Gut microbiota and type 2 diabetes mellitus: a focus on the gut-brain axis

Type 2 diabetes mellitus (T2DM) has become one of the most serious public healthcare challenges, contributing to increased mortality and disability. In the past decades, significant progress has been made in understanding the pathogenesis of T2DM. Mounting evidence suggested that gut microbiota (GM) plays a significant role in the development of T2DM. Communication between the GM and the brain is a complex bidirectional connection, known as the "gut-brain axis," via the nervous, neuroendocrine, and immune systems. Gut-brain axis has an essential impact on various physiological processes, including glucose metabolism, food intake, gut motility, etc. In this review, we provide an outline of the gut-brain axis. We also highlight how the dysbiosis of the gut-brain axis affects glucose homeostasis and even results in T2DM.

Evaluating the Role of Postbiotics in the Modulation of Human Oral Microbiota: A Randomized Controlled Clinical Trial

There is a lack of clinical data to support the effectiveness and safety of postbiotics in the modulation of human oral microbiota and oral health care. Here, volunteers were recruited and randomly assigned to two cohorts: a placebo group (n = 15) and a postbiotic group (n = 16). The placebo group used toothpaste that did not contain postbiotics, while the postbiotic group used toothpaste with postbiotics (3 × 1010 CFU inactivated Lactobacillus salivarius LS97, L. paracasei LC86, and L. acidophilus LA85). Saliva samples were collected at different time points and the immunoglobulin A (IgA) and short-chain fatty acid (SCFA) levels were determined, while the salivary microbiota was analyzed by 16S rRNA amplicon sequencing. The results showed that salivary IgA levels and acetic and propionic acid levels were notably higher in the postbiotic group (P < 0.05), accompanied by an increase in the level of alpha diversity of the salivary microbiota, and these indexes remained high 1 month after discontinuing the use of toothpaste with or without postbiotics. A notable decrease in the relative abundance of the unclassified_Enterobacteriaceae, Klebsiella, Escherichia, etc. in the postbiotic group was accompanied by a notable increase in Ruminofilibacter and Lactobacillus. However, both groups did not cause significant changes in the overall structure of the host salivary microbiota. In conclusion, postbiotics dramatically and consistently improved oral immunity levels and SCFA content in the host. In addition, postbiotics were able to increase the level of microbial alpha diversity and down-regulate the abundance of some harmful microbes without significantly altering the structure of the host salivary microbiota. Chinese Clinical Trial Registry (ChiCTR) ( www.chictr.org.cn ) under the registration number ChiCTR2300074088.

Baicalin circumvents anti-PD-1 resistance by regulating the gut microbiota metabolite short-chain fatty acids

Baicalin is a small molecule medication used to treat hepatitis. Our research group discovered that administering baicalin orally to mice following fecal microbiota transplantation from patients resistant to ICIs supported anti-PD-1 activity. However, the precise mechanisms behind this effect are presently unknown. In this present study, ATB-treated C57BL/6 J mice received FMT from patients with advanced NSCLC amenable to αPD-1. Additionally, subcutaneous LLC cells were injected into the mice. Baicalin oral gavage and αPD-1 injection were administered to the mice on days 3 and 9 after tumour inoculation. 16 S rRNA, metabolomics, and flow cytometry were utilized to clarify the mechanisms of baicalin's relief of immunosuppression. The results indicated that oral administration of baicalin enriched bacteria such as Akkermansia and Clostridia_UCG-014, resulted in an increase in SCFAs, which improved the ratio of PD-1+ (CD8+ T cell/Treg) and promoted the levels of IFN-γ+ CD8+ T cells and TNF-α+ CD8+ T cells within the tumour microenvironment. In conclusion, baicalin regulates the metabolites of the gut microbiota to improve the PD-1+ (CD8+ T cell/Treg) balance and circumvent anti-PD-1 resistance. This is achieved through the regulation of short-chain fatty acids.

Tailored short-chain fatty acids conversion from waste activated sludge fermentation via persulfate oxidation and C3-C5 io-SRB metabolizers

Boosting acetate production from waste activated sludge (WAS) fermentation is often hindered by the inefficient solubilization in the hydrolysis step and the high hydrogen pressure ( [Formula: see text] ) during the acidogenesis of C3-C5 short-chain fatty acid (SCFAs), i.e., propionate (HPr), butyrate (HBu) and valerate (HVa). Therefore, this study employed persulfate (PS) oxidation and C3-C5 incomplete-oxidative sulfate reducing bacteria (io-SRB) metabolizers to tailor SCFAs conversion from WAS fermentation. The decomposition efficiency, performance of SCFAs production was investigated. Results showed that the PS significantly promoted WAS decomposition, with a dissolution rate of 39.4%, which is 26.0% higher than the un-treated test. Furthermore, SCFAs yields were increased to 462.7 ± 42 mg COD/g VSS in PS-HBu-SRB, which was 7.4 and 2.2 times higher than that of un-treated and sole PS tests, respectively. In particular, the sum of acetate and HPr reached the peak value of 85%, indicating that HBu-SRB mediation promoted the biotransformation of HBu and macromolecular organics by reducing the [Formula: see text] restriction. Meanwhile, sulfate radical (SO4∙-)-based oxidation (SR-AOPs) was effective in the decomposition of WAS, the oxidative product, i.e., sulfate served the necessary electron acceptor for the metabolism of io-SRB. Further analysis of Mantel test revealed the cluster of the functional genus and their interaction with environmental variables. Additionally, molecular ecological network analysis explored the potential synergistic and competitive relationships between critical genera. Additionally, the potential synergistic and competitive relationships between critical genera was explored by molecular ecological network analysis. This study provides new insights into the integration of SR-AOPs with microbial mediation in accelerating SCFAs production from WAS fermentation.

Gut microbiota might mediate the benefits of high-fiber/acetate diet to cardiac hypertrophy mice

Continuously prolonged cardiac hypertrophy results in maladaptive myocardial remodeling, which affects cardiac function and can eventually lead to heart failure. Short-chain fatty acids (SCFAs), including acetate, propionate, and butyrate, have been reported to be associated with cardiovascular diseases (CVD). Gut microbiota may mediate between dietary fiber and SCFA effects on cardiac hypertrophy. The mice model of isoproterenol (ISO)-induced cardiac hypertrophy was constructed and verified for physiological, functional, and fibrotic alterations in this study. Both high-fiber and acetate diet improved physiological indexes, ameliorated cardiac functions, and relieved fibrotic alterations in model mice hearts; collectively, cardiac hypertrophy in mice receiving both high-fiber and acetate diet improved. Following 16s rDNA sequencing and integrative bioinformatics, analyses indicated that both high-fiber and acetate diet caused alterations in mice gut microbiota compared with the ISO group, including OTU composition and abundance. In conclusion, high-fiber and acetate diet improve the physiological status, cardiac functions, and fibrotic alterations in ISO-induced hypertrophic mice. Besides, considering the alterations in mice gut microbiota in response to single ISO, both high-fiber and acetate diet treatment, gut microbiota might mediate the favorable benefits of both high-fiber and acetate diet on cardiac hypertrophy.

