Gut microbiota consists of pathogenic bacteria and beneficial bacteria, and the balance between them will be critical to maintain gut-healthy status. Here, we tried to explore the effects of AOS on gut probiotics and pathogenic bacteria.

AOS strengthen gut health via the metabolites of probiotics.

Probiotics produce large amounts of postbiotic metabolites, which play important roles in regulating human health (Pelton, 2020). Vitamin K has been regarded as an underappreciated mediator of gut microbiota community dynamics (Ellis et al., 2021). B vitamins are responsible of crucial microbial bioactivities, metabolism and signaling. Vitamins C, E and B2 are widely reported antioxidants, which affect luminal redox balance (Pham et al., 2021). On the other hand, most gut probiotic, are capable of synthesizing vitamin K and most of B vitamins, including biotin, cobalamin, folates, nicotinic acid, pantothenic acid, pyridoxine, riboflavin, and thiamine (Figure 1A; Gu and Li, 2016). GSH is a major antioxidant and capable of eliminating ROS-caused damage to the most cells, and can be synthesized by Lactobacillus salivarius (Yuan et al., 2022). Antimicrobial peptides (AMPs) are a class of small peptides, which play a key role in the innate immune system of gut health (Zong et al., 2020). Some probiotic lactic acid bacteria produce bacteriocins (Figure 1A), a kind of small cationic peptides that kill the pathogen cells via pore formation (Tiwari, 2022). Defensins belong to cationic antimicrobial peptides, which prevent bacterial infection (Pero et al., 2019) and are critical elements of innate immunity in gut health (Chung and Raffatellu, 2019; Shulman et al., 2021). Phenyllactic acid, a product of phenylalanine catabolism, is the main bioactive metabolite produced by Saccharomyces boulardii (Fu et al., 2022). Phenyl lactic acid improves Samonella Typhimurium-induced colitis by modulating regulating the components of gut microbiota, SCFA production and inflammatory activities (Zhou et al., 2021). Volatile organic compounds (VOCs) are well-known biomarkers of gastrointestinal diseases and nutritional situation (Rondanelli et al., 2019). Bacillus amyloliquefaciens and some yeasts synthesize high level of VOCs (Ngo et al., 2020; Shruthi et al., 2022). AOS has been report to affect the most related lactic acid bacteria (Zhuge et al., 2020; Le et al., 2021; Yudiati et al., 2021), yeasts (Chávez-Falcón et al., 2022), and other probiotics (Guleria et al., 2016) under simulated gastrointestinal conditions.

Akkermansia muciniphila is considered to be favorable intestinal probiotics by improving gut metabolic activities and immune capacity (Zhang et al., 2019; Xu et al., 2020). Both probiotics Lactobacillus gasseri and Lactobacillus reuteri strains have clinically proven to have health-promoting effects by improving lipid metabolism and inflammation (De Gregorio et al., 2020; Wang et al., 2020). AOS intervention promotes the growth of A. muciniphila (Table 1), L. reuteri (Table 2), and L. gasseri (Table 2), which play important roles in the improvement of lipid metabolism (Wang et al., 2020). Meanwhile, AOS diet increases concentrations of the metabolites, such as SCFA via gut microbiota (Tables 1, 2; Mizuno et al., 2020), including acetic acid, propionic acid, and butyric acid (Wang et al., 2020). SCFAs are the main metabolites of gut microbiota, and closely associated with intestinal barrier integrity. SCFAs exert protective functions for intestinal homeostasis by strengthening gut barrier (Figure 1A; De Gregorio et al., 2020; Liu P. et al., 2021). The receptors and transporters of SCFA regulate the antibiotic long-term effects on the colonic mucosa and main the insusceptibility to experimental colitis (Holota et al., 2019). SCFAs exert protective effects on intestinal barrier function by inhibiting NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3, expressed predominantly in macrophages) inflammasome and autophagy (Feng et al., 2018).

