Effect of Lineage-Specific Metabolic Traits of Lactobacillus reuteri on Sourdough Microbial Ecology

This study determined the effects of specific metabolic traits of Lactobacillus reuteri on its competitiveness in sourdoughs. The competitiveness of lactobacilli in sourdough generally depends on their growth rate; acid resistance additionally contributes to competitiveness in sourdoughs with long fermentation times. Glycerol metabolism via glycerol dehydratase (gupCDE) accelerates growth by the regeneration of reduced cofactors; glutamate metabolism via glutamate decarboxylase (gadB) increases acid resistance by generating a proton motive force. Glycerol and glutamate metabolisms are lineage-specific traits in L. reuteri; therefore, this study employed glycerol dehydratase-positive sourdough isolates of human-adapted L. reuteri lineage I, glutamate decarboxylase-positive strains of rodent-adapted L. reuteri lineage II, as well as mutants with deletions in gadB or gupCDE. The competitivenesses of the strains were quantified by inoculation of wheat and sorghum sourdoughs with defined strains, followed by propagation of doughs with a 10% inoculum and 12-h or 72-h fermentation cycles. Lineage I L. reuteri strains dominated sourdoughs propagated with 12-h fermentation cycles; lineage II L. reuteri strains dominated sourdoughs propagated with 72-h fermentation cycles. L. reuteri 100-23ΔgadB was outcompeted by its wild-type strain in sourdoughs fermented with 72-h fermentation cycles; L. reuteri FUA3400ΔgupCDE was outcompeted by its wild-type strain in sourdoughs fermented with both 12-h and 72-h fermentation cycles. Competition experiments with isogenic pairs of strains resulted in a constant rate of strain displacement of the less competitive mutant strain. In conclusion, lineage-specific traits of L. reuteri determine the competitiveness of this species in sourdough fermentations.     

Analysis of antimicrobial and immunomodulatory substances produced by heterofermentative <Emphasis Type="Italic">Lactobacillus reuteri</Emphasis>

Antimicrobial and immunomodulatory potential of various Lactobacillus reuteri strains is closely connected to their metabolite production profile under given cultivation conditions. We determined the in vitro production of antimicrobial substances such as organic acids, ethanol, and reuterin by four strains of L. reuteri (L. reuteri E, L. reuteri KO5, L. reuteri CCM 3625, and L. reuteri ATCC 55730). All studied L. reuteri strains showed the ability to produce lactic acid, acetic acid, and ethanol with concominant consumption of glucose and together with phenyllactic acid—a potent antifungal compound—with concominant consumption of phenylalanine. The reuterin production from glycerol was confirmed for all analyzed lactobacilli strains except L. reuteri CCM 3625. Production of organic acids, ethanol, and reuterin is significantly involved in antimicrobial activity of lactobacilli which was determined using the dual-culture overlay diffusion method against six indicator bacteria and five indicator moulds. In comparison to the referential L. reuteri ATCC 55730, the highest inhibition potential was observed against Escherichia coli CCM 3988 and Pseudomonas aeruginosa CCM 3955. Among analyzed indicators of moulds, the growth of Alternaria alternata CCM F-128 was the most inhibited by all four analyzed L. reuteri strains. Finally, the immunomodulatory potential of analyzed lactobacilli were proven by the determination of the in vitro production of biogenic amines histamine and tyramine. L. reuteri CCM 3625 was able to produce tyramine, and L. reuteri E and L. reuteri KO5 were able to produce histamine under given cultivation conditions.     

Distinct Histone Modifications Modulate <Emphasis Type="Italic">DEFB1</Emphasis> Expression in Human Vaginal Keratinocytes in Response to <Emphasis Type="Italic">Lactobacillus</Emphasis> spp.

