Gut microbiota interacts with inflammatory responses in acute pancreatitis

Abstract

Acute pancreatitis (AP) is one of the most common acute abdominal conditions, and its incidence has been increasing for years. Approximately 15–20% of patients develop severe AP (SAP), which is complicated by critical inflammatory injury and intestinal dysfunction. AP-associated inflammation can lead to the gut barrier and function damage, causing dysbacteriosis and facilitating intestinal microbiota migration. Pancreatic exocrine deficiency and decreased levels of antimicrobial peptides in AP can also lead to abnormal growth of intestinal bacteria. Meanwhile, intestinal microbiota migration influences the pancreatic microenvironment and affects the severity of AP, which, in turn, exacerbates the systemic inflammatory response. Thus, the interaction between the gut microbiota (GM) and the inflammatory response may be a key pathogenic feature of SAP. Treating either of these factors or breaking their interaction may offer some benefits for SAP treatment. In this review, we discuss the mechanisms of interaction of the GM and inflammation in AP and factors that can deteriorate or even cure both, including some traditional Chinese medicine treatments, to provide new methods for studying AP pathogenesis and developing therapies.

Introduction

Multiple organ dysfunction may occur during the early phase of severe acute pancreatitis (SAP), resulting in a high fatality rate. However, over time, the patient enters a second stage, which accompanies the infection and is another cause of the high mortality rate of SAP. ¹ Studies have shown that most pancreatic and extra-pancreatic organ infections are caused by the translocation of intestinal bacteria; such infections result in pancreatic necrosis and sepsis, causing late death in patients with SAP. ² The gut microbiota (GM) is mutualistic with the human body under certain steady states; some gut bacteria can ferment dietary fiber to form short-chain fatty acids (SCFAs), which are then absorbed by the host. ³ The intestinal mucosa can also maintain the stability of the intestinal environment through its barrier function. Once this stability is disrupted by a persistent inflammatory response in SAP, this can lead to intestinal mucosal damage and a change in the status of the intestinal microbiota. ⁴ Studies have also indicated that various types of intestinal microbiota participate in different pathological conditions, including pancreatic diseases. ⁵ The role of the intestinal microbiota in the progression of SAP has gradually been clarified in previous studies.

We have searched articles or other types of manuscripts related to the regulatory mechanism of the intestinal microbiota, inflammation, and pathogenesis of SAP or acute pancreatitis (AP) in PubMed and the China National Knowledge Infrastructure, to describe the interactions between the GM and inflammatory responses in AP. We have identified some new methods of AP pathogenesis and the development of therapies. All of our findings are described in the following chapters.

The influence of intestinal flora changes on the occurrence and development of AP

Intestinal flora migration influences the pancreatic microenvironment in AP

Impairment in microcirculation and blood volume reduction during AP can lead to ischemia and reperfusion damage in the intestinal mucosa, causing loss of intestinal barrier integrity and intestinal bacterial translocation and causing local and systemic infections.^{6 –8} Fewer antimicrobials secreted by the pancreas in AP can also lead to bacterial overgrowth in the small intestine, which further disrupts the balance of the intestinal microbiota. ⁹ The imbalance of the intestinal microbiota or mucosal damage can increase intestinal permeability, causing the translocation of bacteria from the gut to the blood or nearby tissues, such as the pancreas, increasing the risk of pancreatic infection and aggravating inflammation. ¹⁰ A study has found more than one type of bacterial DNA in the peripheral blood of patients with AP, and these DNA molecules are mainly derived from conditional pathogenic bacteria from the gut, such as Escherichia coli, Shigella flexneri, Acinetobacter lwoffii, Bacillus coagulans, and Enterobacter faecalis. ¹¹ Thus, the transfer of bacteria from the gut to the blood may cause infection of necrotic parts of the pancreas.

Recent studies have revealed that nucleotide-binding oligomerization domain 1 (NOD1), an intracellular innate immune receptor, plays a critical role in host defense functions and inflammation. This is because NOD1 can detect small peptide components derived from bacterial wall peptidoglycan and can be excited by intestinal bacteria. ¹² On the other hand, NOD1 has been reported to activate innate responses and produce nuclear factor-kappa B (NF-κB) and type 1 interferon-inducing pancreatitis and contribute to the development of pancreatitis.^13,14 Thus, NOD1 may be an intermediate regulatory factor of intestinal microbiota interaction with AP.

Previously, the microbial composition of the infected areas of pancreatic necrosis was mainly gram-negative bacteria from the gastrointestinal tract (GIT), such as Enterobacteriaceae. However, recently, Staphylococcus and Enterococcus have become dominant bacteria owing to the widespread use of prophylactic antibiotics. ¹⁵ Meanwhile, the prophylactic use of antibiotics does not reduce the risk of infection, and patients with a higher risk of infection in regional pancreatic necrosis are those who have previously received antibiotics.^16,17

Intestinal microbiota attenuates the severity of AP

A normal intestinal microbiota constitutes the intestinal mucosal biological barrier that affects intestinal peristalsis, regulates host immunity, and strengthens the epithelial barrier. ¹⁸ Studies have shown that intestinal mucosal barrier damage in patients with AP is closely associated with the imbalance of the intestinal microbiota, for example, increased abundances of the intestinal pathogenic bacteria Shigella and Enterococcus and decreased abundances of the beneficial bacteria Lactobacillus and Blautia.^{19 –21} Deng showed that the bacterial translocation rates of E. coli and Bifidobacterium and the pathological damage score of intestinal tissue were significantly higher in the intestines of SAP rats than in those of the control group, suggesting that the intestinal barrier function of SAP rats was impaired, resulting in an intestinal microbiota disorder. ²² Moreover, the Acute Physiology and Chronic Health Evaluation (APACHE)-II score, the length of the hospital stay, complications such as infections and the incidence of multiple organ dysfunction syndrome were significantly higher in patients with SAP with a GM imbalance than in individuals with intestinal microbiota ratios similar to those of healthy individuals. ⁹

