Under physiological conditions, a homeostatic GM supports key host functions including immune regulation, metabolic processes, and the maintenance of intestinal barrier integrity (Yue et al., 2019).

In healthy individuals, the GM is predominantly composed of the phyla Firmicutes and Bacteroidetes, followed by Actinobacteria and Verrucomicrobia (Hollister et al., 2014). Under normal physiological conditions, GM is in homeostasis and plays functions of immunity, metabolism and maintenance of intestinal barrier homeostasis in the body (Yue et al., 2019). However, the composition and diversity of GM is easily influenced by various factors (such as diet, drugs, pathogens, and environmental factors), further affecting human and animal health (Cryan et al., 2019; Lange et al., 2016; Zhang, 2022). Compelling evidence indicates that GM dysbiosis increases host vulnerability to a broad spectrum of pathogens and promotes the development of diverse diseases, including diarrhea, IBS, and allergies (Leong et al., 2018; Shchikota et al., 2021).

Despite improvements in living conditions and healthcare in recent years, diarrhea remains a globally prevalent issue (Wolf et al., 2022). A topic of growing interest currently is the interplay between diarrhea and the GM, which is featured by pathogen-dominated GM dysregulation, encompassing bacterial, fungal, and viral etiologies (Li et al., 2021). Invasive pathogens suppress the proliferation of commensal bacteria, thereby reducing beneficial gut microbiota and increasing pathogenic strains. This imbalance further induces intestinal dysfunction and activates immune responses, ultimately culminating in diarrhea (Czepiel et al., 2019; Jesser et al., 2023).

Furthermore, it has been observed that other types of diarrhea are also closely associated with alterations in the GM (Table 1). Thus, it is evident that dysbiosis of the GM exists across various forms of diarrhea.

SCFAs, including acetate, propionate, and butyrate (Liu et al., 2021), are mainly produced by GM via anaerobic fermentation of indigestible carbohydrates and host-derived substrates (Zhang et al., 2023). SCFAs contribute to the enhancement of intestinal barrier function, exhibit anti-inflammatory effects, and participate in immunomodulation (Parada Venegas et al., 2019). Specifically, acetate and propionate serve as energy sources for peripheral tissues (den Besten et al., 2013). A recent study demonstrated that acetic acid mediates crosstalk between epithelial and immune cells and promotes T cell-dependent immunoglobulin A (IgA) production by stimulating CD4⁺ T cells (Takeuchi et al., 2021). Moreover, propionate has been shown to prevent the reduction of TJ proteins, such as ZO-1 and occludin, in colon tissue, and to suppress the mRNA expression of pro-inflammatory cytokines IL-1β, IL-6, and TNF-α (Tong et al., 2016). Butyrate also exerts anti-inflammatory effects by inhibiting the secretion of IL-8, IL-6, IL-12, and TNF-α, while promoting the production of the anti-inflammatory cytokine IL-10, thereby contributing to the maintenance of the intestinal epithelial barrier (Lee et al., 2017). Additionally, they promote the growth of beneficial bacteria, improve GM composition, and help regulate host immune homeostasis (Fukuda et al., 2011; Mann et al., 2024). Moreover, a separate study demonstrated that propionic acid, secreted by Akkermansia muciniphila, binds to G-protein-coupled receptor 43 on the surface of intestinal epithelial cells. This interaction enhances histone acetylation, which in turn upregulates the expression of TJ proteins occludin and ZO-1 and increases mucin levels, ultimately improving the integrity of the intestinal epithelial barrier (He et al., 2023). In addition, another study demonstrated that, relative to healthy calves, calves with diarrhea induced by bovine rotavirus (BRV) exhibited significantly reduced concentrations of total SCFAs, acetic acid, propionic acid, and isocaproic acid; in contrast, only propionic acid concentrations were markedly decreased in calves with diarrhea caused by bovine coronavirus. Notably, the depletion of Parabacteroides and Ruminococcus was strongly associated with reduced acetic acid levels, while declines in isocaproic acid content were closely linked to the loss of Parabacteroides, Ruminococcus, Fournierella, and Rikenellaceae_RC9_gut_group. Furthermore, a significant reduction in propionic acid concentrations showed a positive correlation with the depletion of Collinsella (Cui et al., 2023). Both propionic and butyric acids are believed to enhance the integrity of epithelial cells, with butyric acid serving as the primary energy source for colonocytes (Furusawa et al., 2013; Tong et al., 2016). Reduced butyrate levels have been shown to elevate intestinal oxygenation, which not only drives gut microbial dysbiosis and promotes the expansion of aerobic pathogens but also disrupts intestinal homeostasis (Handa et al., 2023). SCFAs are absorbed by epithelial cells, which in turn stimulates Na⁺-dependent absorption of water and electrolytes, thereby mitigating diarrhea symptoms (Binder, 2010). Several studies have indicated that ADD-type mice exhibit reduced levels of SCFAs (Min et al., 2024; Yang L. et al., 2021; Zhan et al., 2023). Furthermore, piglets infected with E. coli developed diarrhea and exhibited decreased levels of SCFAs in their feces (Liu et al., 2019).