Vaginal commensal lactobacilli are considered to contribute significantly to the control of vaginal microbiota by competing with other microflora for adherence to the vaginal epithelium and by producing antimicrobial compounds. However, the molecular mechanisms of symbiotic prokaryotic-eukaryotic communication in the vaginal ecosystem remain poorly understood. Here, we showed that both DNA methylation and histone modifications were associated with expression of the DEFB1 gene, which encodes the antimicrobial peptide human β-defensin-1, in vaginal keratinocyte VK2/E6E7 cells. We investigated whether exposure to Lactobacillus gasseri and Lactobacillus reuteri would trigger the epigenetic modulation of DEFB1 expression in VK2/E6E7 cells in a bacterial species-dependent manner. While enhanced expression of DEFB1 was observed when VK2/E6E7 cells were exposed to L. gasseri, treatment with L. reuteri resulted in reduced DEFB1 expression. Moreover, L. gasseri stimulated the recruitment of active histone marks and, in contrast, L. reuteri led to the decrease of active histone marks at the DEFB1 promoter. It was remarkable that distinct histone modifications within the same promoter region of DEFB1 were mediated by L. gasseri and L. reuteri. Therefore, our study suggested that one of the underlying mechanisms of DEFB1 expression in the vaginal ecosystem might be associated with the epigenetic crosstalk between individual Lactobacillus spp. and vaginal keratinocytes.     

<Emphasis Type="Italic">Lactobacillus futsaii</Emphasis> CS3, a New GABA-Producing Strain Isolated from Thai Fermented Shrimp (<Emphasis Type="Italic">Kung</Emphasis>-<Emphasis Type="Italic">Som</Emphasis>)

Kung-Som is a popular traditional Thai fermented shrimp product. It is rich in glutamic acid, which is the major substrate for the biosynthesis of gamma-aminobutyric acid (GABA) by lactic acid bacteria (LAB). In the present study, LAB from Kung-Som were isolated, screened for GABA formation, and the two isolates that transform glutamic acid most efficiently into GABA were identified. Based on the API-CHL50 fermentation profile and a phylogenetic tree of 16S rDNA sequences, strain CS3 and CS5 were identified as Lactobacillus futsaii, which was for the first time shown to be a promising GABA producer. L. futsaii CS3 was the most efficient microorganism for the conversion of 25 mg/mL monosodium glutamate (MSG) to GABA, with a maximum yield of more than 99% conversion rate within 72 h. The open reading frame (ORF) of the glutamate decarboxylase (gad) gene was identified by PCR. It consists of 1410 bp encoding a polypeptide of 469 amino acids with a predicted molecular weight of 53.64 kDa and an isoelectric point (pI) of 5.56. Moreover, a good quality of the constructed model of L. futsaii CS3 was also estimated. Our results indicate that L. futsaii CS3 could be of interest for the production of GABA-enriched foods by fermentation and for other value-added products.     

Changes in the diversity and composition of gut microbiota of weaned piglets after oral administration of <Emphasis Type="Italic">Lactobacillus</Emphasis> or an antibiotic

The gut microbiota plays important roles in the health and well-being of animals, and high-throughput sequencing facilitates exploration of microbial populations in the animal gut. However, previous studies have focused on fecal samples instead of the gastrointestinal tract. In this study, we compared the microbiota diversity and composition of intestinal contents of weaned piglets treated with Lactobacillus reuteri or chlortetracycline (aureomycin) using high-throughput sequencing. Nine weaned piglets were randomly divided into three groups and supplemented with L. reuteri, chlortetracycline, or saline for 10 days, and then the contents of three intestinal segments (jejunum, colon, and cecum) were obtained and used for sequencing of the V3–V4 hypervariable region of the 16S rRNA gene. The microbiota diversity and composition in the jejunum were different from those in the colon and cecum among the three treatments. In the jejunum, treatment with L. reuteri increased the species richness of the microbiota, as indicated by the ACE and Chao1 indexes, compared with the chlortetracycline group, in which several taxa were eliminated. In the colon and cecum, relative abundances of the phylum Firmicutes and the genus Prevotella were higher in the chlortetracycline group than in the other groups. Distances between clustered samples revealed that the L. reuteri group was closer to the chlortetracycline group than the control group for jejunum samples, while colon and cecum samples of the L. reuteri group were clustered with those of the control group. This study provides fundamental knowledge for future studies such as the development of alternatives to antibiotics.     