SCFA-producing bacteria include Lactobacillus (Kusumo et al., 2019), Bifidobacterium (Fang et al., 2021), Clostridiales (Gargari et al., 2018), and Lachnospiraceae species (Vacca et al., 2020). Bacteroides have been regarded as the predominant genus in the gastrointestinal tract and are associated with a higher concentration of beneficial SCFA (Fernandez-Julia et al., 2021). AOS may stimulate the growth of Bacteroides and Lachnospiraceae species (Cherry, 2020). Alginate microcapsules are reported to improve the bioactivities of Bifidobacterium species (Zhang Z. et al., 2021). Other work also shows AOS treatment increases the probiotic species Lactobacillus (Table 2) and Akkermansia species (Table 1), and reduces pathogenic species Bacteroides and Parabacteroides (Figure 1A; Table 1; Li et al., 2020). Moreover, the correlation analysis shows AOS improve gut homeostasis by increasing SCFAs production via the probiotics Roseburia, Bifidobacterium (Table 2), and Akkermansia (Table 1) in the model with fumonisin B1-induced intestinal damage (Figure 1A; Li et al., 2022). Other work indicates that AOS treatment maintains mucosal barrier function and inhibits immune injury by increasing Firmicutes and Actinobacteria and reducing Bacteroidetes species (He et al., 2021). AOS increase the proportions of SCFA probiotic producers by increasing the abundance of Ruminococcaceae, Coprococcus, Roseburia, and Faecalibacterium (Figure 1A; Table 2; Han et al., 2021). AOS has been found to reduce Salmonella colonization and promote the improvement of intestinal barrier in broiler chickens (Table 1; Yan et al., 2011). AOS ameliorate high-salt-induced intestinal injury by increasing barrier and absorption functions by increasing the abundance of Lactobacillus, Bifidobacterium, Faecalibaculum (Table 2) and Mucispirillum (Table 1; Zhang Z. et al., 2022). AOS Administration significantly upregulate the levels of IL-10 and TLR-3, and gut barrier biomarkers claudin-1 and mucin 2 (MUC2; Zhuge et al., 2020). MUC2 is a key secretory protein observed in the human intestinal system (Liu et al., 2020).

AOS improve antibacterial infection of the intestine.

Figure 1A shows AOS may improve gut microbiota by inducing probiotics producing biosurfactants, SCFA, hydrogen peroxide, antimicrobial peptides, expolysaccharides, vitamin and antioxidants so on. Biosurfactants are active compounds that are produced from cell surface and the most biosurfactants obtained from a large number of lactic acid bacteria. Probiotic biosurfactants exert beneficial biological activity on the gut microbiome and against pathogen infection via an immense antimicrobial, anti-adhesive, and antibiofilm potential (Figure 1A; Patel et al., 2021; Satria, 2022). Exopolysaccharides are the long-chain polymers of carbohydrates and can produce a protective surface layer in intestinal environment for gut microbiota (Figure 1A). Exopolysaccharide from L. rhamnosus controls dextran sulfate sodium (DSS) -induced colitis in mice by improving gut microbiota (Wan et al., 2022). Probiotics also produce hydrogen peroxide (Figure 1A), which plays a critical role in the treatment of H pylori infection (Alipour and Mofarrah, 2022; Nabavi-Rad et al., 2022).

Staphylococcus aureus can cause a wide variety of infections from skin to life-threating infections (Scolari et al., 2020). It has been widely reported that S. aureus infections are associated with intestinal symptoms, and its influence may be related to lipid raft-associated trafficking of sucrase–isomaltase and thereby may trigger secondary functional gastrointestinal diseases (Mergani et al., 2021). The film which is made of AOS shows the antimicrobial activity against two common pathogenic bacteria S. aureus and E. coli and two pathogenic fungi Aspergillus niger and Penicillium digitatum (Aloui et al., 2021). AOS films are also found to prevent S. aureus and methicillin-resistant S. epidermidis infections by inducing very high antibacterial activity against these life-threatening pathogens (Martí et al., 2019). AOS exert protective functions against enterotoxigenic E. coli-induced animal intestinal barrier injury and its infection (Wan et al., 2021).

Diarrhea is the main symptom of intestinal bacterial infection. AOS have been found to prevent diarrhea by regulating the abundance of Alloprevotella, Bacteroides, Parabacteroides and Rikenellaceae (Figure 1A; Yao et al., 2021). The probiotic mixtures with L. casei, L. bulgaricus, and Streptococcus thermophiles can prevent diarrhea in the elderly (Figure 1A; Mallina et al., 2018). There is a synergic effect between AOS and probiotics (L. bulgaricus and S. thermophilus; Yudiati et al., 2021).