It has been reported that inter-intestinal probiotics mitigated AP severity by inhibiting the activation of the NOD-like receptor family 3 (NLRP3) inflammasome in the gut,^23,24 which might be the mechanism of regulating the intestinal microbiota to reduce the degree of SAP. E. coli has been reported to induce intestinal mucosal barrier damage and aggravate AP through the activation of the toll-like receptor 4 (TLR4), myeloid differentiation factor 88 (MyD88), and p38 mitogen-activated protein kinase (MAPK) signaling pathways.^25,26 Therefore, MAPK inhibitors and TLR4-dependent Phosphoinositide 3-kinase (PI3K), V-akt murine thymoma viral oncogene homolog (AKT), and NF-kB inflammatory signaling pathway inhibitors are important in correcting GM imbalance and mitigating inflammatory responses.^27,28 SCFAs, a metabolite of intestinal bacteria, can not only provide growth energy for intestinal mucosal cells but also regulate intestinal pH, maintain the integrity of tight junction proteins between intestinal mucosal epithelial cells, improve intestinal mucosal barrier function, and significantly reduce the severity of SAP.^21,24,29 On the other hand, Bacteroides, Escherichia–Shigella and Enterococcus, are the major intestinal microbes in AP, and different levels of AP are associated with different intestinal microbiota disorders. ²⁰ In mild acute pancreatitis (MAP), Finegoldia exhibited the most significant increase, and Brucella was the species of intestinal microbiota that showed the largest decrease. Moderately severe acute pancreatitis (MSAP) patients had the most significant increase in Anaerococcus and the most significant decrease in Eubacterium hallii. The potential biomarkers of MAP are Finegoldia, E. hallii, and Lachnospiraceae. E. hallii and Anaerococcus are potential diagnostic biomarkers for MSAP (Table 1). According to reports, Firmicutes increase while Bacteroidetes decrease in acute patients’ intestines. Enterococcus in Firmicutes can adhere to host cells, invade them, and traverse the epithelial barrier. This can lead to infection and systemic inflammation. Bacteroidetes are capable of producing SCFAs, which have anti-inflammatory effects and help maintain the integrity of the intestinal barrier, thereby protecting it. Certain pathogenic bacteria within Bacteroidetes, such as E. coli and Shigella, can disrupt the intestinal mucosal barrier, resulting in severe colonic inflammation. Therefore, the imbalance between Firmicutes and Bacteroidetes can aggravate the pathogenetic condition of AP. ²⁰ SAP was associated with the most significant increase in the abundance of Enterococcus and the greatest decrease in the abundance of E. hallii. ³⁰ The expression of proinflammatory factors such as interleukin (IL)-1, IL-6, and tumor necrosis factor-α (TNF-α) in the serum of SAP patients was positively correlated with the intestinal aerobic bacteria level but negatively correlated with the level of anaerobic bacteria such as Bifidobacterium. ⁹ Perhaps modulating the gut flora can reduce the body’s inflammatory response and reduce AP severity.

Table 1.

Effects of alteration of bacteria on AP and GM and their mechanisms.