Bile acids (BAs) serve as essential signaling molecules that significantly regulate glucose homeostasis, lipid metabolism, and energy expenditure (Sah et al., 2022; Yu et al., 2023). They consist of primary bile acids (PBAs), which are synthesized by the liver, and secondary bile acids (SBAs), which are metabolized by the GM. Among them, PBAs include chenodeoxycholic acid (CDCA) and cholic acid (CA), while SBAs comprise lithocholic acid (LCA) and deoxycholic acid (DCA). Nearly 95% of luminal BAs are reabsorbed in the distal ileum, while the remainder undergoes microbial modification by the GM prior to excretion or passive absorption. In humans, the GM is instrumental in the generation of SBAs via a series of enzymatic reactions, including deconjugation, 7α-dehydroxylation, oxidation, epimerization, desulfation, and esterification. Of these, deconjugation and 7α-dehydroxylation are the most physiologically significant processes (Ridlon et al., 2016; Wahlström et al., 2017). When intestinal homeostasis is disrupted, dysbiosis of the GM affects BAs metabolism and ultimately alters the host response. In IBS-D, excessive fecal BAs are considered a contributing factor to pathogenesis, and there are higher levels of PBAs in fecal samples of IBS-D patients compared to healthy subjects (Duboc et al., 2012; Wei et al., 2021; Wei et al., 2020). However, research has indicated that a microbiota rich in Clostridia can promote BAs excretion in IBS-D patients (Zhao et al., 2020). It is well established that BAs modulate intestinal mucosal permeability and participate in inflammatory responses. Specifically, CDCA and DCA exert their effects by promoting epidermal growth factor receptor (EGFR) autophosphorylation and occludin dephosphorylation, leading to the reorganization of occludin within TJs and a consequent increase in paracellular permeability (Raimondi et al., 2008). Additionally, CDCA contributes to pro-inflammatory responses by stimulating the release of IL-8 and reactive oxygen species (ROS), as well as amplifying the effects of TNF-α and IL-1β on interferon-γ (IFN-γ) production (Sarathy et al., 2017). Ursodeoxycholic Acid (UDCA) has been shown to reduce the production of inflammatory cytokines by participating in the BA receptor Farnesoid X Receptor (FXR), while inhibiting NF-κB activation in macrophages (Pi et al., 2023). BAs metabolites were found to be excessive in the feces of Primary Sclerosing Cholangitis (PCD) patients and PCD mice, and SBAs [DCA, LCA and Hyodeoxycholic Acid (HDCA)] were found to be associated with the onset of diarrhea. These SBAs shortened the gastrointestinal transit time by 0.6-fold, increased the fecal water content by 1.3-fold and stimulated 5-HT levels in vitro and in vivo (Xu Y. et al., 2023). However, blocking BAs conjugated Takeda G protein-coupled receptor 5/Transient Receptor Potential Ankyrin 1 (TGR5/TRPA1) receptors significantly alleviated PCD GM-induced diarrhea. The present study demonstrates that GM and BA metabolism play a role in diarrhea. These results offer promising biomarkers for diagnosing and treating diarrhea and lay the groundwork for further investigation.

