Gut microbiota metabolites, redox status, and the related regulatory effects of probiotics.

The gut microbiota protects the host from pathogens by competitive exclusion, particularly by occupying attachment sites, consuming nutrient sources, and producing certain metabolites [ 66 ]. When the intestinal microbiota is abnormal, harmful bacteria proliferate excessively, causing significant oxidative stress [ 3 ]. Previous studies have reviewed the role of the gut microbiota in oxidative stress [ 67 , 68 ]. In recent decades, an increasing number of studies have shown that gut microbiota metabolites are involved in the regulation of oxidative stress by gut microbiota. The gut microbiota metabolises food into a series of metabolites, including fatty acids, TMAO, and TRP metabolites, and polyphenolic metabolites. Gut bacteria also metabolise and produce bacterial metabolites such as LPS. These gut microbiota metabolites can regulate the redox status and affect the physiological processes of hosts in a variety of ways ( Table 1 ). Table 1 Relationships between gut microbiota metabolites and oxidative stress. Table 1 Gut microbiota metabolites Oxidative stress Oxidative stress index References SCFA ↓ Proinflammatory cytokines↓ [ 7 ] ROS↓ [ 7 ] NADPH oxidase/ROS signalling↓ [ 69 ] antioxidant enzyme activity↑ [ 64 ] MCFA ↓ Cell viability, T-AOC, and expression of antioxidant-related genes↑, ROS production↓ [ 70 ] H 2 O 2 release↓ [ 71 ] weight gain, cardiovascular disease risk↓ [ 72 ] LCFA ↑ Glutathione depletion in mitochondria↑ [ 73 ] Apoptosis, oxidative stress, chronic inflammatory responses↑ [ 74 ] ROS formation↑ [ 75 ] LPS ↑ Nox2↑ [ 76 , 77 ] NF-κB pathway↑ [ 78 ] ROS↑ [ [61] , [79] ] 8-OHDG↑ [ 79 ] TRP Kynurenine ↑ formation of H 2 O 2 ↑, ROS↑ [ 80 ] cell death, ROS pathway↑ [ 81 ] 5-HT ↓ inflammatory cytokines↓ [ 11 ] ROS↓ [ 82 ] Nrf2↑ [ 83 ] Indole and its derivatives ↓ Nrf2, NQO1, GSTA1/2, UGT1A6↑ [ 84 ] 5-MIAA ↓ Nrf2↑ [ 85 ] free radicals↓ [ 86 ] [ 87 ] TMAO ↑ ROS-TXNIP-NLRP3 signalling↑ [ 10 ] Polyphenolic metabolites Luteolin metabolite ↓ NO synthase↓ [ 88 ] Protocatechuic acid ↓ GPX, CAT, GSH↑, MDA, ROS↓ [ [89] , [90] ] 5-(3′,4′-dihydroxyphenyl)-γ-valerolactone ↓ antioxidant capacity↑ [ 91 ] Abbreviations: SCFA, short-chain fatty acids; MCFA, medium-chain fatty acids; LCFA, long-chain fatty acids; LPS, lipopolysaccharides; 5-HT, 5-hydroxytryptamine; 5-MIAA, 5-methoxyindoleacetic acid; TRP, tryptophan; TMAO, trimethylamine- N -oxide; ROS, reactive oxygen species; NADPH, Nicotinamide Adenine Dinucleotide Phosphate; Nox2, NADPH oxidase 2; NF-κB, nuclear factor kappa B; 8-OHDG, 8-hydroxy-2 deoxyguanosine; Nrf2, erythroid 2-related factor 2; NQO1, NADPH/quinone oxidoreductase; GSTA1/2, glutathione S -transferase; UGT1A6, UDP-glucuronosyltransferase 1A6; NO, nitric oxide; GPX, glutathione peroxidase; CAT, catalase; GSH, glutathione; MDA, malondialdehyde. Relationships between gut microbiota metabolites and oxidative stress. Abbreviations: SCFA, short-chain fatty acids; MCFA, medium-chain fatty acids; LCFA, long-chain fatty acids; LPS, lipopolysaccharides; 5-HT, 5-hydroxytryptamine; 5-MIAA, 5-methoxyindoleacetic acid; TRP, tryptophan; TMAO, trimethylamine- N -oxide; ROS, reactive oxygen species; NADPH, Nicotinamide Adenine Dinucleotide Phosphate; Nox2, NADPH oxidase 2; NF-κB, nuclear factor kappa B; 8-OHDG, 8-hydroxy-2 deoxyguanosine; Nrf2, erythroid 2-related factor 2; NQO1, NADPH/quinone oxidoreductase; GSTA1/2, glutathione S -transferase; UGT1A6, UDP-glucuronosyltransferase 1A6; NO, nitric oxide; GPX, glutathione peroxidase; CAT, catalase; GSH, glutathione; MDA, malondialdehyde. Short-, medium-, and long-chain fatty acids (SCFA, MCFA, LCFA) are natural compounds in animal and plant tissues that participate in cell metabolism. SCFA are mostly generated by colonic bacteria and are predominantly metabolised by enterocytes and liver. MCFA derive mostly from dietary triglycerides [ 92 ]. LCFA, such as myristic acid (14:0), palmitate (16:0), and stearic (18:0) are more abundant. Palmitic acid is the most abundant in human food (average consumption of 20–30 g/day) [ 93 ] ( Fig. 1 ). Fig. 1 Mechanisms of gut microbiota producing fatty acids (SCFA, MCFA, LCFA) and their functions. Fig. 1 Mechanisms of gut microbiota producing fatty acids (SCFA, MCFA, LCFA) and their functions. SCFA, particularly acetic acid, propionic acid, and butyric acid, are metabolites produced by beneficial intestinal bacteria through the digestion and fermentation of dietary fibre [ 94 ]. The colon is the major site of SCFA formation, where it is also rapidly absorbed as an energy source [ 95 , 96 ]. Studies have shown that SCFA can target the mammalian G protein-coupled receptor pair GPR41 and GPR43 and inhibit histone deacetylase activity to maintain the gastrointestinal epithelial barrier, regulate insulin secretion, and inhibit inflammation [ 97 ]. In clinical investigations and animal models, SCFA administration and an increased intake of dietary fibre have been shown to exert protective effects in patients with inflammatory bowel conditions, allergic airway diseases, obesity, type 1 diabetes, and type 2 diabetes due to the inhibition of proinflammatory cytokines and ROS [ 7 ]. The ratio for acetate, propionate, and butyrate to be generated in the intestines is 60:25:15 [ 98 ]. These three SCFA have been shown to possess antioxidant capacities. It has been suggested that acetate promotes β-cell survival under stressful conditions [ 99 ] and ameliorates sepsis-induced acute kidney injury by inhibiting NADPH oxidase/ROS signalling [ 69 ]. Propionate ameliorates colitis in mice and inhibits LPS-induced inflammation by improving the antioxidant system and reducing oxidative stress [ 100 , 101 ]. Butyrate stimulates mitochondrial biogenesis, improves