The role of short-chain fatty acids (SCFAs) in regulating stress responses, eating behavior, and nutritional state in anorexia nervosa: protocol for a randomized controlled trial

OBJECTIVE

This protocol proposes investigating the effects of short-chain fatty acids (SCFAs)-namely acetate, propionate, and butyrate-as mediators of microbiota-gut-brain interactions on the acute stress response, eating behavior, and nutritional state in malnourished patients with anorexia nervosa (AN). SCFAs are produced by bacterial fermentation of dietary fiber in the gut and have recently been proposed as crucial mediators of the gut microbiota's effects on the host. Emerging evidence suggests that SCFAs impact human psychobiology through endocrine, neural, and immune pathways and may regulate stress responses and eating behavior.

METHOD

We will conduct a randomized, triple-blind, placebo-controlled trial in 92 patients with AN. Patients will receive either a placebo or a mixture of SCFAs (acetate propionate, butyrate) using pH-dependent colon-delivery capsules for six weeks. This clinical trial is an add-on to the standard inpatient psychotherapeutic program focusing on nutritional rehabilitation.

HYPOTHESES

We hypothesize that colonic SCFAs delivery will modulate neuroendocrine, cardiovascular, and subjective responses to an acute laboratory psychosocial stress task. As secondary outcome measures, we will assess alterations in restrictive eating behavior and nutritional status, as reflected by changes in body mass index. Additionally, we will explore changes in microbiota composition, gastrointestinal symptoms, eating disorder psychopathology, and related comorbidities.

DISCUSSION

The findings of this study would enhance our understanding of how gut microbiota-affiliated metabolites, particularly SCFAs, impact the stress response and eating behavior of individuals with AN. It has the potential to provide essential insights into the complex interplay between the gut, stress system, and eating behavior and facilitate new therapeutic targets for stress-related psychiatric disorders. This protocol is prospectively registered with ClinicalTrials.gov, with trial registration number NCT06064201.

Feasibility and mechanism of recycling carbon resources from waste cyanobacteria and reducing microcystin toxicity by dielectric barrier discharge plasma

Recycling carbon resources from discarded cyanobacteria is a worthwhile research topic. This study focuses on the use of dielectric barrier discharge (DBD) plasma technology as a pretreatment for anaerobic fermentation of cyanobacteria. The DBD group (58.5 W, 45 min) accumulated the most short chain fatty acids (SCFAs) along with acetate, which were 3.0 and 3.3 times higher than the control. The DBD oxidation system can effectively collapse cyanobacteria extracellular polymer substances and cellular structure, improve the biodegradability of dissolved organic matter, enrich microorganisms produced by hydrolysis and SCFAs, reduce the abundance of SCFAs consumers, thereby promoting the accumulation of SCFAs and accelerating the fermentation process. The microcystin-LR removal rate of 39.8% was obtained in DBD group (58.5 W, 45 min) on day 6 of anaerobic fermentation. The toxicity analysis using the ECOSAR program showed that compared to microcystin-LR, the toxicity of degradation intermediates was reduced. The contribution order of functional active substances to cyanobacteria cracking was obtained as eaq- > •OH > 1O2 > •O2- > ONOO-, while the contribution order to microcystin-LR degradation was eaq- > •OH > •O2- > 1O2 > ONOO-. DBD has the potential to be a revolutionary pretreatment method for cyanobacteria anaerobic fermentation.

Microbial metabolites regulate social novelty via CaMKII neurons in the BNST

Social novelty is a cognitive process that is essential for animals to interact strategically with conspecifics based on their prior experiences. The commensal microbiome in the gut modulates social behavior through various routes, including microbe-derived metabolite signaling. Short-chain fatty acids (SCFAs), metabolites derived from bacterial fermentation in the gastrointestinal tract, have been previously shown to impact host behavior. Herein, we demonstrate that the delivery of SCFAs directly into the brain disrupts social novelty through distinct neuronal populations. We are the first to observe that infusion of SCFAs into the lateral ventricle disrupted social novelty in microbiome-depleted mice without affecting brain inflammatory responses. The deficit in social novelty can be recapitulated by activating calcium/calmodulin-dependent protein kinase II (CaMKII)-labeled neurons in the bed nucleus of the stria terminalis (BNST). Conversely, chemogenetic silencing of the CaMKII-labeled neurons and pharmacological inhibition of fatty acid oxidation in the BNST reversed the SCFAs-induced deficit in social novelty. Our findings suggest that microbial metabolites impact social novelty through a distinct neuron population in the BNST.

Human umbilical cord mesenchymal stem cells ameliorate colon inflammation via modulation of gut microbiota-SCFAs-immune axis

BACKGROUND

Inflammatory bowel disease (IBD) is a global health problem in which gut microbiota dysbiosis plays a pivotal pathogenic role. Mesenchymal stem cells (MSCs) therapy has emerged as a prospective novel tool for managing IBD, and which can also regulate the composition of gut microbiota. However, the functional significance of MSCs-induced changes in gut microbiome is poorly understood.

METHODS

Here, we investigated for the first time the role of gut microbiota in mediating the protective effect of human umbilical cord MSCs (HUMSCs) on DSS-induced colitis. Gut microbiota alteration and short-chain fatty acids (SCFAs) production were analyzed through 16S rRNA sequencing and targeted metabolomics. Spectrum antibiotic cocktail (ABX), fecal microbiota transplantation (FMT) and sterile fecal filtrate (SFF) were employed to evaluate the protective effect of intestinal flora and its metabolites. Cytokine microarray, Enzyme-linked immunosorbent assay (ELISA), and flow cytometry were conducted to assess the effect on CD4+T homeostasis.

RESULTS

Here, we investigated for the first time the role of gut microbiota in mediating the protective effect of MSCs on DSS-induced colitis. By performing gut microbiota depletion and fecal microbiota transplantation (FMT) experiments, we revealed that MSCs derived from human umbilical cord ameliorated colon inflammation and reshaped T-cells immune homeostasis via remodeling the composition and diversity of gut flora, especially up-regulated SCFAs-producing bacterial abundance, such as Akkermansia, Faecalibaculum, and Clostridia_UCG_014. Consistently, targeted metabolomics manifested the increased SCFAs production with MSCs administration, and there was also a significant positive correlation between differential bacteria and SCFAs. Meanwhile, combined with sterile fecal filtrate (SFF) gavage experiments, the underlying protective mechanism was further associated with the improved Treg/Th2/Th17 balance in intestinal mucosa mediated via the increased microbiota-derived SCFAs production.

CONCLUSION

The present study advances understanding of MSCs in the protective effects on colitis, providing evidence for the new role of the microbiome-metabolite-immune axis in the recovery of colitis by MSCs.

Diet as a modifiable factor in tumorigenesis: Focus on microbiome-derived bile acid metabolites and short-chain fatty acids

Several lines of evidences have implicated the resident microbiome as a key factor in the modulation of host physiology and pathophysiology; including the resistance to cancers. Gut microbiome heavily influences host lipid homeostasis by their modulatory effects on the metabolism of bile acids (BAs). Microbiota-derived BA metabolites such as deoxycholic acid (DCA), lithocholic acid (LCA), and ursodeoxycholic acid (UDCA) are implicated in the pathogeneses of various cancer types. The pathogenic mechanisms are multimodal in nature, with widespread influences on the host immunes system, cell survival and growth signalling and DNA damage. On the other hand, short-chain fatty acids (SCFAs) produced by the resident microbial activity on indigestible dietary fibres as well as during intermittent fasting regimens (such as the Ramazan fasting) elicit upregulation of the beneficial anti-inflammatory and anticancer pathways in the host. The present review first provides a brief overview of the molecular mechanisms of microbiota-derived lipid metabolites in promotion of tumour development. The authors then discuss the potential of diet as a therapeutic route for beneficial alteration of microbiota and the consequent changes in the production of SCFAs, particularly butyrate, in relation to the cancer prevention and treatment.

