Gut microbiota and metabolites in acute lung injury: mechanisms and therapeutic perspectives

Abstract

Acute lung injury (ALI) and its severe form, acute respiratory distress syndrome (ARDS), represent a clinical syndrome with high mortality, characterized by excessive pulmonary inflammation and oxidative stress. Despite advancements in conventional supportive care, mortality rates for ALI/ARDS remain persistently high (30%-50%). In recent years, increasing attention has focused on the regulatory mechanisms of the gut microbiota and their metabolites in ALI through the bidirectional ‘gut-lung axis’ interaction. This paper systematically reviews the mechanisms by which gut dysbiosis exacerbates lung inflammation and barrier damage via immune cell migration, inflammatory pathway activation, and metabolite imbalance. It also explores the potential of microbiome-based interventions—including probiotics, fecal microbiota transplantation (FMT), and dietary modification—for ALI treatment. This review not only elucidates the complex link between the gut microbiota and lung disease but also provides a theoretical basis for developing novel diagnostic and therapeutic approaches targeting the gut-lung axis. These insights hold significant implications for improving ALI patient prognosis and advancing precision medicine.

Graphical abstract

Introduction

Acute lung injury (ALI), also known as acute respiratory distress syndrome (ARDS), is a severe clinical syndrome characterized by an excessive inflammatory response in the lungs. It is important to note that while “ALI”is historically used in preclinical research (often referring to animal models induced by LPS, sepsis, etc.), “ARDS” is the standardized clinical diagnosis based on the Berlin criteria (hypoxemia, bilateral opacities, non-cardiogenic edema). In this review, we use “ALI/ARDS” when discussing general mechanisms applicable across the spectrum, but specify the etiology (e.g., sepsis-induced ALI, viral ARDS) when describing model-specific or clinical findings. The pathogenesis of ALI/ARDS is complex, involving damage to endothelial and alveolar epithelial cells, neutrophil aggregation, inflammatory responses, and oxidative stress. It is primarily characterized by a dramatic decrease in oxygenation, accompanied by patchy infiltrates visible on chest imaging [1, 2]. The morbidity and mortality of ALI/ARDS exhibit significant regional and age-related variations. In the United States, approximately 200,000 individuals are diagnosed with ALI annually, highlighting its prevalence among intensive care unit (ICU) patients [3]. Another multi-center prospective study conducted in China found that 672 of 18,793 ICU patients met the diagnostic criteria for ARDS [4]. Additionally, a single-center clinical study identified that among 715 patients in respiratory isolation in general wards, 62 (9%) were diagnosed with ALI and 12 (2%) met ARDS diagnostic criteria [5]. Despite recent advances in medical technology, the overall mortality rate of ALI remains high, typically ranging from 30% to 50%, with variations across studies [3]. Specific populations, such as ICU patients, the elderly, and those with chronic medical conditions, exhibit increased susceptibility to ALI and higher mortality rates [6]. The prevalence of ARDS in pediatric ICU patients is approximately 3%, with a mortality rate reaching 17% [7]. The rising incidence and mortality rates of ALI underscore its significant public health burden.

The treatment of ALI/ARDS centers on supportive care and cause-specific therapy. Proven effective strategies include anti-inflammatory therapy, oxygen therapy, mechanical ventilation, and appropriate fluid management [8]. Exploring new therapeutic targets and effective treatments for ALI/ARDS is crucial. Notably, an increasing body of research indicates that gut microbes play a significant role in ALI through various mechanisms, including modulation of the inflammatory response and bidirectional signaling via the gut-lung axis [9]. Gut microbiota metabolites, such as short-chain fatty acids (SCFAs), can protect lung tissue by suppressing inflammation and ameliorating oxidative stress, thereby attenuating ALI. Consequently, investigating the functions of the gut microbiota and their roles in ALI may yield vital insights for novel therapeutic strategies [10].

The role of the gut microbiota and its metabolites in ALI/ARDS is garnering increasing interest. The relationship between gut microbial imbalance and lung diseases, particularly through gut-lung axis interactions, has become a major research focus. Several excellent reviews have summarized the gut-lung axis concept in respiratory diseases [11, 12]. This review aims to provide an updated synthesis focusing specifically on ALI/ARDS, integrate microbiota-derived metabolites with immune cell trafficking mechanisms. Besides, we systematically discusses conflicting experimental and clinical findings, and critically evaluates the translational gaps between animal models of ALI and human ARDS. By emphasizing both mechanistic insights and unresolved challenges, this review aims to provide a balanced and forward-looking perspective on the gut–lung axis. Finally, this paper offers new insights into the therapeutic potential of the gut microbiota in ALI and establishes a theoretical foundation for developing innovative therapeutic strategies, such as probiotics and FMT, to improve patient outcomes.

In this review, we explicitly distinguish correlation from causation and clearly indicate whether conclusions are derived from murine models of ALI, large-animal studies, or human clinical data in patients with ARDS. Observational findings from human cohorts are described as associations, whereas causal language is reserved for studies involving experimental manipulation in animal models.

Bidirectional regulation of the gut-lung axis

The gut-lung axis is a network of interactions between the gut and the lungs through a variety of microbiological, immunological, metabolic and neuroendocrine pathways. The gut microbiota helps prevent pathogen infections, provides nutrition, participates in metabolism, shapes the immune system, and functions as a biological barrier [13]. In turn, the immune system influences microbiota composition [14]. Recent studies increasingly confirm that gut microbial dysbiosis is strongly associated with various lung diseases, such as chronic obstructive pulmonary disease (COPD), asthma, and ALI [15].

Gut microbes play a crucial role in maintaining homeostasis. However, when gut microbial homeostasis is disrupted, it may lead to bacterial translocation, inflammatory responses and metabolite changes, which in turn affect lung health [16]. Gut microbiota dysbiosis regulates the Toll-like receptor (TLR) 4/nuclear factor-κB (NF-κB) signaling pathway within the pulmonary immune system, activating pulmonary oxidative stress and mediating lung injury [17]. CD4⁺ T helper 17 (Th17) cells in the intestinal mucosa produce interleukin-17 A (IL-17 A), which contributes to intestinal barrier disruption, inhibits viral particle clearance, and synergizes with the innate immune system to induce systemic inflammation and exacerbate lung inflammation [18]. In contrast, regulatory T (Treg) cells in the intestinal mucosa promote tolerance to gut microbiota and food antigens and mediate systemic immunosuppression [19].

ALI development involves a cascade of inflammatory cytokine activation. The sustained recruitment of inflammatory factors leads to increased production of pro-inflammatory cytokines, which interact and mutually influence each other, causing further damage to the alveolar-capillary membrane and potentially respiratory failure [20]. Elevated cytokine levels can compromise the intestinal barrier, where tight junctions between epithelial cells are vital for maintaining barrier integrity. Increased inflammatory cytokines, along with upregulated claudin-2 and junctional adhesion molecule (JAM) expression and downregulated claudin-5 and zonula occludens-1 (ZO-1) expression in sepsis, can lead to intestinal hyperpermeability [21]. Loss of intestinal epithelial integrity in sepsis results in decreased anaerobic bacteria, further impairing epithelial function and permitting the proliferation of opportunistic pathogens [13]. Clinical studies have identified translocation of intestinal microorganisms, such as Bacteroidetes and Enterobacteriaceae, across the intestinal mucosa into the lungs of patients with sepsis and ARDS [22, 23]. Furthermore, ligation of mesenteric lymphatics inhibits neutrophil activation, attenuates lung injury, and improves survival in endotoxemic mice [24]. The gut-lymph hypothesis suggests that intestinal flora translocation causes local activation of the mucosal immune system (MIS), and inflammatory factors are stimulated by the MIS to increase production and cross the mesenteric lymphatic vessels into the lungs and systemic circulation [25]. Lung flora dysbiosis in ALI is also an important manifestation of the gut-lung axis under crosstalk. In lipopolysaccharide (LPS)-induced ALI, the main trend in the microbial community response is a decrease in the Firmicutes, represented by Alicyclobacillaceae, and the proliferation of Proteobacteria represented by Brucellaceae and Xanthobacteriaceae [26]. A clinical study comparing bronchoalveolar lavage fluid (BALF) bacterial communities in ARDS patients and healthy volunteers found gut-associated Bacteroides operational taxonomic units (OTUs) commonly present in ARDS patient lungs (41%) but absent in healthy controls [27]. Moreover, alterations in visceral vascular tone and visceral microcirculatory oxygenation may be associated with haemodynamic alterations during low tidal volume mechanical ventilation. The hypercapnia induced by protective ventilation is linked to increased myocardial contractility, reduced systemic vascular resistance, and changes in vascular tone. Subsequently, intestinal dysfunction and compromised intestinal integrity exacerbate systemic inflammation and promote bacterial translocation from the gut to the lungs, thereby intensifying pulmonary dysbiosis [28].