Figure 1B shows that AOS also reduce the abundance of some pathogenic bacteria, which include Escherichia, Shigella, and Peptoniphilus species so on (Figure 1B; Han et al., 2021). These pathogens threaten gut health by producing bacterial toxin (Asadpoor et al., 2021a), hyaluronidase (Tomlin and Piccinini, 2018), lipoteichoic acid (Szentirmai et al., 2021), M protein(Kolesinski et al., 2022), and other structures (bacterial capsule and pili) to help them stay in gut tracts (Gupta et al., 2019). Some pathogens are closely associated with intestinal infection and diarrhea (Figure 1B; Peh et al., 2022). Bacterial toxin produced by the pathogenic species may be the main reason for causing diarrhea (Figure 1B; Dubreuil, 2019).

AOS improve antioxidant properties of the intestine.

AOS increase the abundance of Clostridium orbiscindens, Ruminococcus gnavus, Eggerthella lenta, Clostridium spp. and Clostridiales species in intestine by improving their fermentation levels (Figure 1A; An et al., 2013). Clostridium butyricum increases intestinal antioxidant properties and resistance to adverse stress (Duan et al., 2019). Gut Clostridia species display antioxidant activities by producing the antioxidants glutathione, ascorbic acid and uric acid (Million et al., 2020). The probiotics L. casei, L. bulgaricus, and Streptococcus thermophiles also show antioxidant functions in the elderly (Figure 1A; Mallina et al., 2018; Shori et al., 2022). AOS significantly repair FB1-induced intestinal damage, inflammation, and oxidative stress (including T-SOD and MDA) by increasing the probiotic abundance, such as Roseburia, Bifidobacterium, and Akkermansia, and SCFAs production (Li et al., 2022). SCFAs, such as acetic, propionic, and butyric acids, can exert health-promoting properties by increasing antioxidant activities. Acetic acid promotes the enzymatic antioxidant ability and stimulates antioxidant responses (Gurdo et al., 2018). Propionic exerts anti-inflammatory and antioxidant properties in addition to its antimycobacterial activity. It has therapeutic potential for the treatment of the patient populations driven by excessive inflammation and tissue damage (Negatu et al., 2020). Sodium butyrate is found to increase the oxidative status by activating of Nrf2-dependent signaling (Ma et al., 2018). AOS improve oxidative stress in kidney-damaged model by increasing the levels of SOD and CAT, and reducing the levels of MDA by increasing the abundance of L. johnsonii and L. reuteri.

AOS increase anti-inflammatory properties of the intestine by improving gut microbiota.

AOS also reduce the abundance of pathogenic bacteria including Escherichia, Shigella, and Peptoniphilus species (Han et al., 2021), which are closely associated with intestinal inflammation (Peh et al., 2022). AOS protect against intestinal injury by decreasing the abundance of Enterobacteriaceae, Enterococci (Wang et al., 2006), Bacteroidetes (He et al., 2021), Desulfovibrionaceae, Helicobacter, Peptococcus, and Tyzzerella in the intestine (Figure 1B; Huang J. et al., 2021). The abundance of Bacteroides is negatively associated with the amounts of inflammatory monocytes and positively linked with the levels of the metabolites in intestine (Ejima et al., 2021). Most of these species have capsules to resist bacterial phagocytosis (Figure 1B) and can induce intestinal inflammation and affect intestine permeability (Zhang Y. et al., 2020). AMP-activated protein kinase (AMPK) and NF-κB p65 are critical integrators of cytokine signals and have been observed to be reduced after AOS intervention (Tran et al., 2019; Wan et al., 2020, 2021; Huang J. et al., 2021). AOS can improve obesity-related metabolic abnormalities and inflammation. AOS intervention reverses the gut dysbiosis by increasing the relative abundance of Lactobacillus (Table 2) and Akkermansia species (Table 1) and decreasing the abundance of Bacteroides and Parabacteroides species (Table 1; Li et al., 2020b). AOS supplementary diet reduces the levels of inflammatory cytokines IL-1β and CD11c. AOS supplement ameliorates the inflammatory responses in a DSS-induced colitis model by reducing the levels of TNF-α, COX-2, IL-1β, IL-6, and increasing IL-10 level via the regulation of the abundance of Firmicutes, Actinobacteria, and Bacteroidetes (He et al., 2021). AOS reduce gut inflammation by decreasing D-Lactic acid and LPS levels, and TLR-4 and MAPK expression. Furthermore, AOS also considerably improves the abundance of Lactobacillus, Roseburia, and Lachnospiraceae) and reduce the abundance of Helicobacter, Peptococcus, and Tyzzerella (Huang J. et al., 2021).