Valorization of food waste into short-chain fatty acids via enzymatic pretreatment: Effects of fermentation-pH on acid-producing processes and microbial metabolic functions

Food waste (FW) has been widely considered as an essential resource for the production of short-chain fatty acids (SCFAs), an important class of chemicals with wide applications and over 20 million tons of annual market demand, by anaerobic fermentation. Although enzymatic pre-treatment could improve the FW biodegradation efficiency, resulting in enhanced efficiency of solubilization and hydrolysis, the influence of fermentation-pH on the SCFAs production and the metabolic functions, have rarely been reported. This study demonstrated that the uncontrolled pH could efficiently lead to an increase in the SCFAs production (33011 mgCOD/L) during long-term fermentation of FW (mainly consisting of 48.8% carbohydrates, 20.6% proteins, and 17.4% lipids) after enzymatic pre-treatment compared to the control (16413 mgCOD/L). Meanwhile, the acid-producing processes (i.e., solubilization, hydrolysis, and acidification) were synchronously enhanced by the enzymatic pre-treatment and no control over fermentation-pH. Metagenomic analysis revealed that the acid-forming microorganisms (i.e., Olsenella sp. and Sporanaerobacter) were significantly accumulated, and the corresponding genetic expressions related to extracellular hydrolysis (i.e., aspB and gltB), membrane transport (i.e., metL and glnH), and intracellular material metabolism (i.e., pfkA and ackA) were evidently stimulated, thereby promoting ultimate SCFAs generation. Although the alkaline conditions could further slightly increase the SCFAs yield slightly (37100 mgCOD/L) and also stimulate the metabolic activities, it might not be suitable for large-scale practical applications due to additional costs associated with alkaline chemical additives.

Short-chain fatty acids ameliorate spinal cord injury recovery by regulating the balance of regulatory T cells and effector IL-17+ γδ T cells

Spinal cord injury (SCI) causes motor, sensory, and autonomic dysfunctions. The gut microbiome has an important role in SCI, while short-chain fatty acids (SCFAs) are one of the main bioactive mediators of microbiota. In the present study, we explored the effects of oral administration of exogenous SCFAs on the recovery of locomotor function and tissue repair in SCI. Allen's method was utilized to establish an SCI model in Sprague-Dawley (SD) rats. The animals received water containing a mixture of 150 mmol/L SCFAs after SCI. After 21 d of treatment, the Basso, Beattie, and Bresnahan (BBB) score increased, the regularity index improved, and the base of support (BOS) value declined. Spinal cord tissue inflammatory infiltration was alleviated, the spinal cord necrosis cavity was reduced, and the numbers of motor neurons and Nissl bodies were elevated. Enzyme-linked immunosorbent assay (ELISA), real-time quantitative polymerase chain reaction (qPCR), and immunohistochemistry assay revealed that the expression of interleukin (IL)‍-10 increased and that of IL-17 decreased in the spinal cord. SCFAs promoted gut homeostasis, induced intestinal T cells to shift toward an anti-inflammatory phenotype, and promoted regulatory T (Treg) cells to secrete IL-10, affecting Treg cells and IL-17+ γδ T cells in the spinal cord. Furthermore, we observed that Treg cells migrated from the gut to the spinal cord region after SCI. The above findings confirm that SCFAs can regulate Treg cells in the gut and affect the balance of Treg and IL-17+ γδ T cells in the spinal cord, which inhibits the inflammatory response and promotes the motor function in SCI rats. Our findings suggest that there is a relationship among gut, spinal cord, and immune cells, and the "gut-spinal cord-immune" axis may be one of the mechanisms regulating neural repair after SCI.

Short-chain fatty acids: possible regulators of insulin secretion

The benefits of gut microbiota-derived short-chain fatty acids (SCFAs) towards health and metabolism have been emerging since the past decade. Extensive studies have been carried out to understand the mechanisms responsible in initiating the functionalities of these SCFAs towards body tissues, which greatly involves the SCFA-specific receptors free fatty acid receptor 2 (FFAR2) and free fatty acid receptor 3 (FFAR3). This review intends to discuss the potential of SCFAs particularly in regulating insulin secretion in pancreatic β-cells, by explaining the production of SCFAs in the gut, the fate of each SCFAs after their production, involvement of FFAR2 and FFAR3 signalling mechanisms and their impacts on insulin secretion. Increased secretion of insulin after SCFAs treatments were reported in many studies, but contradicting evidence also exist in several other studies. Hence, no clear consensus was achieved in determining the true potential of SCFA in regulating insulin secretion. In this review, we explore how such differences were possible and hopefully be able to shed some perspectives in understanding SCFAs-signalling behaviour and preferences.

Using heat-activated persulfate to accelerate short-chain fatty acids production from waste activated sludge fermentation triggered by sulfate-reducing microbial consortium

Persulfate has been applied extensively for waste activated sludge (WAS) decomposition due to the strong oxidizing sulfate radical generated as a product. However, the efficiency is not improved without activation to produce free radicals. In this study, a novel coupling strategy of heat-activated persulfate (Heat_PS) pretreatment and sulfate-reducing bacteria (SRB) triggering was explored to enhance short-chain fatty acids (SCFAs) produced by WAS fermentation. The remaining sulfate acts as an essential acceptor of electrons for the metabolism of synergistic SRB, thereby boosting WAS acidification by energetic cooperation with anaerobic fermenters. The results showed that SCFAs yield in the Heat_PS + SRB group peaked at 431.89 mg COD/gVSS, with the proportion of acetate reaching 57.8 %. This was 6.33 and 1.75 times higher than that in raw and single Heat_PS treated WAS, respectively. Carbon balance revealed a conversion rate of 26.1 % of carbon content in WAS to SCFAs, with 4.5 % lower CO₂ equivalents emitted than that in raw WAS fermentation by the assessments of environmental impacts. This was partially attributed to the strong decomposition of WAS by SO₄^•- and ^•OH oxidation from heat-activated PS and the SRB trigger. In addition, the synergistic relationship among acidogenic/fermentative bacteria and SRB consortia was further verified by the positive correlation among Desulfovibrio, the hydrolytic Escherichia-Shigella, Morganella and the fermetative Macellibacteroides and Bacteroides, as revealed by molecular ecological networks (MENs) analysis. The results of this study may highlight the cooperation of the synergistic micribial consortia as an additional perspective for the recovery of value-added biological metabolites from complex biotransformation.

