A novel antifungal nanoemulsion based on reuterin-assisted synergistic essential oils: Preparation and in vitro/in vivo characterization

This research aimed to develop, optimize, and evaluate a new antifungal nanoemulsion system based on the crude reuterin-synergistic essential oils (EOs) hybrid to overcome the EOs application limits. At first, the antifungal effects of the Lactobacillus plantarum and Lactobacillus reuteri cell-free extracts (CFE) were tested against the Botrytis cinerea, Penicillium expansum, and Alternaria alternata as indicator fungus using broth microdilution method. The L. reuteri CFE with the MIC of 125 μL/mL for B. cinerea and 250 μL/mL for P. expansum and A. alternata showed more inhibitory effects than L. plantarum. Next, reuterin as a significant antibacterial compound in the L. reuteri CFE was induced in glycerol-containing culture media. To reach a nanoemulsion with maximum antifungal activity and stability, the reuterin concentration, Tween 80 %, and ultrasound time were optimized using response surface methodology (RSM) with a volumetric constant ratio of 5 % v/v oil phase including triple synergistic EOs (thyme, cinnamon, and rosemary) at MIC concentrations. Based on the Box-Behnken Design, the maximum antifungal effect was observed in the treatment with 40 mM reuterin, 1 % Tween 80, and 3 min of ultrasound. The growth inhibitory diameter zones of B. cinerea, P. expansum, and A. alternata were estimated 6.15, 4.25, and 4.35 cm in optimum nanoemulsion, respectively. Also, the minimum average particle size diameter (16.3 nm) was observed in nanoemulsion with reuterin 40 mM, Tween 80 5 %, and 3 min of ultrasound treatment. Zeta potential was relatively high within -30 mV range in all designed nanoemulsions which indicates the nanoemulsion's stability. Also, the prepared nanoemulsions, despite initial particle size showed good stability in a 90-d storage period at 25 °C. In vivo assay, showed a significant improvement in the protection of apple fruit treated with reuterin-EOs nanoemulsions against fungal spoilage compared to free reuterin nanoemulsion. Treatment of apples with nanoemulsion containing 40 mM reuterin showed a maximum inhibitory effect on B. cinerea (5.1 mm lesion diameter compared to 29.2 mm for control fruit) within 7 d at 25 °C. In summary, the present study demonstrated that reuterin-synergistic EOs hybrid with boosted antifungal activities can be considered as a biopreservative for food applications.

Lactobacillus stress protein GroEL prevents colonic inflammation

BACKGROUND

We previously showed that supernatants of Lactobacillus biofilms induced an anti-inflammatory response by affecting the secretion of macrophage-derived cytokines, which was abrogated upon immunodepletion of the stress protein GroEL.

METHODS

We purified GroEL from L. reuteri and analysed its anti-inflammatory properties in vitro in human macrophages isolated from buffy coats, ex vivo in explants from human biopsies and in vivo in a mouse model of DSS induced intestinal inflammation. As a control, we used GroEL purified (LPS-free) from E. coli.

RESULTS

We found that L. reuteri GroEL (but not E. coli GroEL) inhibited pro-inflammatory M1-like macrophages markers, and favored M2-like markers. Consequently, L. reuteri GroEL inhibited pro-inflammatory cytokines (TNFα, IL-1β, IFNγ) while favouring an anti-inflammatory secretome. In colon tissues from human biopsies, L. reuteri GroEL was also able to decrease markers of inflammation and apoptosis (caspase 3) induced by LPS. In mice, we found that rectal administration of L. reuteri GroEL (but not E. coli GroEL) inhibited all signs of haemorrhagic colitis induced by DSS including intestinal mucosa degradation, rectal bleeding and weight loss. It also decreased intestinal production of inflammatory cytokines (such as IFNγ) while increasing anti-inflammatory IL-10 and IL-13. These effects were suppressed when animals were immunodepleted in macrophages. From a mechanistic point of view, the effect of L. reuteri GroEL seemed to involve TLR4, since it was lost in TRL4-/- mice, and the activation of a non-canonical TLR4 pathway.

CONCLUSIONS

L. reuteri GroEL, by affecting macrophage inflammatory features, deserves to be explored as an alternative to probiotics.