Two-step production of gamma-aminobutyric acid from cassava powder using <Emphasis Type="Italic">Corynebacterium glutamicum</Emphasis> and <Emphasis Type="Italic">Lactobacillus plantarum</Emphasis>

Production of gamma-aminobutyric acid (GABA) from crop biomass such as cassava in high concentration is desirable, but difficult to achieve. A safe biotechnological route was investigated to produce GABA from cassava powder by C. glutamicum G01 and L. plantarum GB01-21. Liquefied cassava powder was first transformed to glutamic acid by simultaneous saccharification and fermentation with C. glutamicum G01, followed by biotransformation of glutamic acid to GABA with resting cells of L. plantarum GB01-21 in the reaction medium. After optimizing the reaction conditions, the maximum concentration of GABA reached 80.5 g/L with a GABA productivity of 2.68 g/L/h. This is the highest yield ever reported of GABA production from cassava-derived glucose. The bioprocess provides the added advantage of employing nonpathogenic microorganisms, C. glutamicum and L. plantarum, in microbial production of GABA from cassava biomass, which can be used in the food and pharmaceutical industries.     

Identification and analysis of the function of surface layer proteins from three <Emphasis Type="Italic">Lactobacillus</Emphasis> strains

To identify and investigate the role of surface layer proteins (SLPs) on the probiotic properties of Lactobacillus strains, SLPs were extracted from Lactobacillus bulgaricus fb04, L. rhamnosus fb06, L. gasseri fb07, and L. acidophilus NCFM by 5 mol/L lithium chloride. The molecular masses of the four SLPs were approximately 45–47 kDa as analyzed by SDS-PAGE. Hydrophobic amino acids were the main components of the four SLPs. The secondary structure content of the four SLPs showed extensive variability among different strains. After the SLPs were removed from the cell surface, the autoaggregation ability, coaggregation ability, and gastrointestinal tolerability of the four lactobacilli were significantly reduced as compared with the intact cells (P?<?0.05). When exposed to bile salt stress, L. rhamnosus fb06, L. gasseri fb07, and L. acidophilus NCFM expressed more SLPs as determined by Bradford method. In conclusion, the four lactobacilli all possessed functional SLPs, which had positive contributions to the probiotic properties of the four Lactobacillus strains. This research could reveal the biological contributions of SLPs from Lactobacillus strains and offer a theoretical basis for the application of lactobacilli and their SLPs in food and pharmaceutical industries.     

Autolyse the cell in order to save it? Inducing,then blocking,autolysis as a strategy for delaying cell death in the probiotic <Emphasis Type="Italic">Lactobacillus reuteri</Emphasis>

Objective

To examine whether choline and its derivatives can be used to preserve viable cells of Lactobacillus reuteri in autolytic models.

Results

A phosphate-induced autolytic model in de Man, Rogosa and Sharpe medium (MRS) was used. Viable cell counts were determined by plated on MRS-agar. Choline and hemicholinium-3 (HC-3) significantly blocked autolysis of L. reuteri at 360 mM and 4 mM, respectively. Viable cell counts corroborated these observations. Importantly, autolytically induced cells treated with choline and hemicholinium-3 were significantly more viable then even non-induced cells. Over-production of a known autolytic protein, spirosin, was not attenuated in the presence of choline and hemicholinium-3.

Conclusion

Inducing autolysis and then blocking it with choline and its analogs is a promising approach for retaining the viability of L. reuteri cells.

     

Genome sequences and comparative genomics of two <Emphasis Type="Italic">Lactobacillus ruminis</Emphasis> strains from the bovine and human intestinal tracts

Background

The genus Lactobacillus is characterized by an extraordinary degree of phenotypic and genotypic diversity, which recent genomic analyses have further highlighted. However, the choice of species for sequencing has been non-random and unequal in distribution, with only a single representative genome from the L. salivarius clade available to date. Furthermore, there is no data to facilitate a functional genomic analysis of motility in the lactobacilli, a trait that is restricted to the L. salivarius clade.

