Citrate utilization by homo- and heterofermentative lactobacilli

Complex carbohydrates catabolism capacity of bladder microbiota inhabiting healthy and overactive female bladders

Overactive bladder syndrome (OAB) is a poorly understood symptom complex that affects 40% of females over the age of 40, with clinical features including urinary urgency and incontinence. In addition to inflammation, oxidative stress, nerve damage and reduced blood flow, alterations in the urinary microbiome (urobiome), specifically in bladder bacterial diversity, have been reported to be associated with OAB. Bladder bacteria are members of the urobiome along with viruses, archaea, fungi, and protozoans. The urobiome metabolism, particularly in relationship to host complex sugars (glycans), has been investigated recently in terms of glycosaminoglycan (GAG) utilization. Nevertheless, other urinary free oligosaccharides (FOS) have not yet been explored in both OAB and urobiome contexts. Similarly, a comprehensive search of microbial genes involved in host glycan metabolism in the bladder of adult females with or without OAB has not yet been reported. In this study, we investigated urinary FOS by mass spectrometry in women without OAB (asymptomatic controls), with OAB without incontinence (dry OAB), or with OAB with incontinence (wet OAB or urgency urinary incontinence, UUI). We also questioned the ability of commensal bladder bacteria to digest these FOS and other glycans, using bioinformatic tools to query publicly available bladder genomes isolated from affected and unaffected adult females to identify genes that encode polysaccharide lyases (PL) and glycoside hydrolases (GH). Our results show that FOS are present in a similar level in affected and unaffected controls with a few exceptions: ten FOS were found to differ between the OAB dry groups and either the control (four) or UUI (six) groups. Our results indicate that bladder microbiota from adult females both with and without OAB have the genetic capacity to digest host glycans and dietary sugars with subtle differences. Bladder bacteria isolated from females with OAB possess more GH/PL genes for host mucins, whereas bladder bacteria from controls possess more GH/PL genes for GAG digestion. In the control group, specifically, the genus Streptococcus possessed genes for the PL8 and GH88 enzymes, known to be involved in host GAG digestion. These novel bioinformatic data can enable future biochemical exploration of the urobiome's metabolism toward specific host glycans, such as GAGs, mucins O-glycans and N-glycans.

Dynamic alterations of flavor, functional nutrients, and microbial community during fermentation of different animal milk kefirs

Kefir is a traditional dairy beverage, usually made from cow or goat milk fermented with kefir grains, and has many health benefits. To elucidate the fermentation patterns of animal milk kefirs during the fermentation process and find the optimal milk types, cow, camel, goat, and donkey milk were fermented with kefir grains for 0, 1, 3, 5, and 7 days. Volatile and non-volatile metabolites and microbial changes were dynamically monitored. The results showed that volatile flavor substances were massively elevated in four kefirs on days 1-3. Lipids and carbohydrates gradually decreased, while amino acids, small peptides, and tryptophan derivatives accumulated during fermentation in four kefirs. Besides, four kefirs had similar alterations in Lactobacillus and Acetobacter, while some distinctions existed in low-abundance bacteria. Association analysis of microorganisms and volatile and non-volatile metabolites also revealed the underlying fermentation mechanism. This study found that appropriately extending the fermentation time contributed to the accumulation of some functional nutrients. Furthermore, goat and donkey milk could be the better matrices for kefir fermentation.

Alfalfa Silage Treated With Sucrose Has an Improved Feed Quality and More Beneficial Bacterial Communities

Alfalfa silage is one of the main roughages in the production of dairy cow, which can provide nutrition with high quality to improve milk quality and production. Sucrose additions have been widely used to improve the silage quality. In this study, the effects of sucrose on the fermentation quality and bacterial communities of alfalfa silage were investigated here using 0, 0.5, and 1% sucrose ensiling treatments for 15, 30, and 60 days. The ensiling time significantly decreased the crude fiber content and increased the ammonia nitrogen, acetic acid content, and the relative abundance of Enterococcus in the silages. The 1% sucrose-treated silage at 60 days had the lowest neutral detergent fiber acid, acid detergent fiber, and crude fiber content and the highest relative feed value. Moreover, sucrose-treated silage contained less acetic acid, propionic acid, and butyric acid, and had a lower pH than the controls for each duration. Enterobacteriaceae, Klebsiella, and Enterococcus were the dominant genera in all groups, and the relative abundance of Enterococcus and Lactobacillus was higher in the 1% sucrose-treated group than in the control. These results suggested that sucrose supplementation could improve alfalfa silage quality and increase its beneficial bacterial content.