Resveratrol-mediated attenuation of superantigen-driven acute respiratory distress syndrome is mediated by microbiota in the lungs and gut

Acute Respiratory Distress Syndrome (ARDS) is triggered by a variety of agents, including Staphylococcal Enterotoxin B (SEB). Interestingly, a significant proportion of patients with COVID-19, also develop ARDS. In the absence of effective treatments, ARDS results in almost 40% mortality. Previous studies from our laboratory demonstrated that resveratrol (RES), a stilbenoid, with potent anti-inflammatory properties can attenuate SEB-induced ARDS. In the current study, we investigated the role of RES-induced alterations in the gut and lung microbiota in the regulation of ARDS. Our studies revealed that SEB administration induced inflammatory cytokines, ARDS, and 100% mortality in C3H/HeJ mice. Additionally, SEB caused a significant increase in pathogenic Proteobacteria phylum and Propionibacterium acnes species in the lungs. In contrast, RES treatment attenuated SEB-mediated ARDS and mortality in mice, and significantly increased probiotic Actinobacteria phylum, Tenericutes phylum, and Lactobacillus reuteri species in both the colon and lungs. Colonic Microbiota Transplantation (CMT) from SEB-injected mice that were treated with RES as well as the transfer of L. reuteri into recipient mice inhibited the production of SEB-mediated induction of pro-inflammatory cytokines such as IFN-γ and IL-17 but increased that of anti-inflammatory IL-10. Additionally, such CMT and L. reuteri recipient mice exposed to SEB, showed a decrease in lung-infiltrating mononuclear cells, cytotoxic CD8+ T cells, NKT cells, Th1 cells, and Th17 cells, but an increase in the population of regulatory T cells (Tregs) and Th3 cells, and increase in the survival of mice from SEB-mediated ARDS. Together, the current study demonstrates that ARDS induced by SEB triggers dysbiosis in the lungs and gut and that attenuation of ARDS by RES may be mediated, at least in part, by alterations in microbiota in the lungs and the gut, especially through the induction of beneficial bacteria such as L. reuteri.

Glyceraldehyde-3-Phosphate Dehydrogenase Increases the Adhesion of Lactobacillus reuteri to Host Mucin to Enhance Probiotic Effects

The ability to adhere to the intestinal mucus layer is an important property of probiotic bacteria. Lactobacillus reuteri strains ZJ615 and ZJ617 show low and high adhesion, respectively, to intestinal epithelial cells. In this study, we quantified bacterial cell wall-associated glyceraldehyde-3-phosphate dehydrogenases (cw-GAPDH) and bacterial cell membrane permeability in both strains using immunoblotting and flow cytometry, respectively. Highly adhesive L. reuteri ZJ617 possessed significantly more cw-GAPDH, higher cell membrane permeability, and significantly higher adhesive ability toward mucin compared with low-adhesive L. reuteri ZJ615. In vitro adhesion studies and analysis of interaction kinetics using the Octet, the system revealed significantly decreased interaction between L. reuteri and mucin when mucin was oxidized when bacterial surface proteins were removed when bacteria were heat-inactivated at 80 °C for 30 min, and when the interaction was blocked with an anti-GAPDH antibody. SWISS-MODEL analysis suggested intensive interactions between mucin glycans (GalNAcα1-O-Ser, GalNAcαSer, and Galβ3GalNAc) and GAPDH. Furthermore, in vivo studies revealed significantly higher numbers of bacteria adhering to the jejunum, ileum, and colon of piglets orally inoculated with L. reuteri ZJ617 compared with those inoculated with L. reuteri ZJ615; this led to a significantly decreased rate of diarrhea in piglets inoculated with L. reuteri ZJ617. In conclusion, there are strong correlations among the abundance of cw-GAPDH in L. reuteri, the ability of the bacterium to adhere to the host, and the health benefits of this probiotic.