Branched chain amino acids (BCAAs), which include leucine (Leu), isoleucine (Ile), and valine (Val), are essential amino acids for the human body (Peng et al., 2020). These amino acids exert direct or indirect effects on diverse physiological functions, including energy metabolism, protein synthesis, and immune responses (Ma et al., 2018; Stipanuk, 2007). Similarly, BCAAs function as modulators that promote intestinal development and enhance gut health (Ren et al., 2016; Ren et al., 2015). Currently, the majority of research has concentrated on the function of Leu, with less attention paid to Val and Ile in the gut. Leu can maintain intestinal health by enhancing TJ in fish (Jiang et al., 2015). Additionally, it has been shown to improve intestinal epithelial cell proliferation, increase villus height, and promote growth in the small intestine of pigs. However, intestinal growth was inhibited when Leu levels were as high as 2.57% (Ren et al., 2015). Dietary Ile improves intestinal immune function and microbial population, and regulates gene expression of antioxidant enzymes, TJ, Nuclear factor erythroid 2-related factor 2 (Nrf2), Kelch-like ECH-associated protein 1 (Keap1), p38, and Extracellular Signal-regulated Kinase 1 (ERK1) in the intestine of Jian carp (Zhao et al., 2014). Additionally, BCAAs have been significantly linked to diarrhea; a decreasing trend in BCAA levels was observed in both the functional diarrhea group and the IBS-D group (James et al., 2023). Rotavirus infection induces diarrhea in weaned pigs via systemic protein metabolic disorders and jejunal mucosal dysfunction. However, dietary supplementation with 1% leucine alleviated rotavirus-induced diarrhea in weaned pigs, potentially due to leucine’s roles in enhancing protein metabolism, improving intestinal digestive and absorptive capacities, and reinforcing the non-specific barrier function of the intestinal mucosa (Mao et al., 2015). Furthermore, L-isoleucine supplementation has been found to significantly reduce stool output and fluid intake in children suffering from non-cholera acute watery diarrhea (Alam et al., 2011). More recently, a study demonstrated that supplementation with Ile increased the relative abundance of Prevotella and decreased the relative abundance of Rikenellaceae in the colon of diarrhea piglets infected with rotavirus, increased the secretion of interleukin-4 (IL-4), IL-10, and Secretory Immunoglobulin A (sIgA), and increased the expression of Claudin-3, Occludin, ZO-1 and mucin 1 (MUC-1), improved the immunity, colon barrier function and colon GM of piglets with diarrhea (Jiang C. et al., 2024; Jiang C. Y. et al., 2024). Nevertheless, the existing literature displays a striking imbalance toward Leu, leaving the mechanisms and efficacy of Val and Ile underexplored. Future studies should thus prioritize elucidating the individualized and synergistic contributions of all three BCAAs—particularly Val and Ile—ac different physiological and pathological contexts, to enable more precise and effective nutritional strategies for intestinal health.

The intestinal barrier is a complex physiological structure that serves as a physical, biological, chemical, and immunological barrier. It interacts with the external environment and regulates host health (Zhou et al., 2024). The intestinal barrier, being semi-permeable, serves a dual function: it safeguards the internal milieu against the potential translocation of pathological molecules and microorganisms, while facilitating the absorption of nutrients and water (Martini et al., 2017). However, in pathological conditions, the integrity of the intestinal barrier can be compromised, leading to many local and systemic diseases (Aleman et al., 2023; Wang et al., 2022). TJs serve as a crucial form of connection between intestinal epithelial cells.