The gut-lung axis facilitates bidirectional communication between the gut and lungs through immune, neural, and metabolic pathways (Fig. 1). Alterations in the gut microbiota can disrupt this communication, increasing susceptibility to lung infection and inflammation. For instance, Dessein et al. reported that antibiotic-associated gut dysbiosis induced pulmonary immunosuppression and exacerbated lung infections in mice [29]. Similarly, Ziaka and Exadaktylos found that gut-derived immune cells contribute to ARDS pathogenesis via the gut-lung axis [12].These findings suggest that changes in gut microbiota composition and metabolite profiles can influence ALI severity by modulating local and systemic inflammation, cellular oxidative stress levels, and cellular infiltration/activation. Maintaining a balanced gut microbiota is therefore crucial for pulmonary immune homeostasis and ALI prevention.

Fig. 1.

Bidirectional crosstalk of the gut-lung axis in ALI/ARDS. This overview illustrates the primary pathways (immune, metabolic, neural, barrier) through which the gut and lungs communicate. dashed arrow indicates a process of migration. Gut dysbiosis promotes lung injury via microbial metabolites, bacterial translocation, and immune cell activation. In turn, systemic inflammation and cytokine release impair gut barrier integrity, increasing permeability and fueling a feedback loop of inflammation. Lymphatic transport of immune factors further contributes to lung injury. Thus, maintaining gut microbiota balance is essential for protecting lung function

In recent years, increasing research has focused on the complex interactions between ARDS and extrapulmonary organs. Regardless of whether infection originates within or outside the lungs, pathogens, inflammatory mediators, immune cells, and the extracellular vesicles (EVs) they release collectively form a dynamic network. This network mediates synergistic damage and functional failure across multiple organs including the heart, brain, kidneys, liver, and intestines. Within this systemic pathological process, the microbiome—particularly the gut microbiota—assumes the role of a central regulator [30]. Research confirms that ARDS is accompanied by significant gut dysbiosis, whose effects extend far beyond the digestive system. Breakthrough findings indicate that reduced levels of the probiotic bacterium Akkermansia muciniphila and its metabolite propionic acid are closely associated with long-term depression in ARDS survivors. Mechanistically, propionic acid alleviates neuroinflammation by inhibiting the TLR4/NF-κB pathway in brain microglia, providing direct evidence for the ‘gut-brain axis’ mediating neuropsychiatric sequelae of ARDS. Concurrently, gut dysbiosis exacerbates pulmonary inflammation by disrupting the intestinal barrier and promoting bacterial translocation, thereby establishing a vicious ‘gut-lung’ cycle [31].

Gut microbiota regulates the development of ALI

The gut microbiota, a complex ecosystem of bacteria, fungi, viruses, and protozoa, plays a pivotal role in modulating host immunity and barrier integrity through metabolite production (e.g., SCFAs) and pathogen exclusion. Emerging evidence underscores its critical involvement in ALI pathogenesis via the gut-lung axis. Dysbiosis—characterized by reduced beneficial taxa and expansion of pathobionts—disrupts intestinal homeostasis, promotes systemic inflammation, and exacerbates lung injury. This imbalance alters immune cell dynamics and activates inflammatory pathways (e.g., TLR4/NF-κB, NLRP3), driving ALI progression. Conversely, microbiota-targeted interventions, such as SCFA supplementation, probiotics may attenuate lung inflammation by restoring microbial balance and enhancing barrier function. Thus, the gut microbiota serves not only as a biomarker for ALI risk but also as a therapeutic target for intervention.

Microbiota dysbiosis and ALI

The gut microbiota comprises trillions of microorganisms, including bacteria, fungi, viruses, and protozoa. Gut microbes metabolize indigestible dietary components, producing metabolites such as SCFAs, essential for host energy metabolism and immune function. Additionally, the gut microbiota protects against harmful bacteria and toxins by promoting intestinal epithelial cell integrity and enhancing barrier function [32]. Growing evidence suggests that alterations in the gut microbiota profoundly affect lung function. In a mouse model, antibiotic-induced microbiota depletion was shown to ameliorate LPS-induced ALI via the gut-lung axis, suggesting a causative role of specific commensals in exacerbating injury [33]. Conversely, another preclinical study found that broad-spectrum antibiotic pretreatment worsened LPS-induced lung inflammation, increasing IL-6 and TNF-α levels [34]. This highlights that the net effect of microbiota depletion depends on timing, antibiotic spectrum, and the specific pathogenetic model, underscoring the complexity of translating these correlative observations from mice to causal mechanisms in humans. Human fecal genome sequencing reveals that the intestinal microbiota primarily consists of four bacterial phyla: Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria, with Firmicutes and Bacteroidetes being slightly dominant, accounting for approximately 90% of total intestinal bacteria. Proteobacteria are considered major pathogenic bacteria capable of producing endotoxins. Adherent and invasive Escherichia may exploit genetic abnormalities to evade pathogen recognition and bacterial clearance, leading to inflammation and immune system imbalance [35]. At the class level, Bacteroidia and Clostridia are predominant, constituting the majority of a relatively stable gut microbiota in healthy individuals [36].

Under pathological conditions such as infection, trauma, toxins, and disease, the gut microbiota typically exhibits decreased beneficial bacteria and overgrowth of pathogenic species, promoting disease progression. The black tea component ethanol precipitate fraction (EP) modulates the gut microbiome to prevent particulate matter (PM)-induced inflammation and oxidative stress in mouse lungs; in this process, the Lachnospiraceae_NK4A136_group may be a core taxon mediating EP’s lung-protective effects [37]. Lung and intestinal dysbiosis was observed in Staphylococcal enterotoxin B (SEB)-induced ARDS mice. The protective effect of resveratrol against ARDS may be mediated, at least partially, by increasing lung and intestinal probiotic populations such as Actinobacteria phylum, Tenericutes phylum, and Lactobacillus reuteri [38]. Clinical studies also characterize ALI/ARDS dysbiosis by an imbalance between beneficial and pathogenic bacteria. Beneficial commensal bacteria (e.g., Faecalibacterium prausnitzii and Bifidobacterium spp.) tend to decrease, while pathogenic or opportunistic bacteria (e.g., Escherichia coli and Streptococcus spp.) increase [39–41]. In a study of patients undergoing cardiac surgery assisted by cardiopulmonary bypass (CPB), Escherichia-Shigella were identified as predominant organisms in the intestines of patients developing postoperative ALI, potentially aiding prediction of CPB-induced ALI development and outcome [42]. Escherichia-Shigella are opportunistic pathogens that promote inflammatory episodes and exacerbate disease progression [43]. Specific structural and functional alterations in the intestinal microbiota were identified in ARDS patients, including enrichment of Proteobacteria phylum, Enterobacteriaceae family, and Escherichia-Shigella genus, alongside a decrease in Bifidobacterium genus. Furthermore, the relative abundance of Enterobacteriaceae increases with ARDS progression [44]. Shotgun metagenomic or metatranscriptomic sequencing analysis of fecal samples from 13 patients with coronavirus disease 2019 (COVID-19) and healthy controls revealed that the COVID-19 gut microbiota was characterized by enrichment of opportunistic pathogens. Bacteroides spp. abundance was inversely associated with COVID-19 severity, whereas Actinomyces oris, Escherichia coli, and Streptococcus parasanguinis were positively correlated with disease severity [45]. Analysis of endotracheal aspirate microorganisms in severely traumatized patients indicated that smoking-induced bacterial communities were enriched with potentially pathogenic organisms, increasing susceptibility to post-traumatic ARDS and contributing to lung inflammation and injury during hospitalization [23]. Thus, the increase in pathogenic bacteria disrupts gut microbiota homeostasis, promotes inflammatory factor secretion, exacerbates lung inflammation, and ultimately leads to poor postoperative prognosis in ALI/ARDS patients. These studies suggest that gut microbiota composition can serve as a biomarker for predicting acute respiratory distress syndrome.