Although anti-inflammatory properties of AOS have been widely reported, the roles of AOS in intestinal inflammation networks remain widely unclear. Figure 2A shows AOS may improve intestinal inflammation cells [Th1, Th2, Th9, Th17, Th22 and regulatory T (Treg) cells] and their secreted inflammatory cytokines via SCFA, butyrate and essential metabolites produced from probiotics. These factors are closely associated with various gut inflammatory diseases or gut health. T Helper (Th) Cells (T helper type 1 (Th1; IFN-γ, IL-2, and TNF-β; Wang J. et al., 2018; Pradhan et al., 2019), Th2 (IL-4, IL-5, IL-9, IL-10, IL-13, IL-25, IL-31, and IL-33; Pradhan et al., 2019), Th9 (IL-9, IL-10, IL-21, IL-33, and IL-36; Hoeppli et al., 2019), Th17 (IL-6, IL-8, IL-17A, IL-17F, IL-21, IL-22, and IL-26; Kulkarni et al., 2018; Wang J. et al., 2018; Alrafas et al., 2019), Th22 (IL-13, IL-22, IL-26 and TNF-α; Sanaii et al., 2019; Shohan et al., 2020) and regulatory T (Treg) cells (IL-10, TGF-β and IL-35; Kulkarni et al., 2018; Alrafas et al., 2019) are involved with gut inflammation (Figure 2A). The effects of AOS on these cells or cytokines are explored. Th cell responses may be affected by the metabolites of probiotics during the prevention of gut inflammation (Figure 2A; Liu X.-J. et al., 2019; Di Gangi et al., 2020). Figure 2B shows the protein–protein interaction (PPI) networks of main inflammatory cytokines, which are also closely associated with gut inflammation and health and may be affected by AOS treatment. Among their secreted cytokines, Cytoscape analysis shows the hub genes of protein–protein interaction networks include IL-10, IL-6, IL-4, IL-2, IL-1β, and TNF-α (Figure 2B), which are closely associated with gut inflammation and health. AOS treatment enhances IL-10 secretion by affecting gut microbiota when compared with LPS treated animal models (Zhang et al., 2022a). AOS intervention decreases the levels of IL-1β, IL-6, and TNF-α, and increases the levels of IL-10 (Zhang et al., 2022a). AOS treatment affects the levels of Th1 cytokines (IL-2 and IFN-γ), and Th2 cytokines (IL-4 and IL-6; Wang W. et al., 2018). IL-10-deficiency will induce colitis in an animal model (Kang et al., 2018) while deletion of IL-6 can exacerbate colitis by inducing systemic inflammation (Ye et al., 2020). IL-1β plays an important role in the pathogenesis of IBD (Mao et al., 2018). IL-2 induces colitis by activating STAT5, which is required for optimal IL-22 production (Bauché et al., 2020). Circulating pro-inflammatory cytokine Il-4 is found to be increased in an IBD model (Zhou et al., 2019). TNF-α-producing CD4+ effector memory T cells stimulate intestinal development and regulate inflammatory responses (Schreurs et al., 2019). Anti-TNF-α therapy inhibits proinflammatory activities of mucosal neutrophils in IBD (Zhang et al., 2018).

AOS can affect many different types of T helper cells and regulate immune-induced diseases. AOS and their derivatives have been found to reduce allergic responses by altering Th1/Th2 balance forward to Th1 cells, inhibiting IgE production, and maintaining the amounts of mast cells (Vo et al., 2015). Oral administration of β-d-mannuronic acid (the main component of AOS) down-regulates the levels of Th17 cell, IL-17 and IL-6 in patients with ankylosing spondylitis (Figure 2; Fattahi et al., 2018). AOS can significantly increase the immunosuppressive activity of Treg cells (CHEN and YAO, 2019).