respiratory capacity, and activates antioxidant enzyme activity to improve antioxidant capacity [ 64 ]. MCFA are fatty acids with 8–12 carbon, including octanoic acid, capric acid and lauric acid. Attention has been paid on MCFA due to its different metabolic pathways and low lipid toxicity compared with LCFA [ 102 ]. It is shown that gut microbiota, including Caproiciproducens, Pseudoramibacter, norank_f_Eubacteriaceae, and Oscillibacter catabolised lactate into MCFA [ 103 ]. Moreover, MCFA have been reported to have a variety of physiological functions. Bourque et al. found that MCFA could prevent weight gain and decrease cardiovascular disease risk in overweight women [ 72 ]. MCFA treatment significantly increased the cell viability, total antioxidant capacity (T-AOC), and expression of antioxidant-related genes in AML12 cells under oxidative stress condition, and reduced ROS production [ 70 ]. Mett and Müller (2021) also reported that the saturated MCFA decanoic acid (10:0) reduced the oxidative stress level in neuroblastoma cell lines [ 71 ]. LCFA are essential molecules in signalling pathways such as peroxisome proliferator-activated receptors (PARPs) [ 104 ]. Certain gut bacteria can produce LCFA using the intake lipid, which later is absorbed by the enterocytes. Pujo et al. showed that the LCFA (3-hydroxyoctadecaenoic acid) derived by Escherichia coli Nissle 1917 had anti-inflammatory capacities [ 105 ]. Research in human have revealed that a high consumption LCFA (DHA, EPA) could reduce the risk of breast, prostate and colon cancer [ 106 ]. However, Torres et al. found that he LCFA, such as valproic acid, can induce glutathione depletion in mitochondria, causing mitochondrial dysfunction and oxidative stress [ 73 ]. Wang et al. also found that LCFA (palmitate) induced apoptosis, oxidative stress and chronic inflammatory responses in the hepatic cells with steatosis [ 74 ]. Using skeletal muscle mitochondria, the study by Seifert et al. demonstrated that even a low supply of LCFA is associated with ROS formation in excess of that generated by NADH-linked substrates [ 75 ]. LPS, also known as endotoxins, are a major component of the cell wall of gram-negative bacteria. Bacterial LPS typically consist of a hydrophobic domain known as lipid A, a nonrepeating ‘core’ oligosaccharide, and a distal polysaccharide (or O-antigen) [ 107 , 108 ]. LPS is released during the death and disintegration of gram-negative bacteria, and it is a potent inducer of the innate immune system [ 109 ]. This endotoxin is recognised by the interplay of receptor and serum-secreted proteins, such as LPS-binding protein and toll-like receptor 4 (TLR-4) [ 110 ]. The stimulation of immune cells by pathogens and LPS produces a variety of molecular species, including ROS, via the respiratory burst as part of the host response to infection [ 6 ]. There is much evidence to suggest that LPS can induce oxidative stress both in vitro and in vivo [ 111 ]. Gut-derived LPS causes neurodegenerative diseases and increases postprandial oxidative stress by activating NADPH oxidase 2 (Nox2) [ 76 , 77 ]. LPS from E. coli contributes to atherosclerotic damage via TLR-4-mediated oxidative stress [ 112 ]. Systemically circulating LPS interacts with TLR-4 to activate inflammatory responses and oxidative stress in type 1 cardiorenal syndrome through the nuclear factor kappa B (NF-κB) pathway [ 78 ]. Moreover, LPS significantly increases the production of ROS and 8-OHDG in bovine mammary epithelial cells and also reduces the viability of the cells [ 79 ]. TRP is the least abundant essential amino acid [ 113 ] and the only amino acid that contains an indole structure [ 114 ]. Its main functions are to participate in protein synthesis and to be a precursor for a variety of bioactive compounds, such as serotonin, indole, and indole derivatives [ 115 , 116 ]. There are three major metabolic pathways of TRP in the gastrointestinal tract: the kynurenine, 5-hydroxytryptamine (5-HT), and microbial metabolism pathways [ 117 ]. TRP metabolites, such as kynurenine, 5-HT, indole, and its derivatives, are related to oxidative stress [ 8 , 11 , 118 , 119 ]. The kynurenine pathway accounts for 90 % of TRP catabolism. Wang et al. showed that the deregulated kynurenine pathway plays a key role in the increased prevalence of cardiovascular disease by regulating inflammation, oxidative stress, and immune activation [ 81 ]. Increased levels of kynurenine have been shown to cause cell death through the ROS pathway in nature killer cells [ 120 ]. Moreover, kynurenine is also able to photooxidize cysteine, NADH, and ascorbic acid in vitro. These photooxidation processes are responsible for the age-related depletion of reduced glutathione and/or formation of H 2 O 2 in the lens [ 80 ]. Moreover, a recent study has also unvealed that oxidative stress decreased the fertility and induced gut microbiota dynsbiosis by disrupting the kynurenine pathway [ 121 ]. 5-HT, also known as serotonin, is an indoleamine monoamine produced by TRP metabolism. Tryptophan is metabolised by tryptophan hydroxylase to hydroxytryptophan (5-HTP) and is subsequently metabolised to serotonin by amino acid decarboxylase [ 122 ]. Most 5-HT in the body is synthesised, stored, and released from enterochromaffin cells, a subset of enteroendocrine cells, in the intestinal mucosa. 