In ALI/ARDS, the gut microbiota similarly modulates the lung microbiota via the gut-lung axis (Table 1). Microbiological evidence indicates that gut bacteria (Bacteroides spp.) were abundantly expressed in the BALF of ARDS patients, and TNF-α as a key mediator of alveolar inflammation in ARDS, is closely associated with lung microbiota alterations [46]. Another study found BALF from ARDS patients enriched with various enterogenic bacteria, including Staphylococcus, Streptococcus, and Enterobacteriaceae, the abundance of which significantly correlated with ARDS severity [47]. Enrichment of lung microbiota characterised by Lachnospiraceae and Enterobacteriaceae spp. predicts the prognosis of patients with ARDS [27]. In a study by Ma et al. Erysipelotrichales, Victivallis, Ruminococcaceae UCG014, Eubacterium ruminantium group, Erysipelotrichaceae and Erysipelotrichia were positively associated with the risk of developing ARDS, while Actinobacteria and Enterobacteriaceae were negatively associated [48]. In septic patients, reduced gastrointestinal motility and impaired intestinal epithelial barriers lead to decreased anaerobic bacteria and increased opportunistic pathogens, further exacerbating intestinal and systemic inflammation and promoting opportunistic pathogen proliferation and translocation [13]. Intestinal microbes, primarily from the Actinobacteria phylum and Enterobacteriaceae family, can translocate to the lungs via the intestinal mucosa, potentially causing or exacerbating sepsis-associated ALI/ARDS [46, 49]. These studies suggest a potential causal relationship between specific gut microbiota types and ARDS development, indicating that gut microbiota changes could serve as predictors of ARDS episodes [50].

Table 1.

Changes in the microbial community in ALI/ARDS

Sample	Etiology	Methods	Increased bacteria in abundance	Decreased bacteria in abundance	Ref
Fecal	Cardiopulmonary bypass	16S rRNA sequencing	Proteobacteria, Enterobacteriaceae, Escherichia-Shigella, Klebsiella, Enterococcus sp.	Bactroidota, Bacteroidaceae, Bacteroides vulgatus	[42]
Fecal	Acute pancreatitis	16S rRNA sequencing	Proteobacteria phylum, Enterobacteriaceae family, Escherichia-Shigella genus, Klebsiella pneumoniae	Bifidobacterium genus	[44]
Endotracheal aspirates	Severe blunt trauma	16S DNA sequencing	Enterobacteriaceae, Prevotella, Fusobacterium	not mentioned	[23]
BALF	sepsis	16S DNA sequencing	Bacteroides sp.	not mentioned	[46]
BALF	Not mentioned	16S DNA sequencing	Staphylococcus, Streptococcus, Enterobacteriaceae	Betaproteobacteria	[47]
BALF	Not mentioned	droplet digital PCR and 16S DNA sequencing	Lachnospiraceae, Enterobacteriaceae	not mentioned	[27]
Fecal	SARS-CoV-2	shotgun metagenomic or metatranscriptomic sequencing	E. coli, Akkermansia muciniphila, and Gemmiger formicili	Bacteroides vulgatus, Prevotella copri, Clostridium leptum, Alistipes putredinis	[101]

Regulation of immune cell function

Accumulating evidence indicates that the gut–lung axis modulates pulmonary inflammation through coordinated regulation of immune cell development, trafficking, and effector function rather than through isolated cell populations. Intestinal immune cells, including Th17 cells and regulatory T cells, can be primed by microbiota-derived signals and subsequently migrate to the lung via chemokine-dependent pathways, where they shape local inflammatory responses during acute lung injury [51]. In parallel, innate lymphoid cells and γδ T cells contribute to early cytokine production and epithelial barrier defense, influencing both the initiation and amplification of lung inflammation [52]. In addition, IgA-producing B cells support mucosal immune homeostasis and may indirectly affect pulmonary immunity through regulation of microbial composition and immune signaling [53, 54].The intestinal mucosal immune barrier, as a core site for immune response initiation and activation, plays a pivotal role in maintaining intestinal barrier integrity and regulating the resolution of systemic inflammatory responses [55]. Its core components include intestinal epithelial cells (IECs) forming a physical barrier, gut-associated lymphoid tissue (GALT) for immunosurveillance, diverse functional immune cell populations (including macrophages, antigen-presenting cells, monocytes, and neutrophils), and secretory IgA (SIgA) produced by plasma cells. Additionally, the system contains numerous immunologically active mediators, such as antibodies, cytokines, lysozyme, defensins, and other bioactive substances [9, 56]. The intestinal mucosal barrier is also crucial for maintaining intestinal immune homeostasis and regulating the long-distance migration of immune cells along the gut-lung axis [57, 58]. B-cell populations, known as antibody-secreting cells (ASCs), produce and release large quantities of antibodies. In the gastrointestinal environment, exposure to foreign dietary antigens, commensal microbiota, and intermittent pathogens drives concerted B-cell differentiation into ASCs [59]. In patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, IgA-induced neutrophil extracellular trap (NET) release may be beneficial as an early defense mechanism against viral entry into mucosal regions. However, as the disease progresses, NET release can become deleterious, potentially exacerbating tissue damage [54].

Innate lymphoid cells (ILCs) comprise three main groups (ILC1, ILC2, and ILC3) each containing multiple subsets [60]. Recent studies demonstrate the role of the gut microbiota in directing ILC2 migration from the gut to the lungs, suggesting that lung-resident ILC2s may originate intestinally via circulation [61]. Gray et al. found that IL-22 is predominantly derived from ILC3s and that the gut microbiota may influence the pulmonary immune response in neonatal mice by triggering IL-22 production in the lungs, particularly in combating pneumonia [62].

Crosstalk between the gut microbiota (or its derived small molecules) and intestinal immune cells is essential for regulating immune cell function, maintaining a healthy immune microenvironment, and facilitating bidirectional communication between intestinal cells. This ultimately impacts intestinal barrier integrity and ALI development [63, 64] (Fig. 2). Alveolar macrophages and alveolar type II (AT2) cells express free fatty acid receptors (FFAR)2, FFAR3 and IL-1β, but their expression patterns and responses to LPS differ [65]. In pancreatitis-associated ALI mice models indicate that SCFAs inhibit NF-κB activation and NOD-like receptor family pyrin domain-containing protein 3 (NLRP3) inflammasome assembly, thereby attenuating lung inflammation and injury [66]. Similarly, tryptophan metabolites modulate immune cell function (e.g., macrophages, dendritic cells), attenuating inflammatory injury [67]. These studies confirm the significant role of the gut microbiome and its metabolites in establishing and modulating pulmonary immune activity.

Fig. 2.

Immune cell trafficking and functional modulation along the gut–lung axis in ALI/ARDS. This schematic summarizes the origin, key trafficking signals, and net effects of major immune cell populations involved in the gut-lung axis. Solid arrows indicate mechanisms supported by multiple experimental studies; dashed arrow indicates a process of migration or secretion. Microbial-derived SCFAs regulate macrophage activity and inhibit NF-κB and NLRP3 inflammasome activation, thereby attenuating pulmonary inflammation. B cells differentiate into ASCs that produce IgA, which can promote NET formation—an early protective mechanism against pathogens such as SARS-CoV-2, though potentially pathogenic during later disease stages. Gut-derived ILCs, including ILC2 and ILC3, migrate to the lungs in response to microbial cues. ILC3-derived IL-22 contributes to neonatal lung defense, while γδ T cells migrate via the Wnt/β-catenin pathway and exacerbate lung injury through IL-17A production. T helper Th1, Th17 and regulatory T Treg cells are also modulated by microbial metabolites, influencing systemic immune responses. The balance of Th17/Treg cells, shaped by microbial products and cytokine signaling, is crucial in modulating lung inflammation and injury. ASCs: Antibody-secreting cells; IECs: Intestinal epithelial cells; ILCs: Innate Lymphoid Cells; NETs: Neutrophil Extracellular Traps; SCFAs: Short-Chain Fatty Acids; Treg: Regulatory T cell; Th1(17): T Helper 1(17) cell

Immune cell migration along the gut-lung axis is a key mechanism connecting the gut and lungs. Studies show that intestinal immune cells (e.g., γδ T cells, neutrophils) can migrate to the lungs and participate in the inflammatory response in ALI [68]. In septic mice, memory γδ T17 cells from the small intestine are the primary cellular source of IL-17 A in the lungs. These cells migrate to the lungs via the Wnt/β-catenin signaling pathway, releasing the pro-inflammatory cytokine IL-17 A, which induces inflammation-aggravated lung tissue injury [52]. Additionally, mesenchymal stem cells (MSCs) may mediate immune response regulation in the lungs, intestines, and gut microbiota by reducing Ly6C⁺CD8⁺ T-cell infiltration, attenuating lung and intestinal injuries, and improving ALI mouse survival rates [69]. Macromolecular polysaccharides may ameliorate lung injury in H1N1-infected mice by regulating Th17/Treg cell balance in the gut-lung axis via the C-C motif chemokine receptor 6 (CCR6)/C-C motif chemokine ligand 20 (CCL20) axis. This immunomodulatory effect may stem from bacterial metabolites like SCFAs, which can restore immune tolerance and inflammatory balance by modulating T-cell differentiation towards Th17 or Treg lineages within the gut-lung axis [70]. This finding reveals a novel mechanism whereby intestinal immune cells migrate to the lungs via the gut-lung axis and exacerbate ALI.