Results

The 2.06 Mb genome of the bovine isolate Lactobacillus ruminis ATCC 27782 comprises a single circular chromosome, and has a G+C content of 44.4%. In silico analysis identified 1901 coding sequences, including genes for a pediocin-like bacteriocin, a single large exopolysaccharide-related cluster, two sortase enzymes, two CRISPR loci and numerous IS elements and pseudogenes. A cluster of genes related to a putative pilin was identified, and shown to be transcribed in vitro. A high quality draft assembly of the genome of a second L. ruminis strain, ATCC 25644 isolated from humans, suggested a slightly larger genome of 2.138 Mb, that exhibited a high degree of synteny with the ATCC 27782 genome. In contrast, comparative analysis of L. ruminis and L. salivarius identified a lack of long-range synteny between these closely related species. Comparison of the L. salivarius clade core proteins with those of nine other Lactobacillus species distributed across 4 major phylogenetic groups identified the set of shared proteins, and proteins unique to each group.

Conclusions

The genome of L. ruminis provides a comparative tool for directing functional analyses of other members of the L. salivarius clade, and it increases understanding of the divergence of this distinct Lactobacillus lineage from other commensal lactobacilli. The genome sequence provides a definitive resource to facilitate investigation of the genetics, biochemistry and host interactions of these motile intestinal lactobacilli.

     

<Emphasis Type="Italic">Lactobacillus casei</Emphasis> as a biocatalyst for biofuel production

Microbial fermentation of sugars from plant biomass to alcohols represents an alternative to petroleum-based fuels. The optimal biocatalyst for such fermentations needs to overcome hurdles such as high concentrations of alcohols and toxic compounds. Lactic acid bacteria, especially lactobacilli, have high innate alcohol tolerance and are remarkably adaptive to harsh environments. This study assessed the potential of five Lactobacillus casei strains as biocatalysts for alcohol production. L. casei 12A was selected based upon its innate alcohol tolerance, high transformation efficiency and ability to utilize plant-derived carbohydrates. A 12A derivative engineered to produce ethanol (L. casei E1) was compared to two other bacterial biocatalysts. Maximal growth rate, maximal optical density and ethanol production were determined under conditions similar to those present during alcohol production from lignocellulosic feedstocks. L. casei E1 exhibited higher innate alcohol tolerance, better growth in the presence of corn stover hydrolysate stressors, and resulted in higher ethanol yields.     

Aciduric Strains of <Emphasis Type="Italic">Lactobacillus reuteri</Emphasis> and <Emphasis Type="Italic">Lactobacillus rhamnosus</Emphasis>, Isolated from Human Feces,Have Strong Adhesion and Aggregation Properties

Human feces were streaked onto MRS Agar adjusted to pH 2.5, 3.0, and 6.4, respectively, and medium supplemented with 1.0% (w/v) bile salts. Two aciduric strains, identified as Lactobacillus reuteri HFI-LD5 and Lactobacillus rhamnosus HFI-K2 (based on 16S rDNA and recA sequences), were non-hemolytic and did not hydrolyze mucin. The surface of Lactobacillus reuteri HFI-LD5 cells has a weak negative charge, whereas Lactobacillus rhamnosus HFI-K2 has acidic and basic properties, and produces exopolysaccharides (EPS). None of the strains produce bacteriocins. Both strains are resistant to several antibiotics, including sulfamethoxazole-trimethoprim and sulphonamides. The ability of Lactobacillus reuteri HFI-LD5 and Lactobacillus rhamnosus HFI-K2 to grow at pH 2.5 suggests that they will survive passage through the stomach. EPS production may assist in binding to intestinal mucus, especially in the small intestinal tract, protect epithelial cells, and stimulate the immune system. Lactobacillus reuteri HFI-LD5 and Lactobacillus rhamnosus HFI-K2 may be used as probiotics, especially in the treatment of small intestinal bacterial overgrowth (SIBO).     