5-HT is an important gastrointestinal signal that mediates many GI functions, including peristalsis, secretion, vasodilation, and perception of pain and nausea, through the activation of 5-HT receptors [ 123 ]. The gut microbiota can also metabolise TRP directly to 5-HT and alter the availability of TRP in the host [ 124 ]. For instance, a study by Yano et al. demonstrated that indigenous spore-forming microbes directly stimulated intestinal serotonin synthesis and release in mice [ 125 ]. Additionally, 5-HT is one of the oldest known neurotransmitters that has been associated with numerous disease states, including depression, social phobia, migraine, irritable bowel syndrome, and pulmonary hypertension [ 126 , 127 ]. Research has shown that 5-HT is an effective antioxidant substance that reduces oxidative stress in murine RAW264.7 macrophages and inhibits the production of inflammatory cytokines [ 11 ]. Furthermore, 5-HT can inhibit the production of ROS, thereby reducing oxidative stress in phagocytes [ 82 ]. Agonists of 5-HT receptors can improve cellular antioxidant capacity by upregulating the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) in astrocytes [ 83 ]. TRP is largely absorbed in the small intestine, but some of it reaches the colon where it is broken down by intestinal bacteria to produce indole and indole derivatives. Indole is a well-known synthesised signalling molecule [ 128 ]. Indole can be used as a signalling molecule and antibacterial agent to regulate intestinal communities of bacteria, fungi, and viruses. Indole derivatives such as indole-3-pyruvic acid, indole-3-acetic acid (IAA), and indole acetaldehyde can be used as ligands for aromatic receptors (AHR), which are widely expressed by cells in the immune system and promote beneficial bacteria over pathogenic bacteria in the gut microbiota to regulate host intestinal and systemic homeostasis. The activated TRP-AHR pathway can induce the expression of downstream cytokines, such as interleukin-22 and interleukin-17, thereby regulating intestinal immune function [ 129 ]. One of the best-studied antioxidant responses mediated by AHR is the activation of Nrf2. Evidence suggests that AHR directly activates Nrf2; up-regulates NQO1, GSTA1/2, and/or UGT1A6; and exerts antioxidant effects [ 84 ]. Therefore, the activation of the TRP-AHR pathway may also regulate oxidative stress. Additionally, evidence shows that indole can induce antibiotic detoxification by activating the protective mechanisms of antioxidant stress and improving the antibiotic resistance capabilities of the E. coli population [ 8 ]. IAA, a derivative of indole, has been shown to have anti-inflammatory properties and to alleviate non-alcoholic fatty liver disease in mice by attenuating the effects of hepatic lipogenesis and oxidative and inflammatory stress [ 118 , 119 ]. Furthermore, 5-methoxyindoleacetic acid (5-MIAA), which is produced in the 5-HT metabolic pathway, can scavenge free radicals [ 86 , 87 ]. In a recent study, 5-MIAA activated Nrf2 to reduce hepatic oxidative injury [ 85 ]. Choline, a trimethylamine-containing compound and part of the phosphatidylcholine head group, is metabolised by gut microbiota to produce an intermediate compound known as trimethylamine (TMA) [ 130 ]. TMA is sequentially absorbed and transported through the portal vein to the liver, where it is oxidized to TMAO by flavin-containing monooxygenase (mainly FMO3) [ 131 ]. The gut microbiota is important for TMAO metabolism. Research using gnotobiotic mice suggests that gut bacteria are essential for transforming dietary compounds into TMA [ 132 ]. Moreover, some studies performed with antibiotics in rats and humans have revealed that the production of TMA and TMAO is almost completely suppressed by the use of broad-spectrum antibiotics. However, TMAO levels return to normal one month after antibiotic withdrawal [ [133] , [134] , [135] ]. TMAO had long been thought to be a choline metabolism waste product that had no function in humans; however, there is now convincing evidence suggesting an association between TMAO and various pathophysiological processes, including endothelial dysfunction, arteriosclerosis, and cardiovascular disease [ 136 , 137 ]. Thus, the harmful effects of TMAO may be related to oxidative stress. Studies suggest that HFD-induced obesity leads to the elevated circulation of gut microbiota-generated TMAO, which contributes to renal interstitial fibrosis and dysfunction by promoting renal oxidative stress and inflammation [ 138 ]. Sun et al. reported that TMAO activates ROS-TXNIP-NLRP3 inflammasome signalling in endothelial cells [ 10 ]. Additionally, TMAO can lead to endothelial dysfunction in old-aged rats through vascular inflammation and oxidative stress [ 139 ], and it can accelerate mouse endothelial cell senescence and vascular aging through oxidative stress [ 140 ]. Dietary polyphenols have beneficial roles in decreasing oxidative stress. The gut microbiota can use the unabsorbed polyphenols as substrates for their enzymatic activities [ 141 ]. It is reported that luteolin can be metabolised to luteolin-3-O-β-D-glucuronide and luteolin-3-O-sulfate, and luteolin helps to reduce the NO synthase to maintain the cardiac health by preventing the damage from free radicals. In addition, cyanidin can be metabolised to protocatechuic acid [ 142 , 88 ]. Zhang et al. revealed that protocatechuic acid increased the activities of GPX and CAT and decreased MDA level in aged rats [ 89 ]. Moreover, Yip et al. also demonstrated that protocatechuic acid improved the mitochondrial function by reducing ROS level and increasing GSH [ 90 ]. Furthermore, 5- (3′, 4′-dihydroxyphenyl)-γ-valerolactone, a metabolite of catechin, also had antioxidant capacity in the intestinal epithelial cells [ 91 ]. Taken together, various gut microbiota metabolites are closely associated with oxidative stress. Therefore, regulating these metabolites and related gut bacteria may have antioxidant effects and impove the health of hunman and animals. Since it is well-known that probiotics can regulate gut microbiota, the following focuses on the regulatory effect of probiotics on the gut metabolites to resist oxidative stress.