Grade of AP	Bacteria	Phylum/family	Change in bacteria	Effect on AP	Effect on GM	Mechanism	References
MAP	Faecalibacterium prausnitzii	Firmicutes/Ruminococcaceae	Beneficial and decreased	Aggravation	Negative (migration of intestinal microbiota and overgrowth of intestinal pathogens)	Promote anti-inflammatory properties, such as the production of SCFAs when digesting plant polysaccharides; promotes intestinal epithelial cell proliferation, mucosal repair, and anti-inflammatory responses by inducing IL-10 and regulating T-cell responses in the intestine.	Jandhyala et al., 31 Alkanani et al., 32 Wu et al., 33 Holmes et al., 34 Smith et al., 35 Louis and Flint, 36 Rossi et al., 37 Martín et al. 38
	Bacteroides	Bacteroidetes/Bacteroidaceae	Harmful and increased	Aggravation; infectious pancreatic necrosis	Negative	Aggravate inflammation by stimulating the intestinal mucosa to produce cytokines and several toxins; possess high numbers of antibiotic resistance genes and are risk factors for autoimmunity. High-protein and high-fat diets increase the abundance of Bacteroides. Cause a thinning of the mucus layer by transforming lactate into SCFAs, which contribute to a decrease in mucin synthesis and destroy the tight junctions, finally leading to increased gut permeability; alter the intestinal barrier function, which results in bacterial overgrowth and impaired immunity.	Hamada et al., 5 Wu et al., 33 Sundin et al., 39 de Goffau et al., 40 Smedley et al., 41 Davis-Richardson et al., 42 Mejía-León et al., 43 Brown et al., 44 Tlaskalová-Hogenová et al., 45 Zhang et al. 46
	Bacteroides dorei	Bacteroidetes/Bacteroidaceae	Harmful and increased	Aggravation	Negative	Impair the intestinal epithelial layer, manipulate immune system development, and aggravate inflammation.	Davis-Richardson et al. 42
	Bacteroides vulgatus	Bacteroidetes/Bacteroidaceae	Harmful and increased	Aggravation	Negative	An opportunistic pathogen causing intraabdominal infections and preventing remission.	Davis-Richardson et al., 42 Fujita et al., 47 Rath et al. 48
	Bacteroides fragilis	Bacteroidetes/Bacteroidaceae	Harmful and increased	Aggravation	Negative	Disrupt the tight junctions by proteolytic degradation due to metalloprotease enterotoxins, thus increasing paracellular permeability, with gut inflammation, cell damage, and a loss of microvilli.	Mejía-León et al., 43 Berkes et al. 49
	Streptococcus	Firmicutes/Streptococcaceae	Harmful and increased	Aggravation	Negative	Correlate with malabsorption or decreased levels of pancreatic enzymes associated with pancreatitis.	Hamada et al., 5 Mancabelli et al. 50
	Saccharopolyspora	Actinobacteria/Pseudonocardiaceae	Harmful and increased	Aggravation	Negative	Enhance inflammatory responses by mediating cytokines and chemokines that recruit inflammatory cells and lead to overproduction of interferon-gamma	Riquelme et al., 51 Kim et al. 52
	Proteobacteria	Proteobacteria	Harmful and increased	Aggravation	Negative	LPS-producing bacteria; increase the abundance of certain pathogens and decrease those of probiotics. Induce a tolerogenic immune program by differentially activating select toll-like receptors in monocytic cells and leading to T-cell anergy.	Zhang et al., 46 Ren et al., 53 Pushalkar et al. 54
	Prevotella	Bacteroidetes/Prevotellaceae	Harmful and increased	Aggravation	Negative	Bacteroides-dominant gut communities and LPS-producing bacteria; carbohydrate-rich diets increase the abundance of Prevotella. Impair the intestinal epithelial barrier function and aggravate inflammation.	Wu et al., 33 Mejía-León et al., 43 Ren et al. 53
	Clostridium lavalense	Firmicutes/Clostridiaceae	Beneficial and decreased	Aggravation	Negative	A butyrate-producing bacterium, maintaining intestinal epithelial barrier function and anti-inflammation; negatively correlates with serum IL-6 levels.	Hamada et al., 5 Tan et al., 9 Ren et al., 53 Takahashi et al., 55 Hague et al. 56
	Atopobium	Actinobacteria/Coriobacteriaceae	Beneficial and decreased	Aggravation	Negative	Actinobacteria produce many compounds, including antibiotics, anticancer agents, immunosuppressants, anthelmintics, antiviral agents, and extracellular enzymes.	Zhang et al., 46 van Bergeijk et al., 57 Barka et al. 58
	Bifidobacterium adolescentis or B. pseudocatenulatum	Actinobacteria/Bifidobacteriaceae	Beneficial and decreased	Aggravation	Negative	Increase the production of butyrate, which is the main energy source for intestinal epithelial cells; exert anti-inflammatory effects and inhibit bacterial translocation.	de Goffau et al., 40 Duffy 59
	Megamonas	Firmicutes	Beneficial and decreased	Aggravation	Negative	The main metabolite is acetic acid, which promotes intestinal peristalsis and relieves constipation.	Mejía-León et al., 43 Tian et al. 60
	Acidaminococcus intestini	Firmicutes	Harmful and increased	Aggravation	Negative	Positively associated with LPS-stimulated TNF-α production.	Mejía-León et al., 43 Zheng et al. 61
	Finegoldia magna	Firmicutes	Harmful and increased	Aggravation	Negative	An opportunistic pathogen, binding to histones through its surface and the extracellular adhesion protein FAF Finegoldia magna cell wall adhesion protein (FAF) and producing collagenase and gelatinase enzymes to protect bacteria and finally aggravate infection. An anaerobic gram-positive coccus acts as a virulence factor.	Hamada et al., 5 Martín et al., 38 Sundin et al., 39 Murphy et al., 62 Krepel et al. 63
	Blautia	Firmicutes/Lachnospiraceae	Beneficial and decreased	Aggravation	Negative	Reduction in the abundance of Blautia promotes the overgrowth of intestinal bacteria, increases intestinal permeability, and ultimately leads to higher concentrations of endotoxins and the activation of inflammatory cascades.	de Goffau et al., 40 Smedley et al., 41 Davis-Richardson et al. 42
	Lachnospira	Firmicutes/Lachnospiraceae	Beneficial and decreased	Aggravation	Negative	Produce butyrate and influence SCFA levels. Produce reuterin, inhibit pathogens, and suppress inflammation in the gut.	Mejía-León et al. 43
MSAP	Escherichia–Shigella	Proteobacteria/Enterobacteriaceae	Harmful and increased	Aggravation	Negative	Opportunistic gram-negative bacterial pathogens. Invade the colonic and rectal mucosa, provoking a strong inflammatory response.	Wu et al., 33 Brown et al. 44
	Eubacterium hallii	Firmicutes/Lachnospiraceae	Beneficial and decreased	Aggravation	Negative	A butyrate (SCFA)-producing bacterium. Produces reuterin, inhibits pathogens, and suppresses inflammation in the gut.	Zhang et al., 46 Fujita et al. 47
MSAP and SAP	Atopobium	Actinobacteria/Coriobacteriaceae	Beneficial and decreased	Aggravation	Negative	Actinobacteria produce many compounds, including antibiotics, anticancer agents, immunosuppressants, anthelmintics, antiviral agents, and extracellular enzymes.	Zhang et al., 46 van Bergeijk et al., 57 Barka et al. 58
	Bacteroides	Bacteroidetes/Bacteroidaceae	Harmful and increased	Aggravation	Negative	Aggravate inflammation by stimulating the intestinal mucosa to produce cytokines and several toxins; possess high numbers of antibiotic resistance genes and are risk factors for autoimmunity. High-protein and high-fat diets increase the abundance of Bacteroides. Cause a thinning of the mucus layer by transforming lactate into SCFAs, which contributes to a decrease in the mucin synthesis, destroys the tight junctions, and finally leads to increased gut permeability; alter the intestinal barrier function, which results in bacterial overgrowth and impaired immunity.	Hamada et al., 5 Wu et al., 33 Sundin et al., 39 de Goffau et al., 40 Smedley et al., 41 Davis-Richardson et al., 42 Mejía-León et al., 43 Brown et al., 44 Tlaskalová-Hogenová et al., 45 Zhang et al. 46
	Firmicutes	Firmicutes	Beneficial and decreased	Aggravation	Negative	Promote anti-inflammatory properties by producing SCFAs when digesting plant polysaccharides; promote intestinal epithelial cell proliferation, mucosal repair, and anti-inflammatory responses by inducing IL-10 and regulating T-cell responses in the intestine.	Jandhyala et al., 31 Alkanani et al., 32 Wu et al., 33 Holmes et al., 34 Smith et al., 35 Louis and Flint, 36 Rossi et al., 37 Martín et al. 38
	Proteobacteria	Proteobacteria	Harmful and increased	Aggravation	Negative	LPS-producing bacteria; Increase the abundance of certain pathogens and decrease that of probiotics. Induce a tolerogenic immune program by differentially activating select toll-like receptors in monocytic cells and leading to T-cell anergy.	Zhang et al., 46 Ren et al., 53 Pushalkar et al. 54
	Synergistetes	Synergistetes	Harmful and increased	Aggravation	Negative	Related to LPS biosynthesis, considered a putative periodontal pathogen and is associated with several systemic diseases and inflammatory disorders of the gastrointestinal tract.	Pushalkar et al., 54 Hugenholtz et al., 64 Amado et al., 65 Oliveira et al., 66 Deng et al., 67 Kumar 68
	Enterococcus	Firmicutes/Enterococcaceae	Harmful and increased	Aggravation	Negative	Positively correlate with serum IL-6 and endotoxin levels, provoke systemic inflammatory response syndrome and intestinal bacterial translocation.	Tan et al. 9
	Bifidobacterium	Actinobacteria/Bifidobacteriaceae	Beneficial and decreased	Aggravation	Negative	Negatively correlated with serum IL-6 levels; decrease the population of Bacteroides vulgatus in the gut and ameliorate gut inflammation.	Tan et al., 9 Davis-Richardson et al., 42 Shiba et al., 69 Setoyama et al. 70
	Lactobacillus	Firmicutes/Lactobacillaceae	Beneficial and decreased	Aggravation	Negative	Cause dysfunction of immune homeostasis in the gut and periphery and hypofunction of microbial anti-infection, accompanied by downmodulation of inflammation by dendritic cells by inducing polarization of regulatory T cells.	Alkanani et al., 32 Bron et al., 71 Smits et al., 72 Penders et al. 73
SAP	Staphylococcus	Firmicutes/Staphylococcaceae	Beneficial and decreased	Aggravation	Negative	Promote the growth of anaerobic bacteria, including species of the genera Bifidobacterium, Clostridium, and Bacteroides.	Sahar et al., 15 Alkanani et al. 32
	Enterococcus	Firmicutes/Enterococcaceae	Harmful and increased	Aggravation	Negative	Positively correlate with serum IL-6 and endotoxin levels, provoke systemic inflammatory response syndrome and intestinal bacterial translocation.	Tan et al., 9 Marques et al. 74

AP, acute pancreatitis; GM, gut microbiota; IL, interleukin; LPS, lipopolysaccharide; MAP, mild acute pancreatitis; MSAP, moderate severe acute pancreatitis; SAP, severe acute pancreatitis; SCFA, short-chain fatty acid; TNF-α, tumor necrosis factor-α.

