lactobacillus plantarum ML11-11, an isolate from Fukuyama pot vinegar, and the yeast Saccharomyces cerevisiae formed significant mixed-species biofilms with concurrent inter-species co-aggregation. The co-aggregation did not occur with heated or proteinase K-treated ML11-11 cells, or in the presence of D-mannose, suggesting that surface proteins of ML11-11 and mannose-containing surface substance(s) of yeast were the predominant contributing factors. Sugar fatty acid ester inhibited mixed-species biofilm formation, but did not inhibit co-aggregation, suggesting that the cell-cell adhesion and cell-polystylene adhesion are controlled by different mechanisms. Microscopic observation and microflora analysis revealed that inter-species co-aggregation plays an important role in the formation of the mixed-species biofilm.

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.