Regulation of inflammatory pathways

The gut microbiota and its metabolites also modulate inflammatory pathways, influencing ALI development and progression. For example, SCFAs inhibit the production of inflammatory cytokines, such as TNF-α and IL-1β, reducing lung inflammation and injury [71]. SCFAs also regulate intestinal barrier function and reduce acute pancreatitis-induced ALI via an AMP-activated protein kinase (AMPK)/NF-κB/NLRP3-mediated pathway [9, 66]. Similarly, tryptophan metabolites regulate the nuclear factor erythroid 2-related factor 2 (Nrf2) antioxidant pathway, protecting against oxidative stress and lung injury [67].

Gut microbiota dysbiosis induces Gram-negative bacteria to produce high levels of LPS, which enters the circulation via a “leaky gut,” increasing systemic (especially pulmonary) immune responses and activating the TLR4 signaling pathway. Gut microbiota-derived products influence innate immune receptor activation, such as Toll-like receptors, impacting ALI development. The TLR4/NF-κB signaling pathway induces inflammation and oxidative stress. Intestinal dysbiosis disrupts the mucosal barrier and induces LPS translocation, causing activation of the pulmonary TLR4/NF-κB signaling pathway and exacerbating lung injury [72]. TLR4 is an important pattern recognition receptor that recognizes translocated pathogenic bacteria in the circulation, initiates downstream signal transduction pathways, and activates the NF-κB pathway through both myeloid differentiation primary response 88 (MyD88)-dependent and -independent pathways, upregulating various inflammatory factors [73]. Sinomenine was shown to activate the aryl hydrocarbon receptor (AhR)/Nrf2 pathway, modulating the intestinal microbiota, restoring the intestinal barrier, terminating the inflammatory response, and ultimately protecting against ALI development [74]. In conclusion, regulation of gut microbiota metabolites and compositional differences can influence lung injury development by modulating the intestinal immune barrier or pulmonary inflammatory pathways.

Regulation of ALI by metabolites of intestinal flora

Metabolites produced by gut microorganisms participate in regulating multiple signaling pathways that continuously modulate local and systemic immune cells. Based on source and synthesis mode, gut microbiota metabolites are broadly classified into three categories: (1) metabolites derived from dietary sources (e.g., SCFAs, tryptophan metabolites, trimethylamine N-oxide (TMAO)); (2) host-produced metabolites modified by the gut microbiota (e.g., secondary bile acids (BAs)); and (3) metabolites de novo synthesized by gut flora (e.g., branched-chain amino acids (BCAAs)) [75]. These metabolites play crucial roles in maintaining gut health, regulating metabolic homeostasis, and systemic immunity [76]. Gut microbiota metabolites profoundly impact human health and disease pathogenesis (Fig. 3).

Fig. 3.

Mechanisms of gut microbiota-derived metabolites in regulating ALI/ARDS. This figure details the specific signaling pathways and cellular targets of key metabolite classes. SCFAs (including acetate, propionate, and butyrate) may exert protective effects against ALI through their regulatory effects on immune cells. Dietary fibre intake promotes the enrichment of SCFA-producing gut bacteria and further regulates inflammatory factors to modulate innate lung immune tension, thereby reducing lung injury. Active components in some traditional Chinese medicines have also been found in numerous studies to alleviate lung inflammation by increasing SCFAs production. Amino acid metabolites, as important biomolecules, play a key role in ALI/ARDS, particularly in immune regulation and energy metabolism. Primary bile acids, including CA and CDCA, are synthesised in the liver and released into the intestines via the bile ducts. Under the action of intestinal bacteria, they form secondary bile acids such as DA, UDCA, and taurochenodeoxycholic acid (TCDCA). Additionally, other metabolites such as succinate, TMA, and nicotinic acid—important intermediates in the tricarboxylic acid cycle—can also regulate ALI by modulating the gut microbiota, host immunity, and inflammatory signaling pathways. CA: Cholic acid; CDCA: chenodeoxycholic acid; DA: Deoxycholic acid; TMA: Trimethylamine; UDCA: Ursodeoxycholic acid

SCFAs

SCFAs are key metabolites produced by gut microbes through dietary fiber fermentation. These fatty acids typically have carbon chains shorter than six atoms and primarily include acetate, propionate, and butyrate. SCFAs play a vital role in maintaining intestinal health and overall metabolism, influencing diverse physiological processes, including inflammatory responses, energy metabolism, and intestinal barrier maintenance [75]. Depletion of gut microbes by broad-spectrum antibiotics significantly reduces fecal SCFAs levels, indicating that SCFAs production largely depends on gut microbiota fermenting dietary fiber [33]. Gut microbial community composition and diversity directly impact SCFAs production.

In vitro studies and rodent models indicate that SCFAs act locally in the gut and systemically via the bloodstream. For example, experimental data suggest that butyrate can activate G protein-coupled receptor (GPR)109 A on immune cells, which is associated with anti-inflammatory effects in various disease models [77]. Translating these mechanistic insights to human ALI/ARDS requires further validation. The protective effects of SCFAs against ALI may involve modulating immune cell function (Table 2). Tregs are a distinct CD4⁺ T-cell subset that maintains immune homeostasis [78]. In a murine model of sepsis, Wei et al. reported that sodium butyrate decreased the wet/dry (W/D) ratio and total protein concentration in lung tissue, lowered levels of pro-inflammatory cytokines, surfactant-associated protein D (SP-D), diamine oxidase (DAO), and LPS, while elevating anti-inflammatory cytokines and tight junction proteins. It improved survival rates, oxygenation index, and lung histology by enhancing intestinal and lung barrier function and modulating CD4⁺Foxp3⁺ Tregs, demonstrating a potential causative protective effect mediated through Treg modulation [79]. These studies suggest SCFAs represent a novel therapeutic target for ALI. Butyrate promotes Treg production, helping reduce systemic inflammation [80, 81]. In an ALI/ARDS mouse model, intraperitoneal (i.p.) injection of sodium butyrate reduced high-mobility group box 1 protein (HMGB1) expression, pro-inflammatory cytokines (TNF-α, IL-6), and inhibited NF-κB signaling pathway activation [82]. For SEB-mediated ARDS in mice, induction of antimicrobial peptides (AMPs), tight junction proteins, and SCFAs maintained gut-lung microbial axis stability. Butyrate supplementation reduced inflammatory cell infiltration and lung injury in ARDS mice and improved clinical parameters like specific airway resistance and conductance [83].

Table 2.

Experimental evidence for the role of SCFAs in ALI/ARDS models

Metabolite name	Model(n)	Study design	Mechanism or effect on ALI/ARDS	Ref
Butyrate	Mouse (n = 18)	Cecal ligation and puncture	Butyric acid increased the CD4+/CD8+T cell ratio and the proportion of CD4+Foxp3+ regulatory T cells (Tregs), alleviated lung damage.	[79]
Butyrate	Mouse (n = 60)	7.5 mg/kg LPS, i.t.	By inhibiting the synthesis of inflammatory cytokines, inflammatory cell infiltration, and the TLR4/NF-κB pathway, it alleviates structural damage and inflammation in the lungs.	[82]
Butyrate	Mouse (n = 3–7)	50 µg Staphylococcal enterotoxin B, i.t	Butyrate reduced inflammatory cell infiltration and lung damage in ARDS mice and improved clinical lung parameters.	[83]
Acetate	Human cohort (n = 13); Mouse (not mentioned); Cell line (HBE cell)	100 PFU (50 µl in sterile PBS), intranasally(in nas)	Acetic acid activates the GPR43-AMPK pathway, partially restoring airway epithelial barrier function and reducing inflammatory cytokine levels.	[84]
Acetate	Mouse (n = 10)	LPS (5 mg/kg), i.t.	Acetate exerts anti-inflammatory and antioxidant effects by inhibiting the phosphorylation of the MAPK signalling pathway.	[85]
Propionic	Mouse (n = 8 − 3)	0.6 mg/kg ZnONPs, i.t.	Propionic alleviates ALI by inhibiting macrophage-associated inflammation and oxidative stress through GPR4.	[87]
SCFAs such as propionic acid	Mouse (n = 5)	0.2 µg/µL (5 ug /mouse), intranasal	Inhibiting the production of T cell inflammatory factors and alleviate the inflammatory response.	[88]
Propionate	Mouse (not mentioned); Cell line (MLE 12 and MH-S cell)	Left lung I/R; 10-200ng/mL LPS for 24 h	Reducing lung alveolar macrophage immune tone by suppressing IL-1β and IL-18.	[91]
Propionate and butyrate	Mouse (n = 7)	Caerulein (100 mg/kg, i.p, q1 h,10 times) + LPS (10 mg/kg)	Activating AMPK/NF-kB/NLRP3 signalling pathways, reduce systemic inflammation, restoring intestinal barrier function.	[66]
SCFAs mixture	Mouse (n = 8–12)	2.5 µg LPS/g BW intranasal	Reducing pulmonary inflammation, oxidative stress, metabolic changes, enhancing bone marrow cell activation.	[71]

i.p intraperitoneal injection, i.t intratracheal instillation, i.v intravenous injection