The proteins ZO-1 and occludin are key structural components of TJs. They are essential for maintaining cellular morphology and TJ structural integrity, and are widely used as indicators for assessing intestinal barrier function (Al-Sadi et al., 2011; Haas et al., 2022). Mucins are the primary glycoproteins that constitute the intestinal mucosal barrier. Among them, Mucin 2 (MUC2) is the most secreted mucin in the gastrointestinal tract and maintains the integrity of the mucus barrier, which is closely related to GM homeostasis (Liu et al., 2023; Yao et al., 2021). Tropini et al. demonstrated that diarrhea is closely related to the GM and the intestinal mucus barrier (Tropini et al., 2018). Diarrhea significantly disrupts the GM and is associated with thinning or loss of the intestinal mucus layer. This effect may be linked to dysregulated expression of the tight junction proteins ZO-1 and occludin, which compromises intestinal barrier integrity and increases permeability (Chen H. R. et al., 2024; Tropini et al., 2018).

Impaired intestinal mucosal barrier function serves as the primary pathological basis for the development of IBS-D (Shi et al., 2023). Upon the onset of IBS-D, patients exhibit a significant reduction in the expression of occludin, ZO-1, and other epithelial tight junction proteins, resulting in compromised intestinal epithelial barrier integrity and elevated intestinal permeability (Guo et al., 2023; Wang L. et al., 2023). Recent studies have found that Lactobacillus promotes occludin and ZO-1 expression and improves diarrheal symptoms (Hou et al., 2020). Related studies have further demonstrated that in diarrheic piglets infected with E. coli, increased abundances of Lactobacillus and Cyanobacterium are associated with reduced intestinal permeability and enhanced barrier repair, with Lactobacillus showing a particularly strong correlation with key intestinal barrier markers (Luo et al., 2022). Xu et al. found that MUC2 is a crucial protein in the prevention and treatment of rotavirus infections and diarrhea. It functions by safeguarding the epithelial barrier and enhancing intestinal permeability resistance (Xu et al., 2016). Wang et al. further observed that elevated MUC2 content in the ileum of diarrheal rats enhances intestinal barrier defense and confers intestinal protection (Wang et al., 2019). Furthermore, MUC2 concentration was significantly lower in AAD mice than in normal mice. A significant negative correlation was also identified between MUC2 and two gut microbial taxa, Prevotellaceae_NK3B31_group and Rothia (Li C. et al., 2023; Li X. T. et al., 2023). Collectively, these findings demonstrate a close association between diarrhea development, GM composition, and intestinal barrier function.

The GM intricately interacts with the host immune system. The crosstalk between the GM and enterocytes plays a crucial role in shaping the intestinal environment, thereby profoundly influencing intestinal immune homeostasis (Hold, 2016). Different types of diarrhea induce alterations in GM composition, which in turn modulates the expression of inflammatory factors. For example, in patients with diarrhea-predominant IBS-D, levels of IL-8 and TNF-α are elevated, while IL-10 is reduced; in mice with AAD, GM dysbiosis is observed, characterized by a marked increase in Proteobacteria and decreases in Bacteroidetes and Firmicutes, accompanied by upregulated IL-1 and IL-6 levels (Chen et al., 2022; Zhen et al., 2015; Zhu et al., 2022). It was observed that E. coli O₁ caused diarrhea in calves with disturbances in the GM and an increased abundance of Proteobacteria and Clostridiales. This condition was accompanied by a decreased expression of CD4⁺ T and an elevated expression of Cluster of Differentiation 8 Positive T Lymphocyte (CD8⁺ T) and CD11c-positive T lymphocyte (CD11c⁺ T) in the ileum. Additionally, there were reduced serum levels of IgA and Immunoglobulin G (IgG), alongside heightened levels of IL-6 and TNF-α (Chen H. et al., 2023). T helper cell 17 (Th17) contribute to the maintenance of host intestinal immune homeostasis through interleukin-17A (IL-17A)-induced expression of the epithelial polymeric immunoglobulin receptor (Cao et al., 2012). In colonic tissues of IBS-D mice, the Th17/Tregs ratio was found to be significantly altered, characterized by reduced Tregs and IL-10⁺Foxp3⁺T cells alongside increased Th17 cells. Correlation analysis further revealed positive associations between Ruminococcus_gnavus with the Th17/Tregs ratio (Zhang M. M. et al., 2024; Zhang Y. et al., 2024; Zhang Z. et al., 2024).