Potential of lactic acid bacteria to modulate coffee volatiles and effect of glucose supplementation: fermentation of green coffee beans and impact of coffee roasting

BACKGROUND

Coffee flavor can be significantly influenced by microbial activities in spontaneous fermentation of coffee cherries. The potential of lactic acid bacteria for flavor modulation through controlled fermentation of green coffee beans has not been explored.

RESULTS

Fermentation by Lactobacillus rhamnosus HN001 with and without 1% w/w glucose supplementation led to modification of flavor-related constituents in green coffee beans, which translated into modulation of coffee volatiles upon roasting. The lactic acid bacteria consumed almost all glucose and fructose, leaving sucrose behind. Amino acids and malic, citric, and succinic acids were partially catabolized. Glucose supplementation enhanced lactic acid production but repressed acetic acid formation. After roasting at 235 °C for 9 min, 12 min, and 15 min, the levels of furfurals in glucose-supplemented-fermented coffee were 10.5-, 2.7-, and 1.1-fold higher than those in the controls (nonsupplemented-unfermented coffee); furthermore, the levels of pyrazines in the controls were 11.9-, 10.1-, and 6.5-fold higher than those in the treated coffee. Glucose-supplemented fermentation yielded roasted coffee with stronger caramelic and burnt characteristics but weaker nutty notes. In roasted non-supplemented-fermented coffee, volatile production was generally reduced, resulting in a milder overall aroma.

CONCLUSION

Lactic acid fermentation of green coffee beans is a new strategy for coffee flavor modulation, creating novel aroma characteristics. © 2018 Society of Chemical Industry.

Effects of chemical additives on the fermentation quality and N distribution of alfalfa silage in south of China

In order to better utilize the last cut alfalfa harvested before killing frost in a high moisture environment, the effects of chemical additives on the quality of alfalfa silage were studied in south of China. The alfalfa was freshly harvested at branching stage, and wilted by dry matter content of about 300 g/kg (fresh matter basis). Silage was prepared by using a small-scale silage fermentation system, where sucrose, potassium citrate, sodium carbonate, formic acid, acetic acid and propionic acid were used as silage additives, and no additives served as control. These silos were stored at ambient temperature (5-20°C), and the silage qualities were analyzed after 120 days of fermentation. All additive treatments affected the chemical composition and N distribution, increased the water-soluble content and crude protein contents, decreased non-protein nitrogen (NPN) content, and enhanced the in vitro ruminal dry matter digestibility (except for sodium carbonate). Silages treated with organic acids were preserved with significantly (P < 0.05) lower pH value, ethanol content and NPN content compared with control. When the fermentation quality, chemical composition and N distribution were considered, the treatment with sucrose or organic acids resulted in high quality of alfalfa silage ensiled before killing frost, with formic acid having the best effect.

Contributions of citrate in redox potential maintenance and ATP production: metabolic pathways and their regulation in Lactobacillus panis PM1

Lactobacillus panis PM1 belongs to the group III heterofermentative lactobacilli and can utilize various NADH-reoxidizing routes (e.g., citrate, glycerol, and oxygen) according to environmental conditions. In this study, we investigated the ability of L. panis PM1 to produce succinate, acetate, and lactate via citrate utilization. Possible pathways, as well as regulation, for citrate metabolism were examined on the basis of the genome sequence data and metabolic profiles of L. panis PM1. The presence of citrate led to the up-regulation, at the transcriptional level, of the genes encoding for citrate lyase, malate dehydrogenase, and malic enzyme of the citrate pathways by 10- to 120-fold. The transcriptional regulator of the dha operon coding for glycerol dehydratase of L. panis PM1 repressed the expression of the citrate lyase gene (10-fold). Metabolite analyses indicated that the transcriptional enhancement by citrate stimulated succinate yield. Citrate metabolism contributed to energy production by providing a major alternate pathway for NAD(+) regeneration and allowed acetyl phosphate to yield acetate/ATP instead of ethanol/NAD(+). Additionally, a branching pathway from oxaloacetate to pyruvate increased the pool of lactate, which was then used to produce ATP during stationary phase. However, the redirection of NADH-to-citrate utilization resulted in stress caused by end-products (i.e., succinate and acetate). This stress reduced succinate production by up to 50 % but did not cause significant changes at transcriptional level. Overall, citrate utilization was beneficial for the growth of L. panis PM1 by providing a NAD(+) regeneration route and producing extra ATP.