Butyrate-induced IL-22 expression in fish macrophages contributes to bacterial clearance

IL-22 has been characterized as a critical cytokine in maintaining barrier integrity and host immunity. So far, it has been known that IL-22 is mainly produced by lymphoid lineage cells. In the present study, we have thoroughly investigated butyrate-induced production and function of IL-22 in fish macrophages. Our results demonstrated that short-chain fatty acids (SCFAs), major microbiota-derived metabolites, promoted the expression of IL-22 in head kidney macrophages (HKMs) of turbot (Scophthalmus maximus L.). Interestingly, butyrate-mediated intracellular bacterial killing in HKMs diminished when IL-22 expression was interfered. Furthermore, the turbot fed the diet containing sodium butyrate (NaB) exhibited significantly lower mortality after bacterial infection, compared to the fish fed a basal diet. At the meantime, a higher level of IL-22 expression and bactericidal activity was detected in HKMs from the turbot fed NaB-supplemented diet. In addition, NaB treatment promoted the expression of antimicrobial peptides (AMPs) β-defensins in zebrafish (Danio rerio). However, butyrate-induced expression of AMPs was reduced in IL-22 mutant zebrafish compared to wild-type (WT) fish. Meanwhile, NaB treatment was incapable to protect IL-22 mutant fish from bacterial infection as it did in WT zebrafish. Importantly, our results demonstrated that IL-22 expression was remarkably suppressed in macrophage-depleted zebrafish, indicating that macrophage might be a cell source of IL-22 production in vivo. In conclusion, all these findings collectively revealed that SCFAs regulated the production and function of IL-22 in fish macrophages, which facilitated host resistance to bacterial invasion.

High-altitude exposure decreases bone mineral density and its relationship with gut microbiota: Results from the China multi-ethnic cohort (CMEC) study

BACKGROUND

Geographic altitude is a potent environmental factor for human microbiota and bone mineral density. However, little evidence exists in population-based studies with altitude diversity ranges across more than 3000 m. This study assessed the associations between a wide range of altitudes and bone mineral density, as well as the potential mediating role of microbiota in this relationship.

METHODS

A total of 99,556 participants from the China Multi-Ethnic Cohort (CMEC) study were enrolled. The altitude of each participant was extracted from global Shuttle Radar Topography Mission (SRTM) 4 data. Bone mineral density was measured by calcaneus quantitative ultrasound index (QUI). Stool samples were collected for 16S rRNA gene sequencing (n = 1384). The metabolites of gut microbiota, seven kinds of short-chain fatty acids (SCFAs), were detected by gas chromatography-mass spectrometry (GC-MS, n = 128). After screening, 73,974 participants were selected for the "altitude-QUI" analysis and they were placed into the low-altitude (LA) and high-altitude (HA) groups. Additionally, a subgroup (n = 1384) was further selected for the "altitude-microbiota-QUI" analysis. Multivariate linear regression models and mediation analyses were conducted among participants.

RESULTS

A significant negative association between high-altitude and QUI was obtained (mean difference = -0.373 standard deviation [SD], 95% confidence interval [CI]: -0.389, -0.358, n = 73,974). The same negative association was also observed in the population with microbiota data (mean difference = -0.185 SD, 95%CI: -0.360, -0.010, n = 1384), and a significant mediating effect of Catenibacteriumon on the association between altitude and QUI (proportion mediated = 25.2%, P = 0.038) was also noticed. Additionally, the acetic acid, butyric acid, and total amount of seven SCFAs of the low-altitude group were significantly higher than that of the high-altitude group (P < 0.05).

CONCLUSION

High-altitude exposure may decrease bone mineral density in adults, thus increasing the risk of osteoporosis. The modulation of gut microbiota may be a potential strategy for alleviating the decrease of bone mineral density.

Chinese patent medicine shouhui tongbian capsule attenuated loperamide-induced constipation through modulating the gut microbiota in rat

ETHNOPHARMACOLOGICAL RELEVANCE

Shouhui tongbian capsule (SHTC) is a commercial Chinese patent medicine used in the treatment of constipation. However, its mechanism of action remains unclear.

AIM OF THE STUDY

The present study was undertaken to assess SHTC relieved effects on the clinical symptoms of loperamide (LOP) induced constipation in Sprague Dawley (SD) rat model and to clarify the relationship between the protective effect of SHTC on constipation and the gut microbiota.

MATERIALS AND METHODS

Constipation male SD rats models were induced with solution of LOP (1.5 mg/kg bw), and rats were treated with an oral dose of SHTC (35, 70 mg/kg bw) three times a day after successful modeling. All rats were assessed weekly by change in body weight, gastric emptying rate, fecal moisture content and wet/dry weight. Hematoxylin and eosin (H&E) were used to observe parts of the rats small intestine. The gut microbiota in colonic contents was analyzed using 16SrRNA gene sequencing. Contents of short-chain fatty acids (SCFAs) were analyzed by gas chromatography-mass spectrometer (GCMS).

RESULTS

The results confirmed the therapeutic effects of SHTC on constipation. Specifically, SHTC could alleviate the decrease in body weight, gastric emptying rate and fecal moisture content caused by LOP-induced constipation. The pathological damage of small intestine was significantly improved by H&E staining. Notably, SHTC increased the relative abundances of Lactobacillus and the ratio of Firmicutes to Bacteroides (F/B). In addition, the content of acetic acid and propionic acid was significantly increased in constipated rats fed with SHTC.

CONCLUSION

SHTC could ameliorate the development of LOP-induced constipation in rats by remodeling the structure of gut microbial community and regulating production of intestinal metabolites.

Unraveling the behavior of nitrite on promoting short-chain fatty acids accumulation from waste activated sludge by peracetic acid pretreatment: Extracellular polymeric substance decomposition and underlying mechanism

Peracetic acid (PAA) is an emerging oxidant for waste activated sludge (WAS) treatment due to its strong oxidization and few toxic byproducts. Nitrite which can be in-situ recovered from WAS fermentation liquor, its protonated form (free nitrous acid, FNA) is regarded as the cost-effective inactivator. The stubborn extracellular polymeric substance (EPS) is the rate-limiting step for energy/resource recovery from WAS. This work found that the co-pretreatment of PAA and FNA can effectively promote short-chain fatty acids (SCFAs) production during anaerobic fermentation. Higher PAA dosage (100 mg/g VSS, FP4WAS) in co-pretreatment was beneficial for organics release (1976.9 mg COD/L), remarkably increased by 10.3- 96.5 % than that of low PAA dosage (25- 75 mg/g VSS), and promoted by 105.1 % and 62.1 % than FNA (FWAS)/PAA (100 mg/g VSS, P4WAS)-pretreated WAS. Effective release of soluble organics contributed to the SCFAs accumulation (7679 ± 86 mg COD/L, 4 d), enhanced by 200.0 % and 19.0 % than FWAS and P4WAS, respectively. Acetic (HAc) and propionic acid (HPr) peaked at 6344.7 mg COD/L in FP4WAS (accounted for 82.6 %), which increased by 10.6- 899.0 % than other groups. Moreover, OH and O₂^- were detected in co-pretreatment, may play the synchronous effect with the crucial intermediates of NO, NO₂ and ONOO^-/ONOOH on EPS decomposition.