The influence of AP on intestinal flora changes

Acute pancreatitis-associated gut barrier and functional damage facilitate intestinal flora migration

Intestinal barrier dysfunction was found in both animal models and clinical patients with AP. ⁷⁵ The mechanism of intestinal microbiota migration in AP is as follows: intestinal barrier damage and a variety of gastrointestinal polypeptide secretions can destroy Cajal mesenchymal cells, ⁷⁶ decrease gastrointestinal movement, ⁷⁷ and impair intestinal motility, ⁷⁸ resulting in the overgrowth of intestinal bacteria in AP. ⁷⁹ Early fasting in patients with AP can cause intestinal ischemia–reperfusion injury, which can lead to intestinal mucosal microcirculation disorders and abnormal release of inflammatory factors and reactive oxygen species (ROS). These substances can cause the oxidative stress response in the intestinal mucosa, ⁸⁰ the apoptosis of intestinal epithelial cells, and increased permeability of intestinal capillaries,^81,82 ultimately leading to intestinal barrier function disorders and increased intestinal permeability.^83,84 The intestinal immune barrier function is compromised in patients with AP, ⁸⁵ and the level of secretory immunoglobulin A is decreased, ⁸⁶ which allows bacteria to pass through the intestinal barrier more easily. In addition to these three effects, long-term fasting and the obstruction of the lower bile duct in patients with SAP can result in a significant decrease in bile secretion or ineffective secretion into the intestine. ⁷⁵ Deoxycholic acid in the bile can selectively inhibit gram-positive bacilli (Bacillus, Clostridium, Lactobacillus, and Streptococcus pneumoniae). The reduction in bile secretion impairs the normal balance of the intestinal microbiota, resulting in the activation of an oxidative stress response and intestinal epithelial cell apoptosis, thus increasing bacterial migration. ⁸⁷

Effects of secretion of cell damage factors on intestinal flora in AP

Intestinal barrier dysfunction is the most common complication of SAP. Previous clinical studies have shown that elevated serum levels of many inflammatory cytokines in SAP, including TNF-α, ⁸⁴ IL-1, ⁸⁸ IL-6, ⁸⁹ neutrophil elastase (NE), and myeloperoxidase (MPO), ⁹⁰ are associated with intestinal barrier dysfunction. One of the main cytokines associated with AP is TNF-α, a proinflammatory cytokine, which is found to have elevated levels both locally, in the intestine, and systemically in patients with intestinal barrier dysfunction.^91,92 An increase in the TNF-α levels can lead to inflammation in the intestinal mucosa and to intestinal epithelial cell apoptosis,^93,94 which can lead to intestinal epithelial mechanical barrier damage and facilitate bacterial displacement. ⁹⁵ In addition to direct injury, TNF-α can initiate a positive feedback loop that induces the secretion of other cytokines, such as IL-1 and IL-6, to further injure the intestinal mucosa. ⁹⁶ An increase in the IL-1 levels in AP and the risk associated with IL-1 and the IL-1 receptor (IL-1R) in the pathogenesis of pancreatitis have been reported. ⁹⁷ IL-1R-deficient mice pretreated with an IL-1R antagonist recombinant human interleukin-1-receptor antagonist (rhIL-1Ra) experience milder pancreatitis after cerulein induction. The activation of IL-1β can also stimulate the local mucosal immune response and cause mucosal injury by stimulating T-cell proliferation and neutrophil entry to the site of injury or infection through the binding of IL-1β and IL-1R.^98,99 Serum IL-6 is another reliable indicator of AP severity that can predict both organ failure and SAP. ¹⁰⁰ The production of IL-6 can activate several different pathways in the adaptive immune system, thereby exacerbating inflammation and negatively affecting barrier function. ¹⁰¹ Tan et al. ⁹ also found that serum IL-6 levels in patients with AP were positively correlated with the abundance of Enterobacter and Enterococcus in the intestinal microbiota and negatively correlated with the abundance of XI groups of Bifidobacterium and Clostridium. In pancreatic tissue from a mouse model of SAP, neutrophil extracellular traps (NETs) decorated with MPO and NE were shown to aggravate tissue damage. ¹⁰² Many lethal complications of SAP have been shown to be closely related to NETs. According to previous reports, NETs can disrupt the balance of the intestinal microbiota, cause intestinal epithelial cell damage, and even induce apoptosis, leading to gut barrier damage, increased intestinal mucosal permeability, elevated endotoxin secretion, and imbalances in the GM.^{90,103 –105}

Pancreatic exocrine deficiency affects the composition and diversity of GM in AP

Patients with AP exhibit complications such as pancreatic exocrine impairment (PEI) and acinar cell dysfunction, which significantly impact changes in intestinal microbiota composition, ¹⁰⁶ and the secretion of many enzymes, such as lactate and bile acids, declines to a certain level. ¹⁰⁷ In animal models of PEI, the intestinal microbes E. coli, Lactobacillus, and Bifidobacterium were increased, and the levels of Fusobacterium and Clostridium hiranonis were decreased, inducing a significant difference in the dysbiosis index between affected animals and healthy individuals. ¹⁰⁸ Stool samples from PEI patients were analyzed and showed that pancreatic elastase levels significantly correlated with intestinal flora diversity compared with those of normal individuals, and significant differences were found in the abundances of 22 taxa, such as an increase in Pseudomonas spp. and a decrease in Bacillus spp. ¹⁰⁶ These results revealed that changes in pancreatic fluid secretion were also significantly correlated with flora diversity.