Acetate pretreatment attenuates barrier dysfunction in influenza A virus (IAV)-infected mice by activating the GPR43-AMPK pathway, protecting airway epithelial tight junctions, partially restoring impaired barrier function, and decreasing inflammatory cytokine levels (TNF-α, IL-6, IL-1β) [84]. Acetate attenuates lung edema, inhibits oxidative stress and inflammatory cell recruitment/mediator production, and modulates mitogen-activated protein kinase (MAPK) pathway activation by inhibiting alveolar-capillary barrier permeability in mice, thereby mitigating lung injury [85]. Acetate and butyrate inhibit the NF-κB pathway, reducing pro-inflammatory factor production [86]. These molecules exert anti-inflammatory and protective effects in ALI.

Oral administration of propionate (another microbiota-derived metabolite) suppresses macrophage-mediated inflammation during ALI in metal fume fever [87]. Mohammed et al. evaluated changes in lung inflammation, gut/lung microbiota, and SCFAs production in ARDS mice; their results indicated that propionate inhibited the ARDS-induced inflammatory response [88]. SCFAs and AMPs can attenuate lung inflammation by modulating immune responses. For instance, SCFAs enhance intestinal epithelial barrier function and reduce pulmonary inflammatory responses by inhibiting pro-inflammatory cytokine release [89, 90]. Dietary fiber intake promotes enrichment of propionate-producing gut bacteria, modulates innate lung immune tone, reduces lung injury by further modulating IL-1β and IL-18, and in vitro propionate supplementation reduces alveolar macrophage immune tone and promotes metabolic reprogramming [91].

Active components in some traditional Chinese medicines also attenuate lung inflammation by increasing SCFAs production. Pretreatment with astragalus polysaccharide, an immunomodulatory bioactive ingredient, modulates alveolar macrophages by altering gut microbiota composition and increasing SCFAs levels, conferring anti-inflammatory activity and acting prophylactically in ALI [87]. Houttuynia cordata polysaccharides locally modulate Th17/Treg cell balance in the lung-intestinal mucosa of influenza-infected mice by regulating gut microbiota composition [70]. Thus, SCFAs, as microbiota metabolites, restore immune tolerance and inflammatory homeostasis, serving as a bridge for traditional Chinese medicines modulation of gut microbiota in ALI treatment. Reducing the Firmicutes/Bacteroidetes ratio resulted in increased relative abundance of SCFA-producing bacteria.

SCFAs (especially butyrate and propionate) reach significant concentrations in feces, the gut lumen, serum, and lung. Modulation by gut microbes and SCFAs can restore lung ultrastructure, reduce α-amylase and systemic inflammation levels, and attenuate ALI via activation of the AMPK/NF-κB/NLRP3 signaling pathway [66]. In aged rats, SCFAs contribute to reducing inflammatory aging, oxidative stress, and metabolic alterations, while enhancing bone marrow cell activation. Demonstrating that SCFAs play a beneficial role in the gut-lung axis of aging organisms by reducing pulmonary inflammatory aging and ameliorating ALI exacerbation severity in aged mice suggests a role for SCFAs in age-related lung disease [71]. With an improved understanding of SCFA biological activities, future studies are expected to reveal their potential applications in systemic diseases like ALI and further clarify the complex relationship between gut microbes and systemic health.

Amino acids

Beyond SCFAs, other gut microbiota metabolites—such as amino acids, secondary BAs, and succinate—are also being investigated for their roles in regulating ALI/ARDS progression (Table 3). Among these, amino acid metabolism plays a significant role in ALI, particularly through the gut-lung axis. In ALI research, amino acid metabolite mechanisms are gaining increasing attention. Amino acids, as essential biomolecules, play key roles in various physiological processes, especially immunomodulation and energy metabolism.

Table 3.

Potential mechanisms by which other metabolic products mediate ALI/ARDS

Metabolite name	Model (n)	Study design	Key Finding	Effect on ALI/ARDS	Ref
Tryptophan	Mouse (n = 3–16)	The superior mesenteric arteries I/R	The levels of tryptophan metabolites indole-3-lactic acid in feces are negatively correlated with intestinal dysfunction which may alleviates intestinal I/R injury by regulating YAP and Nrf2.	Alleviate	[67]
Arginine	Rat (n = 3); Cell line (RAW264.7 cells)	1 mg/kg LPS by pulmonary fluid quantification atomizer; 0.5 µg/ml LPS, 16 h.	L-arginine-modifie.d effectively targets M1 macrophages, promotes the accumulation of curcumin in the lungs, enhances the anti-inflammatory effects of curcumin.	Alleviate	[98]
Arginine	COVID-19 patients (n = 26)	Phenotypic and functional investigations	Alleviating immunosuppression, shorten the duration of mechanical ventilation, and reduce mortality.	Alleviate	[99]
Deoxycholic acid	Mouse (not mentioned)	20 µg of LPS, i.t	The ternary complex consisting of plasmid DNA, lipopolysaccharide-binding peptide, and deoxycholic acid-conjugated polyethylenimine can induce HO-1 expression and reduce proinflammatory cytokine levels in ALI mice.	Alleviate	[108]
Taurochenodeoxycholic acid	Mouse (n = 6)	In nas, S. aureus (2 × 109 CFU, 20 µL)	Deoxycholic acid-conjugated polyethylenimine, as one of the components of the complex, has anti-inflammatory effects on ALI mice.	Alleviate	[110]
Ursodeoxycholic acid	Rat (n = 8–10)	500 µL/kg perinephric fat, i.v	Reducing the PaO2/FiO2 ratio, improve pulmonary edema, and significantly alleviate the pathological and biochemical changes induced by fat embolism-induced ALI/ARDS.	Alleviate	[115]
Ursodeoxycholic acid	Mouse (n = 5–8)	10 mg/kgLPS, i.p	Blocking panoptos-like cell death through the STING pathway.	Alleviate	[117]
Succinate	Mouse (n = 3–10)	100 µg LPS (100 µl), i.t	In HFD mice, SUCNR1 may promote alveolar macrophage inflammatory factor release and proinflammatory signaling pathway activation by activating HIF-1α and NF-κB.	Exacerbate	[129]
Succinate	Mouse(n = 3–20)	The superior mesenteric arteries I/R	Succinate promotes macrophage polarization, alveolar epithelial cell apoptosis, and exacerbates lung injury through the SUCNR1 receptor and PI3K/AKT/HIF-1α signaling pathway.	Exacerbate	[124]
TMAO	Mouse (n = 8–10)	30 mg/kg LPS i.p	Yazhicao reduces TMAO synthesis by improving metabolic remodeling and regulating the intestinal microbiota, inhibits the NF-κB/NLRP3 signaling pathway.	Exacerbate	[131]
AI-2	Mouse (n = 3–20)	1 mg/kg LPS dissolved in 50 µl PBS, intranasal	Increasing the secretion of inflammatory molecules and chemokines, reshaping the intestinal microbiota, inducing the secretion of inflammatory molecules, and exacerbating pulmonary inflammation.	Exacerbate	[134]

i.p intraperitoneal injection, i.t intratracheal instillation, i.v intravenous injection, TMAO Trimethylamine N-oxide, AI-2 autoinducer-2

Certain gut bacteria, such as Lactobacillus murinus, can produce tryptophan metabolites including indole-3-lactic acid (ILA), which have been implicated in mucosal immune regulation [92]. Tryptophan metabolites often act as aryl hydrocarbon receptor (AhR) ligands, regulating innate and adaptive immunity, cell proliferation, inflammation, and apoptosis [93–95]. A clinical study found preoperative fecal levels of the tryptophan metabolite ILA negatively correlated with intestinal impairment in patients undergoing CPB surgery. ILA attenuated intestinal ischemia-reperfusion (I/R) injury by modulating Yes-associated protein (YAP) and Nrf2. ILA increased YAP and Nrf2 expression following intestinal I/R. The YAP inhibitor verteporfin (VP) reversed ILA’s anti-inflammatory effects both in vivo and in vitro [67]. Glutamine is a primary energy source for immune cells like lymphocytes and macrophages. It plays a protective role in inflammatory responses and helps reduce lung tissue damage by supporting immune cell proliferation and function [96]. Arginine is a precursor for nitric oxide (NO) synthesis, an important signaling molecule regulating vasodilation and immune responses. Arginine deficiency may impair immune function, exacerbating ALI [97]. Specific targeting of lung M1 macrophages with L-arginine-modified liposomes enhanced the anti-inflammatory effects of curcumin, exerted immunomodulatory effects, and attenuated ALI/ARDS in cellular and animal models [98]. The important role of arginine in ameliorating ALI was similarly demonstrated clinically. A study in COVID-19 patients showed that myeloid-derived suppressor cell (MDSC) expansion was directly associated with lymphopenia and enhanced arginase activity; arginine supplementation attenuated immunosuppression, shortened mechanical ventilation duration, and reduced mortality [99].