Mucosal-associated invariant T-Cell (MAIT) activation is altered by chlorpyrifos- and glyphosate-treated commensal gut bacteria

Mucosal-associated invariant T-cells (MAIT) can react to metabolites of the vitamins riboflavin and folate which are produced by the human gut microbiota. Since several studies showed that the pesticide chlorpyrifos (CPF) and glyphosate (GLP) can impair the gut microbiota, the present study was undertaken to investigate the impact of CPF and GLP treatment on the metabolism of gut microbiota and the resulting bacteria-mediated modulation of MAIT cell activity. Here, Bifidobacterium adolescentis (B. adolescentis), Lactobacillus reuteri (L. reuteri), and Escherichia coli (E. coli) were treated with CPF (50-200 µM) or GLP (75-300 mg/L) and then used in MAIT cell stimulation assays as well as in vitamin and proteome analyses. All three bacteria were nonpathogenic and chosen as representatives of a healthy human gut microflora. The results showed that E. coli activated MAIT cells whereas B. adolescentis and L. reuteri inhibited MAIT cell activation. CPF treatment significantly increased E. coli-mediated MAIT cell activation. Treatment of B. adolescentis and L. reuteri with CPF and GLP weakened the inhibition of MAIT cell activation. Riboflavin and folate production by the test bacteria was influenced by CPF treatment, whereas GLP had only minor effects. Proteomic analysis of CPF-treated E. coli revealed changes in the riboflavin and folate biosynthesis pathways. The findings here suggest that the metabolism of the analyzed bacteria could be altered by exposure to CPF and GLP, leading to an increased pro-inflammatory immune response.

Resveratrol protects Lactobacillus reuteri against H2O2- induced oxidative stress and stimulates antioxidant defenses through upregulation of the dhaT gene

Understanding of the mechanisms implicated in the protective role of probiotic bacteria is of the utmost scientific interest. This study provides original insight into the genetic and molecular basis of the responses of Lactobacillus reuteri PL503 against hydrogen peroxide (H₂O₂)-induced oxidative stress. Six experimental groups were considered depending on the addition and concentration of H₂O₂ and resveratrol: 1. CONTROL (L. reuteri in MRS broth); 2. H₂O₂ (L. reuteri in MRS broth + 0.5 mM H₂O₂); 3. LRES (L. reuteri in MRS broth + 20 μM resveratrol); 4. HRES (L. reuteri in MRS broth + 100 μM resveratrol); 5. H₂O₂-LRES (L. reuteri in MRS broth + 0.5 mM H₂O₂ + 20 μM resveratrol); 6. H₂O₂-HRES (L. reuteri in MRS broth + 0.5 mM H₂O₂ + 100 μM resveratrol). Three replicates were incubated at 37 °C for 24 h in microaerophilic conditions sampled at 12, 16, 20 and 24 h. The NADH-dependent-oxidoreductase encoded by the dhaT gene is a plausible candidate to be strongly implicated in the antioxidant response of L. reuteri. Resveratrol (100 μM) is found to protect L. reuteri against protein carbonylation plausibly through various mechanisms including direct scavenging of reactive oxygen species (ROS), upregulation of the dhaT gene and promoting the synthesis of sulfur containing compounds. The hypothesis formulated on the ability of L. reuteri to detoxify H₂O₂ and its underlying mechanism needs to be clarified. Furthermore, the consequences of protein carbonylation as a reflection of oxidative damage to bacteria and its role in the responses of bacteria to oxidative stress need to be further investigated.

Resilience of small intestinal beneficial bacteria to the toxicity of soybean oil fatty acids

Over the past century, soybean oil (SBO) consumption in the United States increased dramatically. The main SBO fatty acid, linoleic acid (18:2), inhibits in vitro the growth of lactobacilli, beneficial members of the small intestinal microbiota. Human-associated lactobacilli have declined in prevalence in Western microbiomes, but how dietary changes may have impacted their ecology is unclear. Here, we compared the in vitro and in vivo effects of 18:2 on Lactobacillus reuteri and L. johnsonii. Directed evolution in vitro in both species led to strong 18:2 resistance with mutations in genes for lipid biosynthesis, acid stress, and the cell membrane or wall. Small-intestinal Lactobacillus populations in mice were unaffected by chronic and acute 18:2 exposure, yet harbored both 18:2- sensitive and resistant strains. This work shows that extant small intestinal lactobacilli are protected from toxic dietary components via the gut environment as well as their own capacity to evolve resistance.