Core fluxome and metafluxome of lactic acid bacteria under simulated cocoa pulp fermentation conditions

In the present work, simulated cocoa fermentation was investigated at the level of metabolic pathway fluxes (fluxome) of lactic acid bacteria (LAB), which are typically found in the microbial consortium known to convert nutrients from the cocoa pulp into organic acids. A comprehensive (13)C labeling approach allowed to quantify carbon fluxes during simulated cocoa fermentation by (i) parallel (13)C studies with [(13)C6]glucose, [1,2-(13)C2]glucose, and [(13)C6]fructose, respectively, (ii) gas chromatography-mass spectrometry (GC/MS) analysis of secreted acetate and lactate, (iii) stoichiometric profiling, and (iv) isotopomer modeling for flux calculation. The study of several strains of L. fermentum and L. plantarum revealed major differences in their fluxes. The L. fermentum strains channeled only a small amount (4 to 6%) of fructose into central metabolism, i.e., the phosphoketolase pathway, whereas only L. fermentum NCC 575 used fructose to form mannitol. In contrast, L. plantarum strains exhibited a high glycolytic flux. All strains differed in acetate flux, which originated from fractions of citrate (25 to 80%) and corresponding amounts of glucose and fructose. Subsequent, metafluxome studies with consortia of different L. fermentum and L. plantarum strains indicated a dominant (96%) contribution of L. fermentum NCC 575 to the overall flux in the microbial community, a scenario that was not observed for the other strains. This highlights the idea that individual LAB strains vary in their metabolic contribution to the overall fermentation process and opens up new routes toward streamlined starter cultures. L. fermentum NCC 575 might be one candidate due to its superior performance in flux activity.

RNA sequencing to study gene expression and single nucleotide polymorphism variation associated with citrate content in cow milk

The technological properties of milk have significant importance for the dairy industry. Citrate, a normal constituent of milk, forms one of the main buffer systems that regulate the equilibrium between Ca(2+) and H(+) ions. Higher-than-normal citrate content is associated with poor coagulation properties of milk. To identify the genes responsible for the variation of citrate content in milk in dairy cattle, the metabolic steps involved in citrate and fatty acid synthesis pathways in ruminant mammary tissue using RNA sequencing were studied. Genetic markers that could influence milk citrate content in Holstein cows were used in a marker-trait association study to establish the relationship between 74 single nucleotide polymorphisms (SNP) in 20 candidate genes and citrate content in 250 Holstein cows. This analysis revealed 6 SNP in key metabolic pathway genes [isocitrate dehydrogenase 1 (NADP+), soluble (IDH1); pyruvate dehydrogenase (lipoamide) β (PDHB); pyruvate kinase (PKM2); and solute carrier family 25 (mitochondrial carrier; citrate transporter), member 1 (SLC25A1)] significantly associated with increased milk citrate content. The amount of the phenotypic variation explained by the 6 SNP ranged from 10.1 to 13.7%. Also, genotype-combination analysis revealed the highest phenotypic variation was explained combining IDH1_23211, PDHB_5562, and SLC25A1_4446 genotypes. This specific genotype combination explained 21.3% of the phenotypic variation. The largest citrate associated effect was in the 3' untranslated region of the SLC25A1 gene, which is responsible for the transport of citrate across the mitochondrial inner membrane. This study provides an approach using RNA sequencing, metabolic pathway analysis, and association studies to identify genetic variation in functional target genes determining complex trait phenotypes.

Understanding the adaptive growth strategy of Lactobacillus plantarum by in silico optimisation

In the study of metabolic networks, optimization techniques are often used to predict flux distributions, and hence, metabolic phenotype. Flux balance analysis in particular has been successful in predicting metabolic phenotypes. However, an inherent limitation of a stoichiometric approach such as flux balance analysis is that it can predict only flux distributions that result in maximal yields. Hence, previous attempts to use FBA to predict metabolic fluxes in Lactobacillus plantarum failed, as this lactic acid bacterium produces lactate, even under glucose-limited chemostat conditions, where FBA predicted mixed acid fermentation as an alternative pathway leading to a higher yield. In this study we tested, however, whether long-term adaptation on an unusual and poor carbon source (for this bacterium) would select for mutants with optimal biomass yields. We have therefore adapted Lactobacillus plantarum to grow well on glycerol as its main growth substrate. After prolonged serial dilutions, the growth yield and corresponding fluxes were compared to in silico predictions. Surprisingly, the organism still produced mainly lactate, which was corroborated by FBA to indeed be optimal. To understand these results, constraint-based elementary flux mode analysis was developed that predicted 3 out of 2669 possible flux modes to be optimal under the experimental conditions. These optimal pathways corresponded very closely to the experimentally observed fluxes and explained lactate formation as the result of competition for oxygen by the other flux modes. Hence, these results provide thorough understanding of adaptive evolution, allowing in silico predictions of the resulting flux states, provided that the selective growth conditions favor yield optimization as the winning strategy.