Furthermore, gut microbiota composition and function directly influence amino acid metabolism. Specific microorganisms break down unabsorbed amino acids, producing metabolites like SCFAs, which not only provide energy to intestinal cells but also influence pulmonary inflammation via circulation [100]. Oxidative stress plays a key role in ALI development. Amino acid metabolites modulate oxidative stress levels by regulating antioxidant enzyme expression and activity. Metagenomic sequencing analysis of feces from 13 COVID-19 patients revealed elevated metabolic levels of neutral amino acids, lipopolysaccharides, sphingolipids, and ketones, alongside reduced butyrate synthesis. This suggests the gut microbiota experiences oxidative stress during COVID-19 infection, potentially triggering further lung inflammation [101]. Studies indicate branched-chain amino acids (e.g., leucine, isoleucine, valine) play important roles in regulating oxidative stress and apoptosis. They enhance cellular antioxidant capacity by activating the AMPK/Nrf-2 pathway, thereby attenuating ALI [102].

Predictive functional analyses of different gut microbiota have shown that dysbiosis leads to dysregulated amino acid metabolism. In LPS-induced ALI, amino acid metabolism is dysregulated in lung and colonic tissues, characterized by aberrant expression of glycine, serine, glutamate, and cysteine. Ameliorating this dysregulation significantly protects against ALI [103]. The mechanisms of amino acid metabolites in ALI are complex and diverse, involving immune regulation, oxidative stress, and metabolic reprogramming. In-depth study of these mechanisms can provide new therapeutic insights for ALI and important scientific foundations for improving overall health.

Bile acids

BAs are derived from cholesterol, primarily synthesized in the liver and secreted into the small intestine [104]. Beyond their critical role in lipid and fat-soluble vitamin digestion/absorption, BAs act as important signaling molecules regulating host metabolism. The interaction between BAs and the gut microbiota has been extensively studied. Gut microbiota composition is regulated by BAs, while the microbiota also modulates BA metabolism [105]. Primary BAs, including cholic acid (CA) and chenodeoxycholic acid (CDCA), are synthesized in the liver and released into the intestine via bile ducts [106]. Deoxycholic acid (DCA) is a secondary BA formed by bacterial action on primary BAs. DCA coupled with low molecular weight polyethylenimine (PEI)-2k forms PEI-DA, serving as a highly efficient gene carrier [107]. The ternary complex formed by PEI-DA with heme oxygenase-1 (HO-1) plasmid and can deliver therapeutic genes and peptides to induce HO-1 expression and reduce lung inflammation in ALI animals [108]. Before release, primary BAs are conjugated to form taurine-conjugated BAs like taurochenodeoxycholic acid (TCDCA) [109]. TCDCA modulates pro-inflammatory cytokine secretion in macrophages, activation of MAPK and NF-κB signaling pathways, and TLR2, decreasing inflammatory mediator secretion and attenuating lung injury in Staphylococcus aureus-infected mice [110].

Ursodeoxycholic acid (UDCA), a naturally occurring secondary BA (the 7β isomer of CDCA), possesses anti-inflammatory, anti-apoptotic, anti-tumor, and antioxidant activities [111]. UDCA interacts with and influences the gut microbiota. Various colonic bacteria metabolize primary BAs to produce UDCA, indicating host-microbiota co-metabolism [112]. UDCA exerts protective effects in diseases like fatty liver, diabetes mellitus, and hyperbilirubinemia [113, 114]. Pretreatment with UDCA (40 mg/kg, administered 1 h before fat embolization) inhibited the release of inflammatory cytokines (TNF-α, IL-1β), ameliorating fat embolism-induced ALI [115]. He et al. further found that UDCA’s protective effect against fat embolism syndrome-induced ALI may be mediated by inhibiting p38 MAPK/NF-κB signaling and activating the [116]. In sepsis-induced ALI, UDCA attenuates inflammation, oxidative stress, and lung barrier impairment, thereby mitigating lung injury. This protective effect is associated with inactivation of the stimulator of interferon genes (STING) pathway and inhibition of PANoptosis-like cell death [117]。.

Bile acid metabolism is dysregulated in ARDS patients, with elevated primary BA levels (e.g., CA, CDCA) accompanied by decreased secondary BA levels (including DCA, lithocholic acid (LCA), and UDCA). Dysregulation of plasma BA metabolites may represent an adaptive physiological response in ARDS pathogenesis [118]. Another clinical study found ARDS patients exhibited elevated levels of taurine-conjugated BAs and a significantly lower glycine-to-taurine conjugation ratio (G/T ratio) [119]. Pediatric ARDS patients exhibit gut microbiota dysbiosis characterized by decreased Bifidobacterium genus and increased Veillonella genus. Decreases in deoxycholic acid and hydrogen ions, along with elevated hydroxylated hydrophobic BAs, mark this concurrent disturbance in BA metabolism [120].

Therefore, current findings reveal the potential roles of gut microbiota and BA metabolism in ARDS pathophysiology, opening new avenues for diagnostic and therapeutic biomarkers and laying the groundwork for subsequent scientific exploration. In-depth analysis of the precise roles of these metabolic pathways in ALI/ARDS pathogenesis will be key to disease prevention and treatment.

Other metabolites

Succinate, a key tricarboxylic acid (TCA) cycle intermediate, participates in various physiopathological processes by activating the succinate receptor (SUCNR1) and regulating cellular signaling pathways [121]. Under pathological conditions like inflammation, ischemia, and hypoxia, intracellular succinate is released extracellularly by mitochondria and accumulates in tissues, participating in local and systemic inflammation activation and mediating inflammation-associated disorders, including obesity, diabetes mellitus, and inflammatory bowel disease (IBD) [122, 123]. Lung succinate levels in mice during intestinal I/R positively correlate with the ratio of intestinal succinate-producing to succinate-degrading bacteria, while succinate accumulation in the lungs is associated with intestinal mucosal barrier disruption [124]. This implies an important role for succinate as a host-microbiota co-metabolite intermediate in regulating gut microbial homeostasis and lung injury. Succinate accumulation during I/R may be linked to reversed succinate dehydrogenase activity during ischemia and mitochondrial reactive oxygen species (ROS) production during reperfusion [125]. In obese patients, gut microbiota changes and metabolic disturbances lead to elevated serum succinate levels [126]. SUCNR1, a G protein-coupled receptor, is widely expressed in tissues like the heart, kidneys, liver, nervous system, and lungs [127, 128]. One study found increased SUCNR1 expression in alveolar macrophages and heightened lung inflammation in high-fat diet (HFD) mice. Mechanistically, exacerbation of obesity-induced lung injury is closely related to succinate-SUCNR1 pathway overactivation and downstream HIF-1α and NF-κB signaling pathway activation [129]. Wang et al. found that increased plasma succinate concentration and SUCNR1 expression during intestinal I/R further activated the PI3K-AKT signaling pathway, promoting alveolar macrophage M1 polarization and aggravating lung injury [124]. Therefore, maintaining gut microbial homeostasis and inhibiting succinate accumulation could be therapeutic targets for ALI secondary to obesity or I/R.