Insight into the enhancing short-chain fatty acids (SCFAs) production from waste activated sludge via polyoxometalates pretreatment: Mechanisms and implications

Polyoxometalates (POMs), a versatile and environmentally-friendly inorganic material, have been extensively studied and applied in chemical catalytic oxidation and biological nutrients removal processes. However, little is known about effects of POMs pretreatment on anaerobic sludge fermentation. This study thereby filled such knowledge gap and provided insights into the underlying mechanisms. Results demonstrated the maximal short-chain fatty acids (SCFAs) production increased by 6.18 times with POMs rising from 0 to 0.05 g/g TSS. Mechanistic investigations revealed that the oxidation stress of POMs as well as reactive oxygen species (ROS) activated by POMs were responsible for the disintegration of waste activated sludge (WAS). More importantly, POMs pretreatment improved the biodegradability of organics released, providing more biodegradable substrates for SCFAs generation. Furthermore, the inhibition of POMs to SCFAs producers was less severe than that to SCFAs consumers, leading to SCFAs accumulation. Microbial community analysis exhibited that increased the population of hydrolysis (i.e., Longilinea) and SCFAs generation microbes (i.e., Acinetobacter and Fusibacter). Further evaluation showed that the POMs-based technology is economically and environmentally attractive for the pretreatment of WAS. Finally, a "closed-loop" concept of the reutilization of renewable POMs may provide an important implication of WAS management in the future.

Sodium Butyrate Protects Against Ethanol-Induced Toxicity in SH-SY5Y Cell Line

Alcohol use disorder (AUD), brought about by excessive alcohol use, is associated with damages to several organs including the brain. Chronic excessive use of alcohol can compromise intestinal integrity, leading to changes in gut microbiota (GM) composition known as dysbiosis. Dysbiosis, by disruption of the gut-brain axis (GBA), further exacerbates the deleterious effects of alcohol. One of the fermentation by-products of GM is butyrate (BUT), a short-chain fatty acid (SCFA) that plays an important role in maintaining homeostasis of the GBA. Alcohol metabolism results in formation of acetaldehyde, a highly reactive compound that reacts with dopamine in the brain to form toxic adducts such as salsolinol. Recent studies indicate potential neuro-protective effects of BUT against various toxicants including salsolinol. Here, we sought to investigate whether BUT can also protect against alcohol toxicity. Pretreatment of neuroblastoma-derived SH-SY5Y cells with 500 mM ethanol (ETOH) for 24 h resulted in approximately 40% reduction in cell viability, which was totally blocked by 10 µM of either BUT or AR 420,626 (AR), a selective fatty acid 3 receptor (FA3R) agonist. The neuro-protective effects of both BUT and AR were significantly (80%) attenuated by beta-hydroxy butyrate (BHB), a selective FA3R antagonist. Interestingly, combination of BUT and AR resulted in synergistic protection against ETOH, which was totally blocked by BHB. These findings suggest potential utility of butyrate and/or FA3R agonists against ETOH-induced toxicity.

Classroom microbiome, functional pathways and sick-building syndrome (SBS) in urban and rural schools - Potential roles of indoor microbial amino acids and vitamin metabolites

Sick building symptoms (SBS) are defined as non-specific symptoms related to indoor exposures, including mucosal symptoms in eye, nose, throat, and skin, and general symptoms as headache and tiredness. Indoor microbial composition is associated with SBS symptoms, but the impact of microbial functional genes and potential metabolic products has not been characterized. We conducted a shotgun microbial metagenomic sequencing for vacuum dust collected in urban and rural schools in Shanxi province, China. SBS symptoms in students were surveyed, and microbial taxa and functional pathways related to the symptoms were identified using a multi-level linear regression model. SBS symptoms were common in students, and the prevalence of ocular and throat symptoms, headache, and tiredness was higher in urban than in rural areas (p < 0.05). A significant higher microbial α-diversity was found in rural areas than in urban areas (Chao1, p = 0.001; ACE, p = 0.002). Also, significant variation in microbial taxonomic and functional composition (β-diversity) was observed between urban and rural areas (p < 0.005). Five potential risk Actinobacteria species were associated with SBS symptoms (p < 0.01); students in the classrooms with a higher abundance of an unclassified Geodermatophilaceae, Geodermatophilus, Fridmanniella luteola, Microlunatus phosphovorus and Mycetocola reported more nasal and throat symptoms and tiredness. Students with a higher abundance of an unclassified flavobacteriaceae reported fewer throat symptoms and tiredness. The abundance of microbial metabolic pathways related to the synthesis of B vitamins (biotin and folate), gamma-aminobutyric acid (GABA), short-chain fatty acids (SCFAs), and peptidoglycan and were protectively (negatively) associated with SBS symptoms (FDR < 0.05). The result is consistent with human microbiota studies, which reported that these microbial products are extensively involved in immunological processes and anti-inflammatory effects. This is the first study to report the functional potential of the indoor microbiome and the occurrence of SBS, providing new insights into the potential etiologic mechanisms in chronic inflammatory diseases.

Interactions of functional microorganisms and their contributions to methane bioconversion to short-chain fatty acids

Methane bioconversion to value-added liquid chemicals has been proposed as a promising solution to augment the petroleum-dominated chemical market. Recent investigations have reported that various electron acceptors (e.g., nitrite and nitrate) are available to drive methane bioconversion to short-chain fatty acids (SCFAs). However, little is known about effects of the rate electron acceptor supplied on liquid chemical production from methane. Herein, three independent membrane biofilm reactors (MBfRs) feeding with respective nitrate, nitrite, combined nitrate and nitrite were operated under high and low rate condition in succession, to study whether feeding rate of electron acceptors could impact the methane bioconversion to SCFAs and the associated microbiological features. Long-term operation showed that all tested electron acceptors with a high supply rate were favorable for methane bioconversion to SCFAs (990.9 mg L^-1d^-1, 1695.7 mg L^-1d^-1, and 2425.7 mg L^-1d^-1), while under a low electron acceptor feeding rate, the SCFA production rate decreased to 8.9 mg L^-1d^-1, 16.8 mg L^-1d^-1, and 260.1 mg L^-1d^-1, respectively. Microbial community characterization showed that the biofilm was predominated by Methanosarcina, Methanobacterium, Propionispora and Clostridium. On the basis of the known metabolism characteristics of these microorganisms, it was assumed that these methanogens and fermenters contributed jointly to methane bioconversion to SCFAs. The findings could be helpful to understand the role of electron acceptor rate in methane bioconversion to liquid chemicals.

Resistant starch intake alleviates collagen-induced arthritis in mice by modulating gut microbiota and promoting concomitant propionate production

Gut dysbiosis precedes clinic symptoms in rheumatoid arthritis (RA) and has been implicated in the initiation and persistence of RA. The early treatment of RA is critical to better clinical outcome especially for joint destruction. Although dietary interventions have been reported to be beneficial for RA patients, it is unclear to whether diet-induced gut microbiome changes can be a preventive strategy to RA development. Here, we investigated the effect of a high fiber diet (HFD) rich with resistant starch (RS) on collagen-induced arthritis (CIA) and gut microbial composition in mice. RS-HFD significantly reduced arthritis severity and bone erosion in CIA mice. The therapeutic effects of RS-HFD were correlated with splenic regulatory T cell (Treg) expansion and serum interleukin-10 (IL-10) increase. The increased abundance of Lactobacillus and Lachnoclostridium genera concomitant with CIA were eliminated in CIA mice fed the RS-HFD diet. Notably, RS-HFD also led to a predominance of Bacteroidetes, and increased abundances of Lachnospiraceae_NK4A136_group and Bacteroidales_S24-7_group genera in CIA mice. Accompanied with the gut microbiome changes, serum levels of the short-chain fatty acid (SCFA) acetate, propionate and isobutyrate detected by GC-TOFMS were also increased in CIA mice fed RS-HFD. While, addition of β-acids from hops extract to the drinking water of mice fed RS-HFD significantly decreased serum propionate and completely eliminated RS-HFD-induced disease improvement, Treg cell increase and IL-10 production in CIA mice. Moreover, exogenous propionate added to drinking water replicated the protective role of RS-HFD in CIA including reduced bone damage. The direct effect of propionate on T cells in vitro was further explored as at least one mechanistic explanation for the dietary effects of microbial metabolites on immune regulation in experimental RA. Taken together, RS-HFD significantly reduced CIA and bone damage and altered gut microbial composition with concomitant increase in circulating propionate, indicating that RS-rich diet might be a promising therapy especially in the early stage of RA.