Jinshan Zhao: Conceptualization, Resources, Writing – original draft. Fan Zhao: Investigation, Writing – original draft. Junmeng Yuan: Formal analysis, Investigation. Huawei Liu: Investigation. Yang Wang: Conceptualization, Writing – review & editing.

This study was supported by the 10.13039/501100001809 National Natural Science Foundation of China (Grant No. 32102586), the 10.13039/501100007129 Shandong Provincial Natural Science Foundation (Grant No. ZR2020QC183) and the State Key Laboratory of Animal Nutrition .

Oxidative stress is a process by which excess free radicals attack macromolecules and cause tissue damage. Most living organisms can utilise antioxidant enzymes and endogenous antioxidants to remove excess free radicals in a timely manner and maintain the redox balance. However, when stimulated by stressors or pathogens, the antioxidant system is often insufficient at resisting high levels of free radicals, and oxidative stress occurs. ROS are the most active free radicals, and they include superoxide radicals, hydroxyl radicals, and hydrogen peroxide. Excessive ROS can induce DNA hydroxylation, protein denaturation, and lipid peroxidation, leading to apoptosis and tissue damage [ 14 ]. Oxidative stress and oxidative damage are ubiquitous. Many factors, such as environmental pollution, infection, cigarette smoking, and excessive exercise, can lead to oxidative stress [ 15 ], which consequently contributes to aging [ 16 ], intestinal damage [ 17 ], heart disease [ 18 ], metabolic syndrome [ 19 ], diabetes [ 20 ], and cancer [ 21 ]. Oxidative stress is related to metabolic diseases, including obesity, type 2 diabetes mellitus (T2DM), and diabetic kidney disease [ 15 ]. Oxidative stress has been reported to trigger obesity by increasing preadipocyte proliferation and adipocyte differentiation [ 22 ]. In individuals with obesity, chronic oxidative stress and associated inflammation cause insulin resistance, metabolic dysfunction, diabetes, and cardiovascular diseases by disrupting signalling and metabolism [ 23 ]. In addition, obesity-associated insulin resistance greatly increases oxidative stress and the risk of hypertension, dyslipidaemia, T2DM, atherosclerosis, and non-alcoholic fatty liver disease [ 24 ]. Increasing evidence on the pathophysiology of T2DM has revealed that oxidative stress is the main causative factor responsible for the pathogenesis of insulin resistance, impaired glucose utilisation, and abnormal hepatic glucose production, ultimately leading to T2DM [ 25 ]. Additionally, intrarenal oxidative stress plays an important role in the initiation and progression of diabetic kidney disease. Excessive ROS production triggers renal fibrosis and inflammation by promoting lipid peroxidation, DNA damage, protein modification, and mitochondrial dysfunction, leading to significant tissue damage [ 26 , 27 ]. Oxidative stress is closely related to the formation and progression of cancer [ 21 ]. Oxidative stress causes DNA strand breakage and incomplete enzymatic repair mechanisms, which ultimately lead to chromosomal changes. In turn, these changes result in genetic amplification, alterations in gene expression, and loss of heterozygosity, leading to carcinogenesis [ 28 ]. Breast cancer is the leading type of cancer in women and the leading cause of death among female cancers. An increased level of 8-hydroxy-2 deoxyguanosine (8-OHDG) was found in the DNA of breast cancer tissues compared to that of late-stage cancer tissue [ 29 ]. Moreover, the mitochondrial manganese-dependent antioxidant enzyme superoxide dismutase 2 (SOD2) is considered a tumour suppressor gene. The expression of this gene is frequently decreased in human cancers, as its overexpression leads to severe inhibition of cell proliferation for various cell types, including breast cancer cells [ 30 ]. SOD2 expression in the breast metastatic tumour cell lines MDA-MB-435 and UACC-893 is 2–3 times lower than that in the non-tumoral epithelial cell line MCF-10 A, suggesting that oxidative stress may play an important role in the early phases of carcinogenesis [ 31 ]. Prostate cancer is the most common type of cancer and the second leading cause of cancer-related death in men after lung cancer [ 32 ]. Oxidative stress may disrupt the physiologically active proteome of the cell, serve as a stimulus to trigger stress-response signal transduction pathways, and modulate processes favouring cell survival by activating continuous proliferation. Together, these factors enable prostate cancer to progress [ 33 ]. In addition to ROS-induced DNA damage, other factors induced by oxidative stress can also lead to prostate cancer. Studies have found that androgens can cause oxidative stress and lead to benign prostatic hyperplasia or prostate cancer if benign prostatic hyperplasia overgrowth occurs [ 34 ]. Moreover, the prostate cancer epigenome is characterised by global changes in DNA methylation [ 35 ]. Of all the genes known to be methylated in prostate cancer, glutathione S-transferase P1 ( GSTP1 ) is the most frequently methylated. GSTP1 is a detoxifying enzyme that catalyses conjugation reactions between potentially damaging oxidants, electrophiles, and glutathione [ 36 , 37 ]. GSTP1 expression is diminished or absent in prostate cancer, and this absence is tightly regulated by hypermethylation of the CpG island in the promoter [ 38 ]. Additionally, exposure to cigarette smoke is a primary risk factor for lung cancer. Many carcinogens and free radicals have been identified in cigarette smoke. Moreover, cigarette smoking leads to the influx and activation of phagocytes, which release ROS [ 39 ]. In a study by Gęgotek et al. [ 40 ], disturbances in antioxidant capacity and enhanced DNA