Trimethylamine (TMA) is a gut bacterial-derived metabolite, and TMAO, derived from TMA, induces inflammatory responses [130]. Yazhicao (Commelina communis L., YZC) YZC has a protective effect against sepsis-induced ALI by inhibiting the NF-κB signaling pathway and reducing the expression of NLRP3 inflammatory vesicles in lung tissue. This effect is closely related to the inhibition of TMA production and reduction of TMAO synthesis by YZC remodelling of the gut microbiota [131]. Recently, TMAO levels have been associated with mortality in patients with community-acquired pneumonia and chronic obstructive pulmonary disease [132, 133]. As a quorum-sensing molecule, the bacterial metabolite autoinducer-2 (AI-2) modulates host immunity. In an ALI mouse model, AI-2 remodeled the gut flora, inducing inflammatory molecule secretion and exacerbating pneumonia; this effect was attenuated by AI-2 inhibitors. Thus, gut flora-derived AI-2 modulates lung inflammation via the gut-lung axis [134]. Choline is an inter-organ metabolite derived from the gut microbiota. In vivo and in silico studies reveal that the human gut microbiota synthesizes choline de novo via the acetamide pathway, identifying Enterococcus faecalis as the major choline-synthesizing strain [135]. Clinical studies found reduced choline-producing beneficial bacteria and general choline deficiency in the gut microbiota of cystic fibrosis patients [136]. Consequently, choline not only maintains intestinal barrier function but also reduces inflammation caused by bacterial translocation. Further research on the effects of TMAO and choline on ALI pathophysiology will help identify new therapeutic targets.

Treatment strategies and future research directions

The gut microbiota and its metabolites have been associated with the development and severity of ALI/ARDS. Intensive microbiome research provides new perspectives and tools for ALI diagnosis and treatment, aiding a deeper understanding of the mechanisms underlying the roles of gut microbiota and metabolites in ALI. Intervention strategies targeting the gut-lung axis, such as probiotics, prebiotics, FMT, and dietary interventions, may offer new directions for improving ALI therapy.

Novel microbiome-based approaches to ALI diagnosis

Recent advances in microbiome research have enabled emerging diagnostic methods based on microbiome characteristics to effectively identify and evaluate ALI pathological states. The microbiome encompasses the collection of microorganisms, including bacteria, fungi, and viruses, residing within the host [137]. Gut microbiome balance is essential for host health, and its disruption may lead to various diseases, including ALI [138]. A study analyzing oral, lung, and gut microbes from 479 mechanically ventilated acute respiratory failure patients using advanced DNA sequencing technologies (Illumina amplicon sequencing and nanopore metagenomics of lung microbiota) found decreased lung microbial diversity, characterized by reduced beneficial anaerobes and increased pathogens, correlating with disease severity [139]. As technology advances, microbiome-based diagnostics are becoming a research hotspot. Analysis of gut and lung microbes using high-throughput sequencing (e.g., 16S rRNA sequencing, metagenomic sequencing) provides information on microbial diversity and abundance. This approach effectively identifies specific microbial communities associated with ALI, providing a basis for diagnosis [140]. A large multicenter study based on 16 S rRNA sequencing and RNA sequencing found COPD severity associated with decreased airway microbiota abundance linked to Prevotella and increased Moraxella catarrhalis abundance, aligning with downregulation of genes promoting epithelial defense and upregulation of pro-inflammatory genes associated with inhaled corticosteroid use [141]. Therefore, microbiomics technology development can also be a powerful tool for assessing lung disease therapeutic strategies.

Biomarker development can aid ALI early diagnosis and prognostic assessment [142]. For example, bioinformatics and machine learning analysis of two GEO datasets (GSE2411 and GSE18341) identified 690 differentially expressed genes between ARDS/ALI and healthy populations, which could serve as novel molecular biomarkers for ALI to explore new diagnostics and prognostics [143]. BALF is crucial for assessing the lung microbiome. Methods like metagenomics show promise for ALI diagnosis and treatment by analyzing the BALF microbiome to determine lung infection status and ALI severity. One study used metagenomics to analyze BALF from septic ALI patients, finding significant alterations in the lung microbiome of immunocompromised hosts compared to controls, with increased proportions of commensal and opportunistic pathogens [140]. Thus, microbiome-based diagnostic methods provide novel perspectives for early ALI identification and management. Future efforts require establishing uniform sample collection and processing protocols to ensure cross-study comparability. Based on microbiomics, the relationship between microbiome changes and ALI progression should be investigated to identify potential early warning signs. Translating research findings into clinical applications could improve ALI early diagnosis rates and treatment efficacy.

Therapeutic potential of gut microbiota interventions

The roles and mechanisms of the gut microbiota and its metabolites in ALI have received increasing attention. As understanding of the gut-lung axis deepens, researchers are exploring potential therapeutic approaches for ALI by intervening in the gut microbiota.

Probiotics and prebiotics

Probiotics and prebiotics are important tools for regulating the gut flora. Probiotics are an effective way to modulate the gut microbiota, and specific probiotics (e.g., Lactobacillus and Bifidobacterium) have been shown to improve the composition of the intestinal flora, enhance the intestinal barrier function, and reduce systemic inflammation levels. The protective effects of probiotics and prebiotics on gut barrier function and the mucosal immune system have been associated with up-regulation of TJ proteins, induction of mucin 2 (MUC2), AMPs and SCFAs, and reestablishment of the gut microenvironment [9, 144, 145]. These changes may improve ALI prognosis. Studies show probiotics improve sepsis-induced ALI. Specifically, the probiotic Lactobacillus rhamnosus GG attenuated PM2.5-induced lung injury by regulating gut microorganism composition, enhancing intestinal barrier function, and regulating Th17/Treg balance to inhibit inflammation [138]. The probiotic L. reuteri reduces lung inflammation and mortality in ARDS [146]. Intranasal administration of Lactobacillus johnsonii modulates the gut microbiota and protects against hyperoxia-induced lung injury [147]. Saccharomyces boulardii CNCM I-745 inhibits lung iron deposition and attenuates ALI after CPB in rats by upregulating the ratio and maturation of intestinal conventional dendritic cells (DCs) [148]. Lactobacillus reuteri, Saccharomyces boulardii, and Bifidobacterium inhibit gut dysbiosis and reduce acute pancreatitis-induced lung injury incidence [148, 149]. Akkermansia muciniphila (A. muciniphila) abundance was significantly negatively correlated with TNF-α, IL-1β, and IL-6 in a model of sepsis-induced severe ALI, suggesting its important role in ALI [150]. Additionally, probiotics inhibit harmful microorganism overgrowth, helping restore gut microecological balance and reduce lung inflammation [147]. The potential therapeutic effects of probiotics on ALI, via modulating gut microbiota and attenuating inflammation, have been demonstrated in several clinical settings [151, 152]. Future studies should further explore the efficacy of different probiotic types and combinations, as well as optimal intervention timing and dosage.

Prebiotics, such as inulin and oligosaccharides, promote beneficial bacterial growth and improve the intestinal microecological environment, subsequently affecting lung health [153]. Most current studies utilize antibiotic-treated or germ-free mouse models or specific probiotics (e.g., Lactobacillus, Bifidobacterium breve) as therapeutics. While effective in animal models, probiotic interventions have shown variable efficacy in clinical patients. This discrepancy may arise because specific probiotics require particular environments for efficacy, and complex microbial community preparations might be more therapeutically effective than single strains. Therefore, the clinical utility of probiotics and prebiotics as effective adjunctive strategies for ALI needs further exploration.

Fecal microbiota transplantation

FMT involves processing and transferring healthy donor feces into a patient’s gastrointestinal tract to re-establish gut microbiota structure and restore host function [154]. As an emerging intervention, FMT offers new ideas for treating various diseases, including irritable bowel syndrome (IBS), IBD, autism, and hepatic encephalopathy, showing potential for improving gut microbiota and health [155]. Recent studies demonstrate promising effects of FMT in modulating immunity and reducing lung inflammation and injury. FMT has been demonstrated in ARDS rat models to reduce lung damage while improving metabolic and immune responses by altering the gut microbiota, including Akkermansia and Lactobacillus [156]. Yin et al. found that in rats with LPS-induced acute pancreatitis, FMT attenuated lung injury by inhibiting the AKT/NF-κB signaling pathway [157]. FMT in human umbilical cord mesenchymal stem cell-treated mice alleviated ALI by inhibiting ileal and lung inflammation and remodeling intestinal flora through the TLR4/NF-κB and Nrf2/HO-1 pathways [158]. In the rat ALI model, FMT modulated the gut microbiota, suppressed immune inflammation, reduced inflammatory cytokines, and attenuated LPS-induced lung injury by modulating the transforming growth factor-β1 (TGF-β1)/Smad/extracellular signal-regulated kinase (ERK) pathway [159, 160].