It is thus clear that GM plays an important role in regulating intestinal immune homeostasis during diarrhea.

In recent years, this concept has expanded to include BGMA, prompted by the growing recognition of the gut microbiota’s critical role in human health and disease (Lee et al., 2023). The BGMA acts as a bidirectional communication pathway between the central nervous system (CNS) and the gastrointestinal tract. It mediates interactions involving the CNS, enteric nervous system (ENS), neuroendocrine system, and immune system, with signals being transmitted either directly or indirectly between the CNS and ENS (Arneth, 2018; Grenham et al., 2011; Morais et al., 2021).

Notably, microbial metabolic activity profoundly shapes brain–gut signaling. For example, disruptions in tryptophan metabolism impact the synthesis of serotonin (5-HT), a key intermediate, which can activate brain–gut neural circuits and precipitate diarrheal responses (Morais et al., 2021; Spencer and Hu, 2020). Furthermore, central processes modulate gut function via the hypothalamic–pituitary–adrenal (HPA) axis: psychological stress triggers cortisol release, altering intestinal permeability and compounding gut dysfunction (Chen H. et al., 2023; Chen X. et al., 2023; Chen J. et al., 2023; Morais et al., 2021). Evidence underscores a robust association between functional diarrhea and impairment of the BGMA. Notably higher rates of this condition occur in patients with mental disorders, with GM-CNS crosstalk serving as a potential mediator of this comorbidity (Zhang et al., 2021). The pathogenesis of IBS-D involves multifaceted interactions among brain–gut peptides, immune activation, and microbial composition (Li et al., 2020). For instance, Gao et al. demonstrated that dampening HPA axis activity via CRHR1 downregulation alleviates diarrheal symptoms in IBS models, underscoring the therapeutic relevance of BGMA modulation (Gao et al., 2023). Wu et al. provided further mechanistic insight, identifying correlations between specific microbial genera and neuro-immune markers in IBS-D rats. The genus Paraprevotella was positively associated with elevated 5-HT, CRF, and NPY, suggesting its potential role in modulating the HPA axis via serotonergic pathways (Wu et al., 2022). Additionally, microbial metabolites such as SCFAs and 5-HT are implicated in bidirectional BGMA communication, and their aberrant levels have been consistently reported in IBS-D patients (Dinan and Cryan, 2017; Luo et al., 2021). Interventions including probiotic supplementation have shown promise in reducing 5-HT levels and ameliorating IBS-D symptoms, highlighting the translational potential of targeting microbial components (Gu et al., 2022; Wu et al., 2024). Another compelling example comes from Chen et al. reported that alkaline mineral complex (AMC) water improved diarrhea resistance in stressed piglets by rebalancing the HPA axis and enriching beneficial bacteria such as Lactobacillus helveticus and Ruminococcus gnavus. This reinforces the notion that BGMA-oriented interventions can restore gut homeostasis through multifactorial mechanisms (Chen H. et al., 2023; Chen X. et al., 2023; Chen J. et al., 2023).

Overall, research on the BGMA provides critical insights into the mechanisms underlying diarrhea and reveals promising therapeutic potential. Current evidence suggests that targeting the BGMA—through modulation of microbial metabolites, neuroendocrine pathways, and immune signaling—may alleviate both intestinal and psychiatric symptoms. However, most studies to date remain correlative or reliant on animal models, highlighting a need for causal validation and clinical translation. Future work should integrate multi-omics approaches to elucidate precise molecular targets within the BGMA, ultimately facilitating the development of personalized therapies and bridging the gap between mechanistic discovery and clinical application.