oxidative modifications were observed in 88 % of adenocarcinoma patients and 81 % of squamous cell lung carcinoma patients. Moreover, the inhibition of oxidative stress has a significant anticancer effect on lung cancer [ 41 , 42 ]. Oxidative stress has also been identified to play a key role in the pathogenesis of subfertility in human and animals. Excessive production of ROS has been associated with impaired sperm motility, concentration, and morphology [ 43 ]. A study conducted by du Plessis et al. revealed that H 2 O 2 could induce human sperm motility and increase the overall ROS and NO levels [ 44 ]. ROS also affect a variety of physiologic functions of the ovary, including ovarian steroid genesis, oocyte maturation, ovulation, leading to polycystic ovary syndrome, endometriosis and preeclampsia [ 45 ]. In poultry, oxidized fish oil in feed caused oxidative stress in laying hens, and induced the decreased number of dominant follicles [ 46 ]. Dexamethasone injection also elevated the oxidative stress in laying hens and decreased the laying rate, feed-egg ratio, and increased the expressions of apoptosis-related genes in preovulatory follicle, small white follicle, medium white follicle and ovary [ 47 ]. In Guinea pigs, cypermthrin treatment led to oxidative stress, and decreased sperm count, mobility, and the percentage of spermatozoa with normal plasma membrane [ 48 ]. The gut not only is the main organ for food digestion, absorption, and metabolism, but it also plays an important role in secretion and immunoregulation. The gastrointestinal (GI) tract is the body's largest interface between the internal and external environments. Thus, the external environment, resident immune cells, gut microbiota, and dietary factors make the GI tract prone to ROS attack. The intestinal barrier, which plays a fundamental role in overall health, is composed of four major lines of defence. The first is a mechanical barrier, consisting of lined intestinal epithelial cells and capillary endothelial cells. The epithelial and endothelial cells come into the closest possible contact in the most apical part of the lateral cell membranes at tight junctions, which interconnect the cells and restrict the passage of ions, molecules, and cells through the paracellular space. The second is an immune barrier that consists of gut-associated lymphoid tissue, effector and regulatory T cells, IgA-producing B (plasma) cells, group 3 innate lymphoid cells, resident macrophages, and dendritic cells in the lamina propria. The third line of defence is a chemical barrier, which comprises digestive fluid, antimicrobial peptides, mucin, and other compounds secreted by the intestinal epithelial cells to prevent bacterial adhesion. The final line of defence is a biological barrier, which consists of the gut microbiota responsible for resistance to pathogenic bacteria colonisation [ 49 , 50 ]. Excessive ROS levels can damage the tight junctions of the intestinal epithelial cells and disrupt the GI tract barrier, thereby increasing intestinal permeability. Studies have shown that H 2 O 2 -induced oxidative stress decreases the expression of tight junctions in porcine intestinal epithelial and Caco-2 cells, leading to systemic endogenous stress syndrome [ 17 , 51 ]. In addition, oxidative stress can cause colitis and increase intestinal permeability [ 52 , 53 ]. The cytoskeleton plays an important role in maintaining the structure and function of tight junctions in the epithelium. In a previous study, exposure to 0.5 mM H 2 O 2 for 30 min almost completely destroyed the connection between the actin cytoskeleton and tight junction proteins in Caco-2 cells, resulting in the destruction of the actin skeleton structure [ 54 , 55 ]. Banan et al. [ 56 ] found that oxidants decompose the cytoskeleton and destroy the integrity of the epithelial barrier. Furthermore, excessive ROS levels impair the intestinal immune barrier. Bhattacharyya et al. [ 57 ] suggested that oxidative stress stimulates polymorphonuclear neutrophils, contributing to inflammation in a variety of GI diseases. In glucocorticoid-stressed rats, the level of secreted IgA decreased, while intestinal permeability and bacterial adherence to the mucosa increased [ 58 ]. Chang et al. [ 59 ] demonstrated that trichlorfon exposure decreased the activity of antioxidants and upregulated the levels of the pro-inflammatory cytokines TNF-α and IL-1β. Furthermore, oxidative stress can inhibit mucin synthesis and secretion, which damages the chemical barrier function of the intestine. Zhao et al. [ 60 ] reported that bisphenol A causes oxidative stress by significantly increasing the accumulation of ROS and inhibiting the secretion of mucin 2. Cervantes-García et al. [ 61 ] found that indomethacin induced enteropathy by elevating intestinal oxidative stress, and it decreased occludin and mucin 2 expression in the intestinal tissue. Moreover, the gut microbiota constitutes a natural defensive barrier against infection. It is involved in numerous protective, structural, and metabolic roles in the intestinal epithelium and plays a major role in maintaining gut homeostasis [ 62 ]. Studies have revealed that increased mitochondrial ROS production decreases intestinal microbial diversity and lowers antimicrobial defences [ 63 , 64 ]. In addition, a long-term high-fat diet (HFD) influences the gut microbiota by changing the redox state. Increased ROS and malondialdehyde (MDA) levels in HFD-induced obese mice showed a strong positive association with Escherichia coli and Enterococcus but a negative association with Lactobacillus [ 65 ]. Due to the adverse effects of oxidative stress, it is urgent to clarify the mechanisms and methods for the antioxidation.