Probiotic Lactobacillus sp. inhibit growth, biofilm formation and gene expression of caries-inducing Streptococcus mutans

Streptococcus mutans contributes significantly to dental caries, which arises from homoeostasic imbalance between host and microbiota. We hypothesized that Lactobacillus sp. inhibits growth, biofilm formation and gene expression of Streptococcus mutans. Antibacterial (agar diffusion method) and antibiofilm (crystal violet assay) characteristics of probiotic Lactobacillus sp. against Streptococcus mutans (ATCC 25175) were evaluated. We investigated whether Lactobacillus casei (ATCC 393), Lactobacillus reuteri (ATCC 23272), Lactobacillus plantarum (ATCC 14917) or Lactobacillus salivarius (ATCC 11741) inhibit expression of Streptococcus mutans genes involved in biofilm formation, quorum sensing or stress survival using quantitative real-time polymerase chain reaction (qPCR). Growth changes (OD600) in the presence of pH-neutralized, catalase-treated or trypsin-treated Lactobacillus sp. supernatants were assessed to identify roles of organic acids, peroxides and bacteriocin. Susceptibility testing indicated antibacterial (pH-dependent) and antibiofilm activities of Lactobacillus sp. against Streptococcus mutans. Scanning electron microscopy revealed reduction in microcolony formation and exopolysaccharide structural changes. Of the oral normal flora, L. salivarius exhibited the highest antibiofilm and peroxide-dependent antimicrobial activities. All biofilm-forming cells treated with Lactobacillus sp. supernatants showed reduced expression of genes involved in exopolysaccharide production, acid tolerance and quorum sensing. Thus, Lactobacillus sp. can inhibit tooth decay by limiting growth and virulence properties of Streptococcus mutans.

A new method to bio-preserve sea bass fillets

In this work a bio-preservation technique was applied to sea bass fillets in order to preserve their quality. The preservation consisted in the application of two kinds of active coatings on the product surface differing in the fermentation time of alginate solution by L. reuteri plus glycerol (24 and 48 h). This technological strategy was chosen because it has been demonstrated that L. reuteri produces the reuterin as an intermediate metabolite during the anaerobic fermentation of glycerol. To assess the antimicrobial effects of sodium alginate with L. reuteri and glycerol, both in vitro and in vivo tests were carried out. The active films, in particular at 48 h fermentation, showed a good antibacterial activity, confirmed also by the major reuterin concentration. To prove the effectiveness of the treatments, microbial and sensory attributes were monitored by in vivo test on fish fillets. Results highlighted that the two active sodium alginate coatings showed a good antibacterial activity. In sea bass fillets stored at 4 °C, proliferation of main spoilage microorganisms was delayed with a consequent preservation of sensory attributes. In particular, it was found that improving the fermentation time (48 h) a better microbiological and sensory quality was achieved.

Catechol glucosides act as donor/acceptor substrates of glucansucrase enzymes of Lactobacillus reuteri

Previously, we have shown that the glucansucrase GtfA-ΔN enzyme of Lactobacillus reuteri 121, incubated with sucrose, efficiently glucosylated catechol and we structurally characterized catechol glucosides with up to five glucosyl units attached (te Poele et al. in Bioconjug Chem 27:937-946, 2016). In the present study, we observed that upon prolonged incubation of GtfA-ΔN with 50 mM catechol and 1000 mM sucrose, all catechol had become completely glucosylated and then started to reappear. Following depletion of sucrose, this glucansucrase GtfA-ΔN used both α-D-Glcp-catechol and α-D-Glcp-(1→4)-α-D-Glcp-catechol as donor substrates and transferred a glucose unit to other catechol glycoside molecules or to sugar oligomers. In the absence of sucrose, GtfA-ΔN used α-D-Glcp-catechol both as donor and acceptor substrate to synthesize catechol glucosides with 2 to 10 glucose units attached and formed gluco-oligosaccharides up to a degree of polymerization of 4. Also two other glucansucrases tested, Gtf180-ΔN from L. reuteri 180 and GtfML1-ΔN from L. reuteri ML1, used α-D-Glcp-catechol and di-glucosyl-catechol as donor/acceptor substrate to synthesize both catechol glucosides and gluco-oligosaccharides. With sucrose as donor substrate, the three glucansucrase enzymes also efficiently glucosylated the phenolic compounds pyrogallol, resorcinol, and ethyl gallate; also these mono-glucosides were used as donor/acceptor substrates.