Variations in the energy metabolism of biotechnologically relevant heterofermentative lactic acid bacteria during growth on sugars and organic acids

Heterofermentative lactic acid bacteria (LAB) such as Leuconostoc, Oenococcus, and Lactobacillus strains ferment pentoses by the phosphoketolase pathway. The extra NAD(P)H, which is produced during growth on hexoses, is transferred to acetyl-CoA, yielding ethanol. Ethanol fermentation represents the limiting step in hexose fermentation, therefore, part of the extra NAD(P)H is used to produce erythritol and glycerol. Fructose, pyruvate, citrate, and O2 can be used in addition as external electron acceptors for NAD(P)H reoxidation. Use of the external acceptors increases the growth rate of the bacteria. The bacteria are also able to ferment organic acids like malate, pyruvate, and citrate. Malolactic fermentation generates a proton potential by substrate transport. Pyruvate fermentation sustains growth by pyruvate disproportionation involving pyruvate dehydrogenase. Citrate is fermented in the presence of an additional electron donor to acetate and lactate. Thus, heterofermentative LAB are able to use a variety of unusual fermentation reactions in addition to classical heterofermentation. Most of the reactions are significant for food biotechnology/microbiology.

Glycerol metabolism of Lactobacillus rhamnosus ATCC 7469: cloning and expression of two glycerol kinase genes

Lactobacillus rhamnosus ATCC 7469 was able to grow in glycerol as the sole source of energy in aerobic conditions, producing lactate, acetate, and diacetyl. A biphasic growth was observed in the presence of glucose. In this condition, glycerol consumption began after glucose was exhausted from the culture medium. Glycerol kinase activity was detected in L. rhamnosus ATCC 7469, a characteristic of microorganisms which catabolize glycerol in aerobic conditions. Genetic analysis revealed that this strain possesses two glycerol kinase genes: gykA and glpK, that encode for two different glycerol kinases GykA and GlpK, respectively. The glpK geneis associated in an operon with alpha-glycerophosphate oxidase (glpO) and glycerol facilitator (glpF) genes. Transcriptional analysis revealed that only glpK is expressed when L. rhamnosus was grown on glycerol.

Effect of preculturing conditions on growth of Lactobacillus rhamnosus on medium containing glucose and citrate

Lactobacillus rhamnosus can metabolize citrate through a citrate inducible transport system. The growth curves of L. rhamnosus on medium containing glucose and citrate was found to be highly dependent on preculturing conditions. It exhibited diauxic growth when precultured on glucose, but demonstrated simultaneous consumption when cultured on citrate. The maximum specific growth rate for cells growing on glucose + citrate was 0.38 h-1, which was higher than the growth rate on individual substrates (0.28 h-1). Simultaneous consumption also yielded higher net flavour compounds, diacetyl and acetoin. Flux analysis indicated that L. rhamnosus requires oxygen for balancing excess NADH through NADH oxidase. The flux analysis provided insights into the metabolic network of L. rhamnosus.

Influence of temperature on flavour compound production from citrate by Lactobacillus rhamnosus ATCC 7469

The citrate utilization by Lactobacillus rhamnosus ATCC 7469 was found to be temperature-dependent. The maximum citrate utilization and incorporation of [1,5-14C]citrate rate were observed at 37 degreesC. At this temperature, maximum citrate lyase activity and specific diacetyl and acetoin production (Y(DA%)) were observed. The high levels of alpha-acetolactate synthase and low levels of diacetyl reductase, acetoin reductase and L-lactate dehydrogenase found at 37 degreesC led to an accumulation of diacetyl and acetoin. Optimum lactic acid production was observed at 45 degreesC, according to the high lactate dehydrogenase activity. The NADH oxidase activity increased with increasing culture temperature from 22 degreesC to 37 degreesC. Thus there are greater quantities of pyruvate available for the production of alpha-acetolactate, diacetyl and aceotin, and less diacetyl and acetoin are reduced.