In summary, the occurrence of diarrhea can alter the composition of the gut microbiota and the levels of its metabolites, regulate immune function, affect the gut-brain axis, and impair intestinal barrier integrity. The potential mechanisms mediating these effects are illustrated in Figure 1.

Probiotics are defined as beneficial, viable microorganisms, and a growing body of evidence has shown that numerous probiotic strains alleviate diarrhea by modulating the GM (Wieërs et al., 2019), regulation of inflammatory factor production (Wang F. et al., 2023), and enhancement of the intestinal mucosal barrier function (Camilleri, 2021; Su et al., 2022).

Lactobacillus and Bifidobacterium are widely employed as probiotics in the treatment of diarrhea, owing to their crucial functions in alleviating inflammation and promoting the balance of the intestinal microbiota. Lactoferricin produced by Lactobacillus reuteri CO21 was found to be able to modulate the intestinal physical barrier function by inhibiting the TLR4, Myd88 and Myosin light-chain kinase (MLCK) pathways and up-regulating the expression of the TJ proteins ZO-1 and claudin-2, thereby increasing piglets’ resistance to Enterotoxigenic E. coli and alleviating diarrhea (Xie et al., 2021). Lactobacillus and Saccharomyces boulardii have also shown effectiveness in the prevention or treatment of AAD (Doar and Samuthiram, 2023; Yang Y. et al., 2023; Yang Q. et al., 2023). Saccharomyces boulardii alleviates GM disorders and improves intestinal barrier function (Bustos Fernández et al., 2023). Saccharomyces boulardii mitigates mucosal injury by modulating intestinal mucin composition and secretion, strengthening the mucin barrier, and reducing SN-38 penetration into epithelial cells (Sezer et al., 2009).

In conclusion, probiotics exert beneficial effects on both the prevention and treatment of diarrhea, with such effects being strain-and dose-dependent. Thus, further studies are required to identify and optimize the selection and application of probiotics for managing different types of diarrhea.

FMT is a procedure that involves transferring GM from healthy donors into the gastrointestinal tract of patients to restore a balanced microbial community and treat diseases, particularly those associated with gut dysbiosis such as diarrhea (Almeida et al., 2022; Li et al., 2022).

The therapeutic efficacy of FMT in diarrhea alleviation is closely linked to the modulation of GM composition. On one hand, FMT reintroduces a healthy microbial community that competes for ecological niches in the gastrointestinal tract, thereby suppressing pathogen colonization-this process further facilitates the restoration of immune function and mitigates host tissue damage. On the other hand, FMT aids in replenishing essential metabolites for host metabolism, such as SCFAs, antimicrobial peptides, bacteriocins, and BAs (Ademe, 2020). IBS-D is a common gastrointestinal disorder and is characterized by altered GM, especially involving Firmicutes and Bacteroidetes (Mei et al., 2021; Zhen et al., 2021). However, FMT can reduce intestinal permeability and alleviate the diarrheal effects of IBS-D by modulating GM disorders and affecting GM-produced metabolites such as increasing the production of SCFAs (Lin et al., 2021; Singh et al., 2022; Song et al., 2023). Clostridium difficile infection (CDI) is considered a common cause of AAD (Bosnjak et al., 2023; Tubau-Juni et al., 2023). One study demonstrated that FMT administration to children with recurrent CDI enhanced GM diversity while driving shifts in GM composition and function toward those of the donor (Fareed et al., 2018). During the weaning transition, piglets are prone to diarrhea, which is related to the damaged state of the microbiome and immature immune system (Han et al., 2024). In diarrheal piglets infected with E. coli K88, the application of FMT increased the number of beneficial bacteria in the gut and reduced the number of harmful bacteria, and further research found that FMT triggered intestinal mucosal autophagy and reduced the damage of E. coli K88 to the intestinal barrier (Cheng et al., 2018).