Characterization of Extracellular Yeast Peptide Factors and Their Stress-Protective Effect on Probiotic Lactic Acid Bacteria

Protective effect of the extracellular peptide fraction (reactivating factors, RF) produced by yeasts of various taxonomic groups (Saccharomyces cerevisiae, Kluyveromyces lactis, Candida utilis, and Yarrowia li- polytica) on probiotic lactic acid bacteria (LAB) Lactobacillus casei, L. acidophilus,'and L. reuteri under bile salt (BS)-induced stress was shown. RF of all yeasts were shown to be of peptide nature; the active component of the S. cerevisiae RF was identified as a combination of low-molecular polypeptides with molecular masses of 0.6 to 1.5 kDa. The protective and reactivating effects of the yeast factors were not species-specific and were similar to those of the Luteococcusjaponicus subsp. casei R. In BS-treated cells of the tester bacteria, a pro- tective effect was observed after 10-min preincubation of the LAB cell suspension with yeast RE: the number of surviving cells (CFU) was 2 to 4.5 times higher than in the control. The reactivating effect was observed when RF was added to LAB cell suspensions not later than 15 min after stress treatment. It was less pro- nounced than the protector effect, with the CFU number I to 3 times that of the control. Both the protector and the reactivating effects were most pronounced in the S. cerevisiae and decreased in the row: C. utilis > K. lactis > Y lipolytica. The efficiency of protective action of yeast RF was found to depend on the properties of recepient LAB cells, with the L. casei strain being most sensitive to BS treatment. In both variants, the highest protective effect of RF (increase in the CFU number) was observed for L. acidophilus, while the least pronounced one, for L. casei. The reasons for application of the LAB strains combining high stress resistance and high response to stress-protecting metabolites, including RF factors, as probiotics, is discussed.

A simulated mucus layer protects Lactobacillus reuteri from the inhibitory effects of linoleic acid

Lactobacillus reuteri is a commensal, beneficial gut microbe that colonises the intestinal mucus layer, where it makes close contact with the human host and may significantly affect human health. Here, we investigated the capacity of linoleic acid (LA), the most common polyunsaturated fatty acid (PUFA) in a Western-style diet, to affect L. reuteri ATCC PTA 6475 prevalence and survival in a simulated mucus layer. Short-term (1 h) survival and mucin-agar adhesion assays of a log-phase L. reuteri suspension in intestinal water demonstrated that the simulated mucus layer protected L. reuteri against the inhibitory effects of LA by lowering its contact with the bacterial cell membrane. The protective effect of the simulated mucus layer was further evaluated using a more complex and dynamic model of the colon microbiota (SHIME®), in which L. reuteri survival was monitored during 6 days of daily exposure to LA in the absence (L-SHIME) and presence (M-SHIME) of a simulated mucus layer. After 6 days, luminal L- and M-SHIME L. reuteri plate counts had decreased by 3.1±0.5 and 2.6±0.9 log cfu/ml, respectively. Upon supplementation of 1.0 g/l LA, the decline in the luminal L. reuteri population started earlier than was observed for the control. In contrast, mucin-agar levels of L. reuteri (in the M-SHIME) remained unaffected throughout the experiment even in the presence of high concentrations of LA. Overall, the results of this study indicate the importance of the mucus layer as a protective environment for beneficial gut microbes to escape from stress by high loads of the antimicrobial PUFA LA to the colon, i.e. due to a Western-style diet.