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In conclusion, this review describes how the disruption of redox homeostasis may lead to gastrointestinal and systemic diseases. The recent surge in gut microbiota studies due to advancements in sequencing techniques and metabolomics has enriched our knowledge of the role of gut microbiota and related metabolites in oxidative stress. A subset of gut microbiota metabolites, such as fatty acids, LPS, TRP metabolites, TMAO and polyphenolic metabolites have been shown to be relevant to redox regulation. Moreover, the use of probiotics appears to be a reasonable approach for modulating gut microbiota metabolites, thereby preventing or avoiding oxidative stress and related diseases ( Fig. 2 ). However, studies on this topic have been limited and further research is required. In future studies, the combination of microbiome analysis with other omics, such as proteomics and metabolomics, will help us clarify the interactions between gut microbiota metabolites and host redox status and will illuminate the mechanisms of probiotics in regulating oxidative stress through gut microbiota metabolites. Fig. 2 Probiotics modulate oxidative stress through gut microbiota metabolites. Fig. 2 Probiotics modulate oxidative stress through gut microbiota metabolites.

Oxidative stress is a stress reaction that occurs when the dynamic equilibrium between oxidants and antioxidants is destroyed. Under normal physiological conditions, free radicals are dynamically balanced. However, under certain conditions, the redox balance of the body is destroyed and the production rate of reactive oxygen species (ROS) exceeds that of the antioxidant system, leading to the accumulation of oxygen free radicals and oxidative stress and ultimately resulting in intestinal damage, metabolic syndrome, cancer, and other diseases [ 1 , 2 ]. The gut microbiota, or the microbial population in the intestines, plays a key role in immune function, digestion, and metabolism. Microbial contact-induced epithelial ROS generation is an extremely conserved phenomenon across different phyla. This mechanism is the general means by which bacterial communities can affect redox homeostasis in the host [ 3 ]. In recent years, oxidative stress has been shown to affect the structure and function of the gut microbiota [ 4 ] and alter the composition of gut microbiota metabolites [ 5 ]. Moreover, the metabolites of the microbiota also play an important role in gut microbiota-mediated redox regulation [ 6 , 7 ]. Specific metabolites, such as fatty acids, lipopolysaccharides (LPS), tryptophan (TRP) metabolites, trimethylamine- N -oxide (TMAO) and polyphenolic metabolites, have been reported to regulate oxidative stress [ [8] , [9] , [10] , [11] ]. Given the above evidence, scientists have begun to believe that targeting the gut microbiota and its metabolites may reveal novel strategies for the prevention and intervention of oxidative stress. Among these strategies, probiotics have received the most attention [ 12 ]. In our previous study, we summarised the antioxidant mechanisms of probiotics and found that probiotics may exert the antioxidant ability by various mechanisms such as up-regulating the antioxidases of the host, down-regulating the activities of enzymes producing ROS and regulation gut microbiota [ 13 ]. Here, we mainly focus on the role of gut microbiota metabolites in redox homeostasis and evaluate the current understanding of the effects of probiotic-related gut microbiota metabolite regulation on oxidative stress. In this review, articles examined in open literature containing the probiotics, oxidative stress, gut microbiota metabolites are reviewed in terms of fatty acids, LPS, TRP metabolites, TMAO and polyphenolic metabolites.

Previous studies have shown that probiotics have antioxidant potential. Eating probiotics alone or in foods containing probiotics can reduce oxidative damage, improve the efficiency of free radical scavenging, and alter the activities of key antioxidant enzymes [ 143 ]. In particular, lactic acid bacteria have significant functional properties, such as antioxidant and immunoregulatory capacities, and are considered as ideal probiotics [ 144 ]. There is evidence that lactic acid bacteria can be used as a supplement to contribute to the downregulation of ROS-forming enzymes, modulation of Nrf2 and NF-κB transcription factors, and improvement of gastrointestinal inflammation and other chronic diseases caused by oxidative stress [ 145 ]. Lactobacillus plantarum 9 (LB-9) has been reported to ameliorate dextran sodium sulfate-induced colitis in mice by inhibiting the TNF-α-mediated apoptosis of intestinal epithelial cells and effectively inhibiting oxidative stress responses and inflammation [ 146 ]. Lactobacillus delbrueckii increases intestinal integrity by improving the intestinal structure and tight junctions, while also enhancing antioxidant functions via activation of the TLR-Btk-Nrf2 signalling pathway [ 147 ]. Bifidobacterium is an extensively studied probiotic that is an important member of the gut microbiota of humans and animals. Supplementation of Bifidobacterium animal can increase the jejunal T-AOC level and decreased the jejunal MDA level in pigs [ 148 ], also increased the SOD, catalase (CAT), and gluthathione peroxidase (GPX) activites and decreased the MDA in the plasma and livers of d -galactose-treated rats [ 149 ]; these effects exemplify the potent antioxidant properties of these probiotics. Moreover, the culture supernatant, intact cells, and intracellular cell-free extracts of Bifidobacterium animalis can effectively scavenge free radicals, significantly enhance the activities of antioxidative enzymes, and reduce the MDA content, lipofuscin level, and monoamine oxidase activity in mice [ 150 ]. Bacillus is widely used as feed additive, as it can grow under harsh environmental conditions (e.g., acidic, alkaline, or high-temperature