Alginate oligosaccharide improves lipid metabolism and inflammation by modulating gut microbiota in high-fat diet fed mice

Alginate oligosaccharides are associated with some beneficial health effects. Gut microbiota is one of the most recently identified factors in the development of several metabolic diseases induced by high-fat diet. Our objective was to evaluate how alginate oligosaccharides impact on high-fat diet‑induced features of metabolic disorders and whether this impact is related to modulations in the modulation of the gut microbiota. C57BL/6J mice were fed with chow diet, high-fat diet, or high-fat diet supplemented with alginate oligosaccharides for 10 weeks. Alginate oligosaccharide treatment improved lipid metabolism, such as reducing levels of TG and LDL-C and inhibiting expression of lipogenesis genes. Alginate oligosaccharide administration reduced the levels of fasting blood glucose and increased the levels of serum insulin. Alginate oligosaccharide treatment was found to lower the expression of markers of inflammation, including IL1β and CD11c. Alginate oligosaccharide treatment modulated gut microbial communities and markedly prompted the growth of Akkermansia muciniphila, Lactobacillus reuteri, and Lactobacillus gasseri. Additionally, alginate oligosaccharide intervention significantly increased concentrations of short-chain fatty acids, such as acetic acid, propionic acid, and butyric acid, as well as decreased levels of endotoxin. Alginate oligosaccharides exert beneficial effects via alleviating metabolic metrics induced by high-fat diet, which is associated with increase in A. muciniphila, L. reuteri, and L. gasseri, as well as the release of microbiota-dependent short-chain fatty acids and inhibition of endotoxin levels.

Development and validation of a GC-FID method for the analysis of short chain fatty acids in rat and human faeces and in fermentation fluids

Short-chain fatty acids (SCFAs) are gut microbiota metabolites recognized for their beneficial effects on the host organism. In this study, a simple and rapid sample preparation method combined to SCFAs analysis by direct injection and gas chromatography coupled with flame ionization detection (GC-FID), for the determination and quantification of eight SCFAs (acetic, propionic, i-butyric, butyric, i-valeric, valeric, i-caproic and caproic acids) in rat, mice and human faeces and in fermentation fluids samples, has been developed and validated. The method consists of extraction of the SCFAs by ethyl ether after acidification of the samples. The effect of the number of extractions has been assessed in order to optimize the procedure and to obtain a satisfactory yield for all the analyzed SCFAs. The increase of the extracted analytes quantity was significant passing from 1 to 2 and from 2 to 3 extractions (P < 0.05), while no significant differences were found performing 3, 4 or 5 extractions (P > 0.05). The SCFAs extracted are directly analyzed by GC-FID without derivatization and separated on a polyethylene glycol nitroterephthalic acid modified coated capillary column, with a chromatographic run time of 13 min. The proposed method showed good sensitivity, with limits of quantifications in the range 0.14-0.48 µM for SCFAs from propionic to caproic acids and 2.12 µM for acetic acid; recovery was between 80.8 and 108.8% and intraday and interday repeatability in the range 0.6-5.0% of precision (RSD, %) The optimized method is suitable for the quantitative analysis of SCFAs in real samples of rat, mouse and human faeces and in fermentation fluids, and it can be applied also to very small amount of faecal sample (20 mg).

Promotion of short-chain fatty acids production and fermented sludge properties via persulfate treatments with different activators: Performance and mechanisms

This study explored the influences of peroxydisulfate (PDS) and peroxymonosulfate (PMS) activated with different catalysts on the anaerobic fermentation of waste activated sludge (WAS). All the treatments were effective in promoting short-chain fatty acids (SCFAs) production, particularly acetic acid, in the order of PMS/MnO₂ > PMS/Zn > PDS/Zn > PMS/Fe > PDS/Fe > PDS/MnO₂. Mechanistic investigations demonstrated that WAS disintegration was intensely induced by the free radicals (i.e., SO₄^- and OH) generated in PDS and PMS treating reactors. It significantly promoted the solubilization and hydrolysis processes and thereby provided sufficient bioavailable substrates for further acidogenic metabolisms. Additionally, it enlarged the abundance of functional bacteria responsible for SCFAs production. The simultaneous promotion of bioavailable substrates and fermentative microorganisms markedly contributed to the SCFAs enhancement. Moreover, the dewaterability and stabilization of fermented sludge were both improved with the PDS and PMS treatments, which were beneficial to the final disposal of WAS.

Enhanced production of short-chain fatty acids from waste activated sludge by addition of magnetite under suitable alkaline condition

Alkaline fermentation technology is an effective method for resource recovery, e.g., short-chain fatty acids (SCFAs), but the acidification process needs to be further enhanced. To improve the release and acidification of organic matters in waste activated sludge (WAS) fermentation simultaneously, a novel method was proposed with magnetite addition under alkaline condition in this study. Compared with the control, SCFAs and acetic acids yields increased by 21.2% ± 3.1% and 31.0% ± 1.2% in the 0.6 g/gVSS magnetite-based system, respectively. Besides, the activities of α-glucosidase, protease and acetate kinase (AK) were enhanced with magnetite addition, and the abundance of acidogenic microbes was improved obviously. Furthermore, magnetite reduced the release of PO₄^3--P significantly via the precipitation reaction. Of all, a novel side-stream WAS hydrolysis and acidification process based on magnetite addition under suitable alkaline condition is proposed, realizing the efficient recovery of carbon and phosphorus resources along with wastewater and WAS treatment.

New method for algae comprehensive utilization: Algae-derived biochar enhances algae anaerobic fermentation for short-chain fatty acids production

Interest in the resource utilization of algae has gradually increased due to the frequent occurrence of harmful algal blooms. Here, biochar derived from algae was applied to algae anaerobic fermentation for short-chain fatty acids (SCFAs) production. In the presence of algae-derived biochar, the concentration of SCFAs within 4 d (4334 mg COD/L) was approximately doubled compared to the control (2016 mg COD/L), and the fermentation time for maximal SCFAs yield was shortened. Biochar improved the disruption of algae to release more intracellular macromolecular organics. Altering algae hydrolysis, the activity of hydrolase and the contents of functional gene were advantageous to SCFAs accumulation by providing more micromolecular organics in the presence of biochar. Additionally, the relative abundance and survival of acid-forming bacteria were enhanced significantly. Furthermore, biochar accelerated the electron transport and energy synthesis in the biological system, driving the biological reactions that allow microorganisms to function and life to flourish.