Glycerol supplementation enhances L. reuteri's protective effect against S. Typhimurium colonization in a 3-D model of colonic epithelium

The probiotic effects of Lactobacillus reuteri have been speculated to partly depend on its capacity to produce the antimicrobial substance reuterin during the reduction of glycerol in the gut. In this study, the potential of this process to protect human intestinal epithelial cells against infection with Salmonella enterica serovar Typhimurium was investigated. We used a three-dimensional (3-D) organotypic model of human colonic epithelium that was previously validated and applied to study interactions between S. Typhimurium and the intestinal epithelium that lead to enteric salmonellosis. Using this model system, we show that L. reuteri protects the intestinal cells against the early stages of Salmonella infection and that this effect is significantly increased when L. reuteri is stimulated to produce reuterin from glycerol. More specifically, the reuterin-containing ferment of L. reuteri caused a reduction in Salmonella adherence and invasion (1 log unit), and intracellular survival (2 log units). In contrast, the L. reuteri ferment without reuterin stimulated growth of the intracellular Salmonella population with 1 log unit. The short-term exposure to reuterin or the reuterin-containing ferment had no observed negative impact on intestinal epithelial cell health. However, long-term exposure (24 h) induced a complete loss of cell-cell contact within the epithelial aggregates and compromised cell viability. Collectively, these results shed light on a potential role for reuterin in inhibiting Salmonella-induced intestinal infections and may support the combined application of glycerol and L. reuteri. While future in vitro and in vivo studies of reuterin on intestinal health should fine-tune our understanding of the mechanistic effects, in particular in the presence of a complex gut microbiota, this the first report of a reuterin effect on the enteric infection process in any mammalian cell type.

Inhibition of growth of Trichophyton tonsurans by Lactobacillus reuteri

AIM

The aims of this study were to identify antifungal lactic acid bacteria (LAB) and characterize their activity against the dermatophyte Trichophyton tonsurans.

METHODS AND RESULTS

A total of 165 different LAB were isolated and initially screened for anti-Penicillium expansum activity. Five strains, which exhibited strong inhibitory activity, were then tested against the dermatophyte T. tonsurans DSM12285, where they also caused inhibition as observed by large fungal clearing on agar surface. The strongest inhibition was seen with Lactobacillus reuteri R2. When freeze-dried cell-free supernatant powder from this strain was incorporated in culture medium at concentrations >1%, growth of fungal colony was inhibited. Conidia germination was also inhibited under these conditions as determined by microscopy. The anti-T. tonsurans activity of Lact. reuteri R2 was not affected neither by heat treatment nor by proteolytic treatment using pronase E and proteinase K, indicating that the responsible agent(s) were nonproteinaceous in nature.

CONCLUSIONS

Lactobacillus reuteri R2 was identified as having strong inhibitory activity against the dermatophyte T. tonsurans DSMZ12285.

SIGNIFICANCE AND IMPACT OF THE STUDY

LAB are naturally associated with many foods and are well recognized for their biopreservative properties. The use of these and/or their products may well provide alternative safe approaches for the inhibition of dermatophytic fungi.

Transferability of a tetracycline resistance gene from probiotic Lactobacillus reuteri to bacteria in the gastrointestinal tract of humans

The potential of Lactobacillus reuteri as a donor of antibiotic resistance genes in the human gut was investigated by studying the transferability of the tetracycline resistance gene tet(W) to faecal enterococci, bifidobacteria and lactobacilli. In a double-blind clinical study, seven subjects consumed L. reuteri ATCC 55730 harbouring a plasmid-encoded tet(W) gene (tet(W)-reuteri) and an equal number of subjects consumed L. reuteri DSM 17938 derived from the ATCC 55730 strain by the removal of two plasmids, one of which contained the tet(W) gene. Faecal samples were collected before, during and after ingestion of 5 x 10(8) CFU of L. reuteri per day for 14 days. Both L. reuteri strains were detectable at similar levels in faeces after 14 days of intake but neither was detected after a two-week wash-out period. After enrichment and isolation of tetracycline resistant enterococci, bifidobacteria and lactobacilli from each faecal sample, DNA was extracted and analysed for presence of tet(W)-reuteri using a real-time PCR allelic discrimination method developed in this study. No tet(W)-reuteri signal was produced from any of the DNA samples and thus gene transfer to enterococci, bifidobacteria and lactobacilli during intestinal passage of the probiotic strain was non-detectable under the conditions tested, although transfer at low frequencies or to the remaining faecal bacterial population cannot be excluded.