Safety,probiotic and technological properties of <Emphasis Type="Italic">Lactobacilli</Emphasis> isolated from unpasteurised ovine and caprine cheeses

Eleven Lactobacillus plantarum from Slovak ovine and caprine lump and stored cheeses, and from four commercial probiotic and yogurt cultures (Lactobacillus plantarum, Lactobacillus reuteri, Lactobacillus acidophilus) identified using a Maldi-TOF MS analysis were screened in vitro for selected aspects correlated with safety (antibiotic susceptibility patterns, biochemical and haemolytic activity, presence of genes responsible for biogenic amines production), functional traits (including acid, bile tolerance and antimicrobial activity), ecological roles (ability to produce biofilms), and technological applications (acidification and milk coagulation capacity) for assurance of their quality and diversity. The antibiotic susceptibility showed two L. plantarum strains, 19l5 and 18l4, with the presence of the non-wild-type ECOFFs (epidemiological cut-off) for clindamycin and/or gentamicin. All these strains expressed a high acid tolerance at pH 2.5 after a 4 h exposure (bacteria viability varied between 60% and 91%), and bile resistance at 0.3% oxgall ranged from 60% to 99% with no haemolytic activity. Three wild L. plantarum strains, 17l1, 16l4, 18l2, had no harmful metabolic activities, and formed strong biofilms that were measured by a crystal violet assay. Simultaneously, the acid cell-free culture supernatant (ACFCS) from L. plantarum 18l2 had a marked inhibitory effect on the viability of the pathogens as evaluated by flow-cytometry, and also exhibited fast acidification and milk coagulation. As a result, we conclude that L. plantarum 18l2 can be included as part of the created lactobacilli collection that is useful as a starter, or starter adjunct, in the dairy industry, due to its desirable safety and probiotic characteristics, together with rapid acidification capacity compared with other investigated strains from commercially accessible products.     

Antagonistic Activity of Lactic Acid Bacteria <Emphasis Type="Italic">Lactobacillus</Emphasis> spp. against Clinical Isolates of <Emphasis Type="Italic">Klebsiella pneumoniae</Emphasis>

The screening of three strains of lactic acid bacteria identified as Lactobacillus rhamnosus, Lactobacillus reuteri, and Lactobacillus helveticus showed significant antagonistic activity against Klebsiella pneumoniae strains characterized by multiple antibiotic resistance. Lactobacilli cocultivated with the Klebsiella strains inhibited their growth 20 to 86% on the first and second days, respectively. Exoproteome analysis of L. rhamnosus cocultivated with K. pneumoniae revealed the induction of peptidoglycan hydrolases, including extracellular lytic transglycosylases, family II (MltA), and endopeptidases capable of disrupting the peptidoglycan bacterial cell wall.     

Accumulation of gamma-aminobutyric acid in the halotolerant cyanobacterium <Emphasis Type="Italic">Aphanothece halophytica</Emphasis> under salt and acid stress

γ-Aminobutyric acid (GABA) is known as an inhibitory neurotransmitter in human, while in plants, GABA is an intermediate for amino acid metabolism and also is accumulated in response to a wide range of environmental stress. In the present study, GABA accumulation in Aphanothece halophytica was increased 2-fold in mid-log phase cells grown under salt stress (2.0 M NaCl). When mid-log phase cells were subjected to changes in NaCl concentrations and pH for 4 h, the highest GABA accumulation was observed in cells adapted in medium that contained 2.0 M NaCl and that was adjusted to pH 4.0, respectively. The increase of GABA accumulation was accompanied by an increased glutamate decarboxylase activity. Addition of glutamate to growth medium stimulated GABA accumulation under acid stress but had no effect under salt stress. However, the highest GABA accumulation was detected in cells exposed to both high salt and acid stresses combined with the 5 mM glutamate supplementation with an approximately 3-fold increase as compared to the control. The unicellular A. halophytica showed a similarly high content of GABA to that of a filamentous Arthrospira platensis suggesting the possibility of genetic manipulation of the genes of A. halophytica involved in GABA synthesis to increase GABA yield.     