AOS can increase the abundance of most Lactobacillus species in intestine. L. casei CRL431 and L. paracasei CNCMI-1518 have been found to show protective function against Salmonella typhimurium, which can cause Th1-type cell-mediated immunity by increasing IFN-γ/IL-4 ratio (Figure 2; Lemme-Dumit et al., 2021). The over-expression of IL-12p40 contributes to Th2-type inflammatory responses in the large intestine of mice with allergic diarrhea (Hino et al., 2004). IBD, including ulcerative colitis (UC) and Crohn’s disease (CD), are related to the imbalances of gut microbiota. B. adolescentis regularly treatment may improve the therapeutic effects for IBD by stimulating protective Treg/Th2 response and gut microbiota remodeling (Fan et al., 2021). Th9 cells also promote IBD risk by increasing IL-9 levels and Bifidobacterium species may be related to the prevention of IBD (Vyas and Goswami, 2018; Jakubczyk et al., 2020). The pro-inflammatory responses induced by Th1, Th2, and Th17 is also associated with IBD pathogenesis. IL-25, an IL-17 regulate Th2- and Th9-type immune responses, and IL-25 is important cytokine against T. spiralis infection (Angkasekwinai et al., 2017). AOS may also affect Th9- and Th17-type inflammatory responses by increasing the abundance of Bifidobacterium species (Figure 2; Li et al., 2022).

Th22 cells are closely associated with the severity of IBD, and may be involved in the inflammatory process of IBD (Xia and Li, 2019). Th2/Th17/Th22 responses are common to E. coli-derived vesicles but specific differences are found in Th1 and Treg cell responses (Diaz-Garrido et al., 2019). The balance of Th17/Treg cells plays an important role in the prevention of IBD progression and development (Yan et al., 2020). Lactobacillus acidophilus has been found to inhibit IBD development by regulating the balance between Th17 and Treg cells (Park et al., 2018). Bifidobacterium longum and L. plantarum alleviate allergic rhinitis in mice by revering Th2/Treg balance (Kim et al., 2019). AOS extend the viability of Lactobacillus species (Tiani et al., 2018), increase the abundance of Bifidobacterium species (Li et al., 2022), reduce the abundance of E. coli (Han et al., 2021), and may also affect Th2/Th17/Th22/Treg cell response (Figure 2). All the results suggest that there may exist a linkage between AOS and Th cells or their cytokines via the regulation of gut microbiota.

AOS show protective roles for intestinal homeostasis by increasing the abundance of probiotics (Table 1), which are linked with reducing intestinal inflammation (Zhang T. et al., 2021), improved gut barrier (Ottman et al., 2017), reduced bacterial infection (Peng et al., 2020), against tissue injury (Si et al., 2022), the balance of gut microbiota (Aabed et al., 2019), and protective surface layers (Meng et al., 2021; Figure 1A). However, the most other functions are not deeply explored in the present study. For instance, AOS show anti-adhesive properties by hampering pathogenic bacteria E. coli (Asadpoor et al., 2021b). Bacterial adhesion pili is also associated with pathogenic adhesion in intestine (Figure 1B; Shori et al., 2022) but the possible association with AOS intervention has been seldom reported. There are many great challenges to elucidate the mechanism for the effects of AOS on intestinal homeostasis and inflammation networks. The exact mechanism for the direct interaction between AOS and gut probiotics or pathogenic bacteria has not been established yet. There may be some different receptors between probiotic and pathogenic bacteria, which show different susceptibilities with AOS. Further investigation in IPEC-J2 cells found that AOS acts its function through mannose receptor signaling pathway (Zhao et al., 2020). The polysaccharide utilization loci have been found in Bacteroides species, which may include secreted glycosidases, a complement of cell surface glycan-binding proteins, oligosaccharide receptor or transporters, and a series of metabolic enzymes (Zafar and Saier, 2021). Such a receptor is expected to be discovered in some gut probiotics or gut pathogens to address the important issue.