FMT antagonized LPS-induced ALI by restoring gut microbiota composition, improving diversity, and increasing SCFA-producing beneficial bacteria. FMT’s protective effect against pathological lung injury extends beyond microbiota restoration. FMT counters LPS-induced lung injury by restoring the gut flora and increasing SCFA-producing bacteria. This protection may relate to inhibition of TLR4/NF-κB signaling pathway activation, attenuating inflammatory and oxidative stress factor release [72]. Combining FMT with antibiotics significantly alleviated Klebsiella pneumoniae pneumonia in mice, restored gut flora structure, increased beneficial metabolite (fatty acids, secondary BAs) levels, repaired alveolar epithelial barrier function, and attenuated systemic inflammation and lung pathological damage [161]. Treating antibiotic-induced dysbiosis and lung immune cell suppression with FMT attenuated lung infection in Pseudomonas aeruginosa (PAO1 strain) mice [29]. Thus, FMT effectively restores dysregulated intestinal microecology, improves systemic and local immune responses, and consequently benefits lung health. Although FMT shows potential therapeutic effects on ALI in animal studies, identifying beneficial strains, detrimental strains, and antibiotic-resistant strains remains a major challenge hindering FMT clinical translation. More evidence is needed to evaluate FMT safety and efficacy in clinical applications.

Dietary interventions

Diet directly impacts gut microbiota composition and function. Fiber-rich diets promote SCFA production, accompanied by changes in gut and respiratory tract microbial composition [162]. These metabolites exhibit anti-inflammatory properties by inducing regulatory T-cell and prostaglandin E2 production or modulating dendritic cell function, contributing to reduced lung inflammation [163, 164]. A study following 120,000 individuals for approximately 12–16 years showed that a high-fiber diet reduced respiratory disease mortality [165]. High-fiber diet and acetate supplementation increased Bacteroides acidifaciens abundance, reduced gut dysbiosis, exerted anti-inflammatory effects by decreasing free radical production and inhibiting NF-κB signaling pathway activation, and decreased inflammation in hyperoxia-induced ALI [166]. Compared to a no-fiber diet, mice on a high-fiber diet exhibited reduced lung immunopathology, increased propionate levels, enrichment of specific fecal microbiota, and thus a lung-protective effect [91]. Early gut microbiota composition is determined by factors including delivery mode, feeding practices, antibiotic exposure, and environment. For example, the gut microbiota of breastfed infants (higher Bifidobacteria, Lactobacilli, Staphylococci) differs significantly from formula-fed infants (higher Bacteroides, Clostridia) [167]. The gut microbiota of infants on a solid food diet is largely dependent on dietary intake [167]. Early microbiota composition is multifactorial; environmental changes, including birth and feeding practices, affect offspring gut microbiota and correlate with disease development [168]. During pregnancy and lactation, a maternal low-fiber diet (LFD) predisposes offspring to respiratory infections by remodeling the microbiota and regulating plasmacytoid dendritic cell development and function [50]. These studies suggest dietary interventions are closely associated with gut microbiota changes and the prevention/treatment of respiratory diseases, particularly ALI.

Summary and perspective

In conclusion, the gut-lung axis facilitates bidirectional communication between the gut and lungs through immune, neural, and hormonal pathways, influencing ALI onset and pathological changes. This review summarizes the potential pathogenic mechanisms by which the gut microbiota and its metabolites impact ALI. Intervention targeting the gut microbiota may represent a novel therapeutic strategy for ALI. Concurrently, microbiota metabolites, including SCFAs, amino acids, and bile acids, influence ALI development through multiple mechanisms. Microbiome profiling-based diagnostic methods effectively identify and assess ALI pathological states. The application of probiotics/prebiotics, fecal transplants, or dietary interventions can remodel the gut microbiota, improve gut barrier function and immune responses, and reduce lung inflammation as potential ALI/ARDS treatments. However, this review also highlights significant knowledge gaps and contradictory findings—such as the opposing effects of antibiotic-induced dysbiosis on ALI outcomes in different models—that must be resolved. The field must move beyond correlative associations in human studies and establish causal mechanisms through integrated multi-omics and carefully designed interventional trials.

Although accumulating preclinical studies suggest that targeting the gut microbiota or its metabolites may ameliorate experimental ALI, the translational applicability of these strategies remains uncertain. Several challenges need to be considered, including strain-specific effects of probiotics, substantial inter-individual variability in FMT, and safety concerns in critically ill patients with ARDS. Moreover, clinical evidence is currently limited to small-scale observational studies or pilot trials, with inconsistent outcomes reported across different interventions. These limitations underscore the need for rigorously designed, adequately powered randomized controlled trials before microbiota-targeted therapies can be considered for clinical application.

The role of gut microbiota and their metabolites in ALI/ARDS pathogenesis is an emerging field with significant therapeutic potential. Future studies should focus on elucidating the mechanisms underlying gut-lung axis interactions, including specific microbial products and signaling pathways involved. Additionally, clinical trials are needed to evaluate the efficacy and safety of targeting the gut microbiota in ALI/ARDS management. Potential therapeutic strategies include using probiotics, prebiotics, and FMT to restore gut microbiota balance and ameliorate lung inflammation and injury. Moreover, developing novel diagnostic tools by integrating genomics, metabolomics, microbiomics, and other multi-omics approaches to assess gut microbiota composition and function in ALI patients will help identify new biomarkers and therapeutic targets. These tools could identify patients at risk for ALI/ARDS and guide therapeutic interventions to restore gut microbiota balance and prevent lung injury.

Abbreviations

AhR

Aryl hydrocarbon receptor

AI-2

Autoinducer-2

ALI

Acute lung injury

AMPK

AMP-activated protein kinase

AMPs

Antimicrobial peptides

ARDS

Acute respiratory distress syndrome

ASCs

Antibody-secreting cells

AT2

Alveolar type II

BALF

Bronchoalveolar lavage fluid

BAs

Bile acids

BCAAs

Branched-chain amino acids

CA

Cholic acid

CCL20

C-C motif chemokine ligand 20

CCR6

C-C motif chemokine receptor 6

CDCA

Chenodeoxycholic acid

COPD

Chronic obstructive pulmonary disease

COVID-19

Coronavirus disease 2019

CPB

Cardiopulmonary bypass

DAO

Diamine oxidase

DCA

Deoxycholic acid

DCs

Dendritic cells

EP

Ethanol precipitate fraction

ERK

Extracellular signal-regulated kinase

EVs

Extracellular vesicles

FFAR

Free fatty acid receptors

FMT

Fecal microbiota transplantation

FXR

Farnesoid X receptor

G/T ratio

Glycine-to-taurine conjugation ratio

GALT

Gut-associated lymphoid tissue

GPR

G protein-coupled receptor

HFD

High-fat diet

HMGB1

High-mobility group box 1 protein

HO-1

Heme oxygenase-1

I.p.

Intraperitoneal

I/R

Ischemia-reperfusion

IAV

Influenza A virus

IBD

Inflammatory bowel disease

IBS

Irritable bowel syndrome

ICU

Intensive care unit

IECs

Intestinal epithelial cells

IL-17A

Interleukin-17 A

ILA

Indole-3-lactic acid

ILCs

Innate lymphoid cells

JAM

Junctional adhesion molecule

LBP

Lipopolysaccharide-binding peptide

LCA

Lithocholic acid

LFD

Low-fiber diet

LPS

Lipopolysaccharide

MAPK

Mitogen-activated protein kinase

MDSC

Myeloid-derived suppressor cell

MIS

Mucosal immune system

MUC2

Mucin 2

MyD88

Myeloid differentiation primary response 88

NET

Neutrophil extracellular trap

NF-κB

Nuclear factor-κB

NLRP3

NOD-like receptor family pyrin domain-containing protein 3

NO

Nitric oxide

Nrf2

Nuclear factor erythroid 2-related factor 2

OTUs

Operational taxonomic units

PEI

Polyethylenimine

PM

Particulate matter

ROS

Reactive oxygen species

SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2

SCFAs

Short-chain fatty acids

SEB

Staphylococcal enterotoxin B

SIgA

Secretory IgA

SP-D

Surfactant-associated protein D

STING

Stimulator of interferon genes

SUCNR1

Succinate receptor

TCA

Tricarboxylic acid

TCDCA

Taurochenodeoxycholic acid

Th17

T helper 17

TLR

Toll-like receptor

TMA

Trimethylamine

TMAO

Trimethylamine N-oxide

Treg

regulatory T

UDCA

Ursodeoxycholic acid

YAP

Yes-associated protein

ZO-1

Zonula occludens-1

Authors’ contributions

Shuyuan Yi wrote original draft, drew the figures and investigated the articles. Xiaoli Zhuang and Lan Luo, Supervision, reviewed and edited draft. Lin Fu and Ziyuan Dong Drew the figures and tables. Yu Jiang and Kan Wang provided the comments and additional scientific information. Xiaofang Yang and Feilong Hei reviewed and revised the text. All authors read and approved the final manuscript and approved the submitted version.

Funding

This work was supported by the Chinese Institutes for Medical Research, Beijing (Grant CX24PY21).

Data availability

Not applicable.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.