A physiological comparative study of acid tolerance of <Emphasis Type="Italic">Lactobacillus plantarum</Emphasis> ZDY 2013 and <Emphasis Type="Italic">L. plantarum</Emphasis> ATCC 8014 at membrane and cytoplasm levels

This study aimed to disclose the acid tolerance mechanism of Lactobacillus plantarum by comparing L. plantarum ZDY 2013 with the type strain L. plantarum ATCC 8014 in terms of cell membrane, energy metabolism, and amino acid metabolism. L. plantarum ZDY 2013 had a superior growth performance under acidic condition with 100-fold higher survival rate than that of L. plantarum ATCC 8014 at pH 2.5. To determine the acid tolerance physiological mechanism, cell integrity was investigated through scanning electron microscopy. The study revealed that L. plantarum ZDY 2013 maintained cell morphology and integrity, which is much better than L. plantarum ATCC 8014 under acid stress. Analysis of energy metabolism showed that, at pH 5.0, L. plantarum ZDY 2013 enhanced the activity of Na⁺/K⁺-ATPase and decreased the ratio of NAD⁺/NADH in comparison with L. plantarum ATCC 8014. Similarly, amino acid metabolism of intracellular arginine, glutamate, and alanine was improved in L. plantarum ZDY 2013. Correspondingly, the activity of arginine deiminase and glutamate decarboxylase of L. plantarum ZDY 2013 increased by 1.2-fold and 1.3-fold compared with L. plantarum ATCC 8014 in acid stress. In summary, it is demonstrated that the special physiological behaviors (integrity of cell membrane, enhanced energy metabolism, increased amino acid and enzyme level) of L. plantarum ZDY 2013 can protect the cells from acid stress.     

Precultivation of <Emphasis Type="Italic">Bacillus coagulans</Emphasis> DSM2314 in the presence of furfural decreases inhibitory effects of lignocellulosic by-products during <Emphasis Type="SmallCaps">l</Emphasis>(+)-lactic acid fermentation

By-products resulting from thermo-chemical pretreatment of lignocellulose can inhibit fermentation of lignocellulosic sugars to lactic acid. Furfural is such a by-product, which is formed during acid pretreatment of lignocellulose. pH-controlled fermentations with 1 L starting volume, containing YP medium and a mixture of lignocellulosic by-products, were inoculated with precultures of Bacillus coagulans DSM2314 to which 1 g/L furfural was added. The addition of furfural to precultures resulted in an increase in l(+)-lactic acid productivity by a factor 2 to 1.39 g/L/h, an increase in lactic acid production from 54 to 71 g and an increase in conversion yields of sugar to lactic acid from 68 to 88 % W/W in subsequent fermentations. The improved performance was not caused by furfural consumption or conversion, indicating that the cells acquired a higher tolerance towards this by-product. The improvement coincided with a significant elongation of B. coagulans cells. Via RNA-Seq analysis, an upregulation of pathways involved in the synthesis of cell wall components such as bacillosamine, peptidoglycan and spermidine was observed in elongated cells. Furthermore, the gene SigB and genes promoted by SigB, such as NhaX and YsnF, were upregulated in the presence of furfural. These genes are involved in stress responses in bacilli.     

Characterization of a <Emphasis Type="Italic">Lactobacillus brevis</Emphasis> strain with potential oral probiotic properties

Background

The microflora composition of the oral cavity affects oral health. Some strains of commensal bacteria confer probiotic benefits to the host. Lactobacillus is one of the main probiotic genera that has been used to treat oral infections. The objective of this study was to select lactobacilli with a spectrum of probiotic properties and investigate their potential roles in oral health.