Proteomic analysis of the effect of bile salts on the intestinal and probiotic bacterium Lactobacillus reuteri

Lactobacillus reuteri is a resident of the human and animal intestinal tracts. The ability of L. reuteri to survive passage through the intestinal tract is a key point in its function as a probiotic. In order to examine the nature of bile salt tolerance by L. reuteri, its protein synthesis was analyzed in liquid cultures containing two different bile salt conditions. Significant cell growth inhibition was observed in the presence of 1.2g/L (higher concentration) bile salts. Two-dimensional gel electrophoresis allowed us to identify 28 proteins spots that were consistently and significantly altered in the presence of bile in the growth medium. Peptide mass fingerprinting was used to identify these 28 proteins, and functional annotation revealed their involvement in carbohydrate metabolism, transcription-translation, nucleotide metabolism, amino acid biosynthesis, pH homeostasis and stress responses, oxidation-reduction reactions, and unknown functions. These findings, which suggest that bile salts induce complex physiological responses in L. reuteri may provide early new insights into the inducible mechanisms underlying the capacity of intestinal L. reuteri to tolerate bile stress.

D-alanyl ester depletion of teichoic acids in Lactobacillus reuteri 100-23 results in impaired colonization of the mouse gastrointestinal tract

The dlt operon of Gram-positive bacteria encodes proteins required for the incorporation of D-alanine esters into cell wall-associated teichoic acids (TA). D-alanylation of TA has been shown to be important for acid tolerance, resistance to antimicrobial peptides, adhesion, biofilm formation, and virulence of a variety of pathogenic organisms. The aim of this study was to determine the importance of D-alanylation for colonization of the gastrointestinal tract by Lactobacillus reuteri 100-23. Insertional inactivation of the dltA gene resulted in complete depletion of D-alanine substitution of lipoteichoic acids. The dlt mutant had similar growth characteristics as the wild type under standard in vitro conditions, but formed lower population sizes in the gastrointestinal tract of ex-Lactobacillus-free mice, and was almost eliminated from the habitat in competition experiments with the parental strain. In contrast to the wild type, the dlt mutant was unable to form a biofilm on the forestomach epithelium during gut colonization. Transmission electron microscope observations showed evidence of cell wall damage of mutant bacteria present in the forestomach. The dlt mutant had impaired growth under acidic culture conditions and increased susceptibility to the cationic peptide nisin relative to the wild type. Ex vivo adherence of the dlt mutant to the forestomach epithelium was not impaired. This study showed that D-alanylation is an important cell function of L. reuteri that seems to protect this commensal organism against the hostile conditions prevailing in the murine forestomach.

Influence of oligosaccharides on the viability and membrane properties of Lactobacillus reuteri TMW1.106 during freeze-drying

Freeze-drying is a process commonly used in starter culture preparation. To improve the survival rate of bacteria during the process, cryoprotectives are usually added before freezing. This study investigated the influence of the addition of sucrose, fructo-oligosaccharides (FOS), inulin and skim milk on the viability and membrane integrity of Lactobacillus reuteri TMW1.106 during freezing, freeze-drying and storage. The effect of drying adjuncts on survival was correlated to their interaction with bacterial membrane by determination of the parameters membrane fluidity and membrane lateral pressure. Sucrose, FOS and skim milk significantly enhanced survival of exponential-phase cells of L. reuteri during freeze-drying. Cellular viability during storage of exponential-phase cells remained highest for cells dried in the presence of skim milk and inulin. Membranes of these cells were completely permeabilized after freeze-drying. The application of FOS significantly improved survival of stationary phase cells of L. reuteri TMW1.106 after freeze-drying and storage. This increased viability of L. reuteri TMW1.106 in the presence of FOS correlated to improved membrane integrity. Fructo-oligosaccharides and fructans, but not gluco-oligosaccharides interacted with membrane vesicles prepared from L. reuteri TMW1.106 as indicated by increased membrane lateral pressure in the presence of FOS and fructans. Increased membrane integrity of stationary phase L. reuteri TMW1.106 was attributed to direct interactions between FOS and the membrane which leads to increased membrane fluidity and thus improved stability of the membrane during and rehydration.