Antimicrobial peptide changes in AP-affected intestinal flora

Antimicrobial peptides (AMPs) are oligopeptides that are arranged linearly or circularly and are composed of amino acid residues of different lengths (up to 100). AMPs usually form L-amino acids through secondary structures containing alpha helices, beta sheets, or both. ¹⁰⁹ These biomolecules exhibit diverse biological activities against gram-positive and gram-negative bacteria, viruses, fungi, protozoa, and even tumors. ¹¹⁰ The Data Repository of AMPs (DRAMP) database includes over 4800 peptides that are antiproteins ¹¹¹ and contribute to the maintenance of intestinal bacterial homeostasis and intestinal barrier function. ¹¹² AMPs such as the cathelicidin-related AMP (CRAMP) have been reported to be secreted by pancreatic acinar cells, and reduced secretion of pancreatic AMPs can lead to the abnormal growth of intestinal bacteria and the disruption of the intestinal microbiota balance. Moreover, CRAMP deficiency worsens pancreatic inflammation. ¹¹³ Decreased expression of ileal terminal AMPs was found in necrotizing pancreatitis. ¹¹⁴ Hypertriglyceridemia (HTG) affects the expression of AMPs, including α-defensin, lysozyme, phospholipase A2, and regenerating islet-derived protein 3α (Reg3A), ¹¹⁵ in Paneth cells, which may exacerbate HTG-related acute necrotizing pancreatitis in intestinal barrier dysfunction. Pancreatic cells secrete a variety of AMPs to regulate the structure of the intestinal microbiota. ¹¹⁶ Lysozyme and α-defensins have activities against gram-negative and gram-positive bacteria, and some experts believe that fecal levels of α-defensins are a surrogate marker for gut microbial homeostasis.^117,118 On the other hand, Reg3A, which has powerful bactericidal activity, can antagonize gram-positive bacteria^119,120 by limiting the number of mucosal-adherent bacteria to separate the GM from the epithelium and reduce bacterial translocation ¹²¹ (Table 2).

Table 2.

AMP changes in AP-affected intestinal microbiota.

AMPs	Characteristics	Mechanism	Associated with AP	Associated with GM	References
CRAMP	Reduced	The Orai1 Ca2+ channel, which is needed in pancreatic exocytosis, can suppress the inflammation-associated alteration of intestinal bacteria	Increased mortality in AP	Gastrointestinal inflammation, intestinal bacterial overgrowth or dysbiosis, and systemic infection; impaired immunomodulatory effects	Ahuja et al., 10 Deng et al. 113
RegIIIγ and β-defensins	Reduced	The GM metabolites SCFAs (including butyrate) activate mTOR in IECs and promote IEC RegIIIγ and β-defensins in a GPR43-dependent manner	Serious pancreatic damage and systemic inflammation	Increased intestinal inflammatory responses; decreased SCFA-induced AMP production	Zhao et al. 114
RegIIIγ and β-defensins	Reduced	The GM metabolites SCFAs (including butyrate) activate STAT3 in IECs and promote IEC RegIIIγ and β-defensins in a GPR43-dependent manner	Serious pancreatic damage and systemic inflammation	Inhibition of intestinal immune regulation and intestinal organoid stemness proliferation; decreased AMP secretion	Zhao et al. 114
RegIIIγ and RegIIIβ	Reduced	Microbiota can directly affect AMP production by interfering with TLR-TLR ligand interactions	Serious pancreatic damage and systemic inflammation	Decreased AMP secretion	Zhao et al., 114 Menendez et al., 122 Brandl et al., 123 Vaishnava et al. 124
C-type lectins of the RegIII family	Reduced	Bactericidal activity by binding membrane phospholipids and killing bacteria by forming a hexameric membrane-permeabilizing oligomeric pore	Serious pancreatic damage and systemic inflammation (bactericidal for gram-positive but not for gram-negative bacteria)	Antibacterial effects against enteric pathogens; promoting mutualism with the resident microbiota in orthergasia	Wong 112 , Mukherjee et al., 119 Cash et al. 120
α-Defensin and lysozyme	Reduced	Intestinal microbiota dysbiosis and decreased levels of AMPs in Paneth cells may participate in the pathogenesis of intestinal barrier dysfunction	Serious pancreatic damage and systemic inflammation	Increased intestinal proinflammatory cytokine (TNF-α, IL-1β, and IL-17A) levels in plasma and tissue; weakened resistance against enteric pathogens; dysbiosis of intestinal microbiota structure and aggravated intestinal barrier dysfunction	Huang et al., 115 Clevers and Bevins, 117 Eriguchi et al., 118 Salzman et al., 125 Satoh et al. 126
AMPs secreted by Paneth cells	Reduced	Increase in the concentration of cytosolic Ca2+ accompanies granule secretion responding to bacteria or bacterial products, such as lipopolysaccharide	Serious pancreatic damage and systemic inflammation	Dysbiosis of intestinal microbiota and aggravated intestinal barrier dysfunction	Satoh et al., 126 Ayabe et al. 127

AMP, antimicrobial peptide; AP, acute pancreatitis; CRAMP, cathelicidin antimicrobial peptide; GM, gut microbiota; GPR, G protein-coupled receptor; IEC, intestinal epithelial cell; IL, interleukin; MAP, mild acute pancreatitis; MSAP, moderate severe acute pancreatitis; mTOR, mammalian target of rapamycin; Reg, regenerating islet-derived protein; SAP, severe acute pancreatitis; SCFA, short-chain fatty acid; STAT3, signal transducer and activator of transcription 3; TLR, toll-like receptor; TNF-α, tumor necrosis factor-α.

Risk factors that influence the GM and AP

Trillions of microbes live in the gut, and this community plays a vital role in the regulation of both intestinal and pancreatic functions. The underlying common causes of AP, such as biliary obstruction, alcohol misuse, HTG, and a high-fat/sugar diet, may also cause changes in the intestinal flora. ¹²⁸ These risk factors affect both AP and the intestinal microflora; thus, interactions between the intestinal microflora and the occurrence of AP can be inferred.

Obesity and hyperlipidemia

Obesity typically presents with low-level systemic inflammation, such as increased leukocyte counts and TNF-α, IL-6, and C-reactive protein levels^129,130; furthermore, it is characterized by increased secretion of biomarkers by adipocytes and is associated with AP. Moreover, macrophages in adipose tissue have been reported to participate in inflammation in obesity via the secretion of proinflammatory cytokines, such as TNF-α and IL-6, ¹³¹ both of which have been proven to affect AP and GM.