environments) due to the formation of spores. Dietary supplementation with different concentrations of Bacillus subtilis has been reported to enhance the antioxidative status and digestive enzyme activities of quails [ 151 ]. Tang et al. [ 152 ] showed that Bacillus amyloliquefaciens SC06 induced AKT-FOXO signalling pathway-mediated autophagy to alleviate oxidation-induced apoptosis and cell damage in IPEC-J2 cells. Bacillus amyloliquefaciens SC06 can also alleviate IPEC-1-induced oxidative stress by modulating the Nrf2-Keap 1 signalling pathway and decreasing ROS production [ 153 ]. Furthermore, Bacillus amyloliquefaciens SC06 increased the hepatic capacity of HFD-induced obese mice by regulating the Nrf2-Keap1 signalling pathway, decreasing the Firmicutes/Bacteroidetes ratio, and increasing TM7 abundance to reduce HFD-related lipid accumulation and liver injury [ 154 ]. The antioxidant capacity of probiotics is determined based on numerous processes, including the chelating of metal ions, production of antioxidases and antioxidant metabolites, increases in the antioxidase activities and antioxidant metabolites of the host, the mediation of antioxidant signalling pathways, and the reduction of ROS-producing enzyme activities [ 13 ]. Moreover, studies have shown that probiotics can regulate the oxidative status of hosts by affecting their gut microbiota [ 155 ]. Additionally, recent reports have indicated that the antioxidant role of probiotics in the gut microbiota is closely related to gut microbiota metabolites [ 156 ]. The regulations of gut microbiota metabolites by probiotics are discussed below and summarised in Table 2 . Table 2 Probiotics regulating gut microbiota and related metabolites. Table 2 Probiotics Altered gut microbiota Gut microbiota metabolites Functions References Mixture of 10 Lactobacillus strains and 4 yeast strains Lactic acid bacteria↑ Bifidobacterium ↑ Clostridium leptum ↑ Roseburia ↑ Escherichia coli ↓ SCFA↑ Alleviate type 2 diabetes in db/db mice [157] Lactobacillus paracasei ssp.  Paracasei BCRC 12188 Lactobacillus plantarum BCRC 12251 Streptococcus thermophilus BCRC13869 Bifidobacterium ↑ Lactobacillus ↑ Enterobacteriaceae↓ SCFA↑ Improve the memory of learning abilities in aging mice [156] Lactobacillus fermentum  296 Lactobacillus  sp.↑ SCFA↑ Attenuate cardiometabolic disorders in HFD-treated rats [158] DBR (a mixture of 9 probiotics and 10 digestive enzymes) – SCFA↑ Decrease LDL and increase HDL levels [159] – Lactobacillus ↑ SCFA↑ Improve the reproduction performance of laying hens [ 157 ] multispecies probiotic – MCFA↑ Decrease depressive disorder [162] Lactobacillus rhamnosus GG – MCFA in feces↑ Attenuate alchoholic fatty liver disease [163] Enterococcus faecalis – LCFA↑ Reduce adiposity and activate beige fat formation in HFD-fed mice [165] Enterococcus faecalis AG5 – LCFA↑ Mitigate HFD induced obesity in Wistar rats [166,167] Bifidobacterium infantis Lactobacillus acidopilus Bacillus cereus – LPS↓ Delay the progression of NAFLD [ 158 ] Lactobacillus subtilis Streptococcus faecium Escherichia coli ↓ LPS↓ Restore the bowel flore and decrease LPS level in patients with alcoholic hepatitis [ 159 ] Lactobacillus casei Shirota Lactobacillus fermentium Lactobacillus plantarum Lactobacillus salivarius Bifidobacterium breve – LPS↓ Modulate the production of cytokines [171] Bifidobacterium bifidum Lactobacillus rhamnosus Bifidobacterium longum Bifidobacterium longum subsp. Infantis – LPS↓ Decrease colonic LPS concentrations amd reduce the proinflammatory tone [172] Lactobacillus reuteri – Indole derivative↑ – [ 160 ] Lactobacillus rhamnosus GG Lactobacillus ↑ 5-MIAA↑ Protect against liver oxidative injury [ 85 ] Lactobacillus plantarum ZDY04 Lachnospiraceae↑ Erysipelotrichaceae↑ Bacteroidaceae↑ Mucispirillum↓ TMAO, TMA↓ Reduce TMAO-induced atherosclerosis [175] Lactofermented annurca apple puree – TMAO↓ Control of plasma HDL-C and TMAO levels [176] Enterobacter aerogenes  ZDY01 – TMAO, TMA↓ Attenuate choline-induced atherosclerosis [177] Bifidobacterium animalis subsp. Lactis LKM512 – TMA↓ Reduce the risk of arteriosclerosis development [178] Lactobacillus plantarum CLC 17 – phenolic metabolites↑ – [182] Lactobacillus plantarum , Lactobacillus paraplantarum and Lactobacillus pentosus – phenolic acids↑ Regulate glucose metabolism [183] Abbreviations: SCFA, short-chain fatty acids; MCFA, medium-chain fatty acids; LCFA, long-chain fatty acids; MCFA, medium-chain fatty acids; LPS, lipopolysaccharides; 5-MIAA, 5-methoxyindoleacetic acid; TMAO, trimethylamine- N -oxide; TMA, trimethylamine. Probiotics regulating gut microbiota and related metabolites. Abbreviations: SCFA, short-chain fatty acids; MCFA, medium-chain fatty acids; LCFA, long-chain fatty acids; MCFA, medium-chain fatty acids; LPS, lipopolysaccharides; 5-MIAA, 5-methoxyindoleacetic acid; TMAO, trimethylamine- N -oxide; TMA, trimethylamine. Many studies have indicated that probiotics may resist oxidative damage by regulating SCFA levels. Wang et al. [ 161 ] reported that composite probiotics (specifically a mixture of ten Lactobacillus strains and four yeast strains) can improve gut barrier function by increasing the abundance of SCFA-producing bacteria, the SCFA levels, and the expression of Claudin-1 and Mucin-2, while also decreasing the levels of E. coli and LPS. A probiotic mixture (PM-1) containing Lactobacillus paracasei subsp. Paracasei BCRC 12188, Lactobacillus plantarum BCRC 12251, and Streptococcus thermophilus BCRC13869 increased SCFA production and Nrf2 expression in d -galactose-treated aging mice, and its possible antioxidant effect may have been triggered by SCFA [ 156 ]. Acid-producing probiotics modulate vasodilatation and hypotension by increasing SCFA levels [ 162 ]. Ichim et al. reported that DBR, a mixture of nine probiotics and 10 digestive enzymes, reduced serum cholesterol and cardiovascular risk by increasing propionic acid levels [ 163 ]. Moreover, EUBIO-BPSG is a phytogenic