Antibiotic susceptibility profiles of Lactobacillus reuteri and Lactobacillus fermentum

Lactobacillus reuteri and Lactobacillus fermentum, which are commonly used as food processing aids and probiotics, can potentially act as reservoirs of antibiotic resistance genes. Acquired resistance genes may be transferred via the food chain or in the gastrointestinal tract to pathogenic bacteria. Knowledge of the distributions of antibiotic MICs for a species is needed when using a phenotypic method to assess the presence of acquired resistance genes. In the present study, 56 L. reuteri and 56 L. fermentum strains that differed by source and spatial and temporal origin were assessed for antibiotic susceptibility using an Etest kit and a broth microdilution protocol. L. fermentum strains displayed a uniform distribution of MICs for all six antibiotics tested. L. reuteri strains had a bimodal distribution of MICs or a distribution with MICs above the test range for 7 of the 14 antibiotics tested. Genetic relatedness was observed among L. reuteri strains with high MICs for both ampicillin and tetracycline and among strains with high MICs for both erythromycin and clindamycin. Results obtained with the Etest and the broth microdilution method corresponded well with each other. Thus, further research may make it possible to define microbiological breakpoints for distinguishing between strains with and without acquired resistance genes.

Antibacterial activity of the soy isoflavone genistein

Genistein, a radioprotective soy isoflavone and protein kinase inhibitor, blocks the invasion of pathogenic bacteria in mammalian epithelial cells. The purpose of this study was to evaluate the direct effect of genistein on the survival and growth of the probiotic Lactobacillus reuteri and selected opportunistic bacteria in vitro as a prelude to in vivo use for managing postirradiation sepsis. We evaluated the opportunistic bacterial enteropathogens Escherichia coli, Shigella sonnei, and Staphylococcus aureus as well as Klebsiella pneumoniae and the non-pathogenic organism, Bacillus anthracis (Sterne). The latter two bacteria are found in the environment and may be of concern in irradiated individuals. A standard in vitro test was employed to evaluate the direct effect of genistein on the bacteria. This test involved determining bacterial colony forming unit (CFU) counts at a single concentration of genistein. In the CFU assays, significant reductions in CFUs were found for S. aureus and B. anthracis when cultured in the presence of 100 muM genistein. However, L. reuteri, E. coli, S. sonnei, and K. pneumoniae were not altered by in vitro culturing in the presence of 100 muM genistein. These results demonstrate the in vitro antimicrobial activity of genistein. Furthermore, the use of genistein in combination with probiotics may augment the effectiveness of antimicrobial therapies currently used in the management of infections, including those induced by ionizing irradiation.

Effect of membrane lateral pressure on the expression of fructosyltransferases in Lactobacillus reuteri

The effect of environmental conditions on the production of homo-polysaccharides and oligosaccharides from sucrose and the regulation of glycosyltransferase genes responsible for biosynthesis of homo-polysaccharides was determined in Lactobacillus reuteri TMW1.106 (reutericyclin-producer) and LTH5448 (reutericyclin-negative). Strain L. reuteri TMW 1.106 harbours the glycosyltransferase genes gtfA and inu, strain LTH5448 harbours a fructosyltransferase, ftfA. Fructan and fructose-oligosaccharide (FOS) production in both strains was inducible by reutericyclin, trans-isohumulone, and nigericin at the levels of their minimum inhibitory concentrations (MIC) as well as phenylethanol (6mM) and elevated growth temperatures (45 degrees C), but not by nisin, CCCP or gramicidin. Elevated temperature (45 degrees C), reutericyclin or trans-isohumulone but not CCCP furthermore increased enhanced inu and ftfA transcription in L. reuteri TMW1.106 and LTH5448, respectively. Generally, effects of the various agents on fructosyltransferase transcription corresponded to their effect on formation of poly and oligosaccharides from sucrose. The effect of membrane-active agents on fructosyltransferase expression was compared to their effect on membrane biophysical parameters. The ability of chemical and physical agents to induce expression of fructosyltransferases correlated to their effect on the membrane lateral pressure as measured by pyrene-labelled phospholipids in membrane vesicles. Dextran, levan and fructose-oligosaccharides added at 50gL(-1) protected L. reuteri towards the membrane-active inhibitors nisin, reutericyclin, and CCCP. The induction of glycosyltransferases by membrane stress indicates a protective role of fructans and FOS to lactobacilli exposed to physical and chemical environmental stressors.