Results

An oral isolate characterized as Lactobacillus brevis BBE-Y52 exhibited antimicrobial activities against Streptococcus mutans, a bacterial species that causes dental caries and tooth decay, and secreted antimicrobial compounds such as hydrogen peroxide and lactic acid. Compared to other bacteria, L. brevis BBE-Y52 was a weak acid producer. Further studies showed that this strain had the capacity to adhere to oral epithelial cells. Co-incubation of L. brevis BBE-Y52 with S. mutans ATCC 25175 increased the IL-10-to-IL-12p70 ratio in peripheral blood mononuclear cells, which indicated that L. brevis BBE-Y52 could alleviate inflammation and might confer benefits to host health by modulating the immune system.

Conclusions

L. brevis BBE-Y52 exhibited a spectrum of probiotic properties, which may facilitate its applications in oral care products.

     

Assessment of the antifungal activity of <Emphasis Type="Italic">Lactobacillus</Emphasis> and <Emphasis Type="Italic">Pediococcus</Emphasis> spp. for use as bioprotective cultures in dairy products

Fungi are commonly involved in dairy product spoilage and the use of bioprotective cultures can be a complementary approach to reduce food waste and economic losses. In this study, the antifungal activity of 89 Lactobacillus and 23 Pediococcus spp. isolates against three spoilage species, e.g., Yarrowia lipolytica, Rhodotorula mucilaginosa and Penicillium brevicompactum, was first evaluated in milk agar. None of the tested pediococci showed antifungal activity while 3, 23 and 43 lactobacilli isolates showed strong antifungal activity or total inhibition against Y. lipolytica, R. mucilaginosa and P. brevicompactum, respectively. Then, the three most promising strains, Lactobacillus paracasei SYR90, Lactobacillus plantarum O_VI9 and Lactobacillus rhamnosus BIO_III28 at initial concentrations of 10⁵ and 10⁷ CFU/ml were tested as bioprotective cultures against the same fungal targets in a yogurt model during a 5-week storage period at 10 °C. While limited effects were observed at 10⁵ CFU/ml inoculum level, L. paracasei SYR90 and L. rhamnosus BIO_III28 at 10⁷ CFU/ml respectively retarded the growth of R. mucilaginosa and P. brevicompactum as compared to a control without selected cultures. In contrast, growth of Y. lipolytica was only slightly affected. In conclusion, these selected strains may be good candidates for bioprotection of fermented dairy products.     

Effect of <Emphasis Type="Italic">Lactobacillus reuteri</Emphasis> on Cell Viability and PGE<Subscript>2</Subscript> Production in Human Gingival Fibroblasts

Emerging evidence suggests that probiotic therapy can play a role in the prevention and management of oral inflammatory diseases through immunomodulation and down-regulation of the inflammatory cascade. The aim of this in vitro study was to investigate the viability of human gingival fibroblasts (HGF) and its production of prostaglandin E₂ (PGE₂), when exposed to supernatants of two mixed Lactobacillus reuteri strains (ATCC PTA 5289 and DSM 17938). The experiments were conducted in the presence and absence of the pro-inflammatory cytokine IL-1β. L. reuteri strains were grown and the bacterial supernatant was collected. The cell-free supernatant was diluted to concentrations equivalent to the ones produced by 0.5 to 5.0 × 10⁷ CFU/mL bacteria. Cell viability was assessed with the MTT colorimetric assay and the amount of PGE₂ in the cell culture medium was determined using the monoclonal enzyme immune assay kits. Our findings showed that none of the L. reuteri supernatants were cytotoxic or affected the viability of HGF. The most concentrated bacterial supernatant stimulated the production of PGE₂ by the gingival cells in a significant way in the presence of IL-1β (p < 0.05), suggesting that bacterial products secreted from L. reuteri might play a role in the resolution of inflammation in HGF. Thus, our findings justify further investigations on the influence of probiotic bacteria on gingival inflammatory reactions.