Obesity can also unmask primary HTG due to genetic causes and is a risk factor for secondary HTG, which is associated with pancreatitis.^132,133 At present, HTG-induced AP (HTG-AP) has become the second leading cause of AP. ¹³⁴ Indeed, the proportion of Bacteroides in the intestines of lean mice was found to be higher than that in obese mice after administration of the same diet, while the opposite was true for thick-walled Bacteroides. ¹³⁵ Rats fed a high-fat diet showed significant increases in serum low-density lipoprotein, total cholesterol, and triacylglycerol, as well as changes in Bifidobacteria, Lactobacilli, Enterococci, Enterobacteria, and Anaphylactic bacteria in the intestinal flora. ¹³⁶ In an animal model of hyperlipidemic necrotizing pancreatitis, researchers also found intestinal microflora imbalances and decreased AMPs in Paneth cells, further confirming that hyperlipidemia can affect the severity of AP and the intestinal microflora. ¹¹⁵ Unsaturated fatty acids (UFAs) might be an important factor that can affect both AP and GM in obesity and hyperlipidemia, and these factors are mainly transmitted via the lipolysis of circulating triglycerides. ¹³⁷ The insolubility of UFAs in the aqueous environment of the blood can cause microthrombi formation in the pancreatic vasculature, leading to ischemia and pancreatic infarction. As polar molecules, UFAs usually bind with calcium, resulting in their saponification and inactivation in necrotic fat tissue. ¹³⁸ Unbound UFAs can increase the serum levels of TNF-α and other inflammatory cytokines, ¹³⁹ thereby worsening AP and leading to inflammation of the intestinal mucosa and intestinal epithelial cell apoptosis^93,94 (Figure 1).

Figure 1.

The negative effect of obesity and hypertriglyceridemia both on AP and GM.

(i) The adipose tissue itself and macrophages in it can lead to increase level of pro-inflammation cytokines, like TNF-α and IL-6. These cytokines cause intestinal mucosa inflammatory injury, apoptosis of intestinal epithelial cells, and intestinal flora alteration. (ii) In the obesity or hyperlipidemia, the UFAs transmitted from lipolysis of circulating triglycerides can cause microthrombi formation in the pancreatic vasculature resulting in ischemia and pancreatic infarction. (iii) UFAs are normally bound by calcium, resulting in their saponification and inactivation in necrotic fat tissue, ultimately increasing severity of AP and demolish intestinal epithelium.

AP, acute pancreatitis; GM, gut microbiota.

In addition to the harmful effects of obesity and hyperlipidemia on the GM, recent studies have shown that the intestinal flora is one of the important environmental factors affecting the occurrence and development of obesity. The intestinal flora can induce adipocytokine gene expression by affecting intestinal epithelial cell fasting, leading to the increased production of triacylglycerols in the body and causing lipid metabolism disorders and the development of obesity. ¹⁴⁰ Furthermore, disturbances in the intestinal flora in obese mice may lead to abnormal lipid metabolism, energy metabolism, adipokine synthesis, and cell death, leading to the secretion of a large number of proinflammatory cytokines into the blood and resulting in the exacerbation of pancreatitis. ¹¹⁵ Some studies indicated that intake of probiotic preparations could affect serum cholesterol and high-density lipoprotein levels and indirectly lower blood lipids, suggesting that the establishment of normal intestinal flora can help balance lipid metabolism. ¹¹³

Alcohol

In recent years, due to changes in diet and increases in alcohol consumption, heavy drinking has become a risk factor for AP, and this condition easily progresses to SAP. ¹⁴¹ The toxicity of ethanol is mediated by ethanol itself or its oxidative and nonoxidative metabolism. Oxidative ethanol metabolism potentiates cholecystokinin-induced depolarization by sensitizing pancreatic mitochondria to Ca²⁺-induced mitochondrial permeability transition pore (MPTP) activation, resulting in mitochondrial dysfunction in pancreatic acini and necrosis in the pancreas. ¹⁴² Although the main ethanol metabolism in vivo is oxidation, a smaller part undergoes nonoxidative metabolism.^143,144 Incubation of isolated pancreatic acinar cells with fatty acid ethyl esters, one of the nonoxidative ethanol metabolites in vivo, induced mitochondrial depolarization, depletion of cellular adenosine triphosphate, ¹⁴⁵ and sustained elevations of intracellular Ca²⁺ levels ultimately associated with cellular dysfunction and cell death. ¹⁴⁶ Both alcohol and its metabolites can activate digestive enzymes early in pancreatic acinar tissue, resulting in pancreatic tissue autodigestive injury, and activate pancreatic stellate cells, leading to fibrosis of the pancreas. ¹⁴³

On the other hand, alcohol has been shown to have a negative impact on the intestinal flora of healthy people, such as decreasing the biodiversity of the intestinal flora and affecting the overall composition of the microbial community. ¹⁴⁵ Disturbances in the intestinal flora may cause disorders of glycolipid energy metabolism and other potential functional pathway changes in the body. ¹⁴⁷ Alcohol can lead to changes in the composition of the GIT microbiota and metabolic function, contributing to the well-established association between alcohol-induced oxidative stress and intestinal hyperpermeability to luminal bacterial products.^{146 –148} Exposure to ethanol can increase the release of enterogenous gram-negative bacteria-derived lipopolysaccharide (LPS), leading to macrophage activation and the secretion of cytokines, including TNF-α, IL-1β, and IL-6. ¹⁴⁹ IL-22 is mainly involved in maintaining the integrity of the epithelial barrier and linking intestinal immune activation with epithelial repair and barrier protection.^150,151 Under inflammatory conditions, IL-22 can be activated through the IL-23-Janus kinase/signal transducer and activator of transcription signaling pathway, resulting in the production of AMPs. ¹⁵² Ethanol metabolism in vivo produces acetaldehyde and ROS, which can activate NF-κB and ultimately stimulate the immune response, ¹⁵³ decrease intestinal expression of IL-22, and alter gut epithelial integrity, causing an increase in intestinal permeability and bacterial translocation ¹⁵⁴ (Figure 2).

Figure 2.

The mechanism and negative effect of ethanol metabolism on AP and GM.

The toxicity of ethanol is mediated by ethanol itself though its oxidative or nonoxidative metabolism. Exposure to ethanol can increase release of enterogenous gram-negative bacteria-derived LPS leading to macrophages activation and cytokines secretion, including TNF-α, IL-6, etc. and inhibit intestinal expression of IL-22, finally damaging intestinal flora balance and intestinal mucosa integrity. Oxidative metabolism of ethanol produces acetaldehyde and ROS which could activate NF-κB, inhibit intestinal expression of IL-22, and also activate trypsin causing pancreatic tissue autodigestive injury. Nonoxidative metabolism of ethanol induced mitochondrial depolarization to Ca²⁺-induced MPTP activation, resulting in pancreatic mitochondrial dysfunction and trypsin activation.

AP, acute pancreatitis; GM, gut microbiota.