formulation composed of B. pilosa phytochemicals which has antioxidant capacity [ 164 ]. Tran Nguyen Minh et al. revealed that EUBIO-BPSG could promote the growth of Lactobacillus to produce SCFA in the in vitro co-culture experiments, and also improve the egg production of laying hens and increased fecal Lactobacillus in vivo, implying the SCFA induced by EUBIO-BPSG may elevate the antioxidant capacity of laying hens to improve the reproductive performance [ 157 ]. It is reported that MCFA can also be regulated by probiotics. Capric acid, an MCFA, was elevated after 28 days of the multispecies probiotic intake in individuals with major depressive disorder compared with the placebo group [ 165 ]. Besides, compared with the alcohol-treated mice, MCFA were increased in feces, but decreased in liver when the mice were co-exposed to Lactobacillus rhamnosus GG [ 166 ]. Although it is known that alcohol treatmen can lead to oxidative stress [ 167 ], there is a lack of direct evidence for probiotics improving antioxidant capacity by regulating MCFA. In addition, Quan et al. demonstrated that Enterococcus faecalis , a probiotic, produced an unsaturated LCFA (myristoleic acid). Enterococcus faecalis and its metabolite myristoleic acid can reduce adiposity by brown adipose tissue activation and beige fat formation in HFD-fed mice [ 168 ]. Mishara et al. also found that Enterococcus faecalis AG5 produced several LCFA such as tetradecanoic acid, hexadecanoic acid, octadecanoic acid and bis(6-methylheptyl) phthalate, and mitigated HFD induced obesity in Wistar rats [ 169 ]. As the changes in intestinal microbiota following HFD could promote metabolic endotoxemia and trigger metabolic disorders, including obesity and oxidative stress [ 170 ]. Therefore, the above findings imply that Enterococcus faecalis may alleviate obesity and oxidative stress by the production of LCFA. Probiotics can combat LPS to reduce oxidative stress. A probiotic mixture of Bifidobacterium infantis , Lactobacillus acidopilus , and Bacillus cereus has been shown to downregulate serum LPS and liver TLR-4 levels through the LPS/TLR-4 signalling pathway and delay the progression of non-alcoholic fatty liver disease [ 158 ]. Lactobacillus subtilis and Streptococcus faecium can reduce gut-derived microbial LPS by restoring the gut microbiota of patients with alcoholic hepatitis [ 159 ]. Furthermore, selected probiotics can inhibit the binding of LPS to the CD14 receptor, thereby reducing the overall activation of NF-κB and the production of proinflammatory cytokines [ 171 ]. Rodes et al. revealed that specific probiotic strains, such as Lactobacillus rhamnosus , Bifidobacterium bifidum and Bifidobacterium longum subsp. Infantis , can decrease colonic LPS concentrations, which might further reduce the secretion of inflammatory cytokines in macrophages [ 172 ]. As for TRP metabolites, gut microbe-related SCFA promote colonic serotonin production through their effects on enterochromaffin cells [ 173 ]. There is evidence that a reduction in gut Bifidobacterium species is associated with deficient TRP metabolism in the intestine and marked gastrointestinal dysfunction in mice with autism spectrum disorder [ 174 ]. Lactobacillus reuteri provides indole derivatives of dietary L-TRP, such as indole-3-lactic acid, which activates AHR and leads to the downregulation of Thpok and the reprogramming of CD4 + intraepithelial lymphocytes into CD4 + CD8αα+ double-positive intraepithelial lymphocytes. This reprogramming may be useful for treating disorders that may be sensitive to double-positive intraepithelial lymphocytes, such as inflammatory bowel diseases [ 160 ]. Additionally, a recent study has shown that the Lactobacillus -derived molecule 5-MIAA can activate hepatic Nrf2, promote the transcription of genes involved in the cellular antioxidant response, and improve the hepatic antioxidant capacity of mice [ 85 ]. Lactobacillus plantarum ZDY04 has been reported to significantly reduce serum TMAO and caecal TMA levels by modulating the relative abundance of Lachnospiraceae, Erysipelotrichaceae, Bacteroidaceae, and Mucispirillum in mice, and it can also reduce the development of atherosclerosis [ 175 ]. Tenore et al. reported a reduction in TMAO levels in subjects with a risk of cardiovascular disease following the consumption of lactofermented Annurca apple puree [ 176 ]. Enterobacter aerogenes ZDY01 can decrease caecal TMA and serum TMAO levels by utilising caecal TMA as a nutrient [ 177 ]. Additionally, Bifidobacterium animalis subsp. Lactis LKM512 may decrease TMA levels in healthy individuals [ 178 ]. However, other studies have reported contradictory results. Tripolt et al. found that Lactobacillus casei supplementation had no significant effect on the TMAO levels in subjects with metabolic syndrome [ 179 ]. Malik et al. also noted that Lactobacillus plantarum 299v did not significantly alter plasma TMAO levels in 20 men with stable coronary artery disease [ 180 ]. It is reported that polyphenols exert their beneficial effects by elevating the abundance of probiotic bacterial such as Bifidobacteriaceae and Lactobacillaceae and inhibiting pathogenic bacteria such as E. coli , Clostridium perfringens and Helicobacter pylori [ 181 ]. In turn, probiotic bacteria promote the metabolite of polyphenols. Gil-Sánchez et al. revealed that inclusion of Lactobacillus plantarum CLC 17 to a Dynamic Gastronitestinal Simulator (simgi®) led to a greater formation of phenolic metabolites [ 182 ]. Ueda et al. demonstrated that probiotics, such as Lactobacillus plantarum , Lactobacillus paraplantarum and Lactobacillus pentosus , express tannin metabolising enzymes like tannase and gallate decarboxylase to produce phenolic acids that may play an essential role in glucose metabolism and thus affect the development and treatment of obesity [ 183 ].

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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