From gut microbiota dysfunction to obesity: could short-chain fatty acids stop this dangerous course?

Study of the interactions between the gut microbiota and brain-gut axis represents a very appealing approach to increasing our knowledge about the mechanisms leading to obesity and obesity-related diseases. The aim of this review is to focus on the effects of short-chain fatty acids (SCFAs), which are the main products of gut microbial fermentation from non-digestible carbohydrates in the colon, on the gut-brain axis. Evidence is accumulating regarding the role of SCFAs in the fine-tuning of the gut-brain axis, a feedback system which is vital not only for the proper maintenance of gastrointestinal and metabolic functions, but also for the regulation of food intake and energy expenditure. SCFAs are thought to play a key role in increasing the host capacity to harvest excess energy from the diet. SCFAs, however, can exert their effects on the host metabolism via multiple complementary pathways. Metabolic, inflammatory, and neural pathways can be regulated by SCFAs, which can act by sensing nutritional status, thereby maintaining body energy homeostasis. SCFA production from prebiotic consumption is the rationale for targeting intestinal mechanisms to increase energy expenditure and thereby reduce obesity risk.

Effects of prebiotic mixtures on growth performance, intestinal microbiota and immune response in juvenile chu's croaker, Nibea coibor

Prebiotics has been known to be growth promoter and immunostimulant in aquatic animals. In this study, we investigated the effects of prebiotics on growth performance, intestinal microbiota, short-chain fatty acids (SCFAs) production and immune response of the marine fish, juvenile chu's croaker (Nibea coibor). The fish were fed IG (including 0.5% inulin and 0.5% GOS), GS (0.5% GOS and 0.5% D-sorbitol), IGS (0.33% inulin, 0.33% GOS and 0.33% D-sorbitol) or control diets for 8 weeks. The results showed that the growth performance of the fish was promoted by IG and GS, but not by IGS. The intestinal microbiota in NDC (non-digestible carbohydrates, NDC)-supplemented groups was clearly separated from that of the control, and the highest Shannon and Simpson diversity indices were observed in the IGS group. In the intestine of the croaker, Proteobacteria, Firmicutes, and Bacteroidetes were dominant; among them, 24 taxa revealed a significant difference among groups. Most of these bacteria are able to produce SCFAs, which were significantly increased in all NDC-supplemented groups. Moreover, NDCs were found to activate the immune system of the fish by modulating the serum complements, cytokine levels, lysozyme activities and antioxidant capacity. Furthermore, the results of this study revealed correlations among intestinal microbiota, SCFAs production, innate immunity, antioxidant capacity and digestive enzymes in the croaker fed NDCs. Taken together, our results demonstrated that NDC mixtures might promote growth performance, antioxidant capacity and immune responses of the croaker through modulating the composition of intestinal microbiota and the subsequent SCFAs production, which suggest that NDCs were efficient feed additives for marine fish.

Promoting the anaerobic production of short-chain fatty acids from food wastes driven by the reuse of linear alkylbenzene sulphonates-enriched laundry wastewater

An efficient attempt to improve the anaerobic fermentation of food wastes (FW) via the reuse of linear alkylbenzene sulphonates (LAS)-enriched laundry wastewater was reported. The production of short-chain fatty acids (SCFAs) from FW was enhanced by approximate 6-folds with high proportions of butyric and valeric acids at appropriate LAS level. Mechanism investigations demonstrated that the solubilization of macromolecule organics in FW was effectively improved by surface tension reduction. The hydrolysis and acidification processes during FW fermentation were accelerated and enhanced with the stimulation of microbial activities (higher activities of hydrolases and ATP concentrations). Also, the abundances of anaerobic microorganisms responsible for SCFAs production were enriched. Metatranscriptomic analysis indicated that the encoding genes involved in the metabolism of fermentation substrates for SCFAs production were highly expressed in LAS-added reactors. It was the simultaneous increase of bioavailable substrates and metabolic activities that contribute to the efficient SCFAs production in LAS-added reactors.

Improving anaerobic fermentation of waste activated sludge using iron activated persulfate treatment

This study reported a novel and efficient approach to improve the anaerobic fermentation performance of waste activated sludge (WAS) by the indigenous iron activated persulfate (PS/Fe) treatment. Firstly, the production of short-chain fatty acids (SCFAs), especially acetic acid, was remarkably enhanced within shorter fermentation time. Mechanism investigations demonstrated that the PS/Fe treatment could simultaneously accelerate and enhance the hydrolysis and acidification process while inhibit the methanogenesis during WAS fermentation. The activities of key enzymes and the abundances of anaerobic microorganisms responsible for SCFAs production were stimulated in the presence of PS/Fe which would promote the biological processes. Secondly, the PS/Fe treatment improved the quality of fermentation residue by reducing the toxic organic compounds in the residue and enhancing the dewaterability of fermented sludge, which was beneficial to the final disposal of WAS with added economical and environmental values.

Efficient production of short-chain fatty acids from anaerobic fermentation of liquor wastewater and waste activated sludge by breaking the restrictions of low bioavailable substrates and microbial activity

An efficient approach of bioconverting the organic wastes in liquor wastewater (LW) and waste activated sludge (WAS) to valuable short-chain fatty acids (SCFAs) via anaerobic fermentation was explored. The maximal SCFAs concentration was 5400 mg COD/L with approximate 80.0% acetic and propionic acids under optimized conditions (LW/WAS ratio 1:1, pH 8 and fermentation 4 d). Mechanisms investigation found that the fermentation of LW/WAS made up the drawbacks of sole WAS fermentation by improving the bioavailable substrates and low C/N ratio to stimulate the microbial activities. The bioconversion efficiency of substrates for SCFAs generation was therefore enhanced. The humic acids present in LW could also play positive roles in SCFAs promotion. Moreover, the performance of LW/WAS fermentation was highly correlated with appropriate fermentation pH. The fermentative bacteria responsible for SCFAs production were highly enriched and the activities of key hydrolases, acid-forming enzymes and ATP concentration were greatly improved at pH 8.

Fermented non-digestible fraction from combined nixtamalized corn (Zea mays L.)/cooked common bean (Phaseolus vulgaris L.) chips modulate anti-inflammatory markers on RAW 264.7 macrophages

Chronic non-communicable diseases (NCDs) are low-level inflammation processes affected by several factors including diet. It has been reported that mixed whole grain and legume consumption, e.g. corn and common bean, might be a beneficial combination due to its content of bioactive compounds. A considerable amount would be retained in the non-digestible fraction (NDF), reaching the colon, where microbiota produce short-chain fatty acids (SCFAs) and phenolic compounds (PC) with known anti-inflammatory effect. The aim of this study was to estimate the anti-inflammatory potential of fermented-NDF of corn-bean chips (FNDFC) in RAW 264.7 macrophages. After 24 h, FNDFC produced SCFAs (0.156-0.222 mmol/l), inhibited nitric oxide production > 80% and H₂O₂ > 30%, up-regulated anti-inflammatory cytokines (I-TAC, TIMP-1) > 2-fold, and produced angiostatic and protective factors against vascular/tissue damage, and amelioration of tumor necrosis factor signalling and inflammatory bowel disease. These results confirm the anti-inflammatory potential derived from healthy corn-bean chips.

Analysis of short-chain fatty acids in human feces: A scoping review

Short-chain fatty acids (SCFAs) play a crucial role in maintaining homeostasis in humans, therefore the importance of a good and reliable SCFAs analytical detection has raised a lot in the past few years. The aim of this scoping review is to show the trends in the development of different methods of SCFAs analysis in feces, based on the literature published in the last eleven years in all major indexing databases. The search criteria included analytical quantification techniques of SCFAs in different human clinical and in vivo studies. SCFAs analysis is still predominantly performed using gas chromatography (GC), followed by high performance liquid chromatography (HPLC), nuclear magnetic resonance (NMR) and capillary electrophoresis (CE). Performances, drawbacks and advantages of these methods are discussed, especially in the light of choosing a proper pretreatment, as feces is a complex biological material. Further optimization to develop a simple, cost effective and robust method for routine use is needed.

Short-chain fatty acids production and microbial community in sludge alkaline fermentation: Long-term effect of temperature

Sludge alkaline fermentation has been reported to achieve efficient short-chain fatty acids (SCFAs) production. Temperature played important role in further improved SCFAs production. Long-term SCFAs production from sludge alkaline fermentation was compared between mesotherm (30±2°C) and microtherm (15±2°C). The study of 90days showed that mesotherm led to 2.2-folds production of SCFAs as microtherm and enhanced the production of acetic acid as major component of SCFAs. Soluble protein and carbohydrate at mesotherm was 2.63-folds as that at microtherm due to higher activities of protease and α-glucosidase, guaranteeing efficient substrates to produce SCFAs. Illumina MiSeq sequencing revealed that microtherm increased the abundance of Corynebacterium, Alkaliflexus, Pseudomonas and Guggenheimella, capable of enhancing hydrolysis. Hydrolytic bacteria, i.e. Alcaligenes, Anaerolinea and Ottowia, were enriched at mesotherm. Meanwhile, acidogenic bacteria showed higher abundance at mesotherm than microtherm. Therefore, enrichment of functional bacteria and higher microbial activities resulted in the improved SCFAs at mesotherm.

Long-term effect of pH on short-chain fatty acids accumulation and microbial community in sludge fermentation systems

Long-term effect of pH (4, 10, and uncontrolled) on short-chain fatty acid (SCFA) accumulation, microbial community and sludge reduction were investigated in waste activated sludge (WAS) fermentors for over 90days. The average SCFAs accumulation was 1721.4 (at pH 10), 114.2 (at pH 4), and 58.1 (at uncontrolled pH) mg chemical oxygen demand (COD)/L. About 31.65mgCOD/L was produced at pH 10, accounting for 20% of the influent COD. Illumina MiSeq sequencing revealed that Alcaligenes (hydrolic bacteria) and Erysipelothrix (acidogenic bacteria) were enriched at pH 10, while less acidogenic bacteria existed at pH 4 than pH 10, and no acidogenic bacteria were detected at the uncontrolled pH. The ratios of archaea to bacteria were 1:41, 1:16, and 1:9 at the pH of 10, 4, and uncontrolled. This study elucidated the effects of pH on WAS fermentation, and established the correlation of microbial structure with SCFAs accumulations and sludge reduction.

Enhancing the quantity and quality of short-chain fatty acids production from waste activated sludge using CaO2 as an additive

The effect of calcium peroxide (CaO2) addition on anaerobic fermentation of waste activated sludge (WAS) was investigated. The lab-scale experiments were conducted at 35 °C with CaO2 doses ranging from 0.05 to 0.3 g/g VSS. The performances of hydrolysis and acidification of WAS were significantly enhanced by CaO2 addition, whereas the production of methane was inhibited. Maximum total short-chain fatty acids (TSCFA) production (284 mg COD/g VSS) occurred at a CaO2 dose of 0.2 g/g VSS and fermentation time of 7 d, which was 3.9 times higher than the control tests. Further, CaO2 addition led to the conversion of other SCFAs to acetic acid. Acetic acid comprised 60.2% of TSCFA with the addition of 0.2 g CaO2/g VSS compared with 45.1% in the control tests. The mechanism of improved SCFAs generation was analyzed from the view of both chemical and biological effects. Chemical effect facilitated the disintegration of WAS, and improved the activities of both hydrolytic enzymes and acid-forming enzymes. Illumina MiSeq sequencing analysis revealed that bacteria within phylum Firmicutes increased significantly due to CaO2 addition, which played an important role in the hydrolysis and acidification of WAS. In addition, CaO2 oxidized most refractory organic contaminants, which were difficult to biodegrade under the ordinary anaerobic condition. Hydroxyl radicals were the most abundant reactive oxygen species released by CaO2, which played a key role in the removal of refractory organic compounds. We developed a promising technology to produce a valuable carbon source from WAS.

Enhanced short chain fatty acids production from waste activated sludge conditioning with typical agricultural residues: carbon source composition regulates community functions

BACKGROUND

A wide range of value-added by-products can be potentially produced from waste activated sludge (WAS) through anaerobic fermentation, among which short-chain fatty acids (SCFAs) are versatile green chemicals, but the conversion yield of SCFAs is usually constrained by the low carbon-to-nitrogen ratio of the original WAS. Conditioning of the WAS with cellulose-containing agricultural residues (ARs) has been reported to be an efficient and economical solution for balancing its nutrient components. However, contributions of different ARs to SCFAs production are still not well understood.

RESULTS

To optimize SCFAs production through carbon conditioning of WAS, we investigated the effects of two typical ARs [straws and spent mushroom substrates (SMSs)] on WAS hydrolysis and acidification in semi-continuous anaerobic fermentation. Straw-conditioning group showed a threefold increase in short-chain fatty acids yield over blank test (without conditioning), which was 1.2-fold higher than that yielded by SMS-conditioning. The maximum SCFAs yield in straw-conditioning groups reached 486.6 mgCOD/gVSS (Sludge retention time of 8 d) and the highest volumetric SCFAs productivity was 1.83 kgCOD/([Formula: see text]) (Sludge retention time of 5 d). In batch WAS fermentation tests, higher initial SCFAs production rates were achieved in straw-conditioning groups [49.5 and 52.2 mgCOD/(L·h)] than SMS-conditioning groups [41.5 and 35.2 mgCOD/(L·h)]. High-throughput sequencing analysis revealed that the microbial communities were significantly shifted in two conditioning systems. Carbohydrate-fermentation-related genera (such as Clostridium IV, Xylanibacter, and Parabacteroides) and protein-fermentation-related genus Lysinibacillus were enriched by straw-conditioning, while totally different fermentation genera (Levilinea, Proteiniphilum, and Petrimonas) were enriched by SMS-conditioning. Canonical correlation analysis illustrated that the enrichment of characteristic genera in straw-conditioning group showed positive correlation with the content of cellulose and hemicellulose, but showed negative correlation with the content of lignin and humus.

CONCLUSIONS

Compared with SMSs, straw-conditioning remarkably accelerated WAS hydrolysis and conversion, resulting in higher SCFAs yield. Distinct microbial communities were induced by different types of ARs. And the communities induced by straw-conditioning were verified with better acid production ability than SMS-conditioning. High cellulose accessibility of carbohydrate substrates played a crucial role in enriching bacteria with better hydrolysis and acidification abilities.