Currently, research on the underlying mechanisms of FMT remains limited. Available evidence suggests that alterations in the GM following FMT play a significant role in the pathogenesis of diarrhea. However, several studies indicate that the therapeutic efficacy of FMT may be constrained. Additionally, to date, FMT has been predominantly investigated for the treatment of CDI and diarrhea-predominant IBS-D, while its efficacy in diarrhea of other etiologies remains less established. Thus, further clinical trials are warranted to validate the potential benefits of FMT across diverse forms of diarrhea and to better define its role in managing diarrhea-related disorders. It is also important to note that the limited efficacy of FMT in some cases of diarrhea may be attributable to insufficient donor-recipient matching, among other factors, highlighting the need for more personalized approaches in future studies.

Bacteriophages, viruses that specifically infect and lyse bacteria, offer a promising therapeutic strategy by precisely targeting pathogenic bacteria while preserving the commensal GM (Chen H. et al., 2023; Chen X. et al., 2023; Chen J. et al., 2023; Strathdee et al., 2023).

Antibiotics have always been the cornerstone of treating diarrhea, but rising antimicrobial resistance (AMR) has diminished their efficacy (Baran et al., 2023). Moreover, antibiotics disrupt the commensal GM, leading to dysbiosis and increased susceptibility to recurrent infections (Ramirez et al., 2020). Phage therapy, which uses viruses to specifically infect and kill bacteria, has re-emerged as a promising alternative due to its specificity, self-replicating nature, and ability to disrupt biofilms (Duan et al., 2022).

Multiple in vitro studies have demonstrated the effectiveness of phage therapy for treating diarrheal pathogens. Research has shown that a phage cocktail targeting E. coli, such as a combination of six bacteriophages, can reduce bacterial load by 3 log CFU/mL in vitro and effectively inhibit biofilm formation (Youssef et al., 2025). In addition, a 2023 study utilized a resource library termed the gut phage isolate collection (GPIC)—composed of bacteriophages isolated from healthy human guts—to demonstrate that a bacteriophage cocktail targeting Bacteroides fragilis significantly reduced the abundance of the target bacteria in in vitro fecal cultures, highlighting the potential of bacteriophages in modulating the GM (Shen et al., 2023). In animal model studies, phage therapy has also shown promising effects. Research has shown that the microencapsulated bacteriophage A221 is as effective as the antibiotic florfenicol in treating piglet diarrhea models (Youssef et al., 2025). In addition, phage cocktail therapy targeting Klebsiella pneumoniae associated with inflammatory bowel disease can alleviate intestinal inflammation and tissue damage in mouse models (Fuerte-Stone and Mimee, 2022). These studies indicate that bacteriophages can not only effectively reduce the load of pathogenic bacteria, but also alleviate the inflammatory response and tissue damage caused by it.

In recent years, clinical trials of phage therapy for diarrhea have also made some progress. A phase 1 clinical trial in 2022 tested two bacteriophages targeting Klebsiella pneumoniae associated with inflammatory bowel disease on 18 healthy volunteers. The results showed that when taken together with antacids such as CaCO₃, the bacteriophages not only survived at high levels but also remained active throughout the gastrointestinal tract without affecting the resident GM. All participants did not experience any serious treatment-related adverse events, laying the foundation for further research in patients (Federici et al., 2022).

Although these studies indicate that bacteriophages have great potential in treating diarrhea, a critical translational challenge involves phage instability in the harsh gastrointestinal environment. Gastric acidity and digestive enzymes rapidly denature phage particles, compromising therapeutic efficacy. Advanced encapsulation strategies using electrospun fibers, liposomes, or pH-responsive hydrogels are being developed to shield phages during transit and ensure targeted colonic release (Yang Y. et al., 2023; Yang Q. et al., 2023). Additionally, the field must address the complexity of phage ecology, particularly the potential for temperate phages to facilitate horizontal gene transfer of virulence or resistance genes. Careful selection of obligately lytic phages is therefore essential for clinical safety and efficacy (Gummalla et al., 2023).