High glucose and insulin resistance

Pancreatic damage, pancreatitis, imbalances in the GM, and blood sugar imbalances may be interrelated. ¹⁴⁸ AP exhibits hyperglycemia in the early stage, ¹⁵⁵ which can persist as secondary diabetes even after pancreatitis has been resolved. ¹⁵⁶ Chronic hyperglycemia may cause oxidative stress, mitochondrial damage, the production of advanced glycation end products (AGEs), and the expression of the receptor for AGEs (RAGE), leading to tissue injury. ¹⁵⁷

Researchers have shown that hyperglycemia enhances mitochondrial oxidative stress by increasing ROS production, which is a key step in the pathogenesis of AP, ¹⁵⁸ and mediates lipid peroxidation by increasing cytosolic Ca²⁺.^159,160 Furthermore, increased intracellular Ca²⁺ is also required for premature protease activation, which is an early step in the induction of AP. ¹⁵⁸ Elevated glucose levels begin to form covalent conjugates with plasma proteins through a nonenzymatic process called AGE formation. ¹⁶¹ In combination with AGEs, RAGE promotes the development of pancreatitis in part by mediating uninduced nucleosome activation and proinflammatory mediator release via the absence in melanoma 2 (AIM2) inflammasome activation and proinflammatory mediator release in macrophages in an animal model of AP. ¹⁶² Under glycoxidative stress, stimulated macrophages can induce oxidative stress and NF-κB activation through activation of the PR21ras and MAPK signaling pathways. ¹⁶³ Active NF-κB induces the production of TNF-α, which, in turn, leads to enhanced ROS production and more severe damage to tissues. ¹⁶⁴ In addition, hyperglycemia was shown to compromise the integrity of the intestinal barrier through glucose transporter 2 (GLUT2)-dependent reprograming of the intestinal epithelial cell transcriptome and disruption of tight and adherence junctions, leading to intestinal flora disorders.^165,166 The GM composition of patients and animals with elevated blood glucose was also significantly different from that of normal controls.^167,168

Insulin resistance, which is a kind of metabolic dysfunction associated with type 2 diabetes mellitus, is another critical factor that affects both AP and the GM. ¹⁶⁹ Observational studies have shown an increased risk of AP among people with diseases linked to insulin resistance.^{170 –172} Various factors and hormones, such as NF-κB, TNF-α, amylin, leptin, and IL-6, have recently been shown to be increased in patients with insulin resistance, and those factors have been demonstrated to cause AP and intestinal flora disorders.^{173 –177} In addition, insulin resistance often causes hyperinsulinemia, which can inhibit mucus secretion by promoting fatty acid synthase, to break the integrity of the intestinal barrier, leading to GMs.^165,178 Furthermore, insulin resistance has been regarded as a novel risk factor for post-endoscopic retrograde cholangiopancreatography pancreatitis ¹⁷⁹ and an independent prognostic factor in patients with AP. ¹⁸⁰

Intestinal microbes can also increase insulin resistance by influencing host energy metabolism and the integrity of the intestinal barrier; thus, inflammatory mediators can be transmitted into circulation. ¹⁸¹

Curative substances influence both AP and GM

AP often leads to flora disorder, but some protective cytokines play key roles; for example, IL-22 and IL-23 attenuate intestinal flora disorders.^152,182,183 Propolis has recently been reported to reduce the serum levels of proinflammatory cytokines (TNF-α, IL-1β, and IL-6) and increase IL-22 levels, thereby reducing pancreatic neutrophil infiltration and maintaining the intestinal flora in AP rats. ¹⁸⁴ Store-operated calcium entry (SOCE) modulators, ¹⁸⁵ such as Pyrtriazoles, and the Orai Ca²⁺ channel inhibitor CM4620, which can reduce endoplasmic reticulum calcium influx, target both parenchymal and immune cells to reduce inflammation in experimental AP. ¹⁸⁶ By inhibiting immune cells, SOCE inhibitors can treat imbalances in the GM. Okra pectin could relieve the inflammatory response by inhibiting the expression of proinflammatory mediators, preventing intestinal barrier injury, and regulating the intestinal microbiota by upregulating AMPs and occludin in an AP model. ¹⁸⁷ Probiotics have been reported to significantly attenuate pathological injury of the pancreas and reduce the incidence of complications, such as infection, in patients with AP. ¹⁸⁸ However, the elevated levels of lactic acid produced by bacterial overgrowth in the small bowel and fermentation of carbohydrates significantly contributed to the high death rate. When considering substituting supplementation for individuals with AP, it is necessary to assess the time, type, appropriate, effective doses of probiotics, and prevent bacterial overgrowth.^20,189,190 Some traditional Chinese medicine (TCM) treatments also have effects on both AP and the GM. Saponin A, a monomer of total saikosaponins extracted from Bupleuri Radix, has strong antioxidant properties and can affect the composition of GM by increasing the relative abundance of Lactobacillus and Prevotella species to decrease the development of SAP in rat models. ¹⁹¹ Picroside II is one of the main effective components extracted from Picrorhiza scrophulariiflora Pennell that can improve the intestinal microbiota by inactivating oxidant and inflammatory signals to improve intestinal barrier injury in an SAP rat model. ²⁸ Some studies have reported that berberine can not only repair the gut barrier structure to decrease GM diversity but also reduce blood glucose levels and attenuate insulin resistance; moreover, berberine is regarded as a potential therapeutic agent for AP.^{192 –194} Meng et al. used acupuncture and moxibustion to stimulate ST36 points to treat SAP based on conventional treatments and found that adjuvant acupuncture treatment could reduce the permeability of intestinal mucosa capillaries, alleviate intestinal dysfunction, and promote recovery in patients ¹⁹⁵ (Tables 3 and 4).

Table 3.

Curative substances influencing both AP and GM.

Substance	Effect on AP	Effect on GM	Mechanism	References
IL-22	Relieves inflammation and tissue injury	Promotes epithelial repair and barrier protection	Activation of inflammation, mediated through the JAK/STAT signaling pathway, results in the production of AMPs, finally repairing barrier damage or controlling pathogenic bacterial expansion	Sonnenberg et al.,150,151 Li et al., 152 Zheng et al., 196 Zindl et al. 197
IL-23	Relieves inflammation and tissue injury	Promotes epithelial repair and barrier protection	Promotes IL-22 production	Ngo et al., 182 Shih et al. 183
Propolis	Reduces neutrophil infiltration in the pancreas	Reduces intestinal inflammation	Reduces the serum levels of proinflammatory cytokines (TNF-α, IL-1β, and IL-6) and increases that of IL-22	Al-Hariri et al. 184
SOCE modulators
Pyrtriazole	Reduce inflammation in the pancreas	Treat an imbalance of the GM	Reduce calcium influx in the endoplasmic reticulum	Riva et al., 185 Waldron et al. 186
CM4620
Okra pectin	Reduces inflammation in the pancreas	Prevents intestinal barrier inflammatory injury and regulates intestinal microbiota	Relieves inflammatory responses and intestinal barrier injury and regulates intestinal microbiota by inhibiting the expression of proinflammatory mediators or upregulating AMPs and occludin	Xiong et al. 187
Choline	Choline deficiency is related to exocrine pancreatic insufficiency	Choline deficiency is associated with bacterial overgrowth in the small intestine	Choline is a tightly regulated tissue component in the form of phosphatidylcholine and sphingomyelin in all membranes and many secretions	Bernhard 198

AMP, antimicrobial peptide; AP, acute pancreatitis; GM, gut microbiota; IL, interleukin; SOCE, store-operated calcium entry; JAK, Janus kinase; STAT, signal transducer and activator of transcription; TNF-α, tumor necrosis factor-α.

Table 4.

Promising prebiotic agent for the treatment of SAP.

Substance	Effect on AP	Effect on GM	Mechanism	References
SCFAs	Anti-inflammatory effects on protecting against severe AP-associated lung injury	Protecting intestinal barrier, decreasing bacterial translocation	SCFAs produced by gut microbiome and has a protective effect against pathogen proliferation, inflammatory response, and intestinal barrier injury	van den Berg et al., 21 Jia et al., 24 Pan et al., 29 Patel et al., 190 Wang et al.,199,200 Zhang et al. 201
Six different strains of probiotic prophylaxis mixture	More multiorgan failure-related deaths	More bowel ischemia	Combination of probiotics had no beneficial effect on the occurrence of infectious complications and been damaged to bowel wall because of inflammatory injury and enteral feeding aggravating intestinal mucosal ischemia	Bongaerts and Severijnen, 189 Besselink et al., 202 Rahman et al., 203 Besselink et al. 204
Glutathione biosynthesis by multispecies probiotics	Reducing pancreatic oxidative stress	Reducing oxidative stress in the ileum	This probiotics mixture increases the biosynthesis of glutathione and reduces oxidative stress both in pancreas and ileum	Lutgendorff et al.,205,206
Probiotics capsules (such as a mixture of Bacillus subtilis and Enterococcus faecium)	Reducing pancreatic injury	Reducing bacterial translocation and increasing food tolerance	Gut microbiome plays important role in the pathogenesis of AP. Probiotics improve intestinal microecology and food tolerance, decrease the inflammation	Hooijmans et al., 188 Zhu et al., 207 Tian et al. 208
Chitosan oligosaccharides (COS)	COS decrease inflammatory infiltration and oxidative stress	Remodeling gut dysbiosis by increasing probiotics Akkermansia and eliminating pathogenic bacteria Escherichia–Shigella and Enterococcus	Lighting oxidative stress, reducing proinflammatory cytokine, and balancing intestinal homeostasis	Mei et al. 209
Bifidobacterium spp. (B. animalis) metabolite lactate	Reducing pancreatic and systemic inflammation	B. animalis metabolite lactate is the energy source for intestinal epithelial cells and inhibits bacterial translocation	B. animalis colonization and B. animalis metabolite lactate administration could relieve macrophage-associated local and systemic inflammation through its metabolite lactate-related TLR4/MyD88- and NLRP3/Caspase1-dependent pathway	Li et al. 210

AP, acute pancreatitis; COS, chitosan oligosaccharides; SCFA, short-chain fatty acid; TLR, toll-like receptor.

Future research prospects

SAP is a severe inflammatory disease of the pancreas and results in a high mortality rate when accompanied by multiple organ dysfunction or secondary infection. ²¹¹ Studies have shown that most pancreatic and extra-pancreatic organ infections originate in the intestine and induce inflammatory responses, which are major causes of ‘secondary attack’ and increased late death of patients with SAP. ²¹² Changes in the GM play an important role in intestinal homeostasis and aggravate the inflammatory response under intestinal flora dysfunction in AP.^4,5 The migration and proportion of intestinal flora influence the development and severity of AP. However, the molecular mechanism and signaling pathways associated with changes in the intestinal flora in AP are still unclear. ²¹³ The dominant intestinal microbiota species in MAP, MSAP, and SAP were Bacteroides, Escherichia–Shigella, and Enterococcus, respectively. ²⁰ A majority of diseases are accompanied by changes in the microbiota, and whether there is a way to detect GM species could be helpful in predicting or diagnosing SAP. ²¹⁴

Obesity and hyperlipidemia are regarded as chronic and systemic inflammatory states induced by adipocytes, which secrete a variety of proinflammatory cytokines and act as reservoirs of inflammatory factors. ²³ When obesity and hyperlipidemia cause AP and an intestinal microbiota imbalance, ¹¹⁵ the intestinal microbiota also causes disordered lipid metabolism and the development of obesity by mediating adipocytokine gene expression, leading to a vicious cycle. ¹⁴⁰ Pancreatic endocrine cells participate in the regulation of blood glucose metabolism. Hyperglycemia exacerbates mitochondrial oxidative stress, increases intracellular Ca²⁺ levels, and ultimately promotes the progression of AP.^158,160 Patients with AP generally have insulin resistance,^80,83,86 and gut microbes have been reported to increase insulin resistance. ¹⁸¹ Insulin resistance also causes AP and intestinal microbiota disorders.^85,215,216 Glucose and lipids are sources of energy metabolism and are also factors associated with metabolic diseases. The specific GM species in AP combined with the metabolic disorders associated with glucose and lipids need further study. The effect of probiotics on the treatment of AP combined with metabolic disorders associated with glucose and lipids might be worth studying.^31,161

There is currently no specific treatment for AP. The intestinal flora attenuates the severity of AP, and personalized probiotic intervention is considered a future trend. ²¹⁷ The timepoint, dose, and effectiveness of probiotics used for the treatment of AP are worthwhile of further experiments and clinical studies. In addition, the safety issue of probiotic therapy cannot be ignored. ²¹⁸ TCM, with multiple approaches including decoctions, powders, acupuncture, and moxibustion, has been reported to improve inflammatory or metabolic disorders.^{219 –222} Whether treatments combining probiotics and TCM could be beneficial for SAP patients or whether the curative factors mentioned above may be used to prevent pancreatitis are unclear, and few studies have focused on this issue.

In conclusion, the interaction between the GM and inflammatory responses provides a new understanding of AP disease progression and treatment. Further studies on the interaction of GM and